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Articulo Científico, Metal and metallothionein (MT) in mixed zooplankton were investigated as means of monitoring metal availability regarding environmental exposure. Spatial and temporal variability of Cd, Cu, Ni, Zn, Fe, Mn and Pb in zooplankton and seawater were studied in Saronikos Gulf (Aegean Sea, Eastern Mediterranean), once every second month during an annual cycle (2011–2012). Particulate organic carbon and chlorophyll α were also measured in seawater samples. Median zooplankton metal concentrations were 0.65, 32.4, 7.1, 864, 1420, 40.2 and 26.8 μg g-1 dw for Cd, Cu, Ni, Zn, Fe, Mn and Pb, respectively, and 109 μg g-1 ww for MTs. Metal levels in zooplankton and MTs were higher at sites influenced by human-derived pressures. Additionally, metal concentrations in pelagic fish flesh from the Greek MED-POL data base were used for bioconcentration and biomagnification factors calculation. Bioconcentration from water to zooplankton was higher than metal transfer from either seston to zooplankton or zooplankton to fish., SI

Mesozooplankton vertical pattern was studied down to 2000 m along an east-west transect in the oligotrophic Mediterranean Sea in June 1999. Depth-stratified samples were collected by a WP-2 net (200 µm mesh size). In addition, the copepod grazing impact on autotrophs was assessed in the epipelagic zone by measurement of small and large copepod gut content by day-night sampling. An east-west increasing trend of integrated total biomass and abundance over the 0–1000 m was detected. Similar vertical pattern of standing stock was found along the transect, with subtle differences at stations positioned over cyclonic and anti-cyclonic gyres. No significant differences were found between stations in the epipelagic and upper mesopelagic zone, except at the westernmost station (NW Mediterranean); discrimination of assemblages between the eastern and the western basins were revealed in the lower mesopelagic and bathypelagic zones. Estimated copepod grazing impact on primary production was found overall higher in the western than in the eastern basin. The entire Mediterranean Sea was dominated by copepod functional groups characterized by reproduction strategies (mostly sac spawners), trophic preferences (mainly omnivores and detritivores) and feeding modes (ambush and cruise feeding), facilitating their adaptation and efficiency in the oligotrophic to very oligotrophic environment of the Mediterranean Sea.

The Eastern Mediterranean Sea (EMS) is an ultraoligotrophic semi-enclosed sea with low nutrient levels, low primary production rate, impoverished phytoplankton populations and low zooplankton standing stocks. The Cretan Passage in the western Levantine Sea is one of the least explored areas of the EMS. We measured the mesozooplankton biomass, abundance and taxon composition as well as biochemical parameters like Electron Τransport System (ETS) activity and Aminoacyl-tRNA Synthetases (AARS) activities in late spring of 2016 (28 March–17 April 2016). The sampling area is influenced by the presence of the permanent Rhodes Cyclonic Gyre east of Crete and a series of smaller permanent or recurrent gyres south of Crete which influence the pathway of the Atlantic Water from the Ionian Sea towards the Levantine Basin at surface and subsurface layers. Mesozooplankton abundance showed slightly increasing values from western to eastern stations of the Cretan Passage and although subsurface maxima were recorded in the 50–100 m layer, the abundance at all stations generally decreased with depth. The community was dominated by copepods at all stations. Juveniles of Clausocalanus followed by juveniles of Oithona and the Oncaea media group (Oncaea waldemari, Oncaea scottodicarloi, Oncaea media) were the most abundant taxa in the upper layer, whereas Mormonilla minor, Haloptilus longicornis and Subeucalanus monachus were mainly found below 100 m. ETS, AARS and biomass for carbon measurements were conducted in the same area for the 0–100 m and 0–500 m layers. ETS was highest at LV13 with 10.56 ± 1.33 μl O2 h−1 mg protein−1 in 0–500 m and carbon biomass was highest at LV18 with 4.99 mg m−3 in 0–100 m. Lowest values of 4.52 ± 1.46 μl O2 h−1 mg protein−1 at LV18 in 0–100 m were detected for ETS and 0.91 mg m−3 at LV10 in 0–500 m for carbon biomass. Specific AARS activities showed no significant differences among sites. On the basis of ETS and AARS, we calculated that the zooplankton carbon requirements follow the west to east trend of zooplankton biomass, however, the zooplankton is not well fed and the organisms are living under oligotrophic stress.

Combined analyses of Fatty Acid Trophic Markers (FATM) and Stable Isotopes (SI) were used to characterize food preferences among copepod species/taxa and to trace their food sources in the ultra-oligotrophic Cretan Passage of the Eastern Mediterranean Sea (EMS). FATMs are based on the conservation and transfer of specific source Fatty Acids (FAs) through the food web, providing a qualitative indicator of assimilated food types. SI provide information about trophic levels and food sources of specific organisms. Most species/taxa exhibited low δ15N ratios caused by low δ15N-PON base values at the study site. A cluster analysis based on the mean Fatty Acid (FA) compositions of nine copepods from all stations identified three distinct groups (group 1: Clausocalanus lividus and Euchaeta spp.; group 2: Pleuromamma spp., Haloptilus longicornis, Lucicutia spp., Clausocalanus spp. and Corycaeus spp.; group 3: Pareucalanus attenuatus and Oncaea spp.), coinciding with similarities in feeding behavior. H. longicornis, however, revealed similar trophic positions but different food resources by FATM and SIA analyses. Overall, we conclude that omnivory was the prevailing feeding mode, demonstrating a high degree of opportunistic feeding of copepods in these ultra-oligotrophic waters. This study shed first light on the zooplankton taxa/species life strategies with regard to feeding preferences and lipid storage mechanisms in the oligotrophic EMS, where lack of relevant information exists. Although further investigation is required, the good agreement between FA and SI in some species of copepods emphasize the applicability of lipid trophic markers. Results benefit from the coupling of these indices even in oligotrophic regions of the world ocean.

