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Methane is produced microbially in vast quantities in sediments throughout the world’s oceans. However, anaerobic oxidation of methane (AOM) provides a near-quantitative sink for the produced methane and is primarily responsible for preventing methane emissions from the oceans to the atmosphere. AOM is a complex microbial process that involves several different microbial groups and metabolic pathways. The role of different electron acceptors in AOM has been studied for decades, yet large uncertainties remain, especially in terms of understanding the processes in natural settings. This study reports whole-core incubation methane oxidation rates along an estuarine gradient ranging from near fresh water to brackish conditions, and investigates the potential role of different electron acceptors in AOM. Microbial community structure involved in different methane processes is also studied in the same estuarine system using high throughput sequencing tools. Methane oxidation in the sediments was active in three distinct depth layers throughout the studied transect, with total oxidation rates increasing seawards. We find extensive evidence of non-sulphate AOM throughout the transect. The highest absolute AOM rates were observed below the sulphate-methane transition zone (SMTZ), strongly implicating the role of alternative electron acceptors (most likely iron and manganese oxides). However, oxidation rates were ultimately limited by methane availability. ANME-2a/b were the most abundant microbial phyla associated with AOM throughout the study sites, followed by ANME-2d in much lower abundances. Similarly to oxidation rates, highest abundances of microbial groups commonly associated with AOM were found well below the SMTZ, further reinforcing the importance of non-sulphate AOM in this system.

Current knowledge on the seasonality of benthic nitrate reduction pathways in the aphotic, density stratified coastal zone of the Baltic Sea is largely based on data from muddy sediments, neglecting the potential contribution of sandy sediments. To gain a more comprehensive understanding of seasonality in this part of the Baltic Sea coast, we measured rates of benthic denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) monthly in the ice-free period of 2016 in both sandy and muddy aphotic sediments, northwestern Gulf of Finland. No anammox was observed. The seasonal cycle of denitrification in both sediment types was related to the hydrography-driven development of bottom water temperature. The seasonal cycle of DNRA was less clear and likely connected to a combination of bottom water temperature, carbon to nitrogen ratio, and substrate competition with denitrification. Denitrification and DNRA rates were 50-80 and 20% lower in the sandy than in the muddy sediment. The share of DNRA in total nitrate reduction, however, was higher in the sandy than in the muddy sediment, being (by similar to 50%) the highest DNRA share in sandy sediments so far measured. Our data add to the small pool of published studies showing significant DNRA in both cold and/or sandy sediments and suggest that DNRA is currently underestimated in the Baltic coastal nitrogen filter. Our results furthermore emphasize that the various environmental conditions of a coastal habitat (light regime, hydrography, and geomorphology) affect biogeochemical element cycling and thus need to be considered in data interpretation.

Coastal environments are nitrogen (N) removal hot spots, which regulate the amount of land-derived N reaching the open sea. However, mixing between freshwater and seawater creates gradients of inorganic N and bioavailable organic matter, which affect N cycling. In this study, we compare nitrate reduction processes between estuary and offshore archipelago environments in the coastal Baltic Sea. Denitrification rates were similar in both environments, despite lower nitrate and carbon concentrations in the offshore archipelago. However, DNRA (dissimilatory nitrate reduction to ammonium) rates were higher at the offshore archipelago stations, with a higher proportion of autochthonous carbon. The production rate and concentrations of the greenhouse gas nitrous oxide (N2O) were higher in the estuary, where nitrate concentrations and allochthonous carbon inputs are higher. These results indicate that the ratio between nitrate and autochthonous organic carbon governs the balance between N-removing denitrification and N-recycling DNRA, as well as the end-product of denitrification. As a result, a significant amount of the N removed in the estuary is released as N2O, while the offshore archipelago areas are characterized by efficient internal recycling of N. Our results challenge the current understanding of the role of these regions as filters of land-to-sea transfer of N.

In late 2014, a large, oxygen-rich salt water inflow entered the Baltic Sea and caused considerable changes in deep water oxygen concentrations. We studied the effects of the inflow on the concentration patterns of two greenhouse gases, methane and nitrous oxide, during the following year (2015) in the water column of the Gotland Basin. In the eastern basin, methane which had previously accumulated in the deep waters was largely removed during the year. Here, volume-weighted mean concentration below 70 m decreased from 108 nM in March to 16.3 nM over a period of 141 days (0.65 nM d−1), predominantly due to oxidation (up to 79 %) following turbulent mixing with the oxygen-rich inflow. In contrast nitrous oxide, which was previously absent from deep waters, accumulated in deep waters due to enhanced nitrification following the inflow. Volume-weighted mean concentration of nitrous oxide below 70 m increased from 11.8 nM in March to 24.4 nM in 141 days (0.09 nM d−1). A transient extreme accumulation of nitrous oxide (877 nM) was observed in the deep waters of the Eastern Gotland Basin towards the end of 2015, when deep waters turned anoxic again, sedimentary denitrification was induced and methane was reintroduced to the bottom waters. The Western Gotland Basin gas biogeochemistry was not affected by the inflow.

The redoxclines that form between the oxic and anoxic water layers in the central Baltic Sea are sites of intensive nitrogen cycling. To gain better understanding of nitrification, we measured the biogeochemical properties along with potential nitrification rates and analyzed the assemblages of ammonia-oxidizing bacteria and archaea using functional gene microarrays. To estimate nitrification in the entire water column, we constructed a regression model for the nitrification rates and applied it to the conditions prevailing in the area in 2008–2012. The highest ammonia oxidation rates were found in a thin layer at the top of the redoxcline and the rates quickly decreased below detection limit when oxygen was exhausted. This is probably because extensive suboxic layers, which are known to harbor pelagic nitrification, are formed only for short periods after inflows in the Baltic Sea. The nitrification rates were some of the highest measured in the water columns, but the thickness of the layer where conditions were favorable for nitrification, was very small and it remained fairly stable between years. However, the depth of the nitrification layer varied substantially between years, particularly in the eastern Gotland Basin (EGB) due to turbulence in the water column. The ammonia oxidizer communities clustered differently between the eastern and western Gotland Basin (WGB) and the composition of ammonia-oxidizing assemblages correlated with the environmental variables. The ammonia oxidizer community composition was more even in the EGB, which may be related to physical instability of the redoxcline that does not allow predominance of a single archetype, whereas in the WGB, where the position of the redoxcline is more constant, the ammonia-oxidizing community was less even. Overall the ammonia-oxidizing communities in the Baltic Sea redoxclines were very evenly distributed compared to other marine environments where microarrays have been applied previously., final draft, peerReviewed

Sediment resuspension is a frequent phenomenon in coastal areas and a key driver for many ecosystem functions. Sediment resuspension is often linked to biological and anthropogenic activities, which in combination with hydrodynamic forcing initiate sediment erosion and resuspension, if the erosion threshold (τc) is exceeded. Despite its importance to ecosystem functions very few studies have provided measurements on natural assemblages for subtidal sediments. The aim of this study was to determinate key environmental variables regulating sediment resuspension potential across a sedimentary gradient in a subtidal coastal environment. In order to explore this, we sampled 16 sites encompassing a wide variety in environmental variables (e.g., grain size distribution, macrofaunal communities, vegetation) in the Gulf of Finland, Baltic Sea. A core-based erosion device (EROMES) was used to determine sediment resuspension potential measures of erosion threshold, erosion rate (ER), and erosion constant (me). Based on abiotic and biotic properties sampled, sediments diverged into two distinct groups; cohesive (“muddy”) and noncohesive (“sandy”) sediments. Results showed that abiotic sediment properties explained 38–53% and 15–36% of the total variation in resuspension potential measures in “muddy” and “sandy” sediments, respectively. In cumulative models, biota accounted for 12–26% and 6–24% to the total variation in “muddy” and “sandy” sediments, respectively. Sediment erodibility and resuspension potential of natural sediments is highly variable from local habitats to a larger seascape scale. Our results underline the importance of biota to resuspension potential measures in spatially variable environments.

