Ecology of the North Sea: Problems,successes, failures,future needs

After defining ‘ecology’, outlining the basic categories of ecological research and listing examples of modern ecological investigations, this introductory paper focusses on basic considerations; it is, in essence, a programmatic contribution. Research details on the ecology of the North Sea are the subject of the following papers. Theproblems of ecological North Sea research are formidable. Hydrological and biological fluctuations and variabilities are pronounced. Exchange patterns with the Atlantic are complex, and the inputs of rivers and rain defy exact measurement and prediction. Season, weather, climate—and as yet insufficiently known and controlled human-caused impacts—further complicate the situation. All this results in an unusually high degree of uncertainty. New questions and problems arise before the old ones can be answered or solved. Nevertheless, ecological North Sea research has achieved manysuccesses. The North Sea is the most intensively investigated sea area on our planet. Generations of zoologists, botanists and hydrographers — and more recently microbiologists, meteorologists, climatologists, chemists, pathologists and toxicologists — have produced an impressive body of knowledge. Slowly we are beginning to understand the forces that govern energy budgets and balances, material fluxes, and the factors that control and direct ecosystem dynamics. Essential driving forces of ecosystem dynamics result from microbial, especially bacterial, activities. Ecological modelling has paved the way for new theories and insights, and holds promise for progress towards a predictive ecology.Failures and shortcomings include insufficient long-term research, inadequately designed experiments, and misconceptions in environmental protection. Net changes in ecological processes of an heterogeneous and intensely varying environment such as the North Sea can only be comprehended adequately against the background of sustained measurements over decades.Future needs include: more long-term research; and new patterns of management, institutional organization and financial support. Essential breakthroughs in field-work demand more teamwork, in-situ experimentation and surveys from space. Studies on the health status of organisms and ecosystems should receive more attention. Finally, there is need for changes in human behaviour: we must use our insight and willpower to meet the deadly consequences of our self-made scientific-technological evolution by an equally self-made ethical evolution aimed at achieving a re-harmonization with nature.     

Species-specific PCR discrimination of species of the calanoid copepod Pseudocalanus, P. acuspes and P. elongatus, in the Baltic and North Seas

Two species of the calanoid copepod Pseudocalanus, P. acuspes and P. elongatus, have been reported from the Baltic and North Seas. Morphometric analysis based on Frost (Can J Zool 67:525–551, 1989) did not identify the species in samples collected during the German GLOBEC program; the majority of measurements fell outside reported ranges for Baltic and North Sea specimens. A multiplexed species-specific PCR (SS-PCR) protocol was designed from a ~700 bp sequence of mitochondrial cytochrome oxidase I (mtCOI), with primers designed to amplify regions of different sizes for the two species to allow detection by gel electrophoresis. The accuracy and reliability of the multiplexed SS-PCR reaction was confirmed by sequencing the mtCOI region. Preliminary SS-PCR analysis of geographic distributions of the two species indicated that P. acuspes occurred exclusively at sampling sites in the Baltic Sea, whereas P. elongatus was found in the southern North Sea and at one site in the Baltic. The SS-PCR protocol can be used for routine identification of the two species for studies of population, community and ecosystem dynamics.     

Climate,plankton and cod

While a few North Atlantic cod stocks are stable, none have increased and many have declined in recent years. Although overfishing is the main cause of most observed declines, this study shows that in some regions, climate by its influence on plankton may exert a strong control on cod stocks, complicating the management of this species that often assumes a constant carrying capacity. First, we investigate the likely drivers of changes in the cod stock in the North Sea by evaluating the potential relationships between climate, plankton and cod. We do this by deriving a Plankton Index that reflects the quality and quantity of plankton food available for larval cod. We show that this Plankton Index explains 46.24% of the total variance in cod recruitment and 68.89% of the variance in total cod biomass. Because the effects of climate act predominantly through plankton during the larval stage of cod development, our results indicate a pronounced sensitivity of cod stocks to climate at the warmer, southern edge of their distribution, for example in the North Sea. Our analyses also reveal for the first time, that at a large basin scale, the abundance of Calanus finmarchicus is associated with a high probability of cod occurrence, whereas the genus Pseudocalanus appears less important. Ecosystem‐based fisheries management (EBFM) generally considers the effect of fishing on the ecosystem and not the effect of climate‐induced changes in the ecosystem state for the living resources. These results suggest that EBFM must consider the position of a stock within its ecological niche, the direct effects of climate and the influence of climate on the trophodynamics of the ecosystem.     

