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Benthic macrofauna are important and widely used biological indicators of marine ecosystems as they have limited mobility and therefore integrate the effects of local environmental stressors over time. Recently, environmental DNA (eDNA) analysis has provided a potentially more resource‐efficient approach for benthic biomonitoring than traditional morphology‐based methods. Several studies have compared eDNA with morphology‐based monitoring, but few have compared the two approaches using the exact same sediment cores. In addition, the meiofauna and pelagic organisms obtained as 'bycatch' using eDNA have largely been disregarded from comparisons. Here, we address these shortcomings through comparative invertebrate analyses of six sediment sample replicates from each of four stations in Denmark, using eDNA metabarcoding and morphological identification. Our results revealed large variation between the six replicates for both methods and little overlap in taxon compositions between methods. While the morphological dataset was dominated by molluscs and annelids, the eDNA dataset was dominated by arthropods and annelids. Using community composition data, we found that sampling stations could be distinguished both with eDNA and morphology. Finally, we inferred expected total richness from extrapolated accumulation curves of detected taxa from each method. This indicated that eDNA metabarcoding requires less replication than morphology for maximum coverage of diversity to be reached. However, both methods required high levels of replication, and our results on taxonomic composition add to the evidence that morphological and eDNA‐based methods should preferably be used as complimentary tools for marine bioassessment. [ABSTRACT FROM AUTHOR]

• Transport by surface currents of a toxic cyanobacteria (Nodularia spumigena) bloom, from the Baltic Sea, through the Öresund strait, into the Kattegat. • Satellite images confirms a dense bloom of Nodularia spumigena in the central and southern Baltic Sea during the study period, and surface current measurements in the Öresund strait supports the assumption of a transport of the bloom in the more saline waters. • Measurements of plankton at different depths in the water column in the northern part of the Öresund strait (on the border to the Kattegat area) shows that the concentration of Nodularia spumigena cells and the toxin nodularin first increase and then decrease during a one month period in connection to the bloom in the Baltic Sea. • Both Nodularia spumigena and nodularin is tracked well below the strong halocline, both in water samples and in sedimentation traps. • Nodularin is found both in pelagic (copepods, herring) and benthic (blue mussels, flounder) parts of the ecosystem. • A laboratory study shows a decreased buoyancy, and even sinking of Nodularia filaments when they are exposed to a gradually increasing salinity, simulating the transport of Nodularia, from the Baltic Sea into the Kattegat. • The manuscript conclude that Nodularia spumigena blooms and the toxin nodularin is sinking to the bottom, reaching the benthic food web, to a higher extent in more saline waters, than being the case within the brackish Baltic Sea, where the major part of the blooms are believed to decay in the surface waters. Nodularia spumigena is a toxic cyanobacteria that blooms in the Baltic Sea every year. In the brackish water of the Baltic Sea, its toxin, nodularin, mainly affects the biota in the surface water due to the natural buoyancy of this species. However, the fate of the toxin is unknown, once the cyanobacteria bloom enters the more saline waters of the Kattegat. In order to investigate this knowledge gap, a bloom of N. spumigena was followed during its passage, carried by surface currents, from the Baltic Sea into the Kattegat area, through the Öresund strait. N. spumigena cells showed an increased cell concentration through the water column during the passage of the bloom (up to 130 103 cells ml−1), and cells (4.2 103 cells ml−1) could be found down to 20 m depth, below a pycnocline. Sedimentation trap samples from below the pycnocline (10–12 m depth) also showed an increased sedimentation of N. spumigena filaments during the passage of the bloom. The toxin nodularin was detected both in water samples (0.3–6.0 μg l−1), samples of sedimenting material (a toxin accumulation rate of 20 μg m-2 day−1), zooplankton (up to 0.1 ng ind.−1 in copepods), blue mussels (70–230 μg kg−1 DW), pelagic and benthic fish (herring (1.0–3.4 μg kg−1 DW in herring muscle or liver) and flounder (1.3-6.2 μg kg−1 DW in muscle, and 11.7-26.3 μg kg−1 DW in liver). A laboratory experiment showed that N. spumigena filaments developed a decreased buoyancy at increased salinities and that they were even sinking with a rate of up to 1,7 m day−1 at the highest salinity (32 PSU). This has implications for the fate of brackish water cyanobacterial blooms, when these reach more saline waters. It can be speculated that a significant part of the blooms content of nodularin will reach benthic organisms in this situation, compared to blooms decaying in brackish water, where most of the bloom is considered to be decomposed in the surface waters. [ABSTRACT FROM AUTHOR]

Ocean acidification can impair an animal's physiological performance and energetically demanding activities such as swimming. Behavioural abnormalities and changed activity in response to ocean acidification are reported in fish and crustacean species. We studied swimming activity in the calanoid copepod Pseudocalanus acuspes in response to near-future ocean acidification. Water and copepods were sampled from ten mesocosms deployed on the Swedish west coast. The experiments were conducted on animals reared in the mesocosms for 2 months during spring. Copepods were filmed after long-term (chronic) high-CO, and after 20 h acute exposure to CO. There was no significant effect of CO on copepods in chronic high-CO, nor significant effect after the 20 h acute exposure. In addition, we measured prosome length from a large number of adult copepods, but no effect of acidification on body size was found. In this study, P. acuspes did not show sensitivity to near-future pCO levels. Even if a number of papers suggest that copepods seem robust to future ocean acidification, interaction between multiple stress factors, such as elevated temperature, hypoxia and salinity changes may impair a copepod's ability to resist lowered pH. [ABSTRACT FROM AUTHOR]

Recently, there have been reports of increased abundance and landings of European hake in the northern part of the species range. Biological studies are however scarce and information about finer scale population structure important for stock assessments and fishery management is largely lacking. Here, we report on a population genetic study using neutral and outlier SNP loci assessing population structure in hake in the north-eastern parts of its range in the Atlantic. Hake samples from localities along the west coast of Norway, the Kattegat, the northern North Sea, and one locality in the Bay of Biscay were analysed using 53 SNPs, six of which were outliers potentially influenced by natural selection. We detected small-scale structure among northern samples, all of which were also distinct from Bay of Biscay hake, with the exception of a few individuals from the North Sea and the coast of Norway who clustered genetically together with Bay of Biscay hake. Our findings suggest that the present management unit of a single northern stock of hake is not biologically correct, and that there is more detail in the fine-scale population structure indicating that independent population dynamics could be expected in response to fishing patterns or changing environmental conditions. [ABSTRACT FROM AUTHOR]

The aim of the study was to investigate the relative importance of bottom-up and top-down forcing on trophic cascades in the pelagic food-web and the implications for water quality indicators (summer phytoplankton biomass and winter nutrients) in relation to management. The 3D ecological model ERGOM was validated and applied in a local set-up of the Kattegat, Denmark, using the off-line Flexsem framework. The model scenarios were conducted by changing the forcing by ± 20% of nutrient inputs (bottom-up) and mesozooplankton mortality (top-down), and both types of forcing combined. The model results showed that cascading effects operated differently depending on the forcing type. In the single-forcing bottom-up scenarios, the cascade directions were in the same direction as the forcing. For scenarios involving top-down, there was a skipped-level-transmission in the trophic responses that was either attenuated or amplified at different trophic levels. On a seasonal scale, bottom-up forcing showed strongest response during winter-spring for DIN and Chl a concentrations, whereas top-down forcing had the highest cascade strength during summer for Chl a concentrations and microzooplankton biomass. On annual basis, the system was more bottom-up than top-down controlled. Microzooplankton was found to play an important role in the pelagic food web as mediator of nutrient and energy fluxes. This study demonstrated that the best scenario for improved water quality was a combined reduction in nutrient input and mesozooplankton mortality calling for the need of an integrated management of marine areas exploited by human activities. [ABSTRACT FROM AUTHOR]