Estuaries are important conduits between terrestrial and marine aquatic systems and function as hot spots in the aquatic methane cycle. Eutrophication and climate change may accelerate methane emissions from estuaries, causing positive feedbacks with global warming. Boreal regions will warm rapidly in the coming decades, increasing the need to understand methane cycling in these systems. In this 3-year study, we investigated seasonal and spatial variability of methane dynamics in a eutrophied boreal estuary, both in the water column and underlying sediments. The estuary and the connected archipelago were consistently a source of methane to the atmosphere, although the origin of emitted methane varied with distance offshore. In the estuary, the river was the primary source of atmospheric methane. In contrast, in the adjacent archipelago, sedimentary methanogenesis fueled by eutrophication over previous decades was the main source. Methane emissions to the atmosphere from the study area were highly variable and dependent on local hydrodynamics and environmental conditions. Despite evidence of highly active methanogenesis in the studied sediments, the vast majority of the upwards diffusive flux of methane was removed before it could escape to the atmosphere, indicating that oxidative filters are presently still functioning regardless of previous eutrophication and ongoing climate change.

Determining accurate rates of benthic nitrogen (N) removal and retention pathways from diverse environments is critical to our understanding of process distribution and constructing reliable N budgets and models. The whole-core 15 N isotope pairing technique (IPT) is one of the most widely used methods to determine rates of benthic nitrate-reducing processes and has provided valuable information on processes and factors controlling N removal and retention in aquatic systems. While the whole core IPT has been employed in a range of environments, a number of methodological and environmental factors may lead to the generation of inaccurate data and are important to acknowledge for those applying the method. In this review, we summarize the current state of the whole core IPT and highlight some of the important steps and considerations when employing the technique. We discuss environmental parameters which can pose issues to the application of the IPT and may lead to experimental artifacts, several of which are of particular importance in environments heavily impacted by eutrophication. Finally, we highlight the advances in the use of the whole-core IPT in combination with other methods, discuss new potential areas of consideration and encourage careful and considered use of the whole-core IPT. With the recognition of potential issues and proper use, the whole-core IPT will undoubtedly continue to develop, improve our understanding of benthic N cycling and allow more reliable budgets and predictions to be made.

Estuaries are often seen as natural filters of riverine nitrate, but knowledge of this nitrogen sink in oligotrophic systems is limited. We measured spring and summer dinitrogen production (denitrification, anammox) in muddy and non-permeable sandy sediments of an oligotrophic estuary in the northern Baltic Sea, to estimate its function in mitigating the riverine nitrate load. Both sediment types had similar denitrification rates, and no anammox was detected. In spring at high nitrate loading, denitrification was limited by likely low availability of labile organic carbon. In summer, the average denitrification rate was similar to 138 mu mol N m(-2) d(-1). The corresponding estuarine nitrogen removal for August was similar to 1.2 t, of which similar to 93% was removed by coupled nitrification-denitrification. Particulate matter in the estuary was mainly phytoplankton derived (> 70% in surface waters) and likely based on the riverine nitrate which was not removed by direct denitrification due to water column stratification. Subsequently settling particles served as a link be tween the otherwise uncoupled nitrate in surface waters and benthic nitrogen removal. We suggest that the riverine nitrate brought into the oligotrophic estuary during the spring flood is gradually, and with a time delay, removed by benthic denitrification after being temporarily ` trapped' in phytoplankton particulate matter. The oligotrophic system is not likely to face eutrophication from increasing nitrogen loading due to phosphorus limitation. In response, coupled nitrification-denitrification rates are likely to stay constant, which might increase the future export of nitrate to the open sea and decrease the estuary's function as a nitrogen sink relative to the load.

Estuaries worldwide are known to act as filters of land-derived N loads, yet their variable environmental settings can affect microbial nitrogen (N) retention and removal and thus the coastal filter function. We investigated microbial N-retention (nitrification, ammonium assimilation) and N-removal (denitrification, anammox) in the aphotic benthic systems (here defined as: bottom boundary layer [BBL] and sediment) of two Baltic Sea estuaries that differ in riverine N loads, trophic state, bottom topography, and sediment type. Contrary to our expectations, nitrification rates (5–227 nmol L−1 d−1) in the BBL neither differed between the eutrophied Vistula estuary and the oligotrophic Öre estuary, nor between seasons. Ammonium assimilation rates were slightly higher in the oligotrophic Öre estuary in spring but did not differ between estuaries in summer (9–704 nmol L−1 d−1). In the sediment, no anammox was found in either estuary and denitrification rates were higher in the eutrophied (349 ± 117 µmol N m−2 d−1) than in the oligotrophic estuary (138 ± 47 µmol N m−2 d−1). Irrespective of their differences, in both estuaries the quality of the mainly phytoplankton-derived particulate organic matter (POM) – evaluated by means of C : N and POC : Chl.a ratios – seemed to control N-cycling processes through the availability of particulate organic N and C as substrate sources. Our data suggest, that in stratified estuaries, phytoplankton-derived POM is an essential link between riverine N loads and benthic N cycling and may function as a temporary N reservoir via long particle residence time or coastal parallel transport. Even at low process rates, effective coastal filtering would thus be achieved by the increased time available for the recycling of N via microbial retention processes until its permanent removal via denitrification.

Nitrification is a key process in nitrogen cycling. Several nitrification rate measure- ment techniques rely on the use of chemical inhibitors that effectively and specifically inhibit nitri- fying bacteria. The discovery of ammonia-oxidizing archaea has raised the question of whether these inhibitors are also effective for archaea. The efficiency of 2 commonly used chemicals — allylthiourea (ATU) and nitrapyrin (commercial name N-serve) — in inhibiting nitrification was tested in natural seawater samples containing archaea. In addition, the effect of these inhibitors was tested in a pure culture of Pseudomonas chlororaphis, a bacterium that is used in the denitri- fier method to reduce nitrate to nitrous oxide, whose isotopic composition can be analyzed with isotopic ratio mass spectrometers. This method is commonly used in stable isotope based nitrifica- tion rate measurements. ATU inhibited nitrification by 80% at a 100 μM concentration, whereas N-serve inhibited nitrification completely at a 150 μM concentration. Although the inhibition of nitrification by ATU was not complete for the natural seawater ammonia-oxidizing community

Coastal areas are critical in mitigating the impact of nutrient runoffs and downstream eutrophication of aquatic ecosystems. In the Gulf of Finland, the easternmost sub-basin of the Baltic Sea, seasonal and long-term oxygen depletion at the surface of the sediment feeds back the eutrophication loop by promoting the release of nutrients locked in the sediment matrix. In order to understand how the bacterial community responds to the seasonal variations, we sequenced ribosomal gene fragments from the top sediment layer at two coastal sites in southern Finland in spring, summer and late autumn during two consecutive years. Analysis of the samples collected at a shallow (11 m) and deep site (33 m) revealed that the overall community composition was rather constant over time with an extensive collection of shared operational taxonomic units (OTU) between sites. The dominant taxa were related to organoheterotrophs and sulfate reducers and the variation in community structure was linked to the availability of organic matter in the surface sediment. Proteobacteria formed the most abundant and diverse group. The taxa characteristic of spring samples belonged primarily to Actinobacteria , possibly of fresh water origin and linked to humic carbon. Summer communities were characterized by an increase in the number of reads associated with heterotrophic bacteria such as Bacteroidetes which feed on labile organic matter from spring bloom. Taxa typical of autumn samples were linked to Cyanobacteria and other bloom-forming bacteria from the overlying water and to bacteria feeding on organic matter drifting from the phytal zone.