A recovery of harbour porpoises (Phocoena phocoena) in the southern North Sea? A case study off Eastern Frisia, Germany

Detailed information on year-round distribution, seasonal abundance and inter-annual trends of a given species is essential for any conservation effort. However, for most odontocetes this knowledge is rather limited. Therefore, area-specific management or conservation plans are often difficult to argue for. This is also true for the harbour porpoise (Phocoena phocoena), although it is the most common cetacean species in the North Sea. Knowledge of the current status of local stocks as well as fine scale information on the temporal use of certain areas by the species is incomplete. One area of concern is the southern North Sea where the abundance of harbour porpoises has declined in the twentieth century. Recent studies using stranding data and observations from seabird surveys indicate a comeback of the species along the Dutch and Belgian coast. However, data on other regions of the southern North Sea is sparse. Between 2002 and 2004, we undertook 25 aerial line transect surveys (11,000 km on effort; altitude = 250 and 600 ft) in a 2,500 km² coastal area off Eastern Frisia, Germany including a small portion of Dutch coastal waters. The data were g(0) corrected using a double platform approach and analysed with distance sampling software. A total of 426 harbour porpoises were sighted, including eight calves. Densities ranged between <0.1 and 1.62 individuals/km² with peaks in February and July 2003 as well as February and May 2004. The results of our study show that harbour porpoises are present in the coastal part of the southern North Sea even during their reproductive period. However, they seem to appear in lower numbers and much more irregular than in other areas, for example off Northern Frisia. The results of this study support the recent findings that despite a decline in the mid-twentieth century, harbour porpoises are now at times quite abundant in the southern North Sea. The underlying factors of this ‘return’ should be investigated using a combination of surveys and satellite telemetry.     

Phylogeography and cryptic introduction of the ragworm Hediste diversicolor (Annelida,Nereididae) in the Northwest Atlantic

Many benthic marine invertebrates show striking range disjunctions across broad spatial scales. Without direct evidence for endemism or introduction, these species remain cryptogenic. The common ragworm Hediste diversicolor plays a pivotal role in sedimentary littoral ecosystems of the North Atlantic as an abundant prey item and ecosystem engineer, but exhibits a restricted dispersal capacity that may limit connectivity at both evolutionary and ecological time scales. In Europe, H. diversicolor is subdivided into cryptic taxa and genetic lineages whose distributions have been modified by recent invasions. Its origin in the northwest Atlantic has not been adequately addressed. To trace the age and origin of North American ragworm populations, we analyzed mtDNA sequence data (COI) from the Gulf of Maine and Bay of Fundy (n=73 individuals) and compared our findings with published data from the northeast Atlantic. Our results together with previous data indicate that two species of the H. diversicolor complex have independently colonized the northwest Atlantic at least three different times, resulting in two distinct conspecific assemblages in the Bay of Fundy and Gulf of Maine (respectively) that are different from the species found in the Gulf of St. Lawrence. North American populations had significantly lower genetic diversity compared with populations in the northeast Atlantic, and based on patterns of shared identity, populations in the Bay of Fundy originated from the Baltic Sea and North Sea. Populations from the Gulf of Maine were phylogenetically distinct and most likely originated from unsampled European populations. Analyses of the North American populations revealed patterns of post‐colonization gene flow among populations within the Gulf of Maine and Bay of Fundy. However, we failed to detect shared haplotypes between the two regions, and this pattern of complete isolation corroborates a strong phylogeographic break observed in other species.     