A multi-disciplinary study was conducted to clarify stock identity and connectivity patterns in the populations of European plaice ( Pleuronectes platessa ) in the Skagerrak-Kattegat transition area between the Eastern North Sea and the Baltic Sea. Five independent biological studies were carried out in parallel. Genetic markers suggested the existence of different genetic populations in the transition area. Growth backcalculation with otoliths resulted in significant although limited differences in growth rates between North Sea and Skagerrak, indicating weak differentiation or important mixing. Hydrogeographical drift modelling suggested that some North Sea juveniles could settle along the coast line of the Skagerrak and the Kattegat. Tagging data suggested that both juveniles and adult fish from the North Sea perform feeding migrations into Skagerrak in summer/autumn. Finally, survey data suggested that Skagerrak also belongs to the area distribution of North Sea plaice. The outcomes of the individual studies were then combined into an overall synthesis. The existence of some resident components was evidenced, but it was also demonstrated that North Sea plaice migrate for feeding into Skagerrak and might constitute a large share of the catches in this area. The mixing of different populations within a management area has implications for stock assessment and management. Choice must be made to either lump or split the populations, and the feasibility and constraints of both options are discussed. The outcomes of this work have directly influenced the management decisions in 2015. [ABSTRACT FROM AUTHOR]

Bottom-trawl fisheries are widespread and cause mortality of benthic invertebrates, which in turn may lead to a decrease in the availability of prey for target fish species. Exploitation also reduces the abundance of the fish species themselves. Modelling studies have shown that bottom trawling could lead to both increases and decreases in fish production, but so far empirical evidence to test these ideas has been very limited. We hypothesize that the effect of bottom trawling on the food intake and condition of fish depends on how the ratio of prey to consumers changes with increasing fishing pressure., We assessed the impact of bottom trawling on the food availability, condition and stomach contents of three flatfishes and the Norway lobster in an area in the Kattegat that is characterized by a steep commercial bottom-trawling gradient due to the establishment of an area closed to all fisheries, but otherwise has homogeneous environmental conditions., For plaice, prey biomass initially decreased at a slower rate with trawling than the biomass of fish, and as a result, the amount of food available per plaice increased before decreasing at trawling frequencies >5 times year−1. This pattern was mirrored in both the condition and stomach contents of plaice and for long-rough dab., No effect of trawling on dab prey and condition was found. Conversely, the condition of the main target species - Norway lobster - increased as its biomass decreased with increased trawling intensities., Together, these results support the idea that when the abundance of the prey declines in response to exploitation, the ratio of the prey to consumer biomass will determine whether exploitation will result in an increase or a decrease in the food intake and condition of the predator., Synthesis and applications. Our study indicates that fish production may be maximized by keeping bottom-trawling intensities relatively low, although this may negatively affect the economically more important Nephrops fishery. The effects of bottom trawls may be mitigated by switching to gears, which affect prey availability to a lesser extent, such as pots or creels. [ABSTRACT FROM AUTHOR]

Cod stocks in the North Sea, including the Kattegat and the Skagerrak, have declined dramatically since the 1970s. Occasionally there is a high recruitment of juveniles in Kattegat/Skagerrak, without leading to the rebuilding of adult cod stocks despite reduced fishing mortality. In a biophysical model of egg and larval drift, we examined the potential importance of extant and historical spawning grounds for recruitment of cod in the Kattegat/Skagerrak seas using data of spawning stock biomass from the 1970s and from today's reduced stocks. The results suggest that Kattegat in the 1970s relied on largely locally retained (83%) larvae with little annual variation in recruitment. Kattegat also provided a substantial proportion of larvae recruiting in Swedish Skagerrak (72%). This is in contrast to present conditions where the Kattegat spawning stock has been reduced by 94%, and Kattegat only provides 34% of locally retained larvae and 30% to Swedish Skagerrak. Instead, the protected area in the Öresund and the Belt Sea are expected today to provide most larvae recruiting in Kattegat. Also, the inflow of larvae from the North Sea to Skagerrak and Kattegat can be significant although highly variable between years, with a positive correlation to the North-Atlantic Oscillation index ( NAO). The rebuilding of healthy spawning areas in the Kattegat may be key for restoring local cod stocks in both Kattegat and along the Skagerrak coast. This poses a management challenge if cod with local 'Kattegat' adaptations, e.g., in terms of egg density and migration patterns, are lost or reduced to non-resilient densities. [ABSTRACT FROM AUTHOR]

Research into the influence of environmental variables on the behaviour of Norway lobster ( Nephrops norvegicus), and hence catch rates, dates back to the 1960s (e.g., Höglund and Dybern, Diurnal and seasonal variations in the catch-composition of Nephrops norvegicus (L.) at the Swedish west coast. ICES CM 1965/I46; Simpson, Variations in the catches of Nephrops norvegicus at different times of day and night. Rapport et Proés-verbaux des Réunions Conseil permanent international pour 1'Exploration de la Mer 156:186). However, the use of fishery-dependent data in identifying influential factors is relatively limited and only includes a number of papers on a limited dataset (e.g., Redant and De Clark, Diurnal variations in CPUE and length composition of the catches in a Nephrops directed fishery in the Central North Sea. ICES CM 1984/K:3; Maynou and Sardà, Influence of environmental factors on commercial trawl catches of Nephrops norvegicus (L.). ICES J. Mar. Sci. 58:1318). Here, we aimed to dissociate environmental variability in Norway lobster catches to improve resource exploitation efficiency within the Skagerrak and Kattegat trawl fisheries by utilising data collected as part of an extensive at-sea-sampling programme spanning 16 years. Catch rates were modelled using Generalized Additive Mixed Models ( GAMMs) and considered a range of response variables, including depth, temperature, current speed, season, moon phase and time of day. The results obtained herein showed that time of day, season, depth, temperature, year, trawl type and location all significantly affect catch rates of Nephrops. [ABSTRACT FROM AUTHOR]

The dispersal and retention of early life stages are essential factors for the recruitment of fish stocks, and often depend on local meteorological and hydrographical conditions. A BAltic sea Long-Term large Scale Eutrophication Model (BALTSEM) was used to examine interannual variation in the retention and dispersal of fish eggs, Atlantic cod (Gadus morhua) at spawning areas in the Kattegat and Öresund and to investigate the mechanisms responsible for this variation. Model simulations over period 1971–2006 showed that years with high retention were related to variable weak winds and a strong outflow of water from the Baltic Sea, while years with low retention were related to strong westerly winds. Low egg retention was associated with enhanced southward transport from the Kattegat towards the Great Belt and from the Öresund towards the Baltic Sea. This southward transport of eggs was highly correlated with the local westerly wind component. The southbound egg transport and the westerly wind component both increased after 1989, and were connected with a change in the large-scale atmospheric circulation pattern, as represented by the NAO index. Results of this study provide information about the connectivity between neighboring areas and suggest that sedimentation is an important mortality factor. [ABSTRACT FROM PUBLISHER]

Past, present, and possible future changes in the Baltic Sea acid-base and oxygen balances were studied using different numerical experiments and a catchment-sea model system in several scenarios including business as usual, medium scenario, and the Baltic Sea Action Plan. New CO partial pressure data provided guidance for improving the marine biogeochemical model. Continuous CO and nutrient measurements with high temporal resolution helped disentangle the biogeochemical processes. These data and modeling indicate that traditional understandings of the nutrient availability-organic matter production relationship do not necessarily apply to the Baltic Sea. Modeling indicates that increased nutrient loads will not inhibit future Baltic Sea acidification; instead, increased mineralization and biological production will amplify the seasonal surface pH cycle. The direction and magnitude of future pH changes are mainly controlled by atmospheric CO concentration. Apart from decreasing pH, we project a decreasing calcium carbonate saturation state and increasing hypoxic area. [ABSTRACT FROM AUTHOR]

Abstract: An essential prerequisite of sustainable fisheries is the match between biologically relevant processes and management action. Various populations may however co-occur on fishing grounds, although they might not belong to the same stock, leading to poor performance of stock assessment and management. Plaice in Kattegat and Skagerrak have traditionally been considered as one stock unit. Current understanding indicates that several plaice components may exist in the transition area between the North Sea and the Baltic Sea. A comprehensive review of all available biological knowledge on plaice in this area is performed, including published and unpublished literature together with the analyses of commercial and survey data and historical tagging data. The results suggest that plaice in Skagerrak is closely associated with plaice in the North Sea, although local populations are present in the area. Plaice in Kattegat, the Belts Sea and the Sound can be considered a stock unit, as is plaice in the Baltic Sea. The analyses revealed great heterogeneity in the dynamics and productivity of the various local components, and suggested for specific action to maintain biodiversity. [Copyright &y& Elsevier]

