Trophic interactions within the Ross Sea continental shelf ecosystem

The continental shelf of the Ross Sea is one of the Antarctic's most intensively studied regions. We review the available data on the region's physical characteristics (currents and ice concentrations) and their spatial variations, as well as components of the neritic food web, including lower and middle levels (phytoplankton, zooplankton, krill, fishes), the upper trophic levels (seals, penguins, pelagic birds, whales) and benthic fauna. A hypothetical food web is presented. Biotic interactions, such as the role of Euphausia crystallorophias and Pleuragramma antarcticum as grazers of lower levels and food for higher trophic levels, are suggested as being critical. The neritic food web contrasts dramatically with others in the Antarctic that appear to be structured around the keystone species Euphausia superba. Similarly, we suggest that benthic-pelagic coupling is stronger in the Ross Sea than in most other Antarctic regions. We also highlight many of the unknowns within the food web, and discuss the impacts of a changing Ross Sea habitat on the ecosystem.     

North Sea ecosystem change from swimming crabs to seagulls

A recent increase in sea temperature has established a new ecosystem dynamic regime in the North Sea. Climate-induced changes in decapods have played an important role. Here, we reveal a coincident increase in the abundance of swimming crabs and lesser black-backed gull colonies in the North Sea, both in time and in space. Swimming crabs are an important food source for lesser black-backed gulls during the breeding season. Inhabiting the land, but feeding mainly at sea, lesser black-backed gulls provide a link between marine and terrestrial ecosystems, since the bottom-up influence of allochthonous nutrient input from seabirds to coastal soils can structure the terrestrial food web. We, therefore, suggest that climate-driven changes in trophic interactions in the marine food web may also have ensuing ramifications for the coastal ecology of the North Sea.     

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.     

Nutrient emission reduction scenarios in the North Sea: An abatement cost and ecosystem integrity analysis

Economic cost–benefit appraisal (and its sub-set cost-effectiveness) of ecosystem conservation and/or pollution abatement strategies have proved to be powerful decision-making aids. But the monetary economic valuation of ecosystem goods and services (gains and losses) can only provide a good indication of social welfare impacts under certain conditions and in selective contexts. The values derived through this appraisal process will, for a number of measures, be underestimates of the full total system value [Turner, R.K., Paavola, J., Cooper, P. Farber, S., Jessamy, V., Georgiou, S., 2003. Valuing nature: lessons learned and future research directions. Ecol. Econ. 46, 493–510]. The economic analysis is best suited to assessing the value of ‘marginal’ gains and losses in ecosystem goods/services and not the total destruction of whole systems (including life support systems, the value of which is not commensurate with monetary values and/or is infinitely high). In this study economic costs and what we call ‘ecological risk’ analysis are used to appraise the implementation costs and ecological benefits of selected measures for combating eutrophication. Ecological risk is expressed in terms of ecosystem integrity and resilience. The paper presents three regional case studies dealing with the issue of nutrient emission reduction to the southern North Sea, namely the catchments/estuaries of the Humber (UK), the Rhine (Germany and The Netherlands) and the Elbe (Czech Republic and Germany). On the basis of these comparative regional examples, wider implications in the light of international management of the North Sea are presented.A range of nutrient reduction scenarios have been deployed within the overall OSPAR target agreement of 50% nitrogen and phosphorous reduction compared with 1985 levels. Each scenario assumes pollution reduction measures, characterised in terms of their overall implementation costs and nutrient-reduction effects. Specific policy instruments analysed were: the creation of more intertidal habitat via managed coastal realignment in the Humber area, farm-based land cover changes in the Rhine catchment and a mix of agricultural regime and wastewater treatment plant (WWTP) improvements in the Elbe area. The ecological consequences associated with each reduction scenario have been modelled [using the ERSEM model, see Baretta, J.W., Ebenhöh, W., Ruardij, P., 1995. An overview over the European Regional Sea Ecosystem Model, a complex marine ecosystem model. Neth. J. Sea Res. 33 (3/4), 233–246] for the coastal zones supplied from the rivers Elbe, Humber and Rhine. The modelled ecological quality indicators, which describe scenario effects on the coastal zone ecosystem, are then aggregated in terms of ecosystem integrity and ecological risk. The results are presented in terms of two selected key-indicators: implementation costs of the abatement measures and changes in ecological risk status, across the different catchments and assuming different scenarios. They thus provide a possible basis for international agreement negotiations at the North Sea scale.     