Effects of the large-scale uncontrolled fertilisation process along the continental coastal North Sea

In this paper, effects of eutrophication in selected compartments of the North Sea ecosystem are discussed, encompassing the possibly positive effects of nutrient enrichment. Based on a variety of studies, impacts on biomass of phytoplankton, macrozoobenthos, microzooplankton, shrimps and fishes and productivity are presented. Enhanced nutrient concentrations and loadings can be observed in several coastal areas of the North Sea. As a result, increases in the concentration, production and changes in the species composition was observed in the phytoplankton. In addition, there are some indications for an increased biomass of macrozoobenthos, whereas an increase in microzooplankton can only be assumed from mesocosm experiments. A concomitant increase of higher trophic levels such as shrimps and fishes, as observed in some coastal regions of the North Sea, is difficult to link directly to eutrophication due to a lack of conclusive field observations showing the causality of the changes. That the large fertilisation process in the North Sea has led to a series of changes is, however, without doubt. The answer, to what extent these can be claimed as being harmless, positive or negative from the anthropogenic point of view, is hampered by the lack of good assessment criteria for marine ecosystems and requires a thorough analysis of all compartments involved by means of long-term-series long enough to discriminate between man-made and natural variability.     

Habitats and Distribution Patterns of Marine Luminous Bacteria in the Western Baltic Sea

Numbers of luminous bacteria were counted at three stations of the brackish water ecosystem of the western Baltic Sea from July 1985 to July 1986. Additional samples were taken during three cruises from stations at the North Atlantic Ocean, the Norwegian Sea and adjacent marine areas. — In Kiel Bight (western Baltic) values varied between 0 and 68,000 luminous cfu 1⁻¹. With exception of the coastal station a distinct seasonal distribution pattern was shown in a water depth of 20 m: high numbers found in summer were opposed to low numbers in winter, the peaks being rather high in comparison to those of other areas. Statistical analysis showed that the results of 20 m were significantly different from those of 0 and 10 m depth; however, there was no correlation with temperature and salinity. Taxonomic studies revealed that the population consisted primarily of the genus Photobacterium. — The optimum of salinity was not a brackish but a marine one and was about 30% for the majority of the strains tested. A smaller number of strains grew best at a salinity between 10 and 15%. Optima of temperature ranged from 15 to 20 °C for most of the test strains. — Taxonomic analysis was also performed with luminous strains from marine areas adjacent to the western Baltic Sea, Photobacterium being the dominant genus here, too. Luminous bacteria were also enriched from the external surface and the gut contents of whitings (Merlangius merlangus) and cods (Gadus morhua). A model is proposed which explains the distribution pattern found. According to this, the gut-dwelling luminous bacteria are transported by their hosts from the North Sea into the western Baltic Sea. Here they are released into the environment, thus inhabiting another niche.     

Conservation genetics of harbour porpoises, Phocoena phocoena, in eastern and central North Atlantic

We examined polymorphism at 12 microsatelliteloci in 807 harbour porpoises , Phocoenaphocoena, collected from throughout thecentral and eastern North Atlantic to theBaltic Sea. Multilocus tests for allelefrequency differences, assignment tests,population structure estimates (F_ST) andgenetic distance measures (D_LR andD_C) all indicate six geneticallydifferentiated populations/sub-populationsafter pooling sub-samples within regions.Harbour porpoises from West Greenland, theNorwegian Westcoast, Ireland, the British NorthSea, the Danish North Sea and the inland watersof Denmark (IDW) are all geneticallydistinguishable from each other. A sample ofharbour porpoises collected off the Dutch coast(mainly during winter) was geneticallyheterogeneous and likely comprised a mixture ofindividuals of diverse origin. A mixed stockanalysis indicated that most of the individualsin this sample (77%) were likely migrantsfrom the British and Danish North Sea.     