The consequence of elevated ocean temperatures on commercial fish stocks is addressed using time series of commercial landings (1906-2004) and juvenile survey catch data (1904-2006) collected around Denmark. We analyze (i) whether warm-water sole ( Solea solea) has increased relative to Boreal plaice ( Pleuronectes platessa) and (ii) whether two related warm-water species (turbot, Psetta maxima and brill, Scophthalmus rhombus) show similar responses to increasing temperature or, alternatively, whether turbot (which has a broader juvenile diet) has been favored. Since the early 1980s, both sole and turbot have constituted an increasing part of the commercial landings and survey catches, as compared with plaice and brill, respectively. These changes in species composition were linked to sea surface temperatures, Northern Hemisphere temperature anomalies ( NHA) and the North Atlantic Oscillation. NHA was closely related and explained 43% of the observed variation in sole survey catches relative to the plaice catches and almost 38% of the observed variation in the sole landings relative to the plaice landings. For the less common species, turbot and brill, none of the global change indicators explained more than 15% of the variation, although all showed a positive relationship. Survey catch per unit effort increased significantly for both sole and turbot around the early 1980s, whereas catch per unit effort for plaice and brill remained constant. The results indicate that the abundance of warm-water species is likely to increase with increasing temperature but also that species with similar life histories might react differently according to degree of specialization. [ABSTRACT FROM AUTHOR]

The degree to which marine ecosystems may support the pelagic or benthic food chain has been shown to vary across natural and anthropogenic gradients for e.g., in temperature and nutrient availability. Moreover, such external forcing may not only affect the flux of organic matter but could trigger large and abrupt changes, i.e., trophic cascades and ecological regime shifts, which once having occurred may prove potentially irreversible. In this study, we investigate the state and regulatory pathways of the Kattegat; a eutrophied and heavily exploited marine ecosystem, specifically testing for the occurrence of regime shifts and the relative importance of multiple drivers, e.g., climate change, eutrophication and commercial fishing on ecosystem dynamics and trophic pathways. Using multivariate statistics and nonlinear regression on a comprehensive data set, covering abiotic factors and biotic variables across all trophic levels, we here propose a potential regime shift from pelagic to benthic regulatory pathways; a possible first sign of recovery from eutrophication likely triggered by drastic nutrient reductions (involving both nitrogen and phosphorus), in combination with climate-driven changes in local environmental conditions (e.g., temperature and oxygen concentrations). [ABSTRACT FROM AUTHOR]

Increasing partial pressure of atmospheric CO is causing ocean pH to fall-a process known as 'ocean acidification'. Scenario modeling suggests that ocean acidification in the Baltic Sea may cause a ≤3 times increase in acidity (reduction of 0.2-0.4 pH units) by the year 2100. The responses of most Baltic Sea organisms to ocean acidification are poorly understood. Available data suggest that most species and ecologically important groups in the Baltic Sea food web (phytoplankton, zooplankton, macrozoobenthos, cod and sprat) will be robust to the expected changes in pH. These conclusions come from (mostly) single-species and single-factor studies. Determining the emergent effects of ocean acidification on the ecosystem from such studies is problematic, yet very few studies have used multiple stressors and/or multiple trophic levels. There is an urgent need for more data from Baltic Sea populations, particularly from environmentally diverse regions and from controlled mesocosm experiments. In the absence of such information it is difficult to envision the likely effects of future ocean acidification on Baltic Sea species and ecosystems. [ABSTRACT FROM AUTHOR]

Olsson, J., Bergström, L., and Gårdmark, A. 2012. Abiotic drivers of coastal fish community change during four decades in the Baltic Sea – ICES Journal of Marine Science, 69: 961–970.Evidence for long-term change of marine ecosystems is increasing worldwide. Coastal areas harbour the socio-economically and ecologically most vital aquatic ecosystems, but are under increasing anthropogenic pressure. Little is known, however, about how environmental perturbations affect the development of coastal systems. In this paper, datasets of coastal fish communities covering almost four decades (early/mid 1970s to 2008) in three different basins of the Baltic Sea were analysed. There were clear changes in species composition over time in all but one dataset and coherence among basins in the timing of change. Changes were mainly associated with variables related to climate (water temperature, salinity, and North Atlantic Oscillation index), but less so with those reflecting nutrient status (nutrient concentrations and loading). Despite the importance of local water temperature, regional climatic variables were more important for the temporal development of communities. The results indicate that Baltic coastal fish communities have undergone large structural changes governed by processes acting on both local and regional scales. The findings suggest that ecological targets should be set accounting for long-term changes in community structure and that a common management of coastal and offshore ecosystems would be beneficial. [ABSTRACT FROM PUBLISHER]

In this study, we compared the size selectivity of a T90 codend (netting turned by 90 degrees) with that of a standard codend made of similar netting. Sea trials were conducted in a Norway lobster directed fishery in the Kattegat-Skagerrak area, where there is a need for improved selectivity because of a severe discard problem. The codends were tested by fishing simultaneously with them in a twin trawl rig. Codend covers mounted with Minisamplers were used, which made it possible to catch individuals escaping during towing and haul-back separately. Herein we proposed a model to assess the sequential selection during towing and haul back. This model takes into account the parameter Ctow, which can be interpreted as the proportion of fish that comes into contact with the codend meshes during towing and, thereby, has a chance of escape. Compared to the standard codend, the T90 codend retained fewer Norway lobster both below and above the legal minimum landing size (40 mm, cephalothorax length), thereby causing a reduction of commercial catch. The difference was mainly due to a significantly higher escape rate during towing for the T90 codend. For plaice below minimum landing size (27 cm), the retention was slightly but significantly higher for the T90 codend compared to the standard codend. A model developed for both codends showed that not all plaice are able to attempt escapement during the towing process. For cod, the results indicated an increased L50 (the length at which 50% of this species is caught) for the T90 codend, but the effect was not statistically significant, probably due to the limited number of cod retained during the sea trials. The results demonstrated that, for all three species, a significant proportion did escape during haul-back in both codends. [ABSTRACT FROM AUTHOR]

Possible future changes in Baltic Sea acid-base (pH) and oxygen balances were studied using a catchment-sea coupled model system and numerical experiments based on meteorological and hydrological forcing datasets and scenarios. By using objective statistical methods, climate runs for present climate conditions were examined and evaluated using Baltic Sea modelling. The results indicate that increased nutrient loads will not inhibit future Baltic Sea acidification; instead, the seasonal pH cycle will be amplified by increased biological production and mineralization. All examined scenarios indicate future acidification of the whole Baltic Sea that is insensitive to the chosen global climate model. The main factor controlling the direction and magnitude of future pH changes is atmospheric CO2 concentration (i.e. emissions). Climate change and land-derived changes (e.g. nutrient loads) affect acidification mainly by altering the seasonal cycle and deep-water conditions. Apart from decreasing pH, we also project a decreased saturation state of calcium carbonate, decreased respiration index and increasing hypoxic area - all factors that will threaten the marine ecosystem. We demonstrate that substantial reductions in fossil-fuel burning are needed to minimise the coming pH decrease and that substantial reductions in nutrient loads are needed to reduce the coming increase in hypoxic and anoxic waters. [ABSTRACT FROM AUTHOR]

The aims of the study were to analyse the relations between the physics of a water column and the location of the subsurface chlorophyll maximum (SCM) peaks in a strongly stratified estuary. Could extension and depth location of the SCM be explained by the physical conditions in terms of water column stratification and density interface? Questions were addressed by obtaining data on water column density (CTD), chlorophyll a (Chl a), nutrients, ( F/ F), σ and K(PAR) at 15 positions along a 575 km transect in the Kattegat estuary. Results showed that the estuary was strongly stratified with mixed surface and bottom layers intercepted by a layer where density increased with depth. The SCM occurred only in this density interface, and widths of SCM and density interface were highly correlated. The surface waters were nearly depleted of inorganic nitrogen, phosphate and silicate though with significant higher concentrations in the waters below the interface. The Chl a concentration was comparatively higher in the SCM peak as well as maximum quantum efficiency ( F/ F) and functional cross sectional area (σ). The SCM was maintained at very low light levels and by a diapycnal nitrogen flux, with a stratified water column and nutrient depleted surface waters as predecessors. It was concluded that the depth location and vertical extension of the SCM in the estuary were closely linked to the physical structure of the water column in terms of density interface and stratification. [ABSTRACT FROM AUTHOR]

