Eutrophication of the Dutch Wadden Sea: External nutrient loadings of the Marsdiep and Vliestroom basin

The increasing P and N content in the two main tidal basins in the western Dutch Wadden Sea, the Marsdiep and the Vliestroom basin, has been reconstructed from the 50s onwards. The area is enriched with nutrients by two sources both originating from the river Rhine, one being the discharge from Lake IJssel and the other the exchange with the coastal zone of the North Sea. Due to a buffering by Lake IJssel for about 15–20 years, the eutrophication of the western Wadden Sea showed a time lag compared with the continuously increasing nutrient concentrations in the river Rhine and the coastal zone of the North Sea. At present, the primary production in part of the area still seems to be nutrient limited in summer, while loadings have already been decreasing in recent years. So far, no severe, negative effects on the ecosystem have been reported. Some remarks are made on the eutrophication in other parts of the Dutch' Wadden Sea in relation to the hydrographic characteristics of these areas. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988) Publication No. 18 of the project “Ecological Research of the North Sea and Wadden Sea” (EON)     

Long-term changes in Wadden Sea nutrient cycles: importance of organic matter import from the North Sea

The Wadden Sea is a shallow tidal area along the North Sea coast of The Netherlands, Germany and Denmark. The area is strongly influenced by rivers, the most important of which are the rivers Rhine, Meuse and Elbe. Due to the increased nutrient load into the coastal zone the primary production in the Wadden Sea almost tripled during the past few decades. A conceptual model is presented that links nitrogen input (mainly nitrate) via Rhine and Meuse with the annual nitrogen cycle within the Wadden Sea. Three essential steps in the model are: (1) nitrogen limits the primary production in the coastal zone, (2) a proportional part of the primary produced organic matter is transported into the Wadden Sea and (3) the imported organic matter is remineralized within the Wadden Sea and supports the local productivity by nitrogen turn-over. The conceptual model predicts that during years with a high nutrient load more organic matter is produced in the coastal zone and more organic matter is transported into and remineralized within the Wadden Sea than during years with low nutrient loads. As a proxy for the remineralisation intensity ammonium plus nitrite concentrations in autumn were used. Based on monitoring data from the Dutch Wadden Sea (1977–1997) the above mentioned model was statistically tested. In autumn, however, a significant correlation was found between autumn values of ammonium and nitrite and river input of nitrogen during the previous winter, spring and summer. The analysis supports that in years with a high riverine nitrogen load more organic matter is remineralized within the Wadden Sea than in years with a low nitrogen load. A comparison with older data from 1960 to 1961 suggests that the remineralisation intensity in the Wadden Sea has increased by a factor of two to three. This is not reflected by a two to three-fold increase in riverine nitrogen load from 1960 to present. It is suggested that the increased remineralisation rates in the Dutch Wadden Sea between the 1960s and the 1980s/1990s are largely caused by an increased nitrogen flux through the Channel and the Strait of Dover and by an increased atmospheric nitrogen input.     

Management Strategy Evaluation Applied to Coral Reef Ecosystems in Support of Ecosystem-Based Management

Ecosystem modelling is increasingly used to explore ecosystem-level effects of changing environmental conditions and management actions. For coral reefs there has been increasing interest in recent decades in the use of ecosystem models for evaluating the effects of fishing and the efficacy of marine protected areas. However, ecosystem models that integrate physical forcings, biogeochemical and ecological dynamics, and human induced perturbations are still underdeveloped. We applied an ecosystem model (Atlantis) to the coral reef ecosystem of Guam using a suite of management scenarios prioritized in consultation with local resource managers to review the effects of each scenario on performance measures related to the ecosystem, the reef-fish fishery (e.g., fish landings) and coral habitat. Comparing tradeoffs across the selected scenarios showed that each scenario performed best for at least one of the selected performance indicators. The integrated ‘full regulation’ scenario outperformed other scenarios with four out of the six performance metrics at the cost of reef-fish landings. This model application quantifies the socio-ecological costs and benefits of alternative management scenarios. When the effects of climate change were taken into account, several scenarios performed equally well, but none prevented a collapse in coral biomass over the next few decades assuming a business-as-usual greenhouse gas emissions scenario.     