Faecal pellet production and content along with egg production of the dominant copepod species Acartia clausi were studied in the Thermaikos Gulf (NW Aegean Sea) during a pre-bloom and a bloom of the toxic dinoflagellate Dinophysis acuminata. Both faecal pellet production (6.8–8.6 ind−1 d−1) and egg production (15.8–47.6 ind−1 d−1) appeared unrelated to the D. acuminata bloom. Less than 11% of the copepod faecal pellets contained one or two D. acuminata cells, almost intact, whereas the other material in the pellets was broken into small pieces or amorphous shapes. The toxin outflux seemed to be insignificant when compared to the mean toxin concentration from the whole D. acuminata population. Finally, the potential grazing impact of A. clausi on D. acuminata during the study period was low. European Commission | Ref. EVK3-2001-00055 European Commission | Ref. H2020, n. 871153

According to the methodological standards established by Marine Strategy Framework Directive, the assessment for the pelagic habitat under the Biodiversity Descriptor should be carried out at the regional or sub-regional level. In the case of Mediterranean Sea, the sub-regional assessment seems optimal to take into account biogeographic differences in species composition and functional characteristics. Previous research has shown that phytoplankton diversity indicators are efficient for reliable environmental assessments, although more effort has been recommended to test these indicators on a wide spatial scale to cover wider gradients of natural and anthropogenic pressures. In this work, a set of eight diversity indices was tested against the pressure levels within a common data set of the structure and abundance of phytoplankton communities from the Adriatic, Ionian and Aegean Seas. Expert knowledge was used to define four categories of impacts that take into account partial pressures, such as point and non-point pollution, industry, ports and fisheries. At the level of the common data set, most of the diversity, evenness and dominance indices could only distinguish between the highest level of impact and the rest of impact categories. These indices maintained the distinction between two levels of subsequently dichotomised impacts (no to low impact vs. high impact) across latitudinal and longitudinal gradients. On average, the indices were less sensitive to impacts in the northernmost and westernmost areas than in the southernmost and easternmost areas, although they still showed a significant response. The results also suggest that phytoplankton communities become more uniform and less dominated by a single taxon as sampling depth increases at sites with low impact, while evenness and dominance at impacted sites remain similar at all depths. In order to establish meaningful definitions of good environmental status and targets for pelagic habitats in the Mediterranean Sea, it is necessary to establish spatially specific thresholds by additional examination of indices of good performance.

Since 2007, the non-indigenous calanoid copepod Pseudodiaptomus marinus Sato, 1913 has been increasingly recorded in numerous European sites, spreading at an unexpectedly fast pace over a short time-span. This species presents specific biological and behavioural traits which make it of particular interest for ecological and applied research topics. On 29–30 January 2018, 29 scientists from nine European Countries established the EUROBUS (Towards a EURopean OBservatory of the nonindigenous calanoid copepod Pseudodiaptomus marinUS) Working Group (WG). This WG aimed at creating a European network of institutions and researchers working on the various aspects of the biology and ecology of P. marinus, with an open forum where sharing experience and know-how among WG participants. This brought to an updated distribution map of P. marinus in European waters, as well as to the identification of priority research lines and potential joint initiatives under the WGEUROBUS umbrella. This contribution, stemming from the experts participating at the WG, represents the manifesto of the current and future initiatives developed within WGEUROBUS. info:eu-repo/semantics/publishedVersion

In order to identify how ocean acidification will influence biological interactions and fluxes among planktonic organisms and across trophic levels, a large-scale mesocosm experiment was performed in the oligotrophic Northwestern Mediterranean Sea in the framework of the European MedSeA project. Nine mesocosms were deployed in the Bay of Calvi (Corsica, France) in summer 2012. Six mesocosms were subjected to different levels of CO2 partial pressures (pCO2; 550, 650, 750, 850, 1000 and 1250 μatm) covering the range of atmospheric pCO2 anticipated for the end of this century depending on future emission scenarios, and the last three mesocosms were unaltered (ambient pCO2 of ∼450 μatm). During this 21-day experiment, we monitored copepod egg and naupliar stocks, estimated copepod (Acartia clausi and Centropages typicus) feeding rates and determined the abundance and taxonomic composition of the mesozooplankton community at the start and at the completion of the experiment. This community was clearly dominated by copepods and its final composition slightly varied between mesocosms most likely due to natural and experimental variability that cannot be related to CO2 conditions. The abundances of eggs and nauplii as well as feeding rates of A. clausi and C. typicus on diatoms, dinoflagellates and ciliates showed no significant differences among CO2 levels. The above findings suggest that the experimental set-up especially for the specific trophic conditions and the short duration of the experiment did not provide the information on the effect of acidification that was expected. The acidification might have an effect on planktonic communities and even worsen the problems imposed by food limitation, therefore on this short time scale experiment and under the extreme ologotrophic conditions the signal that dominates was the food limitation.

The rapid warming of the wolrd’s oceans during the last few decades has affected distributional patterns of marine planktonic communities. Here, we analyse links between sea warming and changes in copepod community composition over the last 3 decades (1980−2012). We used zooplankton time-series data which included 79 species of copepods collected at 3 sites in the eastern North Atlantic (Bay of Biscay and the Kattegat Sea) and the Mediterranean Sea (Gulf of Saronikos). First, using community β-diversity metrics, we analysed temporal patterns of copepod community composition changes over time and its relation to local environmental conditions. Second, to test whether the changes in copepod community composition correspond to community thermal preferences, we used the community temperature index (CTI) and compared CTI interannual changes with local temperature trends. The β-diversity analysis reveals a high temporal turnover in the copepod community composition at the 3 sites (30−45%), with a significant similarity decrease over time (‘decay’) associated with both niche descriptors and demographic stochastic processes. CTI results reveal that both in the Kattegat and Saronikos, where the ocean warming rate was the highest amongst sites, copepod community changes are linked to temperature variability, suggesting that the community is tracking their thermal niche over time. Our findings unveil the fundamental role of abiotic factors structuring copepod biodiversity over time and reveal that the local velocity of ocean warming and the species thermal thresholds are key to rearranging copepod community composition in coastal ecosystems.