The effects of hypolimnetic aeration on oxygen penetration depth and oxygen flux into the sediment (DOU) were studied during two successive years. The study was conducted by comparing data from the aerated Enonselka basin with the non-aerated Kajaanselka basin of Lake Vesijarvi (southern Finland). Aeration increased the average O2 concentration in near-bottom layers. This was mainly due to shortened stratification period in the aerated basin, which was again attributed to the positive effect of aeration on hypolimnetic water temperature. Aeration did not prevent hypoxia during summer stratification and oxygen penetration depth in the sediment and DOU did not increase. During summer stratification DOU was even lower at the aerated than at the non-aerated basin. The weak effect of aeration on DOU and oxygen penetration was mostly due to the minor effect on hypolimnetic oxygen concentrations. The positive effect of aeration on hypolimnetic O2 concentration was negated by the positive effects of aeration on hypolimnetic temperature and turbulence that increase oxygen consumption in the water column. Due to the generally low effect on DOU, the present aeration effort in Enonselka does not have positive effects on sediment quality.

The diversity and dynamics of ammonia-oxidizing bacteria (AOB) and archaea (AOA) nitrifying communities in the sediments of the eutrophic Gulf of Finland (GoF) were investigated. Using clone libraries of ammonia monooxygenase (amoA) gene fragments and terminal restriction fragment length polymorphism (TRFLP), we found a low richness of both AOB and AOA. The AOB amoA phylogeny matched that of AOB 16S ribosomal genes from the same samples. AOA communities were characterized by strong spatial variation while AOB communities showed notable temporal patterns. At open sea sites, where transient anoxic conditions prevail, richness of both AOA and AOB was lowest and communities were dominated by organisms with gene signatures unique to the GoF. Given the importance of nitrification as a link between the fixation of nitrogen and its removal from aquatic environments, the low diversity of ammonia-oxidizing microbes across the GoF could be of relevance for ecosystem resilience in the face of rapid global environmental changes. (C) 2015 Elsevier Ltd. All rights reserved.

Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM). However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation of DOM along salinity gradients in an estuary of the northern Baltic Sea efficiently transfers Fe from the dissolved phase into particulate material that accumulates in the sediments. Consequently, we observe a decline with distance offshore in both the Fe content of the sediments and proportion of terrestrial material in the sedimentary organic matter pool. Mössbauer spectroscopy and sequential extractions suggest that large amounts of Fe in sediments of the upper estuarine zone are associated with organic matter as unsulfidized Fe (II) complexes, or present in the form of ferrihydrite, implying a direct transfer of flocculated material to the sediments. Accordingly, the contribution of these components to the total sedimentary Fe declines with distance offshore while other Fe phases become proportionally more important. Sediment core records show that the observed lateral distribution of Fe minerals has remained similar over recent decades, despite variable Fe inputs from anthropogenic sources and eutrophication of the coastal zone. Pore water data suggest that the vertical zonation of diagenetic processes in the sediments is influenced by both the availability of Fe and by bottom water salinity, which controls the availability of sulfate (SO42−).

Chemolithoautotrophic denitrification is an important mechanism of nitrogen loss in the water column of euxinic basins, but its isotope fractionation factor is not known. Sulfurimonas gotlandica GD1(T), a recently isolated bacterial key player in Baltic Sea pelagic redoxcline processes, was used to determine the isotope fractionation of nitrogen and oxygen in nitrate during denitrification. Under anoxic conditions, nitrate reduction was accompanied by nitrogen and oxygen isotope fractionation of 23.8 ± 2.5‰ and 11.7 ± 1.1‰, respectively. The isotope effect for nitrogen was in the range determined for heterotrophic denitrification, with only the absence of stirring resulting in a significant decrease of the fractionation factor. The relative increase in δ(18)ONO3 to δ(15)NNO3 did not follow the 1:1 relationship characteristic of heterotrophic, marine denitrification. Instead, δ(18)ONO3 increased slower than δ(15)NNO3, with a conserved ratio of 0.5:1. This result suggests that the periplasmic nitrate reductase (Nap) of S. gotlandica strain GD1(T) fractionates the N and O in nitrate differently than the membrane-bound nitrate reductase (Nar), which is generally prevalent among heterotrophic denitrifiers and is considered as the dominant driver for the observed isotope fractionation. Hence in the Baltic Sea redoxcline, other, as yet-unidentified factors likely explain the low apparent fractionation.

Seasonally changing mechanisms affect the concentrations of dissolved inorganic nitrogen and soluble reactive phosphorus, which differ between the stands of different macrophyte life forms and open water in a eutrophic lake. Macrophytes that take nutrients up for their growth also shelter sediments from resuspension that brings nutrients back to the water and affect denitrification, which removes nitrogen from the water ecosystem. In this study the changes in nutrient concentrations were observed during the open-water period from April to November, and also denitrification rates were measured at different phases of the open-water season. The study was conducted at a shallow eutrophic lake where the effect of macrophytes on water quality is remarkable. The concentration changes of different nitrogen forms during the summer were very similar at the open-water and floating-leaved macrophyte (Nuphar lutea L.) stations. Nitrate was depleted faster among the submerged macrophytes (Myriophyllum verticillatum L.) than among floating-leaved plants or in open water. The decrease in the concentration of nitrate was so significant during the summer that it also affected the total nitrogen concentration in the water. Denitrification was highest in sediments among floating-leaved macrophytes (average 4.3 mg N m(-2) d(-1)) and lowest in sediments of submerged plants (average 1.5 mg N m(-2) d(-1)). Denitrification among submerged macrophytes was limited by low nitrate availability.

Denitrification in lakes significantly reduces the nitrogen (N) load from land to oceans, but the factors controlling it remain poorly under- stood. Therefore, interlake variation of denitrification in sediments of small to medium-sized lakes (from 0.03 to 17.8 km 2 ) was studied across different geo- graphical scales. At the local scale, the denitrification rates and sediment microbial communities were studied in 4 boreal lakes in close proximity (within 20 km) using the isotope pairing technique (IPT) and molecular methods. These local scale data were com- bined with previously published data on denitrifica- tion rates from 10 other European lakes to extend the analysis to the regional (boreal area) and continental (boreal and temperate areas) scales. Denitrification varied considerably among lakes, ranging from ~50 to ~600, ~0 to ~600 and ~0 to ~12900 µmol N m �2 d �1 at the local, regional and continental scales, respec- tively. This variation was primarily due to nitrate availability. The structure of the denitrifier commu- nity studied at the local scale was independent of the denitrification rates but varied among lakes correlat- ing with nitrate availability and sediment organic content. Removal of nitrate and total N load by deni- trification was less efficient in boreal than in temper- ate lakes. In addition, a meta-analysis of published N mass balance data indicates that the total N retention (denitrification + N sedimentation) is less efficient in boreal than in temperate lakes. Anaerobic ammo- nium oxidation (anammox) was studied at the local scale but was not detected, although (based on molecular markers) several anammox genera were present in the sediments.