Distribution patterns of mesozooplankton biomass in the North Sea

During spring 1986 and winter 1987, zooplankton samples were collected over the entire North Sea by means of a multi-closing net-system. Before taxonomic treatment, wet weight estimates and carbon content conversions were carried out. From this data set, 4 962 522 tons zooplankton biomass (dry weight) were estimated for the whole North Sea during the spring survey. High biomasses (more than 100 mg C/m³) were located in areas between the Orkneys and the Shetlands, off the mouth of the Firth of Forth, the Channel and the river Rhine. Considerable zooplankton biomass was also found parallel to the Danish west coast. Furthermore, a narrow tongue of high biomass (partly greater than 200 mg C/m³) intruded from the north, between 1 °E and 4 °E, into the northern North Sea, turning to the east at 56°N, and continuing into deeper water layers to form a left turning “helix” of high biomass in the central part of the North Sea. During the winter survey the carbon content of the zooplankton stock was a factor 10 lower than in summer. Altogether, 519340 tons of zooplankton biomass (dry weight) were estimated in winter. Centres of relatively high biomass were located off the mouth of the rivers Rhine, Weser and Elbe and off the British east coast moving in a cyclic way across the Dogger Bank into the central North Sea. A further maximum of zooplankton abundance was found in the Skagerrak region. However, an intrusion of zooplankton from the shelf edge into the North Sea was not observed in winter. A qualitative analysis of species composition showed that small copepods dominated the zooplankton in the southern and eastern North Sea. The “eddy” of high biomass in the northern North Sea observed in spring, however, was mostly shaped by the large copepodCalanus finmarchicus (70–90%). The distribution of zooplankton biomass in the North Sea is discussed in relation to the hydrographic conditions and to the biology of the dominant species.     

Timing of downstream migration and food uptake of juvenile North Sea houting stocked in the Lower Rhine and the Lippe (Germany)

In 1992 a stocking programme was established to re‐introduce North Sea houting Coregonus oxyrinchus , which disappeared from the River Rhine in the 1940s, in the Lower Rhine. Juvenile North Sea houting have been stocked since 2001 in a gravel pit lake permanently connected to the Lower Rhine, and in the Lippe, a potamal tributary of the Rhine. Monitoring studies showed that the majority of the small North Sea houting left the gravel pit lake within 4 days after stocking. In the Lippe, some of the juveniles were found in the drift immediately after stocking, generally preferring the middle surface areas of the river. Other North Sea houting waited until dawn before they started their downstream migration. Juveniles immediately started to feed on the zooplankton resources in both waters. Pond studies revealed high growth rates of juvenile North Sea houting, ranging from 0·44 to 0·94 mm day⁻¹. This study suggests that the stocking strategies at both waters of the Lower Rhine comply with the requirements of the migration behaviour of juvenile North Sea houting.     

Outmigration pathways of stocked juvenile European sturgeon (Acipenser sturio L., 1758) in the Lower Rhine River,as revealed by telemetry

Working towards a future Rhine Sturgeon Action Plan the outmigration pathways of stocked juvenile European sturgeon (Acipenser sturio L., 1758) were studied in the River Rhine in 2012 and 2015 using the NEDAP Trail system. A total of 87 sturgeon of 3 to 5 years old (n = 43 in 2012, n = 44 in 2015) were implanted with transponders and released in May and June in the river Rhine at the Dutch‐German border, approximately 160 km from the sea. In total three sturgeons (3%) were found dead on river banks within seven days after the release. Based upon their wounds these sturgeons were likely hit by ship‐propellers. Tracking results were obtained from 57 (66%) of the sturgeons, of which 39 (45%) indicated movement into the Port of Rotterdam. Here the sturgeons remained for an average of two weeks, which suggests they spent time to acclimatize to higher salinities before entering the North Sea. Of the 45 (52%) sturgeons that were confirmed to have entered the North Sea, ten (22%) were recaptured (mainly by shrimpers and gill‐nets) close to the Dutch coastline; nine were alive and were released. From the results we obtained the preferred outmigration pathways, movement speeds and an indication of impacting factors (i.e. ship propellers and bycatch). Bycatches provided also localisations information in the coastal area. A next step to complete this work would be to assess habitat selection in freshwater and downstream migration of young of the year (YOY sturgeons) in the Lower Rhine.     