Compliance and enforcement are important issues from an economic point of view because management measures are useless without a certain level of enforcement. These conclusions come from the well-established theoretical literature on compliance and enforcement problems within fisheries. This paper contributes to the literature by investigating compliance and enforcement in the empirical case of a mixed trawl fishery targeting Norway lobster in Kattegat and Skagerrak located north of Denmark with help from a simulated model. The paper presents results from two simulation models of the case study: one for a single output and a single enforcement tool and one for multiple outputs and multiple enforcement tools. The results of the simulation models are compared to the benchmark case, defined as the enforcement situation in the baseline year. The paper discusses the consequences of policy intervention at the level of compliance and the private and social benefits of varying enforcement intensities, penalties and management measures as well as combinations thereof. The empirical investigation of the case study indicates the current level of control is too low compared to the optimal level of enforcement. Another conclusion from the case study is that only small welfare effects are obtained by increasing enforcement efforts to reduce non-compliance. [ABSTRACT FROM AUTHOR]

Abstract: Microreticulate dinoflagellate cysts are a major component of latest Holocene (Subatlantic) sediments from the Skagerrak and Kattegat, off the west coast of Sweden. They first appear about 4000yearsBP, become more prominent from 2000yearsBP to about 300yearsBP, before they decline to their present minor occurrence in modern sediments. In particular they occur in at least two major pulses and one minor pulse, where their numbers exceed 5000 cysts per gram of sediment, seemingly correlating to times toward the end of the Roman Warm Period, to between the Roman and Medieval warm periods, and with the Little Ice Age. The minor pulse occurs within the Medieval Warm Period. Originally thought to be assignable to the species Gymnodinium catenatum Graham 1943 these cysts were interpreted as an indication of warmer waters, warmer climates and significant changes within the hydrology of the region. Opinion amongst palynologists and phycologists now favour assignment of these cysts to Gymnodinium nolleri Ellegaard and Moestrup 1999, based mainly upon their dimensions; this cautions against earlier environmental interpretations. Further study of microreticulate cysts from core material recovered in the Kattegat and Skagerrak (Gullmar Fjord) supports the identification of these cysts as G. nolleri and emphasises an environmental interpretation that does not solely depend upon possible temperature change but that other factors may be involved. This study also emphasises the importance of these microreticulate cysts to the biostratigraphy of the Subatlantic in offshore west coast Sweden. [Copyright &y& Elsevier]

Madsen, N., and Valentinsson, D. 2010. Use of selective devices in trawls to support recovery of the Kattegat cod stock: a review of experiments and experience. – ICES Journal of Marine Science, 67: 2042–2050.The spawning-stock biomass of cod (Gadus morhua) in the Kattegat area is at a historically low level. Throughout the past decade considerable efforts have been devoted to research on improving both species and size selectivity of the trawls used in the mixed demersal fishery in the area, because this provides a valuable management tool for reducing the bycatch of cod and reducing mortality, and thus helping to rebuild the depleted stock. Gear research in the area has been focused on devices that allow for continued exploitation of the Norway lobster (Nephrops norvegicus) and flatfish, but minimizing the bycatch. We review the results of previous and continuing experiments with various codend mesh sizes, mesh configurations, escape windows, sorting grids, sorting frames, and separator panels, but also changes in whole-trawl designs. Based on our review, we compare and discuss the gear-related technical measures and their effectiveness in maintaining a commercial fishery on viable stocks, yet protecting cod. We discuss the results in relation to changes in legislation and experience with implementation of new selective devices in recent years. We also discuss ways to create stronger incentives for fishers to participate in gear research and to increase acceptance of more selective gears. [ABSTRACT FROM PUBLISHER]

Frandsen, R. P., Madsen, N., and Krag, L. A. 2010. Selectivity and escapement behaviour of five commercial fishery species in standard square- and diamond-mesh codends. – ICES Journal of Marine Science, 67: 1721–1731. The Danish fishery for Nephrops (Nephrops norvegicus) is often conducted in a mixed-species setting, characterized by high rates of discards of several target species, including Nephrops and cod (Gadus morhua). Experiments were conducted to investigate and compare the selective properties of a standard 70 mm square-mesh codend (standard SMC) and a standard 90 mm diamond-mesh codend (standard DMC). Selectivity estimates for five commercial species are provided for both codends. The standard SMC yielded higher estimates of length at 50% retention/mesh size (hereafter SF) for Nephrops and roundfish than did the standard DMC, but no effect of codend type on SF was found for plaice (Pleuronectes platessa). Moreover, a novel codend cover design allowed assessment of the preferred direction of escapement in the codend. Whiting (Merlangius merlangus) and Nephrops showed pronounced, but opposite, vertical preference in the direction of escapement, with whiting escaping upwards and Nephrops downwards. A significant (p < 0.05) difference in the direction of escapement between the two codends was found for haddock (Melanogrammus aeglefinus) and whiting. Owing to the relatively small catches, the outcome is probably most applicable to Nephrops-directed fisheries under similar conditions, and caution should be taken not to extrapolate the results to other fisheries. [ABSTRACT FROM PUBLISHER]

Abstract: We estimated the net annual air–sea exchange of carbon dioxide (CO2) using monitoring data from the East Gotland Sea, Bornholm Sea, and Kattegat for the 1993–2009 period. Wind speed and the sea surface partial pressure of CO2 (pCO2 w), calculated from pH, total alkalinity, temperature, and salinity, were used for the flux calculations. We demonstrate that regions in the central Baltic Sea and the Kattegat alternate between being sinks (−) and sources (+) of CO2 within the −4.2 to +5.2molm−2 yr−1 range. On average, for the 1994–2008 period, the East Gotland Sea was a source of CO2 (1.64molm−2 yr−1), the Bornholm Sea was a source (2.34molm−2 yr−1), and the Kattegat was a sink (−1.16molm−2 yr−1). Large inter-annual and regional variations in the air–sea balance were observed. We used two parameterizations for the gas transfer velocity (k) and the choice varied the air–sea exchange by a factor of two. Inter-annual variations in pCO2 w between summers were controlled by the maximum concentration of phosphate in winter. Inter-annual variations in the CO2 flux and gas transfer velocity were larger between winters than between summers. This indicates that the inter-annual variability in the total flux was controlled by winter conditions. The large differences between the central Baltic Sea and Kattegat were considered to depend partly on the differences in the mixed layer depth. [ABSTRACT FROM AUTHOR]

Abstract: The relative importance of nutrient inputs from streams and the open sea for the water quality is a key issue for an effective management of eutrophication problems in estuaries. In order to resolve this question we have developed a coupled 3D hydrodynamic–ecological model with numerical nitrogen tracking for Horsens estuary, which is a typical Danish estuary that is shallow and well ventilated with water from the adjacent open sea. The nitrogen tracking technique revealed that nitrogen discharges from local streams accounted for the major part (40%–90%) of the nitrogen in NO3, NH4 and detritus whereas only 15% to 50% of the nitrogen in phytoplankton originated from streams. Effects of nitrogen loading reductions were most pronounced for nutrient concentrations and Chl a concentrations whereas the light condition was only slightly improved. The impact of stream nitrogen reductions on nitrate concentrations, Chl a concentrations and light attenuation was respectively 7, 2 and 3 times higher than the impact of open sea nitrogen reductions. Hence, even in estuaries that are well ventilated with water from the adjacent sea significant environmental improvements can be expected after reduction in local nutrient discharges. [Copyright &y& Elsevier]