Low levels of oxygen (O) in the hypolimnion layer of lakes are harmful to benthic animals and fish; they may also adversely affect nutrient cycles. Artificial aeration is often used in lake management to counteract these problems, but the effects of aeration on nitrogen (N) cycling are not known. We studied the effects of hypolimnetic aeration on N dynamics and temperature in a eutrophic lake by comparing continuous and pulsed aeration with a nonaerated station. Aeration decreased the accumulation of NH-N deep in the lake (20-33 m) by supplying O for nitrification, which in turn provided substrate for denitrification and promoted N removal. Aeration also increased the temperature in the hypolimnion. Denitrification rate was highest in the nonaerated deep areas (average, 7.62 mg N m d) due to very high rates during spring turnover of the water column, demonstrating that natural turnover provides O for nitrification. During stratification, denitrification was highest at the continuously aerated station (4.06 mg N m d) and lowest at the nonaerated station (3.02 mg N m d). At the periodically aerated station, aeration pauses did not restrict the increase in temperature but resulted in accumulation of NH-N and decreased the contribution of denitrification as a nitrate reduction process. Our findings demonstrate that hypolimnetic aeration can substantially affect N cycling in lakes and that the effect depends on the aeration strategy. Because N is one of the main nutrients controlling eutrophication, the effects of aeration methods on N removal should be considered as part of strategies to manage water quality in lakes.

The influence of an addition of either sand and/or spring-bloom algae on the efflux of nutrients from intact sediment cores from the Baltic Sea was studied in a flow-through experiment. The addition of sand significantly increased the efflux of silicon (Si) from sediment, but the algal addition did not. The effects on phosphorus (P) were not as clear, and fluxes of nitrogen (NH4 and NO2 + 3) remained relatively unaffected by the additions. The small effect of the algal addition was caused by the short time-period covered by the experiment and possibly by adsorption of released Si by the sediment. A follow-up bottle experiment showed that despite the apparently lower content of easily available Si and biogenic silica, BSi, in the sand, the sand-induced Si efflux was caused by release of Si from the sand itself, rather than by indirectly increasing the dissolution of BSi present in the sediment.

Nitrification is a crucial process in sediment nitrogen cycling. We compared two (15)N tracer-based nitrification measurement techniques (isotope pairing technique (IPT) combined with (15)N nitrate pool dilution and (15)N ammonium oxidation) and three different (15)N analyses from bottom water nitrate (ammonia diffusion, denitrifier and SPINMAS) in a sediment mesocosm. The (15)N nitrate pool dilution technique combined with IPT can be used to quantify the in situ nitrification, but the minimum detection limit for the total nitrification is higher than that in the (15)N ammonium oxidation technique. The (15)N ammonium oxidation technique, however, is not applicable for sediments that have high ammonium content. If nitrate concentration and the amount of (15)N label in the sample are low, the (15)N nitrate analysis should be done with the denitrifier method. In higher (15)N concentrations, the less sensitive SPINMAS method can also be applied. The ammonia diffusion method is not suitable for bottom water (15)N nitrate analyses.

Primary production in the eutrophic Baltic Sea is limited by nitrogen availability; hence denitrification (natural transformation of nitrate to gaseous N(2)) in the sediments is crucial in mitigating the effects of eutrophication. This study shows that dissimilatory nitrate reduction to ammonium (DNRA) process, where nitrogen is not removed but instead recycled in the system, dominates nitrate reduction in low oxygen conditions (O(2)110 μM), which have been persistent in the central Gulf of Finland during the past decade. The nitrogen removal rates measured in this study show that nitrogen removal has decreased in the Gulf of Finland compared to rates measured in mid-1990s and the decrease is most likely caused by the increased bottom water hypoxia.

Steep redoxclines form between oxic surface water and the stagnant, sulfidic hypolimnion in the eutrophied, brackish water Baltic Sea. Nitrification, denitrification, and anammox were measured at and below the redoxcline to quantify the role of water-column nitrogen processes in the overall magnitude of nitrogen removal in the Baltic Sea. Rates of nitrification were very high (up to 85 nmol N L21 d21) at the oxic–anoxic interface, but, surprisingly, nitrification was separated from the processes reducing nitrate to N2 (up to 810 nmol N L21 d21 )b y tens of meters in depth. N2 production was dominated by chemolithotrophic denitrification, with anammox playing only a negligible role, and limited to the water layers in which nitrite or nitrate coexisted with sulfide. The alternating oxygen concentrations in the basin induce irregular bursts of nitrogen removal. Nitrification takes place after mixing of ammonium-rich deep water with oxic water, and denitrification uses the formed nitrite and nitrate once anoxic conditions re-establish. Although removal rates can be high, conditions allowing such rates are likely short-lived. While the sedimentary denitrification rates at the shallower, oxic areas are lower, they are more constant in time, highlighting the need to avoid hypoxia, which would prevent sedimentary denitrification. The Baltic Sea is a large and shallow brackish inland sea in northern Europe (415,266 km 2 , average depth 52 m) that suffers from heavy anthropogenic nutrient loading, and large cyanobacterial blooms are a regular phenomenon. It has been suggested that internal feedbacks sustain the eutrophication (Vahtera et al. 2007). One of the key factors in this feedback mechanism is the effect that increased hypoxia has on the biogeochemical cycles of nutrients. The mechanisms that lead to phosphorus release from the sediments during anoxia are well known, but the effects of hypoxia on nitrogen cycling are still uncertain (Conley et al. 2009), despite the critical role nitrogen has as one of the main nutrients regulating not only the extent but also the species composition of phytoplankton blooms (Smith 1983). Two naturally occurring microbial processes, denitrification and anammox, have the potential to remove nitrogen from the water via the production of N2 gas (Codispoti et al. 2001; Kuypers et al. 2005). These anaerobic processes reduce oxidized substrates (nitrate and nitrite), originating from an aerobic nitrification process, to harmless dinitrogen gas. Therefore, nitrification and the processes that mediate the production of N2 are

