Fish feeding guilds in shallow rocky and soft bottom areas on the Swedish west coast

Multivariate analyses of the diet relationships within fish assemblages resulted in the recognition of five major feeding guilds in shallow (0-9 m) rocky- and soft-bottom habitats on the Swedish west coast. The two habitats had similar guilds that could be broadly described as piscivores, decapod feeders, copepod feeders and amphipod and mollusc feeders. The last guild on the rocky bottom was more strictly one of amphipod feeders, whereas the last guild on soft bottoms consisted of infauna feeders (mainly polychaetes and bivalves). A comparison of the diet of the entire fish assemblages showed that amphipods and gastropods were more prevalent as prey taxa on rocky bottoms, whereas bivalves and teleosts were more common on soft bottoms. A large proportion (12 out of 27 species) of the fish species investigated was found in both habitats. These fish species generally seemed to rely on the same major prey groups in the two habitats, but also included prey taxa characteristic for the habitat in which they resided. Sea trout Salmo trutta , mackerel Scombrus scombrus , saithe Pollachius virens , whiting Merlangius merlangus and cod Gadus morhua were found to be the major piscivores in both the investigated habitats. Herring Clupea harengus and members of the family gobiidae were the most prevalent prey species for piscivores within the rocky- as well as the soft-bottom fish assemblage. A comparison of feeding modes suggested that the fish assemblage on rocky bottoms predominantly (60% of the food items) relied on food found in vegetation. On soft bottoms, the average distribution of food categories among fish species was 44% benthic, 35% pelagic and 21% vegetation-associated food items.     

Environmental variables explaining structural and functional diversity of seagrass macrofauna in an archipelago landscape

Thirteen seagrass beds located over a 80-km range in the brackish waters of SW, Finland, northern Baltic Sea were investigated in order to determine the environmental variables important for univariate community measures and for number, composition and redundancy of functional groups of benthic macrofauna. For species assemblages, fetch and shore angle were the best explanatory variables, followed by sediment granulometry (fine gravel) and then sediment organics. Similarly, fetch, shore angle and Zostera marina shoot density were the best explanatory variables for functional group patterns. Small (< 50 m²) inner-archipelago beds were functionally and structurally equal to the most extensive (500 to > 1000 m²) seagrass beds in the study area. Community measures (density, number of species and diversity) and functional diversity (number of functional groups) equalled or exceeded levels previously recorded in deeper, non-vegetated communities in the northern Baltic Sea. In comparison with marine seagrass assemblages, the total number of species and number of species per function were low. However, species density and derived diversity measures (Shannon-Wieners index H′) equalled or exceeded those reported for other seagrass ecosystems. It is concluded that in terms of seagrass infauna, the Baltic Sea should not be regarded species poor, as is often generally stated, and that conservation initiatives and management strategies should consider both minor as well as more extensive occurrences of seagrasses in coastal waters.     

Phylogeographical structure, distribution and genetic variation of the green algae Ulva intestinalis and U. compressa (Chlorophyta) in the Baltic Sea area

The marine algae Ulva intestinalis and U. compressa are morphologically plastic with many overlapping characters and are therefore difficult to distinguish from each other. The present distribution of U. intestinalis and U. compressa is investigated along the salinity gradient in the Baltic Sea area through analyses of internal transcribed spacer (ITS) sequence data. Also, the amount and distribution of intraspecific genetic polymorphism in the ITS region is studied allowing inferences on the phylogeographical pattern and postglacial recolonization of the Baltic Sea area. The data show that of the two species only U. intestinalis occurs in the Baltic Sea. The distribution of U. compressa is more restricted than previously reported, and it was not found in salinities lower than 15 ppt. All of Scandinavia and the Baltic Sea were covered with ice during the last ice age and the organisms in the Baltic Sea must have colonized the area after the ice had started to melt. The genetic diversity of U. intestinalis and U. compressa in the Baltic Sea and the neighbouring area was found to be reduced compared to that in the British Isles. This reduction may be the result of either a historical reduction of diversity or an adaptation of specific clones to the northern environmental conditions.     