Zooplankton plays a fundamental role in oceanic carbon flux as the primary biological mechanism sequestering the carbon transferred from the atmosphere into surface waters to deeper layers and higher trophic levels. Studies of the zooplanktonic organisms of the Aegean Sea have been initiated in the 1900s, whereas the number of cruises has increased since the late 1980s. This review aims at providing an updated and integrated picture of the Aegean Sea mesozooplankton in the offshore waters with emphasis on epipelagic zone, based on studies conducted during a 25-year period. The mesozooplankton standing stock, the species composition, the contribution of the main functional zooplankton groups, the mesozooplankton community structure as influenced by the major hydrological features and the role of mesozooplankton in the pelagic food web are discussed. An overview of the distribution of the mesozooplankton standing stock highlights a distinct pattern of decreasing abundance and biomass; higher abundance values were observed in early autumn than in spring whereas the opposite was true for biomass, due to the important contribution of cladocerans in early autumn, along the north–south axis whereas higher values are observed in spring-early summer than autumn. The bulk of epipelagic mesozooplankton is concentrated in the upper 100 m layer where mesozooplankton plays a major role in biological processes. Our review confirmed that copepods are by far the dominant group in the Aegean Sea and overall 257 species/genera of copepods and 6 cladoceran species are reported in this study. Different copepod species dominate the epipelagic, mesopelagic, and bathypelagic zones. In the South Aegean Sea (SAS) mesozooplankton composition in the 0–100 m layer is different from that in the North Aegean Sea (NAS), presenting a more homogeneous distribution of taxa abundances in the mesopelagic zone, while differentiation reappears in the bathypelagic zone. The compilation of data from studies in NAS revealed the strong impact of the inflowing Black Sea Water and the associated hydrological front on mesozooplankton standing stock as well as on the mesozooplankton composition. Some of the detected maxima are among the highest values observed at the surface layer (0–50 m) of Mediterranean inshore and offshore waters. The significance of small size species has been underlined not only from the Normalized Biomass Size Spectra but also by the functional group analysis coming in line with other findings in the Mediterranean Sea. A “multivorous web” seems to be common in the Aegean Sea as suggested by the great variety of feeding modes and preferences and by the significant and simultaneous grazing impact on phytoplankton and ciliates by mesozooplankton.

Mesozooplankton composition including functional groups in relation to hydrological features is studied in the oligotrophic pelagic ecosystem of the eastern Mediterranean Sea. Combining classic/standard microscopy and advanced digital imaging approach copepod growth rates, ingestion and production rates were estimated using allometric relationships. For this purpose, mesozooplankton samples were taken in the epipelagic zone (0–200 m) in June 2016, over a grid of 16 stations in the Cretan Sea, the Cretan Passage and the eastern and western Cretan Straits including one station in the Mirtoan Sea. The sampling area is influenced by the presence of the permanent Rhodes Cyclonic Gyre east of Crete and a series of smaller permanent or recurrent gyres south of Crete that influence the anti-estuarine pathway of the Atlantic Water from the Ionian Sea towards the Levantine Basin at surface and subsurface layers. Mesozooplankton abundance exhibited almost similar values (mean value 312 ind m−3 ± 100 SD) among transects with a higher value at the western Straits (up to 562 ind m−3). Copepods were the dominant group at all stations with almost the same relative contributions among stations (76%) but slightly higher values in the Cretan Passage. The contribution of copepod functional groups in the mesozooplankton community is also highlighted. The importance of small sized copepods was underlined by small ambush feeding carnivores, prevailing at all stations and demonstrating a high degree of opportunistic feeding of copepods in these ultra-oligotrophic waters, with chlorophyll a ranging from 0.05 to 0.20 μg L−1. Copepod vital rates (production, ingestion and respiration) estimated by a growth rate model were generally low, with higher values in the Cretan Sea.

Naturally occurring microbial decomposition of organic matter (OM) in coastal marine environments cause increased acidity in deeper layers similar or even exceeding the future predictions for global ocean acidification (OA). Experimental studies in coastal areas characterized by increased inputs of OM and nutrients, coping with intermittent hypoxic/anoxic conditions, provide better understanding of the mechanisms affecting nutrients and carbon biogeochemistry under the emerging effects of coastal pH decrease. Laboratory CO2-manipulated microcosm experiments were conducted using seawater and surface sediment collected from the deepest part of Elefsis Bay (Saronikos Gulf, Eastern Mediterranean) focusing to study the co-evolution of processes affected by the decline of dissolved oxygen and pH induced by (a) OM remineralization and (b) the future anthropogenic increase of atmospheric CO2. Under more acidified conditions, a significant increase of total alkalinity was observed partially attributed to the sedimentary carbonate dissolution and the reactive nitrogen species shift towards ammonium. Νitrate and nitrite decline, in parallel with ammonium increase, demonstrated a deceleration of ammonium oxidation processes along with decrease in nitrate production. The decreased DIN:DIP ratio, the prevalence of organic nutrient species against the inorganic ones, the observations of constrained DON degradation and nitrate production decline and the higher DOC concentrations revealed the possible inhibition of OM decomposition under lower pH values. Finally, our results highlight the need for detailed studies of the carbonate system in coastal areas dominated by hypoxic/anoxic conditions, accompanied by other biogeochemical parameters and properly designed experiments to elucidate the processes sequence or alterations due to pH reduction.

Here we use bulk and amino acid-specific stable nitrogen (N) isotope analysis (AA-CSIA) to evaluate seasonal and regional change in mesozooplankton dynamics for the first time in coastal waters of the eastern Mediterranean. Cyprus mesozooplankton δ15N values were significantly higher in late winter (2.3‰) than in summer (1.2‰), and in all cases were less than the δ15N values of mesozooplankton in the northeast Aegean Sea (NEA; 3.4‰). AA-CSIA indicates that these differences can primarily be attributed to seasonal and regional change in mesozooplankton community trophic structure, with overall trophic position increasing by 0.2–0.3 in winter as compared to summer around Cyprus, and trophic position higher in the NEA than in Cyprus by 0.3–0.6. Such differences are most likely related to the larger contribution of carnivorous mesozooplankton observed in winter around Cyprus and in the NEA. Overall, our findings indicate change in bulk mesozooplankton δ15N value in the eastern Mediterranean is primarily driven by change in community trophic position, rather than variability in δ15N value at the base of the food web.