Predictive ecosystem research in the Wadden Sea

Predictive ecosystem research needs a pluralistic approach. Retrospective studies reveal the initial causes of ongoing ecological change. In the Wadden Sea, inherent ecosystem stability may be falsely assumed, because the effects of modern coastal architecture and of anthropogenic eutrophication to some extent complement each other. Expected environmental changes often have corresponding phases in the past which may serve as a model to predict ecological implications. Historically, quantitative ecology entered the Wadden Sea, via fisheries research, from the oceanic side. Quantified material fluxes may reveal imbalances which are indicative of the rough direction of ecosystem change. For ecosystem research to contribute to the maintenance of the Wadden Sea as a centre of coastal organisms, quantitative knowledge of resources and ecosystem metabolism must be supplemented by qualitative knowledge of habitat requirements and species interdependences. Qualitative ecology entered the Wadden Sea from the landward side. Extending this approach to anticipatory field experiments may help to predict ecological changes at the species level.     

Introduction into the ecosystem of the North Sea: Hydrography,biota, and food web relationships

The North Sea, one of the most productive of the earth's seas and oceans, is also surrounded by some of earth's most densely populated and heavily industrialized regions. A growing number of signals are being received which indicate that this valuable ecosystem is increasingly under stress. This has generated a corresponding increase in concern over the steps to be taken to protect the North Sea. While there are divergent views on what constitutes an ‘ideal’ North Sea, there is a general recognition that any decisions that are made should be based on a good understanding of this ecosystem. The intention of this paper is to give an overview of what is presently known, and to identify areas where more studies are needed. A brief summary of the hydrography and the biota of the North Sea is given. Biotic and abiotic structure justify partitioning the North Sea into three ecologically different regions: southern, central, and northern. For the most part, neither the top predators,e.g. marine birds and mammals, nor the macroalgae and sea grasses are included in this overview.     

Bacteriophage Diversity in the North Sea

In recent years interest in bacteriophages in aquatic environments has increased. Electron microscopy studies have revealed high numbers of phage particles (10⁴ to 10⁷ particles per ml) in the marine environment. However, the ecological role of these bacteriophages is still unknown, and the role of the phages in the control of bacterioplankton by lysis and the potential for gene transfer are disputed. Even the basic questions of the genetic relationships of the phages and the diversity of phage-host systems in aquatic environments have not been answered. We investigated the diversity of 22 phage-host systems after 85 phages were collected at one station near a German island, Helgoland, located in the North Sea. The relationships among the phages were determined by electron microscopy, DNA-DNA hybridization, and host range studies. On the basis of morphology, 11 phages were assigned to the virus family Myoviridae, 7 phages were assigned to the family Siphoviridae, and 4 phages were assigned to the family Podoviridae. DNA-DNA hybridization confirmed that there was no DNA homology between phages belonging to different families. We found that the 22 marine bacteriophages belonged to 13 different species. The host bacteria were differentiated by morphological and physiological tests and by 16S ribosomal DNA sequencing. All of the bacteria were gram negative, facultatively anaerobic, motile, and coccoid. The 16S rRNA sequences of the bacteria exhibited high levels of similarity (98 to 99%) with the sequences of organisms belonging to the genus Pseudoalteromonas, which belongs to the γ subdivision of the class Proteobacteria.The marine bacterial community is responsible for a considerable portion of primary production and regeneration of nutrients in the microbial loop and is associated with a great variety of marine bacteriophages (5, 12). These phages are capable of infecting a large portion of the bacterioplankton (32, 34). It is assumed that as part of the marine food web, bacteriophages play important quantitative and qualitative roles in controlling marine bacterial populations (8, 24, 34, 39, 45). The phenotypic diversity and genotypic diversity of the phage populations are related to the interaction between phages and their host organisms, which provides a tool for understanding the interaction itself (13). To estimate the influence of marine bacteriophages on the diversity of bacterioplankton, we investigated phage diversity. The virus species concept proposed by Murphy et al. (37) delineates seven different families of bacteriophages based on morphological criteria and provides criteria for new phage species based on several traits, such as DNA homologies, serological data, protein profiles, and host ranges.In this paper, we describe the diversity and genetic relationships of marine phages based on investigations of 22 representatives from 85 phage-host systems (35, 36) collected between 1988 and 1992 from waters around an island, Helgoland, located in the North Sea. All of the phages were virulent and formed plaques on their host bacteria. We assigned the phages to different virus families, species, and strains based on morphology, DNA homology, and host range. Furthermore, we characterized the phenotypic and genotypic features of the host bacteria.     