Understanding Long-Term Changes in Species Abundance Using a Niche-Based Approach

One of the major challenges to understanding population changes in ecology for assessment purposes is the difficulty in evaluating the suitability of an area for a given species. Here we used a new simple approach able to faithfully predict through time the abundance of two key zooplanktonic species by focusing on the relationship between the species’ environmental preferences and their observed abundances. The approach is applied to the marine copepods Calanus finmarchicus and C. helgolandicus as a case study characterising the multidecadal dynamics of the North Sea ecosystem. We removed all North Sea data from the Continuous Plankton Recorder (CPR) dataset and described for both species a simplified ecological niche using Sea Surface Temperature (SST) and CPR Phytoplankton Colour Index (PCI). We then modelled the dynamics of each species by associating the North Sea’s environmental parameters to the species’ ecological niches, thus creating a method to assess the suitability of this area. By using both C. finmarchicus and C. helgolandicus as indicators, the procedure reproduces the documented switches from cold to warm temperate states observed in the North Sea.     

Resolving the origins of invertebrate colonists in the Yangtze River Estuary with molecular markers: Implications for ecological connectivity

Understanding connectivity over different spatial and temporal scales is fundamental for managing of ecological systems. However, controversy exists for wintertime ecological connectivity between the Yangtze River Estuary (YRE) and inner southwestern Yellow Sea. Here, we investigated ecological connectivity between the YRE and inner southwestern Yellow Sea in wintertime by precisely pinpointing the source of the newly colonized populations of a winter‐spawning rocky intertidal invertebrate, Littorina brevicula (Philippi, 1844), on artificial structures along the coast of the Yangtze River Delta (YRD) using mitochondrial ND6 sequences and microsatellite data. Clear phylogeographic and genetic differentiation were detected between natural rocky populations south and north of the YRE, which resulted from the lack of hard substrate for rocky invertebrates in the large YRD coast. For the newly colonized populations on the coast of YRD, most individuals (98%) to the south of ~33.5°N were from natural rocky populations to the south of the YRE and most of those (94%) to the north of ~33.5°N were from the northern natural rocky populations, which demonstrated strong ecological connectivity between the inner southwestern Yellow Sea and the YRE in winter time. We presented the first genetic evidence that demonstrated a northward wintertime coastal current in the inner southwestern Yellow Sea, and precisely illustrated the boundary of the coastal current recently proposed by numerical experiment. These results indicated that the YRE serves as an important source of materials and energy for the inner southwestern Yellow Sea in winter, which can be crucial for the function of the Yellow Sea ecosystem.     

Local effects of wind speed and direction on the phytoplankton of the Southern Bight

The causes of the exceptionally early blooms of Ceratium spp.in the Southern Bight of the North Sea during the early 1970sare re-assessed. This localised event is found to coincide witha change in the meridional circulation over the North Sea inthe late winter and early spring, with abnormally strong northerliesduring the 1960s giving way to strengthened southerlies duringthe early 1970s. Such a change in wind direction can be expected(both from observation and simulation) to bring about a lesswidespread distribution of turbid coastal water in the SouthernBight and the marked reduction in windfetch may also have servedto decrease watercolumn turbidity through its effect on theincidence of long-period swell. These processes may be reinforcedby associated changes in the quantity of turbid runoff dischargedby the Rhine. The advanced blooms of Ceratium spp. in the early1970s are attributed both to these effects and to an increasedfrequency of light winds (decreased wind mixing) in the earlymonths of the year.     

Disparate movement behavior and feeding ecology in sympatric ecotypes of Atlantic cod

Coexistence of ecotypes, genetically divergent population units, is a widespread phenomenon, potentially affecting ecosystem functioning and local food web stability. In coastal Skagerrak, Atlantic cod (Gadus morhua) occur as two such coexisting ecotypes. We applied a combination of acoustic telemetry, genotyping, and stable isotope analysis to 72 individuals to investigate movement ecology and food niche of putative local “Fjord” and putative oceanic “North Sea” ecotypes—thus named based on previous molecular studies. Genotyping and individual origin assignment suggested 41 individuals were Fjord and 31 were North Sea ecotypes. Both ecotypes were found throughout the fjord. Seven percent of Fjord ecotype individuals left the study system during the study while 42% of North Sea individuals left, potentially homing to natal spawning grounds. Home range sizes were similar for the two ecotypes but highly variable among individuals. Fjord ecotype cod had significantly higher δ¹³C and δ¹⁵N stable isotope values than North Sea ecotype cod, suggesting they exploited different food niches. The results suggest coexisting ecotypes may possess innate differences in feeding and movement ecologies and may thus fill different functional roles in marine ecosystems. This highlights the importance of conserving interconnected populations to ensure stable ecosystem functioning and food web structures.     