This work focuses on the direct measurement of the vertical flux of appendicularian houses in order to assess their importance as a component of vertical carbon flux in coastal areas. For this purpose, arrays of cylindrical sediment traps were deployed for 5 to 8 days at two depths in a coastal area of the northern Aegean Sea (inner Thermaikos Gulf) during spring. The data support the contention that resuspension was minimal. Fecal pellet (FP) production and grazing experiments with the dominant copepods (Acartia clausi) were conducted to provide additional information on the potential FP contribution to the total carbon flux. The magnitude of the vertical flux of particulate organic carbon (POC) ranged between 310 and 724 mg C m−2 day−1. The proportion of phytoplankton carbon in the POC vertical flux was up to 45 %. The contribution of zooplankton FPs to the total carbon never exceeded 5 %. On the contrary, appendicularian houses were an essential component of the biogenic carbon flux contributing up to 55.3 % of the total vertical carbon flux. Consequently, both phytoplankton and appendicularian houses contributed equally to the biogenic carbon flux exceeding 80 % of the total sinking POC. Taking into account the sinking speed of the particles and the environment in the area, all this carbon probably reaches the seafloor, thus indicating a strong pelagic–benthic coupling.

Vertical distribution of the mesozooplankton community down to 1000 m was investigated in the oligotrophic Aegean Sea during March and September 1997. Total biomass and abundance decreased with depth with a sharp decline ,100 m. However, in the northern part, higher biomass values in the 500–1000 m layer than in the 300–500 m layer were observed, due to the abundance of Calanus helgolandicus in deep waters. Species depth preferences and ranges resulted in statistically distinct communities in different zones, as well as in a differentiation along the north–south gradient of oligotrophy in the epipelagic and deep zones. The communities in these zones showed seasonal differences, while in the mesopelagic zone seasonal and horizontal differences were less significant. The discrimination between the north and the south deep communities was mainly due to the dominance of Calanus helgolandicus and Subeucalanus monachus respectively, which had similar seasonal and vertical distribution patterns and were found oversummering in the deep layers. The north–south distinction was related to the gradient of oligotrophy and the hydrological heterogeneity of the Aegean Sea. Spatial community structure and variability between seasons might facilitate an efficient carbon flow in the Aegean Sea under very oligotrophic conditions.

Atmospheric deposition is assumed to stimulate heterotrophic processes in highly oligotrophic marine systems, controlling the dynamics and trophic efficiency of planktonic food webs, and is expected to be influenced by climate change. In the course of an 8-day mesocosm experiment, we examined the channeling, of the Saharan dust (SD) and mixed aerosols (A) effects on microplankton up to the copepod trophic level, in the highly oligotrophic Eastern Mediterranean Sea. Based on mesocosms with SD and A treatments, we evaluated the feeding response of the dominant copepod Clausocalanus furcatus every other day. We hypothesized that increased food availability under atmospheric deposition would result in increased copepod ingestion rates, selectivity and production. Overall, no robust pattern of food selection was documented, and daily rations on the prey assemblage of all mesocosms were very low indicating severe food limitation of C. furcatus. Although increased food availability was not true, after few days ingestion of ciliates was maximized, followed by egg production, in both the SD and A treatments, indicating their importance in the diet of this copepod as well as a response of C. furcatus feeding performance. Our results help in understanding the trophic efficiency of marine food webs in ultra-oligotrophic environments under atmospheric deposition. We suggest that future mesocosm research in oligotrophic waters should consider more than one copepod species., Sí

Ptacnik, Robert ... et al.-- 10 pages, 4 figures, 1 table, supplementary information https://dx.doi.org/10.1038/srep29286, Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light, The experiment was financed by the European Union Seventh Framework Program MESOAQUA (FP7/2009-2013) coordinated by JCN under grant agreement no. 228224. R.P. further acknowledges financial support by the German Research Council (DFG Pt 5/3-1), JMG by Spanish project REMEI (CTM2015-70340) and AC by project PROTOS (CTM2009-08783) from the Spanish Ministry of Economy and Competitiveness

Dispersal plays a key role to connect populations and, if limited, is one of the main processes to maintain and generate regional biodiversity. According to neutral theories of molecular evolution and biodiversity, dispersal limitation of propagules and population stochasticity are integral to shaping both genetic and community structure. We conducted a parallel analysis of biological connectivity at genetic and community levels in marine groups with different dispersal traits. We compiled large data sets of population genetic structure (98 benthic macroinvertebrate and 35 planktonic species) and biogeographic data (2193 benthic macroinvertebrate and 734 planktonic species). We estimated dispersal distances from population genetic data (i.e., FST vs. geographic distance) and from β-diversity at the community level. Dispersal distances ranked the biological groups in the same order at both genetic and community levels, as predicted by organism dispersal ability and seascape connectivity: macrozoobenthic species without dispersing larvae, followed by macrozoobenthic species with dispersing larvae and plankton (phyto- and zooplankton). This ranking order is associated with constraints to the movement of macrozoobenthos within the seabed compared with the pelagic habitat. We showed that dispersal limitation similarly determines the connectivity degree of communities and populations, supporting the predictions of neutral theories in marine biodiversity patterns., RADIALES (IEO)

The Mediterranean and the Black Seas are connected through Bosphorus, Marmara Sea and Dardanelles (Turkish Straits System, TSS). In this study, we examined the spatial distribution of copepods and investigate their production and grazing. The aim was to understand the transfer of phytoplankton/microzooplankton production up the food chain in TSS and Aegean Sea during spring. The phytoplankton and microzooplankton biomass and production showed a clear decreasing trend from Bosphorus to the Aegean Sea, whereas copepod biomass did not reveal any distinct trend and only the number of copepod species increased from Bosphorus to the Aegean Sea. Production of copepods and egg production showed similar trends except for the Bosphorus, where production of copepods was very low due to the low copepod biomass in this area. In all areas, the copepod carbon demand was largely met by phytoplankton and microzooplankton production. However, only a low amount of primary production was consumed by copepods and production appeared to flow mostly through other pathways (microbial loop) and/or sediment on the bottom. The results of this study confirm the hypothesis that there is a substantial differentiation within pelagic food web structure and carbon flow from Bosphorus to the Aegean Sea. © 2010 Elsevier B.V. 36949 We thank the captain and the crew of R/V ‘Aegaeo’ and R/V ‘Bilim’ for shipboard assistance. We also thank T.G. Nielsen for commenting on a draft version of the manuscript and the three anonymous reviewers for their very valuable comments on the manuscript. Research for this paper was supported by the SESAME project (contract no. 036949 ), supported by the European Commission's Sixth Framework Programme on Sustainable Development, Global Change and Ecosystem .