Whether sediments act as sinks or sources of nutrients depends partly on the oxygen conditions at the seafloor. Laboratory experiments on coastal Gulf of Finland (Baltic Sea) sediment tested the sensitivity of denitrification to a 2 wk anoxia exposure and subsequent reoxidation of the bottom waters. At the same time we followed the rapidly (1 d) and more slowly (9 d) emerging changes in different forms of sediment P after oxic conditions were restored. The total denitrification rate (Dtot) did not change during anoxic incubation, but shifted from coupled nitrification-denitrification (Dn) towards water column nitrate dependence (Dw). As the Dn rate did not decrease at the same rate as the Dw rate increased, the overall effect of 2 wk exposure to anoxia was an increase in Dtot rate. Nitrification was enhanced in the manipulated sediment compared to natural conditions, despite anoxia. Anoxia quickly caused a release of dissolved P from its 2 most labile forms. The effect was readily reversible, but in nature the replenishment of oxygen stores is usually linked to an intense mixing of the water column, and it is possible that part of the P released during anoxia reaches the productive layer. In our experiments, anoxia affected P cycling more than N cycling. Whether sediments act as sinks or sources of nutrients depends partly on the oxygen conditions at the seafloor. Laboratory experiments on coastal Gulf of Finland (Baltic Sea) sediment tested the sensitivity of denitrification to a 2 wk anoxia exposure and subsequent reoxidation of the bottom waters. At the same time we followed the rapidly (1 d) and more slowly (9 d) emerging changes in different forms of sediment P after oxic conditions were restored. The total denitrification rate (Dtot) did not change during anoxic incubation, but shifted from coupled nitrification-denitrification (Dn) towards water column nitrate dependence (Dw). As the Dn rate did not decrease at the same rate as the Dw rate increased, the overall effect of 2 wk exposure to anoxia was an increase in Dtot rate. Nitrification was enhanced in the manipulated sediment compared to natural conditions, despite anoxia. Anoxia quickly caused a release of dissolved P from its 2 most labile forms. The effect was readily reversible, but in nature the replenishment of oxygen stores is usually linked to an intense mixing of the water column, and it is possible that part of the P released during anoxia reaches the productive layer. In our experiments, anoxia affected P cycling more than N cycling. Whether sediments act as sinks or sources of nutrients depends partly on the oxygen conditions at the seafloor. Laboratory experiments on coastal Gulf of Finland (Baltic Sea) sediment tested the sensitivity of denitrification to a 2 wk anoxia exposure and subsequent reoxidation of the bottom waters. At the same time we followed the rapidly (1 d) and more slowly (9 d) emerging changes in different forms of sediment P after oxic conditions were restored. The total denitrification rate (Dtot) did not change during anoxic incubation, but shifted from coupled nitrification-denitrification (Dn) towards water column nitrate dependence (Dw). As the Dn rate did not decrease at the same rate as the Dw rate increased, the overall effect of 2 wk exposure to anoxia was an increase in Dtot rate. Nitrification was enhanced in the manipulated sediment compared to natural conditions, despite anoxia. Anoxia quickly caused a release of dissolved P from its 2 most labile forms. The effect was readily reversible, but in nature the replenishment of oxygen stores is usually linked to an intense mixing of the water column, and it is possible that part of the P released during anoxia reaches the productive layer. In our experiments, anoxia affected P cycling more than N cycling.

Benthic processes were measured at a coastal deposition area in the northern Baltic Sea, covering all seasons. The N-2 production rates, 90-400 mu mol N m(-2) d(-1), were highest in autumn-early winter and lowest in spring. Heterotrophic bacterial production peaked unexpectedly late in the year, indicating that in addition to the temperature, the availability of carbon compounds suitable for the heterotrophic bacteria also plays a major role in regulating the denitrification rate. Anaerobic ammonium oxidation (anammox) was measured in spring and autumn and contributed 10% and 15%, respectively, to the total N-2 production. The low percentage did, however, result in a significant error in the total N-2 production rate estimate, calculated using the isotope pairing technique. Anammox must be taken into account in the Gulf of Finland in future sediment nitrogen cycling research. Benthic processes were measured at a coastal deposition area in the northern Baltic Sea, covering all seasons. The N-2 production rates, 90-400 mu mol N m(-2) d(-1), were highest in autumn-early winter and lowest in spring. Heterotrophic bacterial production peaked unexpectedly late in the year, indicating that in addition to the temperature, the availability of carbon compounds suitable for the heterotrophic bacteria also plays a major role in regulating the denitrification rate. Anaerobic ammonium oxidation (anammox) was measured in spring and autumn and contributed 10% and 15%, respectively, to the total N-2 production. The low percentage did, however, result in a significant error in the total N-2 production rate estimate, calculated using the isotope pairing technique. Anammox must be taken into account in the Gulf of Finland in future sediment nitrogen cycling research. Benthic processes were measured at a coastal deposition area in the northern Baltic Sea, covering all seasons. The N-2 production rates, 90-400 mu mol N m(-2) d(-1), were highest in autumn-early winter and lowest in spring. Heterotrophic bacterial production peaked unexpectedly late in the year, indicating that in addition to the temperature, the availability of carbon compounds suitable for the heterotrophic bacteria also plays a major role in regulating the denitrification rate. Anaerobic ammonium oxidation (anammox) was measured in spring and autumn and contributed 10% and 15%, respectively, to the total N-2 production. The low percentage did, however, result in a significant error in the total N-2 production rate estimate, calculated using the isotope pairing technique. Anammox must be taken into account in the Gulf of Finland in future sediment nitrogen cycling research.

Anaerobic ammonium oxidation (anammox) and denitrification were measured in the open sea and coastal accumulation basins of the Gulf of Finland. The different methods used gave conflicting results on the importance of the anammox process in the sediments. Anammox generally contributed less than 20 % to the total N-2 production, and no anammox was found in a shallow inner estuary basin. However, the discovery of the anammox process in the open sea sediments challenges the denitrification measurements made in the area, as the coexistence of anammox and denitrification compromises the central assumptions behind the method used in denitrification measurements and causes overestimates of the N-2 production. The high (NO3-)-N-15 incubation concentration used in Baltic Sea denitrification measurements exacerbates this overestimation, which is likely to have been substantial. Anaerobic ammonium oxidation (anammox) and denitrification were measured in the open sea and coastal accumulation basins of the Gulf of Finland. The different methods used gave conflicting results on the importance of the anammox process in the sediments. Anammox generally contributed less than 20 % to the total N-2 production, and no anammox was found in a shallow inner estuary basin. However, the discovery of the anammox process in the open sea sediments challenges the denitrification measurements made in the area, as the coexistence of anammox and denitrification compromises the central assumptions behind the method used in denitrification measurements and causes overestimates of the N-2 production. The high (NO3-)-N-15 incubation concentration used in Baltic Sea denitrification measurements exacerbates this overestimation, which is likely to have been substantial. Anaerobic ammonium oxidation (anammox) and denitrification were measured in the open sea and coastal accumulation basins of the Gulf of Finland. The different methods used gave conflicting results on the importance of the anammox process in the sediments. Anammox generally contributed less than 20 % to the total N-2 production, and no anammox was found in a shallow inner estuary basin. However, the discovery of the anammox process in the open sea sediments challenges the denitrification measurements made in the area, as the coexistence of anammox and denitrification compromises the central assumptions behind the method used in denitrification measurements and causes overestimates of the N-2 production. The high (NO3-)-N-15 incubation concentration used in Baltic Sea denitrification measurements exacerbates this overestimation, which is likely to have been substantial.

The ecosystem of the Gulf of Finland is currently dominated by internal phosphorus loading from sediments. The internal load is highly redox sensitive, and its successful modelling on basin-wide scale requires a simplified description of the sediment process. We present here an approach in which redox-sensitive sediment processes are directly linked to the decomposition of carbon instead of the oxygen concentration in near-bottom water. Mineralisation of organic carbon is known to be the major factor controlling sediment nutrient cycling, including denitrification and Fe(III) oxide reduction, giving rise to high phosphorus fluxes from anoxic sediments. Our sediment process description requires only four main parameters, which are here identified by using in situ CO 2 , dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) flux measurements carried out by Goteborg University landers. The model was tested with the aid of time series of denitrification and DIP flux rates measured in the western Gulf of Finland. Modelled near-bottom and surface nutrient concentrations were compared with monitoring data from both the eastern and western Gulf of Finland. The model simulations showed that the average net ecosystem production entering the sediment surface from the euphotic layer was 49 g C m − 2 a − 1 . This organic load induced an average denitrification rate of 2.5 g N m − 2 a − 1 and DIP flux of 0.67 g P m − 2 a − 1 , corresponding to 20,200 t P a − 1 for the whole Gulf of Finland. The model was able to describe the seasonality of denitrification and sediment DIP flux with high precision. Further, the modelled near-bottom and surface nutrient concentrations were compatible with the available data. The results indicate that, on the scales important for coastal and open sea conditions, our simple sediment process description works well. The new tool will help us to use 3D models to study the effects of external load on the production and decomposition of organic matter, and on subsequent benthic nutrient fluxes.