Abstract: Gear selectivity with regard to cod (Gadus morhua) needs to be improved in the Kattegat and Skagerrak Norway lobster (Nephrops norvegicus) fishery. One way to achieve this goal is to improve the selectivity of an escape window (henceforth window) in the gear. Our gear development focused particularly on moving the window further back, gaining more stability in the codend to avoid loss of Norway lobster through the window, making a relatively narrow section where the window is located, and testing larger mesh sizes in the window. We designed a four-panel sorting section—the sorting box—where a 300mm window is placed at the top section at about 3–6m from the codline. Acoustic release technology was used to avoid catch loss during gear retrieval. Sea trials were conducted in the Skagerrak and Kattegat from a commercial trawler. The sorting box yielded a high reduction of the cod catch, but no improvement was observed for cod that came into contact with the window after reducing the distance from the window to the codline. The sorting box also showed a high reduction of flatfish and other roundfish species. The retention of Norway lobster above minimum landing size in the sorting box was higher compared to a general selection model. [Copyright &y& Elsevier]

Abstract: Patterns in community structure and functioning of motile epibenthic fauna were investigated in shallow (0–1m) sediment habitats along the Skagerrak–Baltic estuarine gradient (salinity range from 4 to 34). The study area was divided into five regions, reflecting different sea-basins along the 1260km coastline, and fauna was collected at six sites within each region. Ten replicate samples of motile epibenthic fauna were taken randomly at each site with a portable drop trap (bottom area 1m2) in June and September in 2004. All together, 110 taxa were found, of which 45 had a marine and 65 a limnic origin. The marine species decreased along the salinity gradient while the limnic showed the opposite pattern. Number of species and abundance of epibenthic fauna exhibited considerable local and regional variation, with a trend of increase with decreasing salinity. Fauna biomass, on the other hand was significantly higher (six times) in the Skagerrak–Kattegat area compared to the Baltic. There was a significant difference in fauna composition among regions and season, but with high similarity within the five regions, which implies that management of such coastal habitats should preferably be based on scales of a region (ca. 100km) or smaller. Predators were the dominant functional group in all coastal regions, with a species shift from Crustacea to Insecta along the salinity gradient and with gobid fish occurring in all regions. Grazers were the second most important group in the Skagerrak–Kattegat area, but planktovores were more important in two of the Baltic regions. The importance of shallow sediment bottoms as feeding and nursery grounds for coastal fish assemblages is discussed and compared throughout the investigated area. [Copyright &y& Elsevier]

Abstract: This paper presents and discusses the wave climate off the Swedish west coast. It is based on 8years (1997–2004) of wave data from 13 sites, nearshore and offshore, in the Skagerrak and Kattegat. The data is a product of the WAM and SWAN wave models calibrated at one site by a wave measurement buoy. It is found that the average energy flux is approximately 5.2kW/m in the offshore Skagerrak, 2.8kW/m in the nearshore Skagerrak, and 2.4kW/m in the Kattegat. One of the studied sites, i.e. site 9, is the location of a wave energy research site run by the Centre for Renewable Electric Energy Conversion at Uppsala University. This site has had a wave power plant installed since the spring of 2006, and another seven are planned to be installed during 2008. Wave energy as a renewable energy source was the driving interest that led to this study and the results are briefly discussed from this perspective. [Copyright &y& Elsevier]

Abstract: This study illuminates a range of technological options relevant to present legislation for regulating fish by-catch in a small-meshed Nephrops fishery. The selection of cod, haddock, hake, lemon sole, Nephrops, plaice, saithe, witch, and whiting were evaluated using the twin-trawl technique for: (i) a 90mm diamond mesh codend (standard codend); (ii) a standard codend with a 120mm square mesh panel (SMP); and (iii) a standard codend with a 35/80mm grid mounted in the extension piece to hinder access to the codend for large individuals. We used selection models to estimate selection parameters by species and confidence bands to compare the selective properties of different gear types. For cod, haddock, hake, Nephrops, plaice, and whiting we obtained estimates for all three gear variants, whereas we obtained estimates for lemon sole and witch only with the standard and the SMP codends and for saithe only for the SMP codend. The SMP significantly (p <0.05) improved selectivity of haddock in terms of releasing more individuals below minimum landing size (MLS). For a narrow size range of plaice below MLS, the SMP retained more individuals than did the standard codend. No effect of the SMP was detected for any of the other species. The grid codend significantly reduced catches of all fish species above MLS, but the codend also had an increased retention of cod and haddock below MLS. Furthermore, the grid resulted in a 17% loss of marketable Nephrops. [Copyright &y& Elsevier]

Abstract: The transition zone between the North Sea and the Baltic Sea is a highly dynamic region where a general estuarine circulation forms a regional scale frontal system from northern Kattegat to the Arkona Sea. This system is characterized by an upper low saline outflowing Baltic water mass from the inflowing saline Skagerrak bottom water to the Kattegat and Belt Sea area. Large and rapid fluctuations of the frontal system are caused by barotropic transports, forced by changing sea level difference between northern Kattegat and the western Baltic Sea, and this results in high variability of the hydrographic conditions and also in frequent in- and outflow events to the Baltic. The dynamics in the region are here analyzed by a regional model of the transition zone, covering the area from the northern Kattegat to the Arkona Sea. The model is validated against water level, temperature and salinity measurements from the region, and the transports through the Danish straits are related to previous estimates and empirical relations. A sensitivity study quantify the role of bathymetry, the tidally induced mixing and the inflowing Skagerrak bottom water for ventilating the bottom water with Skagerrak water or surface water. Furthermore, the dynamics in the region is analyzed with tracers representing the age of the water. The distribution of age tracers with different boundary conditions are analyzed, and the role of advection and mixing for ventilating the bottom water is quantified in terms of the water age. It is shown that the Great Belt area is a very dynamical area where bottom water is ventilated with surface water. The interannual variation of the ventilation of bottom water in the period 2001–2003 is analyzed by various age tracers and related to observed oxygen conditions, and it is shown that the extreme hypoxic event in the autumn 2002 in the southern Kattegat, the Great Belt and in the western Baltic Sea coincide with an unusual low vertical ventilation rate in the Great Belt area, but normal advection rates of bottom water from the northern Kattegat. This indicates that during this particular event, and probably in general, ventilation of bottom water in the Great Belt has significant influence on oxygen conditions in the southern part of the region and for ventilation of bottom waters in the western Baltic Sea. In contrast, the central Kattegat is primarily ventilated by advection of bottom water from the Skagerrak. An age tracer representing the ventilation rate of bottom water with either Skagerrak water or surface water is shown to be inversely correlated to the observed oxygen distribution in the region. [Copyright &y& Elsevier]

Abstract: We determined 15N/14N ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (δ 15N=5‰) and the imprint of river-borne nitrogen input into the SE North Sea (δ 15N up to 12‰ in estuaries of the SE North Sea) on the other hand. We compare the results with δ 15N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of δ 15N in cores from the Skagerrak – the largest sediment sink for nitrogen in the entire North Sea – was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea. [Copyright &y& Elsevier]

Abstract: Discarding of fish species in the Kattegat-Skagerrak Nephrops directed fisheries remains at high levels. In this work we have tested four sets of codends pair-wise with the catch comparison technique under full commercial conditions to assess their potential in reducing the catch of undersized commercial species, in particular cod. We investigate the position of a 90mm square-mesh panel (SMP), the effect of increasing the mesh size from 90 to 120mm in the SMP, the effect of inserting a 90mm SMP in an 80mm codend and the effect of increasing the codend mesh size from 90 to 120mm in a total of 89 hauls. Inserting the SMP 3–6m from the codline compared to 6–9m from the codline, reduced catches of cod above the minimum landing size (MLS) and Nephrops above and below the MLS. Increasing the mesh size in the SMP from 90 to 120mm increased the catch of cod above the MLS in weight with an overall increase of 12% in the revenue. The effect of installing a 90mm SMP 6–9 m from the codline in a nominal 80mm codend had no effect on the catch of cod. Increasing the nominal codend mesh size from 90 to 120mm reduced the catch of all species below the MLS, except monkfish. Catches of Nephrops above the MLS were, however, reduced by approximately one third and the total loss in revenue was 21%. [Copyright &y& Elsevier]