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.     

Restoration and recovery of shallow eutrophic lake ecosystems in The Netherlands: epilogue

During the symposium Restoration and recovery of shallow lake ecosystems in The Netherlands studies on restoration of eutrophic lakes were addressed and discussed. Many Dutch shallow lakes have received high external loadings of phosphorus through supply water that is influenced by the River Rhine and loadings in The Netherlands. Two important Action Plans (the Rhine Action Plan, the North Sea Action Plan) are now in operation to reduce nutrient emissions. The targets set are not likely to be fully reached, so that supplementary reduction of phosphorus supplied to inland fresh waters will be required. In several shallow lakes such a reduction has been achieved recently, but without leading to discernible recovery. The main causes of delay are phosphorus storage and its subsequent release from sediments and foodweb; however, the remaining extraneous phosphorus supply is often still too high. Supplementary actions are, therefore, called for. A further reduction of phosphorus inputs is suggested, besides supplementary measures proposed, viz. dredging, flushing, biomanipulation, chemomanipulation. Restoration to the past situation via upwelling groundwater appeares to be feasible in some cases. There is a common consensus that each lake behaves differently depending to its morphology, hydrology and history of eutrophication. Therefore each lake has to be studied before restoration measures can be applied. Besides, the ecosystem should not only be studied as a separate entity, but as a part of systems of a higher integration level.     

Top 10 Principles for Designing Healthy Coastal Ecosystems Like the Salish Sea

Like other coastal zones around the world, the inland sea ecosystem of Washington (USA) and British Columbia (Canada), an area known as the Salish Sea, is changing under pressure from a growing human population, conversion of native forest and shoreline habitat to urban development, toxic contamination of sediments and species, and overharvest of resources. While billions of dollars have been spent trying to restore other coastal ecosystems around the world, there still is no successful model for restoring estuarine or marine ecosystems like the Salish Sea. Despite the lack of a guiding model, major ecological principles do exist that should be applied as people work to design the Salish Sea and other large marine ecosystems for the future. We suggest that the following 10 ecological principles serve as a foundation for educating the public and for designing a healthy Salish Sea and other coastal ecosystems for future generations: (1) Think ecosystem: political boundaries are arbitrary; (2) Account for ecosystem connectivity; (3) Understand the food web; (4) Avoid fragmentation; (5) Respect ecosystem integrity; (6) Support nature’s resilience; (7) Value nature: it’s money in your pocket; (8) Watch wildlife health; (9) Plan for extremes; and (10) Share the knowledge.     

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.     

Abiotic drivers of interannual phytoplankton variability and a 1999–2000 regime shift in the North Sea examined by multivariate statistics

The Dutch coastal zone is a region of the North Sea with a marked interannual and long‐term abiotic and phytoplankton variability. To investigate the relationship between abiotic variability and phytoplankton composition, two routine water monitoring data sets (1991–2005) were examined. Multivariate statistics revealed two significant partitions in the data. The first consisted of interannual abiotic fluctuations that were correlated to Rhine discharge that affected the abundance of summer and autumn diatom species. The second partition was caused by a shift in the abiotic data from 1998 to 1999 that was followed by a shift in phytoplankton composition from 1999 to 2000. Important factors in the abiotic shift were decreases in suspended matter (SPM) and phosphate (DIP) concentrations, as well as in pH. The decrease in SPM was caused by a reduction in wind speed. The increase in water column daily irradiance from the decrease in SPM led to increases in the abundance of winter–spring species, notably the prymnesiophyte Phaeocystis globosa. Because wind speed is related to the North Atlantic Oscillation (NAO) index it was possible to correlate NAO index and P. globosa abundance. Only five abiotic variables representing interannual and long‐term variability, including Rhine discharge and NAO index, were needed to model the observed partitions in phytoplankton composition. It was concluded that interannual variability in the coastal phytoplankton composition was related to year‐to‐year changes in river discharge while the long‐term shift was caused by an alternating large‐scale meteorological phenomenon.     