The short-time scale evolution of plankton carbon partitioning and downward flux in the modified Black Sea water (BSW) mass entering the northeast Aegean Sea was studied using a Lagrangian approach (6-10 April 2008). The free-drifting sediment trap positioned at the bottom of the BSW layer and the control drifter, followed the same path within the anticyclone that circulates the BSW in the area. Zooplankton biomass increased (from 159 to 292 mg C m -2 ), as did faecal pellet production (from 5 to 8 mg C m -2 day -1 ), whereas a generally decreasing trend was displayed by particulate organic carbon (POC) (from 2099 to 1440 mg C m -2 ), net primary production and biomass of plankton cells >5 μm (from 32 to 11 mg C m -2 day -1 and from 153 to 124 mg C m -2 , respectively). At the same time, the organic carbon flux increased (from 131 to 311 mg C m -2 day -1 ), due to the contribution of zooplankton detritus (from 30 to 165 mg C m -2 day -1 ). Normalized biomass-size spectra slopes suggest an elevated grazing pressure upon microplankton cells and a non-steady-state ecosystem. Moreover, both the overall shallow slope values and their high correlation to organic carbon flux indicate an increased efficiency of energy transfer to higher trophic levels.

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m − 3 and 14,157 ind m − 3 ) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi , Centropages typicus , Paracalanus parvus . The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera , Oithona copepodites, Oncaea media , Ctenocalanus vanus , Farranula rostrata . During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.

The seasonal distribution of the major components of the zooplankton community, protozooplankton, copepods and cladocerans, along a eutrophication gradient were examined in order to establish if eutrophication through increases in phytoplankton biomass and productivity has an impact on biomass and composition of the zooplankton community. Data on salinity, temperature, inorganic nutrients, chlorophyll a, phytoplankton biomass and production, protozooplankton and mesozoo-plankton biomass were collected during routine sampling at seven stations representing a eutrophication gradient from open to estuarine waters. Mesozooplankton biomass varied seasonally in all areas with a unimodal pattern characterized by a peak between mid-spring and mid-summer. Total mesozooplankton biomass as well as copepod and cladoceran biomass, decreased with DIN concentration. Salinity and the type of ecoystem (well-mixed shallow estuaries versus stratified open waters) were the major factors controlling the biomass proportion of copepod species. Annual production of the total copepod community as well as the total grazing impact of copepods on primary production was higher in open waters than in estuarine waters. In estuarine type ecosystems, phytoplankton production is underexploited by copepod grazing whereas in the open type ecosystems, the phytoplankton production alone could not satisfy the carbon demand of copepods stressing the potential importance of protozoans in the copepod diet.

Aurelia spp. are cosmopolitan scyphozoan species and probably the most studied jellyfish in the world. They inhabit nearshore waters, especially closed basins, such as coastal embayments, fjords and estuaries, occupying a great variety of habitats worldwide. Recent studies have addressed the biogeography of the genus Aurelia and reported that it constitutes a species-complex embracing numerous locally adapted species. The Mediterranean Sea is a hotspot of biodiversity threatened by climate change, which is expected to have a significant influence on the biodiversity and biogeography of marine populations. Here we compiled a comprehensive data set on Aurelia spp. occurrence in the Mediterranean Sea and assessed the thermal niches as well as the phenology of the various populations. Our results indicate that the species biogeography is restricted to temperate areas of the Mediterranean basin, whereas the seasonal pattern generally displayed an unimodal peak that occurs earlier in warmer systems. Our results highlight that the thermal niche of the species, where the bulk of the population (90%) is present, shows a temperature window from 12 to 20°C, although it is further constrained when accounting only for the northern populations of the western and Adriatic basins. Hence, while global warming has been claimed as one of the most important triggers for jellyfish outbreaks, the projected temperature increase of the Mediterranean Sea warns on the shrinking of favorable environmental conditions for the species with the concomitant risk of its potential decline and perhaps extinction in the Mediterranean Sea.

Special issue: Towards a Broader Perspective on Ocean Acidification Research.-- 9 pages, 4 figures, 3 tables, Rising levels of atmospheric CO2 are responsible for a change in the carbonate chemistry of seawater with associated pH drops (acidification) projected to reach 0.4 units from 1950 to 2100. We investigated possible indirect effects of seawater acidification on the feeding, fecundity, and hatching success of the calanoid copepod Acartia grani, mediated by potential CO2-induced changes in the nutritional characteristics of their prey. We used as prey the autotrophic dinoflagellate Heterocapsa sp., cultured at three distinct pH levels (control: 8.17, medium: 7.96, and low: 7.75) by bubbling pure CO2 via a computer automated system. Acartia grani adults collected from a laboratory culture were acclimatized for 3 d at food suspensions of Heterocapsa from each pH treatment (ca. 500 cells ml−1; 300 μg C l−1). Feeding and egg production rates of the preconditioned females did not differ significantly among the three Heterocapsa diets. Egg hatching success, monitored once per day for the 72 h, did not reveal significant difference among treatments. These results are in agreement with the lack of difference in the cellular stoichiometry (C : N, C : P, and N : P ratios) and fatty acid concentration and composition encountered between the three tested Heterocapsa treatments. Our findings disagree with those of other studies using distinct types of prey, suggesting that this kind of indirect influence of acidification on copepods may be largely associated with interspecific differences among prey items with regard to their sensitivity to elevated CO2 levels, The participation of ES andCPwas helped from funding by the Spanish Ministry of Economy and Competitivity through projects CTM2011-23480 and CTM2012-32017, respectively. This work was supported by the project CROA(LS8-1893), implemented within the framework of the Action “Supporting Postdoctoral Researchers” of the Operational Program “Education and Lifelong Learning” (Action’s Beneficiary: General Secretariat for Research and Technology), and is co-financed by the European Social Fund (ESF) and the Greek State