Abstract: This paper presents a summary of all catch-comparison gear tests performed between 2002 and 2006 with Nordmøre-type sorting grids in the Swedish Nephrops trawl fishery in the Skagerrak and Kattegat. The overall aim was to minimise fish by-catch while maintaining catch rates for the target species, i.e. Nephrops. The results showed that a trawl fitted with a grid and a square-mesh cod-end, caught less small and marketable cod, haddock, whiting, hake and plaice than did a standard commercial trawl. For cod larger than 35cm, the catch reduction was close to 100%. For marketable Nephrops no difference in catch amounts was detected. Results of three modified grid variants are also reported here. None of these showed convincing results in terms of better species-selectivity or superior handling characteristics than the standard grid. The results on fish by-catch reduction indicate that the Swedish grid may be a useful tool to reduce non-target mortality and to decouple Nephrops from cod and other demersal fish species under existing and future recovery plans. [Copyright &y& Elsevier]

Abstract: Similar to many other commercial marine fish species, Atlantic cod (Gadus morhua L.) migrate towards specific sites at spawning. These temporal aggregations are generally the most targeted by the commercial fishery. The Kattegat cod has undergone a substantial reduction over the past 25 years and both stock size and spawning stock biomass have remained at very low levels since the end of the 1990s. There is, therefore, an urgent need to map and document spawning grounds still in use. In the present study, spawning sites of Atlantic cod were identified in the Kattegat using a combination of commercial and fishery independent data from 1996 to 2004. Moreover, putative spawning and non-spawning areas were also sampled before and during the reproductive season between 2002 and 2006, and the proportion of mature females together with the individual physiological status were used to validate and strengthen the spatial analyses. The spatial analyses identified several spawning areas in the region and two areas in the southeastern part of the Kattegat which appeared to be the most important. The results showed the presence of cod spawning aggregations, although reduced in size, in areas that have been utilized for more than 25 years according to historical information. Some local spawning grounds may have also disappeared. The proportion of mature females was higher in putative spawning than in non-spawning areas (p <0.001) and females from spawning areas had higher gonadosomatic (p <0.05) and hepatosomatic (p <0.001) indices than those from non-spawning areas. Knowledge of stock spatial and temporal distribution is essential in designing recovery strategies for depleted fish populations. The unambiguous stability of the locations of spawning aggregations over time, as shown in this study, represents a useful aid in order to efficiently implement a recovery plan for the collapsing cod stock in this area. [Copyright &y& Elsevier]

Abstract: Vast amounts of fish bone lie preserved in Denmark''s soil as remains of prehistoric fishing. Fishing was particularly important during the Atlantic period (ca. 7000–3900 bc, i.e. part of the Mesolithic Stone Age). At this time, sea temperature and salinity were higher in waters around Denmark than today. Analyses of more than 100,000 fish bones from various settlements from this period document which fish species were common in coastal Danish waters at this time. This study provides a basis for comparing the fish fauna in the warm Stone Age sea with the tendencies seen and predicted today as a result of rising sea temperatures. One example concerns the anchovy (Engraulis encrasicolus), which lived in the Stone Age sea, and has become more numerous in Danish waters since the mid-1990s. Other warm water fishes represented among the Stone Age bone samples include smoothhound (Mustelus sp.), common stingray (Dasyatis pastinaca), European sea bass (Dicentrarchus labrax), black sea bream (Spondyliosoma cantharus) and swordfish (Xiphias gladius). Surprisingly, Atlantic cod (Gadus morhua), whose biomass in the Kattegat and eastern Baltic Sea is presently at record low levels, was one of the most frequently caught species in the Danish Stone Age sea. These results demonstrate that major changes to the fish fauna near Denmark will occur as climate changes. However, exploitable cod populations can potentially be maintained in waters near Denmark, including the North Sea, but the vulnerability to climate change and the risk of stock collapse will increase at present high fishing mortalities. [Copyright &y& Elsevier]

Abstract: Vast amounts of fish bone lie preserved in Denmark''s soil as remains of prehistoric fishing. Fishing was particularly important during the Atlantic period (ca. 7000–3900 bc, i.e. part of the Mesolithic Stone Age). At this time, sea temperature and salinity were higher in waters around Denmark than today. Analyses of more than 100,000 fish bones from various settlements from this period document which fish species were common in coastal Danish waters at this time. This study provides a basis for comparing the fish fauna in the warm Stone Age sea with the tendencies seen and predicted today as a result of rising sea temperatures. One example concerns the anchovy (Engraulis encrasicolus), which lived in the Stone Age sea, and has become more numerous in Danish waters since the mid-1990s. Other warm water fishes represented among the Stone Age bone samples include smoothhound (Mustelus sp.), common stingray (Dasyatis pastinaca), European sea bass (Dicentrarchus labrax), black sea bream (Spondyliosoma cantharus) and swordfish (Xiphias gladius). Surprisingly, Atlantic cod (Gadus morhua), whose biomass in the Kattegat and eastern Baltic Sea is presently at record low levels, was one of the most frequently caught species in the Danish Stone Age sea. These results demonstrate that major changes to the fish fauna near Denmark will occur as climate changes. However, exploitable cod populations can potentially be maintained in waters near Denmark, including the North Sea, but the vulnerability to climate change and the risk of stock collapse will increase at present high fishing mortalities. [Copyright &y& Elsevier]

Abstract: Plaice (Pleuronectes platessa) has spatially restricted nursery grounds located in shallow soft bottom areas, where the nursery areas only make up a small fraction of the species distribution range. The importance of different coastal areas for recruitment is expected to depend on the quality and size of the nursery grounds. This paper describes the geographical extension of plaice nursery grounds at a regional level along the Swedish west coast. Densities of juvenile plaice were used as the response variable of habitat quality and were compiled based on records from autumn sampling in the various regions. Different aspects of nursery ground quality were related to the density of 0-group plaice, to evaluate the contribution of different factors to the geographical pattern observed. Larval supply was found to be the most important component of nursery quality showing a close relationship with the density of juvenile plaice on a regional scale. The relative contribution of plaice recruits from Swedish nursery grounds to the Skagerrak/Kattegat stock was evaluated using historical data. Swedish nursery grounds were estimated to contribute 77% of the recruits to the adult stock in the area. [Copyright &y& Elsevier]

Abstract: The aim of the study was to estimate yearly macroalgal production in the Kattegat. The estimate was calculated from the abundance and distribution of nine of the most dominant macroalgal species, and from factors important for abundance, distribution and growth (e.g. bottom topography and sediment composition, irradiance, nitrogen concentrations and seawater temperature). The result showed that 6.6% of the Kattegat area is suitable for macroalgal growth. The estimated production was 4–514gCm−2 year−1 depending on depth and sub-area. The total yearly production was estimated to 119×106 kgCy−1. [Copyright &y& Elsevier]

Abstract: Inter-annual variations and long-term trends in phytoplankton primary production (PP) within the Baltic entrance region (the Kattegat and the Belt Sea) are presented and discussed. The study employs the core of Danish monitoring data, with measurements at 6–8 different sites from the past 20–50 years. Temporal development of the annual PP is compared with changes and variations in the land-based nutrient inputs and to other, independent, Swedish and Danish PP data. Spatial and seasonal variations based on annual and monthly PP, respectively, are evaluated. There are large variations on all scales; annual PP ranges from 50 to 500 g C m− 2, with maximum values in some Danish fjords and minimum values in open waters during the 1950s. The Kattegat and the Sound have a lower mean production (135–165 g C m− 2; 1981–2000) than the Great and Little Belts (185–220 g C m− 2). Compared to 1950s and 1960s, the daily PP has changed from being almost constant between March and October, to having two more distinct maxima, one in March and one between July and September. It is obvious that annual mean production has increased considerably since the 1950s, but also that this increase took place before 1980. For data after 1980, we find a co-variation between annual nutrient loads and regional mean PP; years of low total nitrogen (TN) and total phosphorus (TP) input to the region coincide with low PP and vice versa. Waste-water treatment and measures in agriculture have reduced the land-based input of TN by about 1/3 and input of TP by about 2/3 since the 1980s, enough to cause a substantial decrease in the surface water nutrient concentrations. A simultaneous, but weak downward trend in the regional mean PP can be seen up to 1997, after which the trend is broken. Higher production from 1998 is most likely an effect of changes in the method used for determination of PP. [Copyright &y& Elsevier]