Zooplankton research in the North Sea

Summary Water types of the North Sea with different plankton are the thermally stratified northern and central regions with a relatively nutrient rich inflow of Atlantic water, a mixed region in the southern North Sea with a poor inflow of Channel water, and turbid narrow coastal zones with inflow of nutrient rich river water. Plankton studies reveal that the primary production starts early, in February, in the southern region, but is delayed in the coastal zones by turbidity. In stratified areas the algal spring bloom is delayed by mixing towards the greater depth and usually starts with the onset of thermal stratification. The spring bloom soon declines and the algae remain on a low density level in summer, presumably due to depletion of nutrients in the euphotic zone. The coastal zones and the frontal zones between mixed and stratified water have a relatively high summer primary production.The herbivores (mainly planktonic copepodes and the tunicateOikopleura dioica) grow and increase in number when the temperature rises and food is available. There is a considerable mismatch with the algal spring bloom, which comes too early and is too short in most regions. The best coincidence occurs in the coastal zones and maybe the frontal zones. Carnivores build high biomasses in late summer and fall in the coastal regions and compete with fish larvae for food and also kill many fish larvae. The large scyphomedusae are most important in this respect.The overall yearly primary production of the North Sea is estimated to be about 100 mg C.m^–2. The estimates for herbivores and fish are 20 and 1 mg C.m^–2. Considering a growth efficiency of 20%, the herbivores must consume all algae produced. The indication of a low consumption due to bad phenological coincidence in most regions leads to the assumption, that either primary production is underestimated or there is a considerable influx of organic matter from the Atlantic Ocean. During June–July 1979 the carnivore consumption was estimated in the coastal zone of the Southern Bight to be 39 mg C.m^–2.d^–1 at a copepod production of 20 mg C.m^–2.d^–1. Consumption by fish larvae and large jellyfish (Cyanea lamarckii) was 15% and 74%, respectively.It seems clear that the productivity of the North Sea depends highly on coastal and frontal zones, where herbivores find sufficient food at optimal growth conditions. Most organic matter will at the end be consumed by invertebrate carnivores, which urge fish populations to reproduce early in spring or to recruit at remote places.     

Functioning of the Sea of Azov ecosystem

The structure and functioning of the Sea of Azov ecosystem have been studied. Based on the results of an analysis of the principal community components (phytoplankton, bacterioplankton, zooplankton, macrozoobenthos, and fish), the elements of balance equality and food utilization by the ecosystem components were calculated and schemes of their energy flow were constructed. The intensity and trends of production-destruction processes were characterized. It was revealed that the main energy flow in the Sea of Azov ecosystem passes through the detritus food web (81%). Along with eutrophication and sulfide pollution of bottom sediments, the impact of predatory ctenophore Mnemiopsis leidyi (A. Agassiz) determines to a considerable extent the pattern of the ecosystem’s transformation.     

Euthanasia across the North Sea

The United Kingdom and The Netherlands are separated by a narrow stretch of sea but in terms of an understanding of euthanasia they seem to be light years apart. An attempt to bridge the information gap seems in order.The position paper on euthanasia published by the Royal Dutch Medical Association contains 16 typed pages.¹ In contrast, the report on euthanasia issued by a working party of the British Medical Association is 80 pages in small print.²     

Symbiotic interactions in the EVOLVE III ecosystem model.

