Long-term variability of winter nitrate concentrations in the Northern Wadden Sea driven by freshwater discharge,decreasing riverine loads and denitrification

The hypothesis of a recent reversal in the eutrophication of the Wadden Sea and the potential of inshore waters in denitrification is explored. Salinity, temperature and nitrate concentrations in the List Tidal Basin (Northern Wadden Sea) have been measured about twice weekly since 1984. Salinity has a clear seasonal cycle with lowest salinities of about 27 in late winter and highest salinities of about 31 in summer. Mean annual deviations from the long-term mean salinity correlate significantly with riverine freshwater discharge. Winter nitrate concentrations are generally high (about 50 μM on average). The major part of the variability is related to salinity (∼35%). Temperature had a minor impact (∼1%). Superimposed on this, a long-term decrease of about 1 μM per year was found. Together, these processes account for about 45% of the nitrate variability. The long-term decrease of about 2% per year is similar to continental riverine trend in total nitrogen loads. In contrast to the List Tidal Basin, salinity explained more than 90% of nitrate variability in the off-shore German Bight. Salinity (30) normalised winter nitrate data of the German Bight also show a long-term decreasing trend. Most of the List Tidal Basin data are either on or below the nitrate–salinity relation found in the German Bight. This observation suggests that denitrification has a major impact on the winter nitrate concentrations in the Northern Wadden Sea compared to the German Bight. It is hypothesised that a large part of the unexplained variability is related to weather-dependent changes in residence time of tidal water masses in the Wadden Sea and circulation patterns within the German Bight.     

Zooplankton response to a warmer northern Wadden Sea

Weekly measurements of mesozooplankton (>76 μm) and hydrographic parameters have been carried out since 1984 in the List Tidal Basin (northern Wadden Sea). Monthly water temperature significantly increased by 0.04°C year⁻¹. The largest increase by 3°C in 22 years occurred in September, implying an extension of the warm summer period. Mean annual copepod abundance and length of copepod season correlated significantly with mean temperature from January to May. Except for an increasing Acartia sp. abundance during spring (April–May), no long-term trends in copepod abundance were observed. The percentage of carnivorous zooplankton increased significantly since 1984 mainly due to a sudden increase in the cyclopoid copepod Oithona similis in 1997. We expect that global warming will lead to a longer copepod season and higher copepod abundances in the northern Wadden Sea.     

Distribution patterns of bacterioplankton and chlorophyll-a in the German Wadden Sea

 In a first synoptic evaluation, the temporal and spatial distribution of bacterioplankton and chlorophyll-a were determined in the German Wadden Sea. Three surveys were undertaken in winter, spring, and summer of 1994 using up to eight ships simultaneously between the river Ems and Sylt island. Despite intensive hydrodynamic mixing of the Wadden Sea water, spatial gradients were obvious. The abundance of bacterioplankton ranged from 0.4 to 26×10⁵ ml^–1 and chlorophyll-a varied between <0.1 and 79 μg l^–1. In winter, relatively homogeneous distribution patterns of both parameters with small gradients were found. Highest chlorophyll-a values connected with a highly patchy structure were observed in spring, while in summer both total chlorophyll-a values and the complexity of the distribution pattern had decreased. In contrast, bacterial numbers increased steadily from January to July with the highest bacterial densities and greatest patchiness observed in summer. Moreover, in some regions of the Wadden Sea, a trophic succession of algae as carbon producers and bacteria as consumers was evident. Correlation analysis verified the relationship between bacteria and chlorophyll a, indicating bottom-up control of bacterial abundance in the northern part of the German Wadden Sea. Since the observed regression slope is remarkably low (0.12–0.46) compared to literature values (0.5–0.8), we suggest that the link between phytoplankton and bacteria found here is a special characteristic of the Wadden Sea as a transition zone between the coastal region and the outer North Sea. Received: 26 May 1998 / Accepted:12 November 1998     

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)     

Tidal impact on planktonic primary and bacterial production in the German Wadden Sea