Changes of the macrozoobenthos at 3 monitoring stations in the western Baltic Sea and the Sound

The soft bottom fauna of the western Baltic Sea and the Sound has been sampled and analysed every year since 1979 under the Baltic Monitoring Programme. Furthermore, benthos studies have been carried out in the area at intervals from as far back as 1871. In the area a distinct halocline exists between the overlying low saline Baltic water and the high saline North Sea water.The variation in the species richness, abundance and biomass of the soft bottom fauna is mainly related to 3 abiotic factors.First, many species live at the limit of their distribution. The low salinity of the Baltic Sea prevents their penetration into the Baltic proper. However, the marine species may be able to survive and grow but not to reproduce. Consequently, the population will depend on an influx of larvae for it's survival.Second, the distinct halocline prevents the transport of oxygen to the deeper parts of the Baltic Sea. Oxygen will be supplied under special weather conditions where inflow of high-saline oxygen rich North Sea water occur. The incidences of salt water inflow have increases in the last four decades.Third, an increasing load of the Baltic Sea with nutrients and organic matter has influenced the fauna. The result have been an increased biomass of the benthos above the halocline. Below the halocline the result has been a decrease in the biomass and a change in the species composition.     

Evaluating the performance of benthic multi-metric indices across broad-scale environmental gradients

The usefulness of benthic multi-metric indices when assessing seafloor integrity across broad environmental gradients should be deliberated, as their lack of transparency might hide important sources of variation and fail to identify environmental change. This study compares the performance of two multi-metric indices; the Benthic Quality Index (BQI) and the Brackish water Benthic Index (BBI) between three sub-basins in the Baltic Sea. Both indices reflect the salinity-driven gradient in macroinvertebrate diversity and composition as well as changes in bottom water oxygen concentrations. The relative contribution of predictor variables for explaining index variation does, however, differ between sub-basins, resulting in the indices representing different aspects of the benthic community along the environmental gradient. This context-dependency is caused by inherent differences in benthic community characteristics between the sub-basins of the Baltic Sea, and how the communities are portrayed by the indices. An increased transparency of the importance of the different predictors for directing index values is needed for coherent classifications over broad environmental gradients, such as those occurring in large estuarine water bodies. Use of a weight of evidence table to combine multiple indicators would preserve transparency and be more likely to provide a robust assessment method that would detect seafloor degradation at an early stage.     

Marine landscapes and population genetic structure of herring (Clupea harengus L.) in the Baltic Sea

Numerically small but statistically significant genetic differentiation has been found in many marine fish species despite very large census population sizes and absence of obvious barriers to migrating individuals. Analyses of morphological traits have previously identified local spawning groups of herring (Clupea harengus L.) in the environmentally heterogeneous Baltic Sea, whereas allozyme markers have not revealed differentiation. We analysed variation at nine microsatellite loci in 24 samples of spring-spawning herring collected at 11 spawning locations throughout the Baltic Sea. Significant temporal differentiation was observed at two locations, which we ascribe to sympatrically spawning but genetically divergent 'spawning waves'. Significant differentiation was also present on a geographical scale, though pairwise F(ST) values were generally low, not exceeding 0.027. Partial Mantel tests showed no isolation by geographical distance, but significant associations were observed between genetic differentiation and environmental parameters (salinity and surface temperature) (0.001 < P < or = 0.099), though these outcomes were driven mainly by populations in the southwestern Baltic Sea, which also exhibits the steepest environmental gradients. Application of a novel method for detecting barriers to gene flow by combining geographical coordinates and genetic differentiation allowed us to identify two zones of lowered gene flow. These zones were concordant with the separation of the Baltic Sea into major basins, with environmental gradients and with differences in migration behaviour. We suggest that similar use of landscape genetics approaches may increase the understanding of the biological significance of genetic differentiation in other marine fishes.     

Applying macrophyte community indicators to assess anthropogenic pressures on shallow soft bottoms

Vegetated soft bottoms are under pressure due to a number of anthropogenic stressors, such as coastal exploitation and eutrophication. The ecological value of these biotopes has gained recognition through international conventions and the EU directives, which request methods for assessment of the environmental status of coastal areas. However, currently there is no appropriate method for assessing the status of shallow vegetated soft bottoms in the northern Baltic Sea. Therefore, we developed a macrophyte community index and tested its response in relation to important pressures (eutrophication and boating activity) and natural gradients (topographic openness, depth and salinity) on shallow bays in the northern Baltic Sea. The macrophyte index, and hence the proportion of sensitive to tolerant species, decreased with increasing phosphorus concentration, turbidity and level of boating activity, while the cumulative cover of macrophytes only showed a negative trend in response to increasing turbidity. Juvenile fish abundance was positively related to the index, indicating importance of sensitive macrophyte species for ecosystem functioning. As the index was tested in a wide geographic area, and showed a uniform response across natural gradients, it is a promising tool for assessment of environmental status that may be applied also in other vegetated soft-bottom areas.     