A metabarcoding analysis of the wrackbed microbiome indicates a phylogeographic break along the North Sea–Baltic Sea transition zone

Sandy beaches are biogeochemical hotspots that bridge marine and terrestrial ecosystems via the transfer of organic matter, such as seaweed (termed wrack). A keystone of this unique ecosystem is the microbial community, which helps to degrade wrack and re-mineralize nutrients. However, little is known about this community. Here, we characterize the wrackbed microbiome as well as the microbiome of a primary consumer, the seaweed fly Coelopa frigida, and examine how they change along one of the most studied ecological gradients in the world, the transition from the marine North Sea to the brackish Baltic Sea. We found that polysaccharide degraders dominated both microbiomes, but there were still consistent differences between wrackbed and fly samples. Furthermore, we observed a shift in both microbial communities and functionality between the North and Baltic Sea driven by changes in the frequency of different groups of known polysaccharide degraders. We hypothesize that microbes were selected for their abilities to degrade different polysaccharides corresponding to a shift in polysaccharide content in the different seaweed communities. Our results reveal the complexities of both the wrackbed microbial community, with different groups specialized to different roles, and the cascading trophic consequences of shifts in the near shore algal community.     

Protected areas in the North Sea: An absolute need for future marine research

There are many signals that different human activities affect the marine ecosystem on local and sometimes regional scales. There is evidence that in the Dutch sector of the North Sea at least 25 species have decreased tremendously in numbers or have totally disappeared. But what has caused their disappearance: fisheries, pollution, eutrophication, climatic changes, or a combination of causes? On the Dutch Continental Shelf, the fisheries are now so intensive that every square metre is trawled, on an average, once to twice a year. Furthermore, it has been shown that trawling causes direct damage to the marine ecosystem. This indicates that the “natural” North Sea ecosystem we are studying is already a heavily influenced system. And what is the value of data on the diversity and production of benthic animals, if the research area has been raked by beamtrawl gear an unknown amount of times before sampling? To be able to study the natural trends in the marine ecosystem, or to answer the question which human activity has most influenced the ecosystem, there is an absolute and immediate need for protected areas to be established. The size of the protected areas must be determined by the behaviour of that species characteristic for the area. In such areas, where fisheries and local pollution would be forbidden or very limited, scientific research into the species composition and age distribution of different populations should be carried out and trends should be established.     

Benthic isopods (Crustacea,Malacostraca) from the Ross Sea,Antarctica: species checklist and their zoogeography in the Southern Ocean

In this study isopod species of the Ross Sea were investigated. Literature until May 2008 was checked to provide an overview of all known and described species in the Ross Sea. This species checklist was then enlarged through material of the 19th Italica expedition in 2004. During this expedition for the first time a small mesh net (500 μm) was used. Nine thousand four hundred and eighty one isopod specimens were collected during this expedition. Through this material the number of isopod species in the Ross Sea increased from 42 to 117 species, which belong to 20 families and 49 genera. Fifty-six percentage of the isopods species collected during the Italica expedition are new to science. The zoogeography of the 117 species was investigated. A non-transformed binary presence-absence data matrix was constructed using the Bray–Curtis coefficient. The results were displayed in a cluster analysis and by nonmetric multidimensional scaling (MDS). This paper gives a first insight into the occurrence and distribution of the isopod species of the Ross Sea.     

The Holocene occurrence of Acipenser spp. in the southern North Sea: the archaeological record

Archaeological sturgeon remains from the southern North Sea basin used to be automatically attributed to Acipenser sturio, since this was the only acipenserid species believed to occur there. These species identifications, however, were in need of revision after a growing number of indications were found for the historical presence of Acipenser oxyrinchus in western Europe. In this study, morphological and genetic data on sturgeon remains from archaeological sites along the southern North Sea are revised. A large number of Dutch, Belgian, British and some French archaeological sturgeon remains, dating from the Mesolithic up to Late Modern times, are morphologically examined and fish sizes are reconstructed. This study of >7000 acipenserid bones proves the sympatric occurrence of European sturgeon A. sturio and Atlantic sturgeon A. oxyrinchus in the southern North Sea at least since the Neolithic (fourth millennium BC onwards), with A. oxyrinchus remains always outnumbering those of A. sturio. Human influence is documented by the decrease in finds through time, but no clear evidence was found for a diachronic change in fish lengths that could possibly be related to fishing pressure.     