11 pages, 3 figures, 2 tables, Although copepods have been considered tolerant against the direct influence of the ocean acidification (OA) projected for the end of the century, some recent studies have challenged this view. Here, we have examined the direct impact of short-term exposure to a pCO2/pH level relevant for the year 2100 (pHNBS, control: 8.18, low pH: 7.78), on the physiological performance of two representative marine copepods: the calanoid Acartia grani and the cyclopoid Oithona davisae. Adults of both species, from laboratory cultures, were preconditioned for four consecutive days in algal suspensions (Akashiwo sanguinea) prepared with filtered sea water pre-adjusted to the targeted pH values via CO2 bubbling. We measured the feeding and respiratory activity and reproductive output of those pre-conditioned females. The largely unaffected fatty acid composition of the prey offered between OA treatments and controls supports the absence in the study of indirect OA effects (i.e. changes of food nutritional quality). Our results show no direct effect of acidification on the vital rates examined in either copepod species. Our findings are compared with results from previous short-and long-term manipulative experiments on other copepod species. © 2015 The Author, This work was supported by the project CROA (LS8-1893), implemented within the framework of the Action «Supporting Postdoctoral Researchers» of the Operational Program “Education and Lifelong Learning” (Action’s Beneficiary: General Secretariat for Research and Technology), co-financed by the European Social Fund (ESF) and the Greek State. The participation of ES and CP was supported with funding by the Spanish Ministry of Economy and Competitivity through projects CTM2011-23480 and CTM2012-32017, respectively

This study was carried out in the northern Aegean Sea during late summer and spring. The aim was to examine the spatial and temporal distribution of the zooplankton community across a dynamic frontal area and to investigate how the oceanographic heterogeneity structures the composition of the zooplankton assemblages. The low-salinity and cold Black Sea water coming from the Dardanelles Strait is modified by mixing with the underlying warm and saline Aegean water. These hydrological features result in a pronounced thermohaline front in the northern Aegean Sea throughout the year. In both seasons, zooplankton was collected using both 45 and 200 µm mesh plankton nets. A high abundance of zooplankton was observed in the surface layer at the stations closest to the Dardanelles Strait on the stratified side of the front. The zooplankton distribution and community structure in the northern Aegean Sea were strongly influenced by the hydrological features. The frontal structure acts as a boundary for the ...

Two important issues in the studies of harmful algae include ecological role of the toxic compounds and their fate through the food web. The aims of this study were to determine whether the production of domoic acid is a strategy evolved to avoid predation and the role of copepods in the fate of this toxic compound through the food web. The copepod Acartia clausi was fed with single and mixed cultures of the toxic diatom Pseudo-nitzschia multiseries and the non-toxic diatom Pseudo-nitzschia delicatissima. Ingestion rate as a function of diatom abundance was the same for the toxic and non-toxic Pseudo-nitzschia species, indicating no selective feeding behaviour against P. multiseries. The toxins ingested by the copepods did not affect mortality, feeding behaviour, egg production and egg hatching of the copepods. Copepods assimilated the 4.8% of the total domoic acid ingested. Although the amount of toxins daily detoxificated by the copepods was 63.6%, the copepods accumulated domoic acid in their tissues. We conclude that domoic acid is not toxic for copepods and, probably for this reason, this toxin does not act as feeding deterrent for copepods. However, even though the production of domoic acid has apparently not evolved to deter predation, copepods may play an important role on the fate of this toxic compound through the marine food web.

14 pages, 4 figures, 4 tables, Light and nutrient availability are assumed to largely control the dynamics and trophic efficiency of marine planktonic food webs, and are expected to be influenced by climate change (i.e. changes in stratification due to global warming). During an 8-day mesocosm experiment, we investigated the propagation of light energy up to the copepod trophic level in ultra-oligotrophic Eastern Mediterranean waters. Using mesocosms with light treatments corresponding to ca. 40% (L1) and 4.5% (L4) of the incident light intensity (simulating light conditions at 10 and 50–60 m depths, respectively), we assessed the feeding behaviour of the dominant copepod Clausocalanus furcatus every second day. We hypothesized that increased availability of copepod food at higher light levels would result in increased copepod ingestion rates and selectivity. Our results showed that copepod prey (microbial biomass.5 mm) had only a small increase in the L1 and daily rations (DRs) of C. furcatus were similar among treatments (usually ,15% of body carbon). Nevertheless, microplankton was clearly selected in L1, having double the contribution in the DR of C. furcatus compared with L4. A more balanced selective feeding pattern was observed in L4, with nanoplankton also being important in the diet. An increase in the copepod stock in the L1 compared with L4, observed at the end of the experiment, is likely associated with the light-mediated effects on C. furcatus feeding behaviour. We suggest that the importance of light availability on the efficiency of the marine planktonic food web should be further considered by climate change predictive models, This work was supported by the projects LightCopFed, LiMic, LightDynamix and from the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement no. 228224, MESOAQUA, assigned to S.I. (LightCopFed), A.C. (LiMic), R.P., R.P (LightDynamix). R.P. acknowledges additional financial support by the German Research Council (DFG Pt 5/ 3-1)