Potential modification of the fluxes of nitrogen from the Humber Estuary catchment (U.K.) to the North Sea in response to changing agricultural inputs and climate patterns

The catchment of the Humber Estuary drains approximately 20% of the land area of England via two main rivers, the Trent and the Ouse, and a number of tributaries. The catchment is home to major metropolitan and industrial centres, as well as to extensive areas of agricultural land; for this reason, the river and estuarine systems have been subject to considerable anthropogenic inputs. The Humber Estuary is one of the largest U.K. estuaries and the major U.K. freshwater input to the North Sea. The U.K. Natural Environment Research Council (NERC) Land Ocean Interaction Study (LOIS), which combined extensive physical and biogeochemical measurements with an integrated modelling programme, was established to examine the transport and fate of nutrients and other constituents through the land-sea boundary. In this paper, a model of nitrogen (nitrate, nitrite, ammonium, particulate nitrogen) transport and cycling in the Humber Estuary, calibrated on the basis of measured constituent concentrations at its riverine and marine boundaries, is linked off-line to a Humber catchment and rivers model of nitrogen transport, which furnished simulated constituent values at the tidal limits, and the resulting estuarine nitrogen profiles compared to those of the standalone estuarine model. The estuarine model is then re-run using simulated concentration values at the tidal limits from catchment-river model simulations incorporating realistic changes in agricultural fertiliser inputs and climate forcing functions. The standalone estuarine model simulation estimated nitrate+nitrite (total nitrogen) export to the North Sea to be ca. 53000 t in 1994 and 44000 t in 1995. Following linkage of the estuarine and catchment-river models, the estimated fluxes for these years increased by 20–30%, {relative to the standalone simulation}. Higher {winter} riverine flows largely accounted for this difference. The altered flows also markedly changed the simulated concentrations and distributions of suspended particulate matter (SPM) within the estuary, indicating strongly that the transport and fluxes of particle-reactive and particle-associated constituents would show measurable differences. Scatter in the measured SPM data precluded identification of the more precise simulation run, however. Subsequent simulations using the linked models estimated that a 50% reduction in artificial fertiliser applications within the catchment gave a 10–15% decrease in nitrogen loads to the North Sea, relative to the 1994–95 input, whilst forcing the catchment model with a climate perhaps appropriate for the mid-21st century yielded nitrogen fluxes that were similar to those of the mid-1990s.     

Phylogeography of the bullhead Cottus gobio (Pisces: Teleostei: Cottidae) suggests a pre-pleistocene origin of the major central European populations

The bullhead Cottus gobio is a small, bottom-dwelling fish consisting of populations that have not been subject to transplantations or artificial stocking. It is therefore an ideal model species for studying the colonization history of central European freshwater systems, in particular with respect to the possible influences of the Pleistocene glaciation cycles. We sampled Cottus populations across most of its distribution range, with a special emphasis on southern Germany where the major European drainage systems are in closest contact. Mitochondrial D-loop sequencing of more than 400 specimens and phylogenetic network analysis allowed us to draw a detailed picture of the colonization of Europe by C. gobio. Moreover, the molecular distances between the haplotypes enabled us to infer an approximate time frame for the origin of the various populations. The founder population of C. gobio stems apparently from the Paratethys and invaded Europe in the Pliocene. From there, the first colonization into central Europe occurred via the ancient lower Danube, with a separate colonization of the eastern European territories. During the late Pliocene, one of the central European populations must have reached the North Sea in a second step after which it then started to colonize the Atlantic drainages via coastal lines. Accordingly, we found very distinct populations in the upper and lower Rhine, which can be explained by the fact that the lower Rhine was disconnected from the upper Rhine until approximately 1 million years ago (Ma). More closely related, but still distinct, populations were found in the Elbe, the Main and the upper Danube, all presumably of Pleistocene origin. Intriguingly, they have largely maintained their population identity, despite the strong disturbance caused by the glaciation cycles in these areas. On the other hand, a mixing of populations during postglacial recolonization could be detected in the lower Rhine and its tributaries. However, the general pattern that emerges from our analysis suggests that the glaciation cycles did not have a major impact on the general population structure of C. gobio in central Europe.     