Oligotrophic waters of the Northwest Atlantic support taxonomically diverse diatom communities that are distinct from coastal waters

Diatoms are important components of the marine food web and one of the most species-rich groups of phytoplankton. The diversity and composition of diatoms in eutrophic nearshore habitats have been well documented due to the outsized influence of diatoms on coastal ecosystem functioning. In contrast, patterns of both diatom diversity and community composition in offshore oligotrophic regions where diatom biomass is low have been poorly resolved. To compare the diatom diversity and community composition in oligotrophic and eutrophic waters, diatom communities were sampled along a 1,250 km transect from the oligotrophic Sargasso Sea to the coastal waters of the northeast US shelf. Diatom community composition was determined by amplifying and sequencing the 18S rDNA V4 region. Of the 301 amplicon sequence variants (ASVs) identified along the transect, the majority (70%) were sampled exclusively from oligotrophic waters of the Gulf Stream and Sargasso Sea and included the genera Bacteriastrum, Haslea, Hemiaulus, Pseudo-nitzschia, and Nitzschia. Diatom ASV richness did not vary along the transect, indicating that the oligotrophic Sargasso Sea and Gulf Stream are occupied by a diverse diatom community. Although ASV richness was similar between oligotrophic and coastal waters, diatom community composition in these regions differed significantly and was correlated with temperature and phosphate, two environmental variables known to influence diatom metabolism and geographic distribution. In sum, oligotrophic waters of the western North Atlantic harbor diverse diatom assemblages that are distinct from coastal regions, and these open ocean diatoms warrant additional study, as they may play critical roles in oligotrophic ecosystems.     

Microscopic species make the diversity: a checklist of marine flora and fauna around the Island of Sylt in the North Sea

Based on the past 150 years of research and ongoing time-series observations we give a comprehensive overview of marine species composition around the island of Sylt in the eastern North Sea. A total of 2758 species is listed according to the categories microplankton (591 species), zooplankton (137), nekton (118), benthic microflora (158), benthic macroflora (125), benthic micro-and meiofauna (1204), benthic macrofauna (509), birds and mammals (91), and neobiota (39). Plants account for a third of the species, most (85%) of them are microscopic Chromista. Among animals, 60% of the species are micro- and meiofauna though this faunal component is still insufficiently known. These figures are similar to records from the southern North Sea and therefore may by typical for temperate climate sedimentary coastal areas. A comparison with the total of marine species suggests that the small benthic fauna may be severely understudied over most of the world. Analysis of global change depends on sound baseline data and species inventories like this can assist in the detection of biodiversity changes. They emphasise rare species and the full range of local habitats while time-series measurements usually rely on a few selected habitats and biotic components to generate a very general picture of the state of an ecosystem.     

Breaking the ice: large-scale distribution of mesozooplankton after a decade of Arctic and transpolar cruises

Mesozooplankton collected during five summer expeditions to the Arctic Ocean between 1987 and 1991 was analysed for regional patterns in biomass and species distribution, distinguishing between an epipelagic (0–100 m) and a deeper (0–500 m) layer. A total of 58 stations was sampled mainly in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean and in areas of the Greenland Sea, West Spitsbergen Current and Barents Sea. Results from the different expeditions were combined to create a transect extending from the Fram Strait across the Eurasian Basin into the Makarov Basin. Mesozooplankton dry mass in the upper 500 m decreased from 8.4 g m⁻² in the West Spitsbergen Current to less than 2 g m⁻² in the high-Arctic deep-sea basins. In the central Arctic Ocean, biomass was concentrated in the upper 100 m and was dominated by the large copepods Calanus hyperboreus and C. glacialis. In contrast, the mesozooplankton in the West Spitsbergen Current was more evenly distributed throughout the upper 500 m, with C. finmarchicus as the prevailing species. The distribution of abundant mesopelagic species reflected the hydrographic regime: the calanoid copepod Gaetanus tenuispinus and the hyperiid amphipod Themisto abyssorum were most abundant in the Atlantic inflow, while Scaphocalanus magnus was a typical component of the high-Arctic fauna. The relatively high mesozooplankton biomass and the occurrence of boreal-Atlantic species in the central Arctic Ocean are indicators for the import of organic material from allochthonous sources, especially from the northern North Atlantic. Hence, in spite of its enclosure by land masses, the Arctic Ocean is characterized by an exchange of water masses and organisms with the North Atlantic, and advection processes strongly influence the distribution of plankton species in this high-latitude ecosystem. Received: 18 December 1997 / Accepted: 11 April 1998     