 Tidal variation of biological parameters was studied at three anchor stations in selected inlet channels of the northern German Wadden Sea in May and July 1994. Concentrations of bacteria, chlorophyll a and suspended matter as well as primary and bacterial production were assessed over a period of 25 h in the surface and in the bottom water. Diurnal variation in primary production was found both under in situ light conditions and under constant illumination. Tidal turbulence caused the introduction of detritus, bacteria and pigments from the sediment into the water column. The impact of sediment resuspension was most evident in the bottom water, leading to tidally oscillating bacterial production rates which were high during high stream velocity and low during the slack times. Estimations of the areal daily phytoplankton production and corresponding bacterial carbon demands were unbalanced. Primary production accounted for only 25–45% of the total bacterial carbon requirement. This discrepancy is due to the shallow euphotic depth in the Wadden Sea, allowing net primary production only in the upper 2–3 m of the water column, while the relatively high levels of bacterial activity do not show a vertical decline. Assuming that the specific biological activities in the water columns over the tidal flats are similar to those found in the inlet channels, it was found that production processes dominate in shallow areas whereas decomposition processes dominate in the deep channels. Moreover, the predominance of heterotrophic processes in the inlet channels means that additional organic carbon sources must contribute to the heterotrophic metabolism in the deep parts of the Wadden Sea, and that the horizontal flux of material is important in this turbid mesotidal ecosystem. Received: 18 April 1998 / Accepted: 12 November 1998     

The Dutch Wadden Sea: a changed ecosystem

Since 1600 the surface area of the Dutch Wadden Sea decreased considerably by successive reclamations of saltmarshes. In 1932 the Zuiderzee (3200 km²) was closed off from the Wadden Sea causing in the remaining part an increase in tidal range and current velocities. In 1969 the Lauwerszee (91 km²) was closed off and turned into a freshwater lake as well. Man's use of the Wadden Sea changed simultaneously. Dredging in harbours and shipping routes, and extraction of sand and shells became common practice. Extraction of sand increased manifold between 1960 and 1985. These activities did contribute to the turbidity of the Wadden Sea water. Discharge of nitrogen and phosphorus compounds into the western Wadden Sea increased also manifold since 1950, causing an increase in phytoplankton production, duration of phytoplankton blooms, and intertidal macrozoobenthic biomass. Loads of metals and organochlorine contaminants entering the Wadden Sea were hard to estimate. Fisheries changed drastically since the 1930's. Fishing for ‘Zuiderzee’ herring came to an end shortly after closing off the Zuiderzee. The anchovy fishery ceased in the 1960’, that for flounder in 1983. Undersized brown shrimps were fished until 1971. Selective shrimptrawls and sorting devices with flushing seawater were introduced to reduce mortality among young flatfish and shrimps. Oysters became extinct in the 1960's due to over-exploitation of the natural beds. Production of mussels increased more than ten times between 1950 and 1961 due to ‘culturing’, catches of cockles increased slowly between 1955 and 1984. Whelks were fished until 1970. The most important changes in the biotic system of the Wadden Sea, increased production of microalgae and intertidal macrozoobenthos, can be attributed to increased nutrient loads. Eutrophication provided ample food supply for mussels which are harvested mainly by man and eider duck, and may have caused also increased growth in juvenile plaice. Increased turbidity may have impaired life conditions for adult dab, and have presented also recovery of sublittoral eelgrass beds after their disappearance in the 1930's due to the ‘wasting disease’. Increased turbidity in the Wadden Sea is probably caused by closing off the Zuiderzee (1932), a significant increase of dredge spoil disposal near Hoek van Holland between 1970 and 1983, and a more than 10-fold increase of mussel culturing in the Wadden Sea since 1950. Stocks of several bird species breeding in the Wadden Sea area suffered great losses in the early 1960's due to a pesticide accident. Most of the breeding populations have recovered. PCB's caused a reproduction failure among harbour seals in the 1970's. Since 1980 official Dutch policy aims at multiple use of the Wadden Sea, with emphasis on protection and restoration of the natural environment. The 3rd Water Management Plan (1989) aims at a development of the Wadden Sea ecosystem towards the situation of ca. 1930, i.e. without undoing present sea dikes and reclaimed polders. Management of the Dutch Wadden Sea will therefore have to focus mainly on reduction of eutrophication, pollution and turbidity. Some management options are discussed.     