In the last two decades of the twentieth century, the ctenophore Mnemiopsis leidyi (A. Agassiz) has invaded the Black, Azov, Marmara and Aegean Seas, and, recently, the Caspian Sea. Here, we compare its spatial and temporal distribution, seasonal dynamics and the time and duration of reproduction. We also discuss factors that control its abundance throughout its invasive range and its effect on ecosystems. Observations are based on the long-term field data collected by three research institutes. An analysis of the effects of temperature, salinity, prey (zoo- and ichthyoplankton) availability and predation (by ctenophores of the genus Beroe) on M. leidyi population size, and the effects of M. leidyi on zoo- and ichthyoplankton, and on fish populations in the Black and Azov Seas is also provided. With the Black Sea current, M. leidyi spreads to the upper layers of the Sea of Marmara, where it now occurs around the year. At regular intervals, the Black Sea current also takes it to the northern Aegean Sea. In contrast, it has to re-invade the Sea of Azov every spring or summer, dying out during winter when the temperature drops below 4°C. The warm summer and mild winter temperatures, relatively low salinity and abundance of prey in the Black Sea are close to optimal for M. leidyi, while they are suboptimal in the northern Aegean Sea, where salinity and temperature are often too high. In the Black Sea the absence of gelatinous and other predators led to an enormous ctenophore abundance for a decade, but with the appearance of Beroe ovata in 1999, M. leidyi abundance greatly decreased. Analysis of seasonal dynamics of M. leidyi in the Black Sea and in other seas of the Mediterranean basin indicates similarities in the timing of maximum abundance and biomass, in spite of some differences in the initiation and duration of reproduction. A peak biomass and density occurred in 1989 in the Black and Azov Seas and in 1990 in the other seas. The M. leidyi invasion negatively affected the ecosystems of the Black Sea and the Sea of Azov. The zooplankton, ichthyoplankton and zooplanktivorous fish stocks all underwent profound changes. Similar effects, but less pronounced, were recorded in the Sea of Marmara. Effects on Mediterranean food chains have, so far, remained insignificant. Salinity is probably supraoptimal here, and several predators prevent M. leidyi from reaching outbreak levels.

12 pages, 9 figures, 2 tables, Plankton biomass and composition in the pelagic zone of oceans is exposed to changes in availability of light and nutrients due to large-scale ocean circulation and water column stratification. We hypothesized that displacement of plankton from surface to deeper darker waters would not only favor heterotrophy over time, as previously suggested, but also first rapidly affect the level of mixotrophy and, consequently, overall microbial grazing in plankton food webs. To test this in an oligotrophic marine system we incubated Eastern Mediterranean water (from 10 m depth north of Crete in September 2010) in 2.8 m3 mesocosms simulating two different light intensities at the sampling station, surface waters (ca. 10 m; mesocosms L1) and deeper layers (ca. 50–60 m; mesocosms L4). The biomass and abundance of the main planktonic groups were monitored either daily or every second day, depending on the group. Microzooplankton grazing rates and the contribution of mixotrophic feeding were estimated by a combination of dilution experiments and incubations with live fluorescently labeled algae (LFLA). Although no nutrients were added to the mesocosms the chlorophyll a increased during the first 2 days of the experiment in both treatments. This increase resulted from phytoplankton growth in the light L1-mesocosm (autotrophic biomass was ca. doubled in L1 compared to L4), but was mostly due to photoadaptation of the algae in the L4-mesocosm, as indicated by lower carbon to chlorophyll a ratios. By the end of the experiment, the total biomass of protozoan and metazoan grazers in L1 was ca. twofold higher than in L4. The microzooplankton responded within the first 24 h, showing different grazing activity in L1 than in L4. Microzooplankton grazing rates on total Chl a were similar in both treatments; however, phytoplankton instantaneous growth rates were higher in the more illuminated mesocosm. This resulted in a closer coupling between both rates in L4, where all production was grazed daily, than in L1. Nevertheless, the overall flux of carbon through the microzooplankton was 33–60% higher in L1 compared to L4 throughout the experiment. The fraction of mixotrophy in the ciliate community varied in L1 (20–50%), but decreased over time in L4 from 50% to 30%. Our results do not support studies from freshwater, postulating that reduced light and nutrient limitation may increase herbivore production due to stoichiometric effects. Finally, we discuss how mixotrophy may bias rate estimates in dilution experiments, This work was funded through projects CTM2009-08783 from the SpanishMinistry of Science and Innovation assigned to A.C. and projects LightDynamix, LiMic and LightCopFed from the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement no. 228224, MESOAQUA, assigned to R.P. (LightDynamix), A.C., R.A.M., A.S., A.G. (LiMic) and S.I. (LightCopFed)

We analyzed and compared Mediterranean mesozooplankton time series spanning 1957–2006 from six coastal stations in the Balearic, Ligurian, Tyrrhenian, North and Middle Adriatic and Aegean Sea. Our analysis focused on fluctuations of major zooplankton taxonomic groups and their relation with environmental and climatic variability. Average seasonal cycles and interannual trends were derived. Stations spanned a large range of trophic status from oligotrophic to moderately eutrophic. Intra-station analyses showed (1) coherent multi-taxa trends off Villefranche sur mer that diverge from the previous results found at species level, (2) in Baleares, covariation of zooplankton and water masses as a consequence of the boundary hydrographic regime in the middle Western Mediterranean, (3) decrease in trophic status and abundance of some taxonomic groups off Naples, and (4) off Athens, an increase of zooplankton abundance and decrease in chlorophyll possibly caused by reduction of anthropogenic nutrient input, increase of microbial components, and more efficient grazing control on phytoplankton. (5) At basin scale, the analysis of temperature revealed significant positive correlations between Villefranche, Trieste and Naples for annual and/or winter average, and synchronous abrupt cooling and warming events centered in 1987 at the same three sites. After correction for multiple comparisons, we found no significant correlations between climate indices and local temperature or zooplankton abundance, nor between stations for zooplankton abundance, therefore we suggest that for these coastal stations local drivers (climatic, anthropogenic) are dominant and that the link between local and larger scale of climate should be investigated further if we are to understand zooplankton fluctuations, Sí

The percentage contribution of F. rostrata to total copepod abundance was found to be low at the coastal areas. The minima of the contribution of F. rostrata generally occurred during summer, and the maxima in autumn – winter. In offshore waters of the eastern Mediterranean, F. rostrata dominated mainly during the warm period, whereas in the Balearic Sea, higher percentages were found during spring. Its reproductive strategy, trophic modes, and population dynamics should be investigated in order to understand the ecological role of this species within the pelagic foodweb of oligotrophic areas.