The gulf of Bothnia: The northernmost part of the Baltic Sea

Summary The Baltic Sea, one of the largest brackish water areas in the world, can be characterized as a young, cold sea containing an impoverished ecosystem due to salinity stress. The present Baltic Sea was formed as late as 2000 to 2500 years ago when the Danish sounds became more narrow and shallow. The inflow of freshwater from the surrounding land areas caused the Baltic to gradually attain its brackish character. Today the Baltic covers an area of some 366,000 km² as a series of basins separated by shallower areas and filled with about 22,000 km³ of brackish water. These basins are, from north to south, the Gulf of Bothnia, the Gulf of Finland, the Gotland Sea and the Bornholm Sea. The climate gradient ranges from almost arctic conditions in the extreme north to a more maritime climate in the southern parts. The North Sea salt water is connected to the Baltic through the shallow Kattegat and the sills in the Danish sounds. The inflow of salt water occurs in two different ways,viz. as a continuous flow along the bottom due to the salinity gradient and as pulses of salt water generated by the distribution of air pressure and the direction of the wind. The freshwater input (500 km³) from mainly the large rivers equals roughly the net outflow and stresses the south-bound current along the Swedish coast that also compensates for the salt water inflow. Tidal movements can be seen in the southern Baltic, but are of minor importance for the system. The residence time of the total water mass is 25 years and the hydrographical conditions within the different basins are stable and dominated by a permanent halocline, and a thermocline developing every spring. The salinity ranges from about 1–2 per mille in the innermost part of the Gulf of Bothnia to 10–15 per mille in the Bornholm Sea. Total vertical mixing takes place during winter in at least the northern parts of the sea. Due to the climate-gradient, the ice condition differs from about four months of total ice-cover in the inner parts of the Gulf of Bothnia to one month or less of coastal ice in the southern part of the Baltic. Thus, the seasonal effect is more pronounced in the northern parts.The living systems of the Baltic are reduced and adapted to these varying conditions. When comparing the deeper soft bottoms of the Gulf of Bothnia to the rest of the Baltic, the following pattern can be seen. The pelagic primary productivity increases by a factor 6 from north to south. The southern parts of the sea show a pronounced spring peak, while in the north the spring development is delayed or replaced by a summer maximum. The total increase of the macrofauna biomass is striking, from about 1 g.m^–2 (w.wt) in the north to 100 g.m^–2 (w.wt) or more in the south. The meiofauna and the zooplankton biomasses show less variability. The meiofauna increases by a factor of 2–4, giving a biomass of about twice that of the macrofauna in the northernmost part. The extremely low salinity of this area causes the exclusion of bivalves (filter-feeders) from the fauna. Available data, pooled with the high metabolic rate of the meiofauna, roughly follow the changes in primary productivity within the Baltic Sea. The changing ratio of macro- to meiofauna, as well as results from intensive studies of the macrobenthic amphipodPontoporeia affinis (Lindström), suggest that the macrofauna is regulated mainly by food limitation and that the benthic and pelagic systems are closely coupled.     

Zoobenthic diversity-gradients in the Baltic Sea: Continuous post-glacial succession in a stressed ecosystem

The Baltic Sea, formed after the latest glaciation, is an enclosed, low-saline, non-tidal ecosystem and has steep latitudinal and vertical gradients from sub-arctic conditions in the north to temperate in the south. The sea has undergone rapid changes since the glaciation, and the “ecological age” of the present ecosystem is only about 8000 years. Primary successional processes are still ongoing, and numerous ecological niches (e.g. large-bodied sediment bioturbators) remain available for immigration. The system is species-poor and vulnerable to the threat of exotic invasive species, and to date about 50 zoobenthic species have established populations in the Baltic Sea. The present biota is a mixture of species of different ecological and zoogeographical origin (marine to limnic; northern Arctic marine and limnic, to North Sea and Atlantic marine). The current distribution patterns of zoobenthos are illustrated, using marine, limnic and non-indigenous examples of structure and ecosystem functions. The species richness decreases from over 1600 marine benthic species in the open Skagerrak to about 500 in the western parts of the Baltic Sea, approximately 80 in the southern regions, to less than 20 in the northern regions. On the other hand, limnic species increase diversity in the inner reaches of the Gulf of Finland and the Gulf of Bothnia. Polychaetes, molluscs and echinoderms are dramatically reduced in numbers from the south to the north.     