Parasitic feeding mechanisms were introduced into the EVOLVE III ecosystem model. The modified system exhibited a number of interesting behaviors, including obligative and facultative parasitism, transient facultative mutualism and nonsymbiotic feeding. Speciation and massive die-offs were also observed. The introduction of a symbiotic capability altered the co-evolutionary development of traits that exhibited an arms race when the symbiotic capability was not present. Stable mutualisms were not possible in the present version of the model due to the restriction that organisms were not allowed to feed both on other organisms and from the environment at the same time. The ecosystem biomass increased with increased symbiotic activity. Biomass was influenced by the balance of virulence and resistance traits in the population and decreased as these traits became more disparate.     

Algal production in the west-central North Sea

In 1976, 13 cruises were used for a plankton survey of an areaapproximately 300 x 150 km off the north-east coast of Englandfrom March to November. Results are presented of integratedzooplankton samples, surface measured nitrate, phosphate, silicateand chlorophyll, as well as other environmental characteristics.They show that the spring bloom started from near the DoggerBank and spread to most of the area by the end of April; thisdeclined and a nearshore production zone was seen in summer.This eventually disappeared and an offshore autumnal peak wasfound. The data are interpreted with use of a vertically integratedtwo-dimensional model. The model adequately explains the amplitudeand timing of the two-dimensional distributions of algal standingstock for the first half of the year but provides a poor representationafter that. It is inferred that annual primary production inthis region is low, about 40 g C.m^–2. This is due to lowoverwinter levels of nutrients and a shallow mixed layer depthof water. Pelagic fish are considered to have an ecologicalefficiency of 7–10% and demersal fish of about 4%. Thelatter figure may be due to underexploitation of demersal biomass.     

Benthic community structures in the North Sea

Coherent assemblages of marine benthic species have been recognized from the early twentieth century, and the classical papers of Petersen (1914, 1918) were based on studies of limited areas in the North Sea. In 1986, a synoptic survey of the North Sea north to 57°N was undertaken by a group of ten laboratories from seven North Sea countries. The results of this survey have recently been published (Heip et al., 1992a, b; Künitzer et al., 1992; Huys et al., 1992), and some of the results are summarized in this paper. The analysis of the macrofauna is based on slightly more than 700 taxa. In general, the North Sea macrofauna consists of northern species extending south to the northern margins of the Dogger Bank, and southern species extending north to the 100 m depth line. The central North Sea is an area of overlap of southern and northern species, especially around the 70 m depth contour. Consistent groupings of species are recognized that were summarized in seven faunal groupings. Macrofaunal body weight, density and diversity increase linearly towards the north. Macrofaunal biomass for the whole area averages 7 g adwt. m⁻² and decreases from south to north. Distribution patterns and trends within the meiofauna were very different. Nematodes, which are the dominant taxon overall, are least abundant in the sandy sediments of the Southern Bight, then increase to a maximum around 53° 30′ N and slowly decrease again towards the north. Copepod density and diversity are highest in the Southern Bight, due to the presence of many interstitial species. A large number of species new to science were recorded by the North Sea Benthos Survey and about 1500 species are expected to occur. Copepods show very distinct assemblages according to water depth and sediment type. The contrasting patterns in latitudinal gradients of body weight and number of species of macro- and meiofauna can be only partially explained. Latitude and sediment characteristics, such as grain size and content in plant pigments, and water depth, determine part of the variance in species composition, density and biomass of the benthic fauna, but the patterns that are observed are different for different benthic groups, requiring careful consideration as to their use in biological monitoring procedures. Distributions are related to current patterns in the North Sea, annual temperature variations and availability of food. However, large parts of the variance in many parameters remain unexplained.     