Die Bedeutung des Elbe-Ästuars für die Abwasserbelastung der südlichen Nordsee in bakteriologischer Sicht

Zusammenfassung 1. Der Elbe und ihren Nebenflüssen werden an zahlreichen Orten städtische und industrielle Abwässer zugeführt und damit auch große Mengen von Bakterien und Nährstoffen. Von diesen gelangt ein Teil schließlich in die Nordsee und beeinflußt dort das Lebensgeschehen.2. Der Bakteriengehalt des Elbewassers spiegelt sich in dem der Nordsee westlich von Helgoland wider. Die Zusammensetzung der Bakterienflora ist hier allerdings eine ganz andere. Denn die von der Elbe mitgeführten Süßwasser- und Abwasserbakterien werden großenteils im Mündungsgebiet zunächst durch Brackwasserbakterien und schließlich durch halophile Meeresbakterien ersetzt.3. Die Änderung der Bakterienpopulationen in den halinen Zonen des Ästuars wird von der Wassertemperatur beeinflußt; sie erfolgt um so schneller je höher die Temperatur ist.4. Durch die Umstellung der Bakterienflora erfolgte die Selbstreinigung von abwasserbelastetem Brack- und Meerwasser langsamer als die von Süßwasser.5. Da bei niedrigen Wassertemperaturen auch die Aktivität der meisten Bakterien abnimmt, kann es während des Winters zu einer Anreicherung von an sich abbaufähigen Schmutzstoffen in den Küstengewässern kommen.

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The importance of the Elbe estuary for polluting the southern North Sea from the bacteriological point of view

The Elbe river and its affluents are polluted by wastes and sewages. Consequently, nutrient concentrations as well as bacterial numbers are relatively high, and the bacteria contamination of the Elbe estuary is influencing the bacteria distribution of the North Sea near Helgoland. The composition of the microflora is changing in the saline zones of the Elbe estuary; freshwater representatives and sewage forms are replaced by halophilic marine bacteria. These changes in the bacterial population are influenced by water temperature — increasing temperatures cause an acceleration. The change of the bacterial flora is also responsible for the retardation of the rate of self-purification in polluted brackish and seawater. The decrease of bacterial activity at low temperatures may cause an increase in the concentration of decomposible pollutants in coastal waters during the winter.

     

Long-term population dynamics of breeding bird species in the German Wadden Sea area

For no other group of organisms in coastal areas are there so exact and long-term data available as there are for seabirds. Since the beginning of the 20th century, documentation of population size, especially for species breeding in colonies from the groups gulls, terns and auks, is almost complete. These species act as bio-indicators, and data on fluctuations in their population size are useful as they reflect changes in the state of the marine ecosystem. The population development of some of these seabird species (Herring Gull, Guillemot, Common, Arctic and Sandwich Tern) from the German North Sea coast, which primarily feed on fish, is given. Common to all these species is an exponential increase in numbers in recent years (1970–1985). Possible causes for this development, e.g. pressure from enemies or competitors, availability of breeding places, anthropogenic stress and mortality factors, as well as the direct and indirect anthropogenic-influenced changes in the trophic system due to the increasing eutrophication of coastal waters, are evaluated. Signs of a collapse in the stocks of seabrids resulting from environmental pollution are discussed. Consequences resulting from the ecosystem changes, such as reduction of nutrient discharge into the North Sea and the expansion of biological monitoring, are described. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Human transformations of the Wadden Sea ecosystem through time: a synthesis

Todays Wadden Sea is a heavily human-altered ecosystem. Shaped by natural forces since its origin 7,500 years ago, humans gradually gained dominance in influencing ecosystem structure and functioning. Here, we reconstruct the timeline of human impacts and the history of ecological changes in the Wadden Sea. We then discuss the ecosystem and societal consequences of observed changes, and conclude with management implications. Human influences have intensified and multiplied over time. Large-scale habitat transformation over the last 1,000 years has eliminated diverse terrestrial, freshwater, brackish and marine habitats. Intensive exploitation of everything from oysters to whales has depleted most large predators and habitat-building species since medieval times. In the twentieth century, pollution, eutrophication, species invasions and, presumably, climate change have had marked impacts on the Wadden Sea flora and fauna. Yet habitat loss and overexploitation were the two main causes for the extinction or severe depletion of 144 species (~20% of total macrobiota). The loss of biodiversity, large predators, special habitats, filter and storage capacity, and degradation in water quality have led to a simplification and homogenisation of the food web structure and ecosystem functioning that has affected the Wadden Sea ecosystem and coastal societies alike. Recent conservation efforts have reversed some negative trends by enabling some birds and mammals to recover and by creating new economic options for society. The Wadden Sea history provides a unique long-term perspective on ecological change, new objectives for conservation, restoration and management, and an ecological baseline that allows us to envision a rich, productive and diverse Wadden Sea ecosystem and coastal society.