Recent changes in the contributions of river Rhine and North Sea to the eutrophication of the western Dutch Wadden Sea

From 1955 to the mid 1980s the loads of both nitrogen and phosphorus from the river Rhine to the Dutch coastal area, the Wadden Sea included, increased. Since 1985 the phosphorus loads has decreased significantly, while the nitrogen load remained about the same.Annual primary production in the western Dutch Wadden Sea has increased fromc. 40 g C m^–2 (1950) to 150 (mid 1960s) and over 500 g C m^–2 (1986). The biomass of macrozoobenthos has more than doubled since 1970. Simultaneously, the meat yield of cultured blue mussels (Mytilus edulis), has increased since the 1960s. Previously, it was indicated that the increase in primary production of the phytoplankton over the period 1950 to 1986 was stimulated by the load of dissolved inorganic phosphate from Lake IJssel, a reservoir supplied by Rhine water. Since 1990, however, primary production has been higher than was expected from decreased phosphate loads from Lake IJssel. It is argued that this lack of response may have been caused by increased concentrations of dissolved inorganic phosphate at sea originating from increased inflow from a.o. the Strait of Dover, which compensate for the decrease in phosphate from the rivers, possibly in combination with a significant improvement of the light conditions of the water in the Wadden Sea.     

Variability of nutrient limitation in the Archipelago Sea,SW Finland

Over a two year study period, zooplankton was sampledin Gazi Bay, Kenya, using a 335 μm mesh size Bongonet. Two Way Indicator Species Analysis (TWINSPAN)classification technique demonstrated that rainfalland tidal regime had substantial influence on thezooplankton community structure. Samples collectedduring the rainy season months clustered together whentreated with TWINSPAN. Furthermore, theclustering was more pronounced for neap tidesamples than for spring tide ones. Samples obtainedduring spring tide did not give a clear cut pattern. Canonical Correspondence Analysis (C.C.A.) confirmedthese findings, a clustering together of rainy/neaptide samples; and little separation (based onenvironmental variables) between samplingstations. This revised version was published online in August 2006 with corrections to the Cover Date.     

On trends in nutrient concentration in the northern Wadden Sea of Sylt

The mean values on hydrographical parameters in the Wadden Sea of Sylt taken weekly over a period of seven years (1975, 1976 and 1984 to 1988) were compared in order to detect trends in concentrations of nutrients or plankton. An increase in NO₂, NO₃, PO₄ and Si could be found in summer. NH₄ showed a negative trend. Corresponding with the rising nutrient-level, Chlorophyll-a and pH show a positive trend. Salinity decreased significantly. Changes in NH₄ and NO₂ were correlated with rainfall. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Long-term changes in macrozoobenthic abundance on the tidal flats of the western part of the Dutch Wadden Sea

For 20 years (1969–1988), larger bottom animals have been sampled quantitatively once or twice annually at 15 stations scattered over Balgzand (a 50 km² tidal flat area in the westernmost part of the Dutch Wadden Sea). In 29 species, numbers were sufficiently high to allow a statistical evaluation of the fluctuation patterns of their annual densities. The results revealed two main trends: (1) a sensitivity to low winter temperatures in 12 species, causing low densities in these species immediately after a severe winter (1979, 1985, 1986 and 1987) and relatively high densities during a period with some mild winters in succession (1973–1975); (2) an upward long-term trend in 11 (other) species, causing upward trends (viz. roughly a doubling) of total macrozoobenthic biomass and production over the 20-year period of observation, probably as a consequence of increasing eutrophication. By far the major part of the species thus exhibited either of these two patterns, causing total biomass and species number to be governed largely by the above two trends. Results of less frequent sampling (once per 5 or 10 years) of 26 transects scattered over the ≈500 km² of tidal flats of the whole western half of the Dutch Wadden Sea showed that the two trends also represent the changes occurring in a much larger area. Some local departures from the general patterns are discussed and related to specific causes. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