Comparative analysis of ammonia monooxygenase (amoA) genes in the water column and sediment-water interface of two lakes and the Baltic Sea

The functional gene amoA was used to compare the diversity of ammonia-oxidizing bacteria (AOB) in the water column and sediment-water interface of the two freshwater lakes Plusssee and Sch?hsee and the Baltic Sea. Nested amplifications were used to increase the sensitivity of amoA detection, and to amplify a 789-bp fragment from which clone libraries were prepared. The larger part of the sequences was only distantly related to any of the cultured AOB and is considered to represent new clusters of AOB within the Nitrosomonas/Nitrosospira group. Almost all sequences from the water column of the Baltic Sea and from 1-m depth of Sch?hsee were related to different Nitrosospira clusters 0 and 2, respectively. The majority of sequences from Plusssee and Sch?hsee were associated with sequences from Chesapeake Bay, from a previous study of Plusssee and from rice roots in Nitrosospira-like cluster A, which lacks sequences from Baltic Sea. Two groups of sequences from Baltic Sea sediment were related to clonal sequences from other brackish/marine habitats in the purely environmental Nitrosospira-like cluster B and the Nitrosomonas-like cluster. This confirms previous results from 16S rRNA gene libraries that indicated the existence of hitherto uncultivated AOB in lake and Baltic Sea samples, and showed a differential distribution of AOB along the water column and sediment of these environments.     

Spreading and physico-biological reproduction limitations of the invasive American comb jelly Mnemiopsis leidyi in the Baltic Sea

Hydrodynamic drift modeling was used to investigate the potential dispersion of Mnemiopsis leidyi from the Bornholm Basin in the Baltic Sea where it has been observed since 2007 further to the east and north. In the brackish surface layer dispersion is mainly driven by wind, while within the halocline dispersion is mainly controlled by the baroclinic flow field and bottom topography. Model runs showed that the natural spreading via deep water currents from the Bornholm Basin towards north and east is limited by topographic features and low advection velocities. Based on the information on ranges of salinity and temperature, which limit survival and reproduction of this ctenophore within the Baltic Sea, areas have been identified where the American comb jelly, M. leidyi could potentially survive and reproduce. While, we could show that M. leidyi might survive in vast areas of the northern Baltic Sea its reproduction is prevented by low salinity (<10 psu) and temperature (<12°C). Thus, due to the combined effect of low salinity and temperature, it is not probable that M. leidyi could establish permanent populations in the central or northern Baltic Sea. However, it seems that in the southern parts of the Baltic Sea environmental conditions are suitable for a successful reproduction of M. leidyi.     

Evolution at two time frames: ancient structural variants involved in post-glacial divergence of the European plaice (Pleuronectes platessa)

Changing environmental conditions can lead to population diversification through differential selection on standing genetic variation. Structural variant (SV) polymorphisms provide examples of ancient alleles that in time become associated with novel environmental gradients. The European plaice (Pleuronectes platessa) is a marine flatfish showing large allele-frequency differences at two putative SVs associated with environmental variation. In this study, we explored the contribution of these SVs to population structure across the North East Atlantic. We compared genome-wide population structure using sets of RAD-sequencing SNPs with the spatial structure of the SVs. We found that in contrast to the rest of the genome, the SVs were only weakly associated with an isolation-by-distance pattern. Indeed, both SVs showed important variation in haplogroup frequencies, with the same haplogroup increasing both along the salinity gradient of the Baltic Sea, and found in high frequency in the northern-range margin of the Atlantic. Phylogenetic analyses suggested that the SV alleles are much older than the age of the Baltic Sea itself. These results suggest that the SVs are older than the age of the environmental gradients with which they currently co-vary. Altogether, our results suggest that the plaice SVs were shaped by evolutionary processes occurring at two time frames, firstly following their origin, ancient spread and maintenance in the ancestral populations, and secondly related to their current association with more recently formed environmental gradients such as those found in the North Sea–Baltic Sea transition zone.Subject terms: Structural variation, Population genetics     

Linking denitrifier community structure and prevalent biogeochemical parameters in the pelagial of the central Baltic Proper (Baltic Sea)