Recent phytoplankton productivity of the northern Bering Sea during early summer in 2007

Although the northern Bering Sea is one of the most productive regions in the northern North Pacific Ocean and currently considered a declining productivity region, no recent primary productivity measurements have been collected in this region. Phytoplankton productivity was measured in the northern Bering Sea in 2007 using a dual ¹³C–¹⁵N isotope tracer technique to quantify present rates of primary productivity and to assess changes under recent environmental conditions in this area. We found that large diatoms (mostly Fragilaria sp.) dominated the phytoplankton during the initial part of the cruise, whereas unidentified nano + pico phytoplankton largely dominated at the surface about 2 weeks later (at “revisited stations”). At the 1% light depth, diatoms and Phaeocystis sp. were the dominant species, whereas diatoms and unidentified nano + pico cells were dominant at the revisited sites. Based on nitrate and ammonium uptake rates, the estimated f-ratios (the ratio of nitrate uptake rate/nitrate + ammonium uptake rates of phytoplankton) were high (0.65–0.74), indicating that nitrate was an important nitrogen source supporting primary production in the northern Bering Sea during the cruise in 2007. Compared with previous studies performed several decades ago, we found significantly lower chlorophyll-a concentrations and carbon uptake rates of phytoplankton in the northern Bering Sea in 2007. This is consistent with recent studies that have shown lower rates of production in the Chukchi Sea and declines in benthic biomass and sediment oxygen uptake in the northern Bering Sea.     

Patterns of ecological specialization among microbial populations in the Red Sea and diverse oligotrophic marine environments

Large swaths of the nutrient‐poor surface ocean are dominated numerically by cyanobacteria (Prochlorococcus), cyanobacterial viruses (cyanophage), and alphaproteobacteria (SAR11). How these groups thrive in the diverse physicochemical environments of different oceanic regions remains poorly understood. Comparative metagenomics can reveal adaptive responses linked to ecosystem‐specific selective pressures. The Red Sea is well‐suited for studying adaptation of pelagic‐microbes, with salinities, temperatures, and light levels at the extreme end for the surface ocean, and low nutrient concentrations, yet no metagenomic studies have been done there. The Red Sea (high salinity, high light, low N and P) compares favorably with the Mediterranean Sea (high salinity, low P), Sargasso Sea (low P), and North Pacific Subtropical Gyre (high light, low N). We quantified the relative abundance of genetic functions among Prochlorococcus, cyanophage, and SAR11 from these four regions. Gene frequencies indicate selection for phosphorus acquisition (Mediterranean/Sargasso), DNA repair and high‐light responses (Red Sea/Pacific Prochlorococcus), and osmolyte C1 oxidation (Red Sea/Mediterranean SAR11). The unexpected connection between salinity‐dependent osmolyte production and SAR11 C1 metabolism represents a potentially major coevolutionary adaptation and biogeochemical flux. Among Prochlorococcus and cyanophage, genes enriched in specific environments had ecotype distributions similar to nonenriched genes, suggesting that inter‐ecotype gene transfer is not a major source of environment‐specific adaptation. Clustering of metagenomes using gene frequencies shows similarities in populations (Red Sea with Pacific, Mediterranean with Sargasso) that belie their geographic distances. Taken together, the genetic functions enriched in specific environments indicate competitive strategies for maintaining carrying capacity in the face of physical stressors and low nutrient availability.     