Abstract: The study presents high spatial resolution bio-optical, physical, and chemical data obtained inside and outside a sub-surface chlorophyll maxima (SCM). The peak of the SCM was located just above the pycnocline, which separated nutrient rich bottom water from nutrient depleted surface waters. The SCM was dominated (∼90%) by diatoms, in particular, Nitzschia sp. and Cerataulina pelagica and a few phytoflagellates. Chl a concentrations in the SCM reached 7.5mgChl a m−3. The SCM was a genuine maximum in carbon biomass. The 480/665 absorbance ratio in the Chl a samples proved to be a very useful tool regarding the nutritional status of the phytoplankton. Chromomorphic dissolved organic matter (CDOM) showed a clear maximum along with the SCM, which might indicate that in situ organic matter degradation processes were responsible for this relation. The SCM developed because of an inflow of nutrient rich bottom water from the Kattegat, whereby estimated vertical flux was raised from 9.8×10−5 to 2.0×10−4 mMNm−2 s−1. This vertical flux was estimated to sustain a production of 1368mgCm−2 day−1, which equals approximately the spring bloom in Århus Bay. The study indicated that the SCM developed in a balance between nutrient availability and scalar irradiance. Estimations showed that the SCM accounted for 17% of the total primary production during that period in the bay. [Copyright &y& Elsevier]

Abstract: This paper introduces an effective tool for monitoring phytoplankton in open waters. In this study, phytoplankton is the principal component together with salinity and temperature for describing the water dynamics in the Danish marine waters. A major obstacle for accurate monitoring of phytoplankton (chl-a) is the very dynamic nature of phytoplankton and the water masses encompassing the algae. This can be overcome by combining the very coarse temporal resolution data from the traditional monitoring programme to validate hydrodynamic modelling together with remote sensing. DHI Water and Environment''s 3D modelling tool MIKE 3, which is a dynamical ecological model, has been applied to the Danish marine waters covering the Baltic sea through the inner Danish straits and extended to the North Sea. MIKE 3 is a commercially available software system that has been used in a wide variety of applications concerning 3D studies of hydrodynamics in the marine environment. MIKE 3 is an operational system with quick, less expensive and easily accessible assessments of the state and development in phytoplankton with appropriate scaling of resolutions. The paper concludes that a combination of traditional monitoring with ship, dynamical modelling of hydrodynamics and eutrophication in combination with remote sensing of chl-a concentrations of surface waters offers the ideal tools to enhance the temporal and spatial description of large water bodies. The results presented cover the period January to September 1999. The 3D model is verified and calibrated using in situ measurements from various monitoring stations in the Danish marine waters and validated against satellite images from SeaWiFS. The data basis for this study has been two national programmes both related to the monitoring of the coastal environment. [Copyright &y& Elsevier]

Abstract: Based on a large number of observations this paper discusses the baroclinic surface currents in the Kattegat, a sea in the transition area between the brackish Baltic Sea and the saline North Sea. The observations consist of long time series of primarily surface salinity, current direction and speed, measured several times daily (salinity only one time daily) at eight lightvessels once located in the area. Also, observations of wind direction and speed are used. The observations of surface current are grouped in various ways in order to display the dynamical properties of the area. This is done using information about the prevailing wind, the barotropic flow, and/or the characteristics of the water masses. The dynamics in the Kattegat is shown to be much influenced by baroclinic flows, driven primarily by the density gradient across the Kattegat-Skagerrak front, a prominent surface front found to the north, the location of which is believed to be due to wind-driven flows around the headland Skagen. The analyses of the surface currents show evidence of the existence of strong anticyclonic circulation in the upper layer throughout a large part of the Kattegat, in agreement with theoretical considerations involving geostrophic adjustment along the Kattegat-Skagerrak front, propagation of Kelvin waves, and damping due to friction. Also, the analyses show a large degree of spatial and temporal variability, associated with movements of the Kattegat-Skagerrak front and intermittent outflows of low-saline Baltic water. These results shed new light on the dynamics in the Kattegat, which has often been assumed to consist of a basic estuarine circulation modified by the wind. A long-term energy budget for the Kattegat reveals that wind-generated vertical entrainment is a primary driving agent for the anticyclonic circulation and that outflow from the Kattegat-Skagerrak front and frictional damping are the major sinks of energy. The dynamical properties of this area should be taken into account in other studies related to the physical, chemical, or biological conditions, and they can probably explain much of the variability found by previous investigators. For example, it is discussed briefly how the dynamics in the Kattegat may play an important role in relation to dispersal of fish eggs and larvae, with a possible large influence on the recruitment success. [Copyright &y& Elsevier]

Abstract: Review is given about the main results of the oceanographic component of the BALTEX research programme (one of the six continental scale experiments within GEWEX-WCRP to study water and energy cycles in the regional climate system) and related programmes/projects over the last 10 years. Working closely together with two other components – regional meteorology and hydrology of the Baltic Sea drainage basin – oceanographic research has considerably improved the understanding of and ability to model the Baltic Sea marine system. In the Baltic Sea physics seven different broad topics are identified where knowledge has significantly improved. These are reviewed together with a discussion of gaps in knowledge. The focus is on the water and energy cycles of the Baltic Sea, but various aspects of forcing and validation data and modelling are also discussed. The major advances achieved through BALTEX and related programmes are:[•] Meteorological, hydrological, ocean and ice data are now available for the research community. [•] Progress in understanding of the strong impact of large-scale atmospheric circulation on Baltic Sea circulation, water mass exchange, sea ice evolution, and changes in the ocean conditions of the Baltic Sea. [•] Progress in understanding of the importance of strait flows in the exchange of water into and within the Baltic Sea. [•] Progress in understanding of intra-basin processes. [•] Ocean models introduced into Baltic Sea water and energy studies. [•] Development of turbulence models and 3D ocean circulation models for application to the Baltic Sea. [•] Improved Baltic Sea ice modelling and increased understanding of the need for coupled atmosphere–ice–ocean-land models. [Copyright &y& Elsevier]

The study is based on a 1 year record (n=46) of the diffuse attenuation coefficient—Kd(PAR) (m-1)—in combination with CTD-casts at one position in Århus Bay, Kattegat, the North Sea–Baltic Sea estuarine transition. Water samples were collected occasionally for determination of suspended particulate matter (SPM) and chromomorphic dissolved organic matter (CDOM). The Kd(PAR) varied between 0.152 and 0.557 (m-1) with an average of 0.293 (m-1). The depth of the photic zone (1% light level) varied between 8.3 and 15.7 m at maximum and average Kd(PAR). Maximum Kd(PAR) values occurred in periods of high chl-a concentrations. The average CDOM absorption coefficient was 0.232 (m-1) whereas the average chl-a and inorganic SPM concentrations equalled 3.3 (mg m-3) and 4.5 (g m-3). Light attenuation partitioning in percent was: water (9), CDOM (17), SPM (42), and phytoplankton (32) at average conditions. Attenuation by phytoplankton reached up to 74% at high chl-a concentrations whereas SPM light attenuation reached 45% at high SPM concentrations. Results show that light attenuation in the bay is governed by phytoplankton and SPM concentrations whereas CDOM is of less significance. Light absorption in the PAR range—a(PAR)—ranged between 0.262 (m-1) at average conditions and 0.49 (m-1) at maximum phytoplankton concentrations. Scattering was governed by SPM at average conditions whereas phytoplankton scattering was higher than SPM scattering at chl-a concentrations >4.5 (mg m-3). [Copyright &y& Elsevier]

Cruises were carried out in the entire Kattegat in early March of 1998 and 2000 to investigate the spawning of plaice Pleuronectes platessa. The data showed that plaice still spawn in the Kattegat, with a main spawning area in the southern Kattegat and an area of less, but variable importance in the northern coastal Kattegat. The results indicate that spawning in the southern part of Kattegat occurs earlier than in the northern Kattegat. The preferred spawning depth was found to be around 30–40 m. Juvenile recruitment in the nursery areas along the Danish east coast derives mainly from the southern stock in the southern Kattegat with low and variable input of Skagerrak plaice. Maturity ogives for both sexes were obtained and both A50 and L50 were lower than those estimated 10 years previous to this study and for plaice in the North Sea. The shift towards earlier maturity and the absence of the larger age groups in the catches indicate that the Kattegat plaice stock is under severe pressure today. [Copyright &y& Elsevier]