The oxic-anoxic interface of the water column of the Gotland Basin (central Baltic Sea) is characterised by defined biogeochemical gradients and is hypothesised to be a zone of pronounced denitrification. Our aim was to analyse the composition and distribution of pelagic denitrifying microorganisms in relation to the physico-chemical gradients in the water column. PCR-amplified nirS genes--coding for dissimilatory nitrite reductase--were analysed as functional markers by terminal restriction fragment length polymorphism and cloning. The overall nirS diversity was low, with the lowest levels found at the oxic-anoxic interface. Only a few terminal restriction fragments dominated the denitrifier communities throughout the water column, and these could be assigned to several new Baltic Sea clusters that were revealed by phylogenetic analysis. The novel clusters were separated in two groups corresponding to the oxygen concentrations within specific layers of the water column. Gradients of prevalent biogeochemical parameters (H(2)S, NH(4) (+), NO(3) (-) and O(2)) largely determined the composition of the nirS-type denitrifier communities within the water column of the Gotland Basin.     

Modelling spatial patterns in harbour porpoise satellite telemetry data using maximum entropy

The distribution of harbour porpoises in EU waters is poorly understood, and modelled predictions of their distributions could inform the strategic spatial planning of future exploitation of the marine environment to avoid potential conflicts. We analysed satellite telemetry data from 39 harbour porpoises Phocoena phocoena in inner Danish waters using a modelling tool rooted in maximum entropy: Maxent. Maxent does not require absence data and has been shown to be effective for data characterised by small sample size, sampling bias and locational errors. For each season we used an iterative bootstrapping procedure to randomly select among the most precise records from each of the 39 tagged individuals, and ran Maxent on pooled records based on explanatory environmental variables hypothesised to serve as good proxies for harbour porpoise prey abundance. Among our environmental variables, distance to coast and bottom salinity had the most explanatory power, and their response shapes were relatively consistent across most seasons. The predictive power of the models (assessed by ROC‐AUC) ranged from 0.70 to 0.86 within seasons. The southern Kattegat, the Belt Seas, most western part of the Baltic Sea and the Sound were predicted to have relatively high probabilities of occurrence across seasons. In contrast, the central part of Kattegat and the Baltic Sea south and east of Limhamn and Darss Ridge consistently showed low probabilities of occurrence. Areas with the lowest probabilities of occurrence were generally characterised by high predictive uncertainty. Our methods have implications for the analyses of satellite tagged animals in terrestrial and marine environments. By coupling a bootstrapping procedure with Maxent we circumvented some of the statistical challenges presented by satellite telemetry data to generate spatial predictions within the inner Danish waters.     

Environmental Drivers of the Canadian Arctic Megabenthic Communities

Environmental gradients and their influence on benthic community structure vary over different spatial scales; yet, few studies in the Arctic have attempted to study the influence of environmental gradients of differing spatial scales on megabenthic communities across continental-scales. The current project studied for the first time how megabenthic community structure is related to several environmental factors over 2000 km of the Canadian Arctic, from the Beaufort Sea to northern Baffin Bay. Faunal trawl samples were collected between 2007 and 2011 at 78 stations from 30 to 1000 m depth and patterns in biomass, density, richness, diversity, and taxonomic composition were examined in relation to indirect/spatial gradients (e.g., depth), direct gradients (e.g., bottom oceanographic variables), and resource gradients (e.g., food supply proxies). Six benthic community types were defined based on their biomass-based taxonomic composition. Their distribution was significantly, but moderately, associated with large-scale (100–1000 km) environmental gradients defined by depth, physical water properties (e.g., bottom salinity), and meso-scale (10–100 km) environmental gradients defined by substrate type (hard vs. soft) and sediment organic carbon content. We did not observe a strong decline of bulk biomass, density and richness with depth or a strong increase of those community characteristics with food supply proxies, contrary to our hypothesis. We discuss how local- to meso-scale environmental conditions, such as bottom current regimes and polynyas, sustain biomass-rich communities at specific locations in oligotrophic and in deep regions of the Canadian Arctic. This study demonstrates the value of considering the scales of variability of environmental gradients when interpreting their relevance in structuring of communities.     