Breakdown of dissolved organic nitrogen in the ocean had been thought to be the preserve of microbes at the surface. The discovery that these microbes are not up to the task calls for a reassessment of the biogeochemistry of this nitrogen pool.

Six strains of Nodularia spp., both toxic and non-toxic, were tested for their ability to incorporate leucine and thymidine. All axenic cyanobacterial strains studied showed high leucine incorporation (1.4 to 12 pmol μg chl a -1 h -1 at a concentration of 212 nM leucine), whereas thymidine was either taken up at a very low rate or not at all (0 to 0.2 pmol μg chl a -1 h -1 at a concentration of 121 nM thymidine). We therefore recommend using the thymidine incorporation method instead of that of leucine for measuring heterotrophic bacterial production during cyanobacterial blooms.

The effects of addition of algae (340 mg C m−2) and of the amphipod Monoporeia affinis (1500 ind m−2) on denitrification; porewater nutrient profiles; nutrient fluxes between sediment and water; and bacterial production were studied. A laboratory experiment was conducted using cores filled with sediment from the Gulf of Finland (Baltic Sea). Denitrification (isotope pairing method) was dramatically reduced in the cores enriched with algae (from 200–300 to 10–20 μmol N m−2 day−1). However, the addition of M. affinis together with algae negated this effect. Monoporeia affinis alone increased the coupled nitrification-denitrification as compared to the control cores. Bioturbation by the amphipods oxidised the sediment surface layers and consequently, PO43−concentration in the porewater decreased. In the cores enriched with algae, the loss of N from the sediment porewater was dominated by the efflux of NH4+which formed 90% of the loss. In the M. affinis enriched cores, the share of the NH4+efflux was 60% of which the excretion by M. affinis was 5–10%. The effects of M. affinis were found to be caused mainly by the improved O2conditions in the sediment.

Benthic bacterial production was measured with a modified ['4C]leucine incorporation method in the northern Baltic Sea in 1996 and 1997. Samples were incubated and TCA extractions were done in microcentrifuge tubes. Unincorporated label was removed by repeated centrifugation. Pellets were suspended in scintillation cocktail and counted as a gel using internal standardisation, a method that gave highly comparable counts with combustion of the sample. Bacterial production varied between 0.15 and 0.8 mg C 1-' h-' (90 to 350 mg C m-2 d-'). The highest production was found at Stn GF2 in the Gulf of Finland and the lowest in the northern Baltic Proper Bacterial growth rates were 0.07 to 0.84 d-'. Comparison of these results with published data from sedimentation trap measurements suggests that only a part of the benthic bacterial C demand can be explained by sedimentation.

Ecosystem functioning is threatened by an increasing number of anthropogenic stressors, creating a legacy of disturbance that undermines ecosystem resilience. However, few empirical studies have assessed to what extent an ecosystem can tolerate repeated disturbances and sustain its multiple functions. By inducing increasingly recurring hypoxic disturbances to a sedimentary ecosystem, we show that the majority of individual ecosystem functions experience gradual degradation patterns in response to repetitive pulse disturbances. The degradation in overall ecosystem functioning was, however, evident at an earlier stage than for single ecosystem functions and was induced after a short pulse of hypoxia (i.e., three days), which likely reduced ecosystem resistance to further hypoxic perturbations. The increasing number of repeated pulse disturbances gradually moved the system closer to a press response. In addition to the disturbance regime, the changes in benthic trait composition as well as habitat heterogeneity were important for explaining the variability in overall ecosystem functioning. Our results suggest that disturbance-induced responses across multiple ecosystem functions can serve as a warning signal for losses of the adaptive capacity of an ecosystem, and might at an early stage provide information to managers and policy makers when remediation efforts should be initiated.

Nitrification is a crucial process in sediment nitrogen cycling. We compared two (15)N tracer-based nitrification measurement techniques (isotope pairing technique (IPT) combined with (15)N nitrate pool dilution and (15)N ammonium oxidation) and three different (15)N analyses from bottom water nitrate (ammonia diffusion, denitrifier and SPINMAS) in a sediment mesocosm. The (15)N nitrate pool dilution technique combined with IPT can be used to quantify the in situ nitrification, but the minimum detection limit for the total nitrification is higher than that in the (15)N ammonium oxidation technique. The (15)N ammonium oxidation technique, however, is not applicable for sediments that have high ammonium content. If nitrate concentration and the amount of (15)N label in the sample are low, the (15)N nitrate analysis should be done with the denitrifier method. In higher (15)N concentrations, the less sensitive SPINMAS method can also be applied. The ammonia diffusion method is not suitable for bottom water (15)N nitrate analyses.

The Baltic Sea is one of the most eutrophic marine areas in the world. The role of nitro- gen as a eutrophicating nutrient in the Baltic Sea has remained controversial owing to a lack of understanding of nitrogen cycling in the area. We investigated the seasonal variation in sediment nitrification, denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) at 2 coastal sites in the Gulf of Finland. In addition to the estimation of in situ rates, we assessed the potential for nitrification and denitrification in different seasons. The nitrification and nitrogen removal processes were maximal during the warm summer months, when the sediment organic content was highest. In colder seasons, the rates of nitrification and nitrate reduction measured under in situ conditions decreased, but the potential for nitrification remained equal to, or higher than, that during the warm months. The rates of denitrification and nitrification were usually higher in the accumulation basin, where the organic content of the sediment was higher, but the transportation area, despite lower denitrification rates and potential, typically had a higher potential for nitrification than the accumulation basin. Anammox and DNRA were not impor- tant nitrate sinks in any of the seasons sampled. The results also show that the rates of denitrification in the coastal Gulf of Finland sediment have decreased, and that benthic denitrification might be a less important sink for fixed nitrogen than previously assumed.

A cyanobacterial bloom in the Gulf of Finland, Baltic Sea, was sampled throughout the development and senescence of aggregates in August 1999. While conditions inside the aggregates were favourable for denitrification (rich in nitrogen and carbon, with anoxic microzones), essentially none was detected by a sensitive isotope pairing method. Polymerase chain reaction-based methods, targeting functional genes encoding the key enzymes of denitrification and nitrification processes (nirS, nirK, amoA), revealed that the non-aggregated filaments harboured amoA gene fragments with high similarity to Nitrosospira amoA sequences, as well as both types of nitrite reductase genes, nirS and nirK. Only the nirS-type nitrite reductase gene and no amoA was detected in aggregated filaments. Thus, despite optimal environmental conditions and genetic potential for denitrification, the blooms of filamentous nitrogen-fixing cyanobacteria must be seen solely as a source, and not as a sink of nitrogen in the Baltic Sea.

Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia. Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia. Hypoxia, a growing worldwide problem, has been intermittently present in the modern Baltic Sea since its formation ca. 8000 cal. yr BP. However, both the spatial extent and intensity of hypoxia have increased with anthropogenic eutrophication due to nutrient inputs. Physical processes, which control stratification and the renewal of oxygen in bottom waters, are important constraints on the formation and maintenance of hypoxia. Climate controlled inflows of saline water from the North Sea through the Danish Straits is a critical controlling factor governing the spatial extent and duration of hypoxia. Hypoxia regulates the biogeochemical cycles of both phosphorus (P) and nitrogen (N) in the water column and sediments. Significant amounts of P are currently released from sediments, an order of magnitude larger than anthropogenic inputs. The Baltic Sea is unique for coastal marine ecosystems experiencing N losses in hypoxic waters below the halocline. Although benthic communities in the Baltic Sea are naturally constrained by salinity gradients, hypoxia has resulted in habitat loss over vast areas and the elimination of benthic fauna, and has severely disrupted benthic food webs. Nutrient load reductions are needed to reduce the extent, severity, and effects of hypoxia.

The effects of the polychaetes Marenzelleria sp. (Polychaeta, Spionidae), nonindigenous, rapidly increasing species in the Baltic Sea, on benthic nutrient fluxes, denitrification and sediment pore water nutrient concentration were studied in laboratory experiments using a flow-through setup with muddy sediment from coastal regions of the Gulf of Finland. In addition, different forms of sediment phosphorus (P), separated by chemical fractionation, were studied in three sediment layers. At a population density corresponding to about half the highest measured in the northern Baltic Sea, Marenzelleria sp. increased the fluxes of P and ammonium to the water column. No effect could be recorded for denitrification. Since the previously dominant species of the area, Monoporeia affinis, can enhance denitrification and lower the amount of dissolved P in the pore water, the replacement of M affinis with Marenzelleria spp. may lead to increased P flux to the water column and decreased denitrification, further increasing the ammonium flux to the water column. However, sediment reworking by Marenzelleria spp. also oxidizes the surface sediment in the long run, improving its ability to retain P and support nitrification. Therefore, the impact of Marenzelleria spp. on sediment nutrient release may not be as drastic as the initial reactions seen in our experiments suggest. (c) 2007 Elsevier B.V. All rights reserved. The effects of the polychaetes Marenzelleria sp. (Polychaeta, Spionidae), nonindigenous, rapidly increasing species in the Baltic Sea, on benthic nutrient fluxes, denitrification and sediment pore water nutrient concentration were studied in laboratory experiments using a flow-through setup with muddy sediment from coastal regions of the Gulf of Finland. In addition, different forms of sediment phosphorus (P), separated by chemical fractionation, were studied in three sediment layers. At a population density corresponding to about half the highest measured in the northern Baltic Sea, Marenzelleria sp. increased the fluxes of P and ammonium to the water column. No effect could be recorded for denitrification. Since the previously dominant species of the area, Monoporeia affinis, can enhance denitrification and lower the amount of dissolved P in the pore water, the replacement of M affinis with Marenzelleria spp. may lead to increased P flux to the water column and decreased denitrification, further increasing the ammonium flux to the water column. However, sediment reworking by Marenzelleria spp. also oxidizes the surface sediment in the long run, improving its ability to retain P and support nitrification. Therefore, the impact of Marenzelleria spp. on sediment nutrient release may not be as drastic as the initial reactions seen in our experiments suggest. (c) 2007 Elsevier B.V. All rights reserved. The effects of the polychaetes Marenzelleria sp. (Polychaeta, Spionidae), nonindigenous, rapidly increasing species in the Baltic Sea, on benthic nutrient fluxes, denitrification and sediment pore water nutrient concentration were studied in laboratory experiments using a flow-through setup with muddy sediment from coastal regions of the Gulf of Finland. In addition, different forms of sediment phosphorus (P), separated by chemical fractionation, were studied in three sediment layers. At a population density corresponding to about half the highest measured in the northern Baltic Sea, Marenzelleria sp. increased the fluxes of P and ammonium to the water column. No effect could be recorded for denitrification. Since the previously dominant species of the area, Monoporeia affinis, can enhance denitrification and lower the amount of dissolved P in the pore water, the replacement of M affinis with Marenzelleria spp. may lead to increased P flux to the water column and decreased denitrification, further increasing the ammonium flux to the water column. However, sediment reworking by Marenzelleria spp. also oxidizes the surface sediment in the long run, improving its ability to retain P and support nitrification. Therefore, the impact of Marenzelleria spp. on sediment nutrient release may not be as drastic as the initial reactions seen in our experiments suggest. (c) 2007 Elsevier B.V. All rights reserved.

Estuaries have been suggested to have an important role in reducing the nitrogen load transported to the sea. We measured denitrification rates in six estuaries of the northern Baltic Sea. Four of them were river mouths in the Bothnian Bay (northern Gulf of Bothnia), and two were estuary bays, one in the Archipelago Sea (southern Gulf of Bothnia) and the other in the Gulf of Finland. Denitrification rates in the four river mouths varied between 330 and 905 mu mol N m(-2) d(-1). The estuary bays at the Archipelago Sea and the Gulf of Bothnia had denitrification rates from 90 mu mol N m(-2) d(-1) to 910 mu mol N m(-2) d(-1) and from 230 mu mol N m(-2) d(-1) to 320 mu mol N m(-2) d(-1), respectively. Denitrification removed 3.6-9.0% of the total nitrogen loading in the river mouths and in the estuary bay in the Gulf of Finland, where the residence times were short. In the estuary bay with a long residence time, in the Archipelago Sea, up to 4.5% of nitrate loading and 19% of nitrogen loading were removed before entering the sea. According to our results, the sediments of the fast-flowing rivers and them estuary areas with short residence times have a limited capacity to reduce the nitrogen load to the Baltic Sea. Estuaries have been suggested to have an important role in reducing the nitrogen load transported to the sea. We measured denitrification rates in six estuaries of the northern Baltic Sea. Four of them were river mouths in the Bothnian Bay (northern Gulf of Bothnia), and two were estuary bays, one in the Archipelago Sea (southern Gulf of Bothnia) and the other in the Gulf of Finland. Denitrification rates in the four river mouths varied between 330 and 905 mu mol N m(-2) d(-1). The estuary bays at the Archipelago Sea and the Gulf of Bothnia had denitrification rates from 90 mu mol N m(-2) d(-1) to 910 mu mol N m(-2) d(-1) and from 230 mu mol N m(-2) d(-1) to 320 mu mol N m(-2) d(-1), respectively. Denitrification removed 3.6-9.0% of the total nitrogen loading in the river mouths and in the estuary bay in the Gulf of Finland, where the residence times were short. In the estuary bay with a long residence time, in the Archipelago Sea, up to 4.5% of nitrate loading and 19% of nitrogen loading were removed before entering the sea. According to our results, the sediments of the fast-flowing rivers and them estuary areas with short residence times have a limited capacity to reduce the nitrogen load to the Baltic Sea. Estuaries have been suggested to have an important role in reducing the nitrogen load transported to the sea. We measured denitrification rates in six estuaries of the northern Baltic Sea. Four of them were river mouths in the Bothnian Bay (northern Gulf of Bothnia), and two were estuary bays, one in the Archipelago Sea (southern Gulf of Bothnia) and the other in the Gulf of Finland. Denitrification rates in the four river mouths varied between 330 and 905 mu mol N m(-2) d(-1). The estuary bays at the Archipelago Sea and the Gulf of Bothnia had denitrification rates from 90 mu mol N m(-2) d(-1) to 910 mu mol N m(-2) d(-1) and from 230 mu mol N m(-2) d(-1) to 320 mu mol N m(-2) d(-1), respectively. Denitrification removed 3.6-9.0% of the total nitrogen loading in the river mouths and in the estuary bay in the Gulf of Finland, where the residence times were short. In the estuary bay with a long residence time, in the Archipelago Sea, up to 4.5% of nitrate loading and 19% of nitrogen loading were removed before entering the sea. According to our results, the sediments of the fast-flowing rivers and them estuary areas with short residence times have a limited capacity to reduce the nitrogen load to the Baltic Sea.