Historical data on physico-chemical and biological characteristics during the last 30 years were used to examine the seasonal nutrient and plankton dynamics in a semi-enclosed area of the Mediterranean Sea, Elefsis Bay, in phase with the development of intermittently hypoxic and anoxic conditions. Sediment records covering most of the Holocene showed that the area was affected by hypoxia and/or anoxia in the past. However, the occurrence of hypoxia in Elefsis Bay need not necessarily be attributed to anthropogenic activities but could be naturally driven by oceanographic – climate forcing.

Non-calanoid copepods (i.e., Cyclopoida), have been found to be numerically important in oligotrophic seas and to have such ecological impacts that set them apart from most other planktonic crustaceans. Despite the presence of the cyclopoid genus Farranula in the Mediterranean Sea and Atlantic Ocean, the available information regarding its spatial and temporal distribution is rather limited. Based on data collected from open sea as well as coastal regions of the Mediterranean Sea, we present here the first comparative overview of the spatial and seasonal variability of F.rostrata in different Mediterranean regions. We have studied data collected at four ongoing zooplankton time-series stations in the Mallorca Island (Balearic Sea), the Gulf of Naples (Tyrrhenian Sea), the northern Adriatic Sea and the Saronikos Gulf (Aegean Sea) as well as monthly data collected during one year in the Bay of Tunis. At all five areas, the absolute abundance of the species as well as the relative abundance among copepods was found to be low ; thus the species has not been referred among the abundant or even common species in the above areas, either in other Mediterranean coastal areas according to the literature, being almost absent in confined areas. Regarding the offshore waters, the relative abundance of this species is more important among copepods than in coastal waters and it seems to be a key component of the zooplankton in very oligotrophic areas e.g during June 1999, F. rostrata accounted more than 30% of the total copepod community (exceeding that Corycaeus spp. and Oncaea spp.) in the ultra-oligotrophic environment of Levantine Sea, and its contribution decreased gradually towards the West Mediterranean Sea. F.rostrata population was perennial throughout the year in almost all studied coastal areas. Pronounced seasonal signals in abundance and relative abundance were observed for F.rostrata in the Gulf of Naples, the northern Adriatic Sea, the Bay of Tunis and the Saronikos Gulf ; abundance minima occurred during early summer and maxima in fall-winter, when the influence of open sea becomes more intense in coastal waters. No clear seasonal pattern was observed in Balearic Sea, an area largely and continuously affected by offshore waters ; the above patterns confirm the pelagic character of the species. These first observations suggest that aspects of the reproductive strategy and its feeding behavior should be investigated in order in understand the ecological role of this species within the pelagic food web of oligotrophic areas.

The distribution of the ctenophore Mnemiopsis leidyi in the Aegean Sea, influenced by the outflow of Black Sea water through the Dardanelles strait, has been studied since 1990. Swarms were found in the northern Aegean Sea, mainly near coastal areas, but at a lower abundance (max 150 ind. m−2) than in the Black Sea. It has been hypothesized that M. leidyi was introduced to the Aegean Sea by Black Sea water, but its presence in Saronikos Gulf and Elefsis Bay, located in the central Aegean Sea, could equally well be attributed to introduction with ballast water. No impact of M. leidyi on the mesozooplankton communities was detected in the northern Aegean Sea, probably due to its low abundance. During July 2001 the distribution and biology of M. leidyi and of the indigenous Bolinopsis vitrea were studied in the northeastern Aegean Sea. The abundance of both species was extremely low. The M. leidyi of the Aegean Sea were smaller and probably had lower fecundity than those of the Black Sea, but they presented higher metabolic and clearance rates. The metabolic and clearance rates of Bolinopsis vitrea were lower than those of M. leidyi of the same length. These results suggest a regulation of the ctenophore populations in the oligotrophic waters of the Aegean Sea.

This study includes the first information on the combined effect of low pH and raised temperature on egg production rate (EP), hatching success (HS), excretion and respiration of the Mediterranean copepod Acartia clausi. Adult individuals of A. clausi and fresh surface seawater were collected at a coastal station in Saronikos Gulf during April 2012. Four different conditions were applied: two different pH levels (present: 8.09 and future: 7.83) at two temperature values (present: 16°C and present+4 °C= 20°C). EP and HS success decreased significantly over the duration of exposure at future pH at both temperature conditions. However, the analysis of the combined effect of pH, T, chlorophyll α and the duration of the experiments on EP and HS revealed that ocean acidification had no discernible effect, whereas warming; food and the duration of exposure were more significant for the reproductive output of A. clausi. Temperature appeared to have a positive effect on respiration and excretion. Acidification had no clear effect on respiration, but a negative effect on the A. clausi excretion was observed. Acidification and warming resulted in the increase of the excretion rate and the increase was higher than that observed by warming only. Our findings showed that a direct effect of ocean acidification on copepod’s vital rates was not obvious, except maybe in the case of excretion. Therefore, the combination of acidification with the ambient oligotrophic conditions and the warming could result in species being less able to allocate resources for coping with multiple stressors.

The study was carried out in the Skagerrak during late summer when population development in the pelagic cycle culminated in the yearly maximum in zooplankton biomass. The cyclonic circulation of surface water masses created the characteristic “dome-shaped” pycnocline across the Skagerrak. The large dinoflagellate Ceratium furca dominated the phytoplankton biomass. Ciliates and heterotrophic dinoflagellates were the major grazers and, potentially, consumed 43–166% of daily primary production. The grazing impact of copepods was estimated from specific egg production rates and grazing experiments. The degree of herbivory differed between species (14–85%), but coprophagy (e.g. feeding on fecal pellets) and ingestion of microzooplankton were also important. The appendicularian Oikopleura dioica was present in lower numbers than copepods, but cleared a large volume of water. The grazing impact of copepods and O. dioica was estimated to 57±24% and 12±12% of daily primary production, respectively. Sedimentation of organic material (30 m) varied between 169 and 708 mg C m−2 day−1, and the contribution from the mesozooplankton (copepod fecal pellets and mucus houses with attached phytodetritus of O. dioica) was 5–33% of this sedimentation. Recycling of fecal pellets and mucus houses in the euphotic zone was 59% and 36%, respectively. However, there was a high respiration of organic material by microorganisms in the mid-water column, and 34% of the sedimenting material actually reached the benthic community in the deep, central part of the Skagerrak.