Geographic variation in fitness‐related traits of the bladderwrack Fucus vesiculosus along the Baltic Sea‐North Sea salinity gradient

In the course of the ongoing global intensification and diversification of human pressures, the study of variation patterns of biological traits along environmental gradients can provide relevant information on the performance of species under shifting conditions. The pronounced salinity gradient, co‐occurrence of multiple stressors, and accelerated rates of change make the Baltic Sea and its transition to North Sea a suitable region for this type of study. Focusing on the bladderwrack Fucus vesiculosus, one of the main foundation species on hard‐bottoms of the Baltic Sea, we analyzed the phenotypic variation among populations occurring along 2,000 km of coasts subjected to salinities from 4 to >30 and a variety of other stressors. Morphological and biochemical traits, including palatability for grazers, were recorded at 20 stations along the Baltic Sea and four stations in the North Sea. We evaluated in a common modeling framework the relative contribution of multiple environmental drivers to the observed trait patterns. Salinity was the main and, in some cases, the only environmental driver of the geographic trait variation in F. vesiculosus. The decrease in salinity from North Sea to Baltic Sea stations was accompanied by a decline in thallus size, photosynthetic pigments, and energy storage compounds, and affected the interaction of the alga with herbivores and epibiota. For some traits, drivers that vary locally such as wave exposure, light availability or nutrient enrichment were also important. The strong genetic population structure in this macroalgae might play a role in the generation and maintenance of phenotypic patterns across geographic scales. In light of our results, the desalination process projected for the Baltic Sea could have detrimental impacts on F. vesiculosus in areas close to its tolerance limit, affecting ecosystem functions such as habitat formation, primary production, and food supply.     

An environmental gradient dominates ecological and genetic differentiation of marine invertebrates between the North and Baltic Sea

Environmental gradients have emerged as important barriers to structuring populations and species distributions. We set out to test whether the strong salinity gradient from the marine North Sea to the brackish Baltic Sea in northern Europe represents an ecological and genetic break, and to identify life history traits that correlate with the strength of this break. We accumulated mitochondrial cytochrome oxidase subunit 1 sequence data, and data on the distribution, salinity tolerance, and life history for 28 species belonging to the Cnidaria, Crustacea, Echinodermata, Mollusca, Polychaeta, and Gastrotricha. We included seven non‐native species covering a broad range of times since introduction, in order to gain insight into the pace of adaptation and differentiation. We calculated measures of genetic diversity and differentiation across the environmental gradient, coalescent times, and migration rates between North and Baltic Sea populations, and analyzed correlations between genetic and life history data. The majority of investigated species is either genetically differentiated and/or adapted to the lower salinity conditions of the Baltic Sea. Species exhibiting population structure have a range of patterns of genetic diversity in comparison with the North Sea, from lower in the Baltic Sea to higher in the Baltic Sea, or equally diverse in North and Baltic Sea. Two of the non‐native species showed signs of genetic differentiation, their times since introduction to the Baltic Sea being about 80 and >700 years, respectively. Our results indicate that the transition from North Sea to Baltic Sea represents a genetic and ecological break: The diversity of genetic patterns points toward independent trajectories in the Baltic compared with the North Sea, and ecological differences with regard to salinity tolerance are common. The North Sea–Baltic Sea region provides a unique setting to study evolutionary adaptation during colonization processes at different stages by jointly considering native and non‐native species.     

Uptake of dissolved organic nitrogen by size-fractionated plankton along a salinity gradient from the North Sea to the Baltic Sea

The Baltic Sea is known for its ecological problems due to eutrophication caused by high nutrient input via nitrogen fixation and rivers, which deliver up to 70% of nitrogen in the form of dissolved organic nitrogen (DON) compounds. We therefore measured organic nitrogen uptake rates using self produced ¹⁵N labeled allochthonous (derived from Brassica napus and Phragmites sp.) and autochthonous (derived from Skeletonema costatum) DON at twelve stations along a salinity gradient (34 to 2) from the North Sea to the Baltic Sea in August/September 2009. Both labeled DON sources were exploited by the size fractions 0.2–1.6 μm (bacteria size fraction) and >1.6 μm (phytoplankton size fraction). Higher DON uptake rates were measured in the Baltic Sea compared to the North Sea, with rates of up to 1213 nmol N l^?1 h^?1. The autochthonous DON was the dominant nitrogen form used by the phytoplankton size fraction, whereas the heterotrophic bacteria size fraction preferred the allochthonous DON. We detected a moderate shift from >1.6 μm plankton dominated DON uptake in the North Sea and central Baltic Sea towards a 0.2–1.6 μm dominated DON uptake in the Bothnian Bay and a weak positive relationship between DON concentrations and uptake. These findings indicate that DON is an important component of plankton nutrition and can fuel primary production. It may therefore also contribute substantially to eutrophication in the Baltic Sea especially when inorganic nitrogen sources are depleted.