In situ nutrient enrichment experiment in the Bohai and Yellow Sea

Nutrient concentrations and N : P ratios have changed significantlyin the past 40 years in the Bohai and Yellow Sea. How do thesechanges influence or contribute to the growth of phytoplankton?Nutrient enrichment experiments were conducted in 1998 and 1999to shed light on which was the first nutrient to limit algalgrowth and uptake rates of nitrogen and phosphorus. Significantvariance analysis, together with nutrient concentration andratio, demonstrated that phosphorus was the first nutrient tolimit the growth of phytoplankton in the Laizhou Bay (SouthBohai); nitrogen was the first, whilst phosphate might be thepotential, nutrient to limit the growth of phytoplankton inthe West Yellow Sea; the Central Yellow Sea was oligotrophicand any one of nitrogen, phosphorus and silicon would limitthe growth of phytoplankton; and silicon was confirmed not tolimit the growth of phytoplankton, although the silicate concentrationdecreased acutely, in the Laizhou Bay. Moreover, the ratio ofnitrogen to phosphorus in phytoplankton uptake was smaller thanthat in seawater, which suggested that phosphorus was preferentiallyused before nitrogen by the phytoplankton. The preference ofphosphorus over nitrogen indicates a further limitation of phosphorusin the Laizhou Bay, and increases the high possibility thatphosphorus, rather than nitrogen, is the first nutrient to limitthe growth of phytoplankton in the West Yellow Sea. Half-saturationconstants (K_s) of dissolved inorganic nitrogen (DIN) and phosphatein the Bohai and Yellow Sea were 1.80 µM and 0.13 µM,respectively. Compared with other sea areas, the K_s value ofDIN in the Bohai and Yellow Sea was located at the high endof the spectrum.     

Fundamentals of an applied ecosystem research project in the Wadden Sea of Schleswig-Holstein

The aims, content, and organisatory structure of a proposed interdisciplinary ecosystem research project in the Wadden Sea of Schleswig-Holstein (W. Germany) are briefly presented. The project will include research on both fundamental as well as applied aspects of the Wadden Sea ecosystems and their interaction with local human activities. In contrast to most of the other completed or currently running ecosystem research projects on tidal coasts, a considerable part of the scientific work will also deal with aspects of ecosystem management and protection of the various marine and semiterrestrial habitats of the Wadden Sea. Considerable attention is paid to theoretical and methodological aspects of research on ecosystems and landscape units. In particular, the adoption of a hierarchical view of complex biological and environmental systems is recommended. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Biological monitoring of fish and crustaceans in the Wadden Sea — potential and problems

Monitoring of fish and crustaceans in the Wadden Sea (WS) must cope with rapid changes in distribution patterns, access to certain areas and gear efficiency. Application and limitations of a variety of fishing devices (fyke nets, gill nets, enclosures, stow nets, purse seines, beam trawls, push nets, beach seines, bottom trawls, pelagic trawls) are discussed with regard to different objectives of monitoring. Furthermore, the validity of data from three current monitoring programmes is also discussed. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Variations in the mussel population of the Dutch Wadden Sea in relation to monitoring of other ecological parameters

The pattern of distribution of intertidal mussel beds is relatively constant over a number of years although their surface area can vary greatly. The abundance of mussels shows much greater fluctuations. In the western part of the Dutch Wadden Sea, west of the Terschelling tidal divide, the amount of mussels on natural beds fluctuated between 1 and 24 million kg fresh weight during the years 1949 to 1988. In the eastern part of the Dutch Wadden Sea the biomass varied between 5.5 and 180 million kg. The influence of the mussels on the ecosystem therefore can be very different between years. When many mussels are present the whole watermass can be filtered every few days. In years with few mussels present the filtering may take one month. It is argued that monitor programmes for a.o. nutrients, chlorophyll and growth rates of benthic organisms are of limited value if there is no indication about the total amount of mussels in the area. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