Diseases in North Sea fishes

Prior to the studies reviewed here, only lymphocystis and skeletal deformities of a variety of fish species and certain diseases of eel were known to occur in the German Bight (North Sea). From 1977 until now, 9 externally visible lesions on North Sea fishes were observed; in addition to those mentioned before, they comprise: fin rot, ulcerations, epidermal papilloma, hyperplasia, pseudobranchial tumour, eye diseases and gill swellings. With the exception of information on changes in frequencies of vertebral deformities of herring from the 1950's to the 1970's, there are no long-term data characterizing changes in frequencies of the diseases under study. For pseudobranchial tumours of cod and epidermal papilloma of dab, information is provided on occurrence and abundance. The distribution pattern of cod afflicted with pseudobranchial tumours is strongly influenced by the migratory behaviour of the fish. Epidermal papillomas of dab were more frequently found at stations within the inner German Bight than in neighbouring areas. The Bight is used for dumping of wastes from titaniumdioxide production. Further disease hot spots are areas off the Humber estuary and the British coast. Analysis of chromium in dab from the German Bight revealed elevated concentrations in epidermal tissues of specimens from the dumping area compared with that found in dab from neighbouring localities. Particulate iron was demonstrated to occur in mucous cells of dab from the dumping area. From increased levels of heavy metals with cancerogenic potential in sensitive target tissues and from increased prevalences of diseased fish in the dumping area it is concluded that these phenomena are possibly causally linked. In the vicinity of the Humber estuary high disease rates were encountered and areas with high prevalences of dab afflicted with epidermal papilloma extended over regions shown to be transport routes for persistent pollutants such as radioactive materials. It is therefore suggested that the long-range distribution of fish diseases in the southern North Sea might reflect the long-range transport of persistent pollutants.     

Distribution of capsulated bacterioplankton in the North Atlantic and North Sea

In laboratory experiments, bacterioplankton were incubated under different nutrient conditions, and the percentage of bacteria exhibiting a polysaccharidic capsule (capsulated bacteria) and that of CTC (cyanotetrazolium chloride)-positive and therefore metabolically highly active bacteria were determined. In these seawater cultures amended with nutrients more than 95% of the CTC-positive cells exhibited a capsule. During two cruises, one to the North Atlantic and one to the North Sea, we investigated the distribution of capsulated bacteria throughout the water column. Capsulated bacteria were generally more abundant in eutrophic surface waters than in deeper layers or more oligotrophic regions. In the upper 100 m of the North Atlantic, about 6–14% of the total bacterioplankton community was capsulated, while in the layers below 100 m depth, 97% of the bacteria lacked a visible capsule. The percentage of capsulated bacteria correlated with bacterial abundance and production, and chlorophyll a concentration. Also, the bioavailability of DOC (dissolved organic carbon), estimated by the ratio between bacterial production and DOC concentration, significantly correlated with the percentage of capsulated bacteria. In the North Sea, the contribution of capsulated bacteria to the total number of bacteria decreased from the surface (3 m depth) to the near-bottom (25–35 m) layers from 20% to 14% capsulated bacteria. In the nearshore area of the North Sea, about 27% of the bacteria exhibited a capsule. Overall, a pronounced decrease in the contribution of capsulated bacteria to the total bacterial abundance was detectable from the eutrophic coastal environment to the open North Atlantic. Using this epifluorescence-based technique to enumerate capsulated bacterioplankton thus allowed us to routinely assess the number of capsulated bacteria even in the oceanic water column. Based on the data obtained in this study we conclude that almost all metabolically highly active bacteria exhibit a capsule, but also some of the metabolically less active cells express a polysaccharide capsule detectable with this method.     

Across the North Sea and beyond

Increasing numbers of ancient genomes from the Viking period retrieved across the North Sea regions are revealing a complex layer of genetic ancestries and a past cosmopolitanism that was triggered by different mobility patterns.     

Evaluating ecosystem structure and functioning of the East China Sea Shelf ecosystem,China

As China’s second-largest large marine ecosystem, the East China Sea Shelf has suffered from overfishing, eutrophication, and physical disturbance over the last several decades. A trophic mass-balance model of this ecosystem was developed in order to characterize the structure and functioning of its food web, to identify its keystone species, and to quantify the ecological impacts of fishing that it sustained during the early 2000s. Using a multivariate statistical analysis, we identified 38 functional groups for the trophic model, including fish and invertebrate groups targeted and not targeted by fisheries. Pelagic sharks and rays were identified as the keystone species in the ecosystem. Strong benthic–pelagic coupling was indicated in this ecosystem. In particular, this study highlighted the interdependent relationships that exist among plankton, benthic invertebrates, and detritus. Recent fishing activities were characterized by high exploitation rates for various commercially targeted and non-targeted species, leading to the removal of much of the ecosystem’s fishable production. Overall, our findings give a preliminary explanation of the current problems of eutrophication and fishery depletion and other changes in the East China Sea Shelf, and highlight the need for developing ecosystem-based fisheries management.