Phaeocystis colony distribution in the North Atlantic Ocean since 1948, and interpretation of long-term changes in the Phaeocystis hotspot in the North Sea

Monitoring of Phaeocystis since 1948 during the Continuous Plankton Recorder survey indicates that over the last 5.5 decades the distribution of its colonies in the North Atlantic Ocean was not restricted to neritic waters: occurrence was also recorded in the open Atlantic regions sampled, most frequently in the spring. Apparently, environmental conditions in open ocean waters, also those far offshore, are suitable for complete lifecycle development of colonies (the only stage recorded in the survey). In the North Sea the frequency of occurrence was also highest in spring. Its southeastern part was the Phaeocystis abundance hotspot of the whole area covered by the survey. Frequency was especially high before the 1960s and after the 1980s, i.e., in the periods when anthropogenic nutrient enrichment was relatively low. Changes in eutrophication have obviously not been a major cause of long-term Phaeocystis variation in the southeastern North Sea, where total phytoplankton biomass was related significantly to river discharge. Evidence is presented for the suggestion that Phaeocystis abundance in the southern North Sea is to a large extent determined by the amount of Atlantic Ocean water flushed in through the Dover Strait. Since Phaeocystis plays a key role in element fluxes relevant to climate the results presented here have implications for biogeochemical models of cycling of carbon and sulphur. Sea-to-air exchange of CO₂ and dimethyl sulphide (DMS) has been calculated on the basis of measurements during single-year cruises. The considerable annual variation in phytoplankton and in its Phaeocystis component reported here does not warrant extrapolation of such figures.     

Estimation of the load of cyclic organochlorines in North Sea zooplankton

During a North Sea survey in late spring 1986, zooplankton biomass (g dry weight/m²) as well as pollutant concentrations in zooplankton (ng/g dry weight) were determined at 127 stations. On the basis of these data, the load of several cyclic organochlorines in zooplankton (ng/m²) was estimated. It appeared that the highest loads of these pollutants were incorporated in the large zooplankton stocks of the northern and central North Sea, north of 56°N, whereas the highest concentrations were found in zooplankton of the southern North Sea. An explanation for the accumulation of pollutants in sediments as well as in benthos organisms of the central and northern North Sea as found by several authors is discussed in connection with the assumption of a considerably high vertical flux of organic material in these areas. The calculation of total zooplankton content in the North Sea during the late spring survey amounted to about 5 million tons dry weight. This biomass was estimated to contain about 2.6 t PCBs, 19.3 kg p,p′-DDE, 12 kg τ-HCH, 6.4 kg HCB und 5.6 kg α-HCH. These results agree with those on other compartments of the North Sea ecosystem (benthos, fish) found in recent literature. The estimated annual turnover of PCBs in zooplankton (15.2 t) is of the same order of magnitude as the estimated amount transported yearly into the North Sea by rivers and the atmosphere (6–13 t).     

Further expansion of the genus Cercopagis (Crustacea,Branchiopoda, Onychopoda) in the Baltic Sea,with notes on the taxa present and their ecology

The onychopod cladoceran Cercopagis that recently invaded the Baltic Sea is reported from new zones of the northern Baltic proper. Because of successful survival and an expanding distribution range, the addition of Cercopagis to the Baltic fauna is considered to be permanent. What has previously been cited as Cercopagis pengoi encompasses the morphology of several other species, subspecies and forms. Either a number of morphologically similar species is present, or there is a number of spurious species in Cercopagis. The last hypothesis is favoured. The spatial distribution pattern of Cercopagis, as well as that of total zooplankton, was correlated with depth. Deep (>100 m) and shallow (<10 m) stations had significantly lower abundance than stations of intermediate depth (<100 m). An overview of the distribution of C. pengoi group in fresh and brackish waters suggests a high tolerance to environmental factors, but with differences among taxa. Due to this ecological flexibility, the colonization of the Baltic is not unexpected. Increasing salinity may restrict dispersal of cercopagids to the southern areas of the Baltic and to the North Sea, but inland lakes (e.g. in Sweden) present an ecological profile suitable for colonization.