A combined inflow and intrusion event was studied in the Little Belt region in the southwest Kattegat during autumn 1999. A persistent period of outflow from the Baltic Sea towards the North Sea was replaced by a strong southward flow in the opposite direction. This reverse of flow direction was related to a change in dominant wind direction and prevailed for about 14 days as demonstrated by current measurements. The study is based on CTD-casts at seven stations, ADCP current data, water sampling for nutrient analyses ((NH3+NH4), (NO2+NO3), PO4 and SiO2), and phytoplankton species composition. The reversal caused a considerable increase in bottom water salinity reaching up to 10 at several stations. The inflow water reached into a region of strong mixing where surface and bottom water became nearly fully mixed in the Little Belt constriction where high current speeds prevail. An intrusion of mixed water developed when current again turned into a flow out of the Little Belt and the intrusion was clearly recognised as a band of high fluorescence. The increase in fluorescence was related to a strong bloom of the potential toxic diatom Pseudo-nitzschia pseudodelicatissima. Cell concentrations in the intrusion reached up to 3249 cells ml−1. Nutrient concentrations were consistently higher in the inflow bottom water as compared to surface concentrations. The bloom was supposedly fuelled by the mixing of nutrient rich bottom water into the photic zone. The occurrence and frequency of these intrusion events are discussed in relation to general hydrographical conditions in the region, as well as the intrusions contribution to the primary production. A conceptual model for the development of the intrusion is proposed. [Copyright &y& Elsevier]

In aseismic regions the lack of large earthquakes in modern times makes the study of historical earthquakes essential as such events often serve as the yardstick for estimating seismic hazards. Since the largest earthquakes with few exceptions took place hundreds of years ago we may claim that their recurrence times are very large or alternatively their magnitudes are biased upwards. Scandinavia is no exception in this regard and one interesting event in particular is the Kattegat earthquake of 22 December 1759. It is unique in the sense that a macroseismic questionnaire was circulated in Zealand within days of its occurrence. The puzzling feature here is that intensity ranges from VII to I across Zealand but then given I=III for Hamburg and other North German cities. We have chosen two avenues in reassessing the magnitude of the Kattegat earthquake, namely (1) 2D finite-difference (FD) waveform modelling to evaluate possible amplitude amplifications due to sub-surface basin structures and (2) re-assessing the intensity reports of the 1759 Kattegat earthquake in light of the new European Macroseismic Scale (EMS-98).Our 2D FD results show that the presence of the Danish and the North German basins led to shear wave amplitude amplification by factors of 10 and 5, respectively. This is equivalent to a significant increase or overestimation of the area of perception (tied to I=III). The re-assessment of the macroseismic observations in light of the above results and the EMS-98 downgrade the Kattegat earthquake MS magnitude from 5.6 to 5.1. [Copyright &y& Elsevier]

The rates and pathways of anaerobic carbon mineralization processes were investigated at seven stations, ranging from 10 to 56 m water depth, in the Kattegat and Belt Sea, Denmark. Organic carbon mineralization coupled to microbial Mn and Fe reduction was quantified using anaerobic sediment incubation at two stations that were widely separated geographically within the study area. Fe reduction accounted for 75% of the anaerobic carbon oxidation at the station in the northern Kattegat, which is the highest percentage so far reported from subtidal marine sediment. By contrast, sulfate reduction was the dominant anaerobic respiration pathway (95%) at the station in the Great Belt. Dominance of Fe reduction was related to a relatively high sediment Fe content in combination with active reworking of the sediment by infauna. The relative contribution of Fe reduction to anaerobic carbon oxidation at both stations correlated with the concentration of poorly crystalline Fe(III), confirming that the concentration of poorly crystalline Fe(III) exerts a strong control on rates of Fe reduction in marine sediments. The dependence of microbial Fe reduction on concentrations of poorly crystalline Fe(III) was used to quantify the importance of Fe reduction at sites where anaerobic incubations were not applied. This study showed that Fe reduction is an important process in anaerobic carbon oxidation in a wider area of the seafloor in the northern and eastern Kattegat (contribution 60 – 75%). By contrast, Fe reduction is of little significance (6 – 25%) in the more coarse-grained sediments of the shallower western and southern Kattegat, where a low Fe content was an important limiting factor, and in fine-grained sediments of the Belt Sea (4 – 28%), where seasonal oxygen depletion limits the intensity of bioturbation and thereby the availability of Fe(III). A large fraction of the total deposition of organic matter in the Kattegat and Belt Sea occurs in the northern Kattegat, and we estimate 33% of benthic carbon oxidation in the whole area is conveyed by Fe reduction. [ABSTRACT FROM AUTHOR]

Seasonal variations in primary production, phytoplankton biomass, chlorophyll-a, dissolved inorganic phosphorus and nitrogen concentrations in the upper 10 m of the Kattegat were analysed by means of monitoring data from 1993–1997. Spatial optimal analysis, based on a stochastic model, was used to reconstruct weekly constituent fields onto a spatial grid. The reconstructed fields were spatially integrated, resulting in a relatively smooth seasonal variability of the average variables. A simple dynamical model, set up as a periodical boundary problem, is suggested for the average phytoplankton concentration, dissolved inorganic nitrogen and entrainment depth as state variables. The model is forced by the solar radiation, nitrogen load from the land sources and atmosphere as well as by nitrogen supply from the lower nutrient-rich layer. The latter process is modelled proportional to the water entrainment into the upper euphotic layer and is driven by atmospheric forcing, river runoff and the Baltic water inflow. Four model coefficient values were fitted by minimising the root mean square difference between the integrated monitoring data and the model output. The suggested diagnostic model reflects the main features in seasonal variability of phytoplankton and nitrogen concentrations by average values, including the magnitude and timing of such dynamic events as the spring and late summer phytoplankton blooms. The importance of different forcing factors is quantified and estimates of unobserved components such as new primary production can be computed. [Copyright &y& Elsevier]

The dynamics controlling the response of the Baltic Sea to changed atmospheric and hydrologic forcing are reviewed and demonstrated using simple models. The response time for salt is 30 times longer than for heat in the Baltic Sea. In the course of a year, the Baltic Sea renews most of its heat but only about 3% of its salt. On the seasonal scale, surface temperature and ice-coverage are controlled by the atmospheric conditions over the Baltic Sea as demonstrated by e.g. the strong inter-annual variations in winter temperature and ice-coverage due to variations in dominating wind directions causing alternating mild and cold winters. The response of surface temperature and ice-coverage in the Baltic Sea to modest climate change may therefore be predicted using existing statistics. Due to the long response time in combination with complicated dynamics, the response of the salinity of the Baltic Sea cannot be predicted using existing statistics but has to be computed from mechanistic models. Salinity changes primarily through changes in the two major forcing factors: the supply of freshwater and the low-frequency sea level fluctuations in the Kattegat. The sensitivity of Baltic Sea salinity to changed freshwater supply is investigated using a simple mechanistic steady-state model that includes baroclinic geostrophic outflow from the Kattegat, the major dynamical factor controlling the freshwater content in the Kattegat and thereby the salinity of water flowing into the Baltic Sea. The computed sensitivity of Baltic Sea surface salinity to changes of freshwater supply is similar to earlier published estimates from time-dependent dynamical models with higher resolution. According to the model, the Baltic Sea would become fresh at a mean freshwater supply of about 60 000 m3 s−1, i.e. a 300% increase of the contemporary supply. If the freshwater supply in the different basins increased in proportion to the present-day supply, the Bothnian Bay would become fresh already at a freshwater supply of about 37 000 m3 s−1 and the Bothnian Sea at a supply of about 45 000 m3 s−1. The assumption of baroclinic geostrophic outflow from the Kattegat, crucial for the salinity response of the Baltic Sea to changed freshwater supply, is validated using daily salinity profiles for the period 1931–1977 from lightship La¨so¨ Nord. [Copyright &y& Elsevier]