Monitoring harbour seal (Phoca vitulina) in the Danish Wadden Sea

Monitoring of harbour seal (Phoca vitulina) has been carried out in the Danis part of the Wadden Sea during the past 8 years by the Fishery and Maritime Museum. In addition to this, field observations over three breeding seasons have been undertaken. The census shows a steady increase of approx. 14% per yeart. The distribution of the seals in the area can be split up into two different types of haulout localities, and a varied pattern of haulouts is observed during the year. The mass die-off in 1988 is discussed within the framework of the knowledge on the population, and it is recommended that more effort be put into monitoring and population studies of the harbour seal in the entire International Wadden Sea. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, 1–4 November 1988)     

Changes of the tidal water levels at the German North Sea coast

For 10 selected tide gauges at the German North Sea Coast, the time series of the mean high water (MHW) and mean low water (MLW) were analysed. Since about 1950, both levels deviate from earlier trends; MHW shows an accelerated rise, while MLW even fell after an earlier rise. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988).     

Monitoring the fish fauna in the Wadden Sea with special reference to different fishing methods and effects of wind and light on catches

The seasonal and local distribution of fish in the Wadden Sea depends on the various modes of life of each species. To obtain nearly all species of the fish fauna and the stages of development in different spatial regions of the Wadden Sea-shallow and deep channels, puddles and pools, sedimentation fields and tidal flats—it is essential to use different fishing methods synchronously. During a two-year monitoring project in 1986 and 1987 on fish and fisheries in a restricted area of the Wadden Sea of Schleswig-Holstein, five types of fishing gear were used synchronously: handnet, push-net, 2-m beam trawl, commercial shrimp trawl, fyke net. In 822 samples of all gear 33 species were caught. No gear obtained the maximum number of species. Day and night samples show considerably different composition. Furthermore, force and direction of wind have an important effect on the distribution of fish in the coastal area. The advantages and disadvantages of the different gear for monitoring purposes are discussed from scientific and practical viewpoints. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG 1–4 November 1988)     

Seasonal variability of inorganic and organic nitrogen in the North Sea

This study considers the cycling of nitrogen in the waters of the North Sea, particularly focussing on organic nitrogen. Dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were measured in the North Sea over a one-year period (autumn 2004–summer 2005). The surface water concentrations of nitrate, ammonium, DON and PON during the present study ranged from <0.1–7.2 μM, <0.1–2.0 μM, 1.9–11.2 μM and 0.3–5.6 μM, respectively, with DON the dominant fraction of total nitrogen at all times. These nutrients concentrations were significantly lower compared to previous studies in the southern North Sea. The seasonal variations showed high mean surface concentrations of nitrate (4.7 ± 0.6 μM) and DON (8.9 ± 0.9 μM), low ammonium (<0.1 μM) and PON (0.8 ± 0.1 μM) in winter, shifting to low nitrate (0.3 ± 0.3 μM) and DON (4.2 ± 1.2 μM) in summer, with high ammonium (0.8 ± 0.4 μM) in autumn and PON (2.5 ± 1.2 μM) in spring. Highest mean surface DON concentration was measured in winter and may be due to resuspension of the organic matter from the bottom sediments. For autumn and spring, phytoplankton DON release was likely to be the most significant source of DON as shown by high concentrations of low molecular weight (LMW) DON and its positive correlation to chlorophyll a. Low total and LMW DON concentrations during summer were likely to be due to the uptake of the LMW DON fraction by phytoplankton and bacteria and the stratification of the water column. DON is therefore shown to be a potentially important source of nitrogen in shelf seas especially after the spring bloom has depleted nitrate to limiting concentrations. Handling editor: L. Naselli-Flores     

Monitoring activities in the Federal Republic of Germany pursuant to international conventions and national legislation

The main activities of the Federal Republic of Germany regarding the monitoring of the pollution of the North Sea and the Baltic Sea are described. Reference is also made to research projects concerned with the investigation of the processes of transportation and transformation of harmful substances in the marine ecosystem. Political conclusions drawn by the Federal Government are mentioned. Environmental protection measures are only dealt with marginally; combatting measures are not dealt with. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)