The community structure of the bacteria associated with Nodularia spumigena (Mertens) cyanobacterial aggregates in the Baltic Sea was studied with temperature gradient gel electrophoresis (TGGE), using a 16S rRNA gene fragment as a target. Various developmental stages of the aggregates and free-floating cyanobacterial filaments were sampled to reveal possible changes in associated microbial community structure during development and senescence of the aggregates. The microbial community structures of all samples differed, and the communities of young and decaying aggregates were separated by cluster analysis of the TGGE fingerprint data. Sequencing of the TGGE fragments indicated the presence of bacteria from the alpha-, beta-, and gamma-proteobacterial groups, as well as members of Cytophaga-Flexibacter-Bacteroides lineages and gram-positive Actinobacteria spp. The majority of the Nodularia-associated sequences were not closely related to previously reported 16S rDNA sequences from the Baltic Sea or any other environment. The structure of the bacterial assemblage reflects the environmental changes associated with the succession and decay of the cyanobacterial aggregates. In addition, the sequence data suggest that the N. spumigena (Mertens) blooms in the Baltic Sea may host thus far uncharacterized bacterial species.

Fluid motion within and around sinking aggregates is an important factor in particle scavenging and solute exchange between sinking aggregates and the surrounding water and, hence, vertical fluxes and remineralization processes in the ocean. In the present study, we analyzed O-2 uptake rates in >2-mm porous diatom aggregates and in model aggregates impermeable to flow by measuring, on the same aggregates, the interface diffusive uptake rates with microsensors and the total (diffusive + advective) O-2 uptake rate by the Winkler method. The uptake rates were measured in a flow field similar to that experienced by sinking aggregates. The ratio of total O-2 uptake rate to diffusive uptake rate was 0.97 +/- 0.10 (n = 14) in model aggregates impermeable to flow. In contrast, total O-2 uptake was similar to or higher than diffusive uptake rate calculated from the O-2 gradients at the aggregate-water interface in 85% of all field-sampled and roller tank diatom aggregates examined. The highest ratio of total O-2 uptake rate relative to diffusive uptake rate measured in

Denitrification in sediments is of major importance to the Baltic Sea nitrogen (N) budget. However, little is known about denitrification in the water column in the Baltic. We tested whether denitrification could be active in pelagic cyanobacterial aggregates, commonly found in the Baltic Sea during warm summer months. In these aggregates, anoxic microniches may form. Such micro-zones suggest a possibility for anaerobic processes, such as denitrification by the heterotrophic bacteria associated with the cyanobacteria. Denitrification and nitrogen (N 2 ) fixation in a cyanobacterial bloom were measured on a 3 wk cruise in the Baltic Sea in order to determine whether the Baltic Sea cyanobacterial blooms act as sources or sinks of N. Experimental conditions analogous to formation of anoxic microniches within cyanobacterial aggregates did not activate the denitrification process, even when anoxic conditions prevailed for several hours. Only in 3 cases was denitrification, measured using the 15 N-isotope pairing method, detectable, giving rates of 0.8 to 1.8 nmol N 2 l -1 h -1 . Nitrogen fixation, determined using the acetylene reduction assay, varied from 0.03 to 1.85 μmol N 2 l -1 h -1 . According to this study, the blooms of N 2 -fixing cyanobacteria in the Baltic Sea must be seen solely as sources, not sinks for N in the marine pelagic environment.

Size, chemical composition, bacteria, flagellate, and ciliate abundance, bacterial pro- duction and growth rates as well as community respiration rates were measured on natural diatom aggregates of different sizes collected by SCUBA divers on 5 subsequent days offshore from north- ern Zealand, Denmark. Aggregate size was highly variable (0.16 to 524 mm 3 ) throughout the sampling period, whereas aggregate (agg.) dry mass (70 to 390 µg agg. -1 ) and organic carbon content (48 to 130 µg agg. -1 ) varied less. The composition of particulate organic amino acids on aggregates was very different from that of total dissolved amino acids in the matrix water of the aggregates. Bac- teria, flagellates, and ciliates were 10- to 10 000-fold enriched on aggregates compared to their abun- dance in the surrounding water. Enrichment factors of bacteria, flagellates, and ciliates decreased significantly with increasing aggregate size. Ciliates showed the highest and flagellates the lowest enrichment on aggregates. Absolute rate of bacterial production was significantly correlated with aggregate size, and it constantly increased on aggregates of similar size throughout the sampling period. Cell-specific production rates were consistently higher for aggregate-associated bacteria than for free-living bacteria, and 3 to 20% of all bacteria were produced on aggregates. High release of dissolved organic matter and bacteria into the surrounding water was indicated by the release of amino acids into the surrounding water and high calculated detachment rates of aggregate- associated bacteria. Marine snow, thus, should be regarded as comprising integral components rather than isolated microenvironments of the pelagic zone. Its nature has important consequences for organic matter cycling from small scales up to the global scale.

Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM) and as colloidal Fe oxyhydroxides, principally ferrihydrite. However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation processes along a salinity gradient in an estuary of the northern Baltic Sea efficiently transfer Fe and OM from the dissolved phase into particulate material that accumulates in the sediments. Flocculation of Fe and OM is partially decoupled. This is likely due to the presence of discrete colloidal ferrihydrite in the freshwater Fe pool, which responds differently from DOM to estuarine mixing. Further decoupling of Fe from OM occurs during sedimentation. While we observe a clear decline with distance offshore in the proportion of terrestrial material in the sedimentary particulate organic matter (POM) pool, the distribution of flocculated Fe in sediments is modulated by focusing effects. Labile Fe phases are most abundant at a deep site in the inner basin of the estuary, consistent with input from flocculation and subsequent focusing. The majority of the labile Fe pool is present as Fe (II), including both acid-volatile sulfur (AVS)-bound Fe and unsulfidized phases. The ubiquitous presence of unsulfidized Fe (II) throughout the sediment column suggests Fe (II)-OM complexes derived from reduction of flocculated Fe (III)-OM, while other Fe (II) phases are likely derived from the reduction of flocculated ferrihydrite. Depth-integrated rates of Fe (II) accumulation (AVS-Fe + unsulfidized Fe (II) + pyrite) for the period 1970–2015 are greater in the inner basin of the estuary with respect to a site further offshore, confirming higher rates of Fe reduction in near-shore areas. Mössbauer 57Fe spectroscopy shows that refractory Fe is composed largely of superparamagnetic Fe (III), high-spin Fe (II) in silicates, and, at one station, also oxide minerals derived from past industrial activities. Our results highlight that the cycling of Fe in boreal estuarine environments is complex, and that the partial decoupling of Fe from OM during flocculation and sedimentation is key to understanding the role of Fe in sedimentary diagenetic processes in coastal areas.