Bio-availability of phosphorus in sediments of the western Dutch Wadden Sea

The purpose of this study was to make a prognosis of the effects of extended purification of terrestrial waste water, reaching the Wadden Sea by the River Rhine and Lake IJssel, on the phosphate concentration in the western Wadden Sea.The quantities of different phosphorus fractions in intertidal and subtidal sediments of the Marsdiep tidal basin (western Dutch Wadden Sea) were measured. Different methods are applied to determine the amount of phosphorus that can be released from these sediments. The direct bioavailability is determined by inoculating sediment suspensions with a natural mixture of precultured micro-organisms from the sampling area. A second approach is the measurement of the phosphate release under different redox conditions. Sequential extraction of sediment samples with different solvents is also applied. Under the present conditions and compared to the nutrient loads from fresh water (Lake IJssel) and from the North Sea, the phosphorus stored in the sediments of the western Dutch Wadden Sea plays a minor role in the total supply to micro-algae and bacteria. The bulk of the biologically available phosphorus in the sediments originates from the metal-associated fraction. Releasable phosphate may contribute to the local annual primary production to an extent of ca 45 to ca 150 g C m^–2 a^–1. The total amount of phosphorus in the sediment (mainly calcite associated) is twice to 6 times the biologically available amount.     

Response of the Dutch Wadden Sea ecosystem to phosphorus discharges from the River Rhine

The primary production in the western part of the Dutch Wadden Sea (Marsdiep tidal basin) has been reported to have increased by a factor of two since the late 1970s. This doubling of the phytoplankton and the microphytobenthos production has been ascribed to the eutrophication of that area: mean annual phosphate concentrations have increased, whereas the nitrogen concentrations have not. Analysis of the available production data indicates a more than tenfold increase of the production of the phytoplankton in the Marsdiep tidal channel since the early 1950s. The increase in the phytoplankton production in the inner area seems to be of the same order of magnitude. The primary production of the phytobenthos and the secondary production of the macrozoobenthos seems to have increased proportionally to that of the phytoplankton. It is known that the nutrients in the Marsdiep tidal basin originate from the River Rhine. However, there are two possible routes for nutrients to reach that area. One is via the River IJssel and Lake IJssel (varying amounts of fresh water are sluiced out from the latter into the Wadden Sea). The other is along the coast of the North Sea, where the water transport is driven northeastwards by the residual current. Analysis of available data shows that the annual primary production in the Marsdiep tidal basin is mainly determined by the phosphate discharge from Lake IJssel which directly reach that part of the Wadden Sea, rather than from the River Rhine of which the water must run northward along the coast before reaching the western Dutch Wadden Sea. The close relation between fluctations in annual phosphate discharges and fluctations in annual phytoplankton primary production up to more than ten times the 1950 values indicates that primary production in the Marsdiep tidal channel and probably even in the main part of the whole basin, was and is phosphate limited.The mineralization of North Sea particulate organic matter inside the Wadden Sea, previously thought to be the main phosphate source, is questioned. The repercussions of this finding for management are discussed.     

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.     

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)     

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.     

Eutrophication and mussel culture in the western Dutch Wadden Sea: Impact on the benthic ecosystem; a hypothesis

Since 1950, two large-scale changes have taken place in the western Dutch Wadden Sea, namely the eutrophication of the area and the introduction of an extensive mussel culture. Although eutrophication in the fresh waters started already around 1950, nutrient concentrations in the western Wadden Sea remained fairly constant until about 1970, due to the retention of nutrients in Lake IJssel, the main source. From 1970–1980 concentrations increased strongly, and during the last years the situation has stabilized. Mussel culture was introduced in 1950 and expanded during the next decade to an area of 70 km², all situated in the sublittoral area. From 1960 the area of mussel culture remained about constant with fluctuating yields of between 35 and 120 millions of kg fresh weight. Due to a lack of data for the period until 1970 the impact of eutrophication and mussel culture on the ecosystem cannot be assessed. From 1970 onwards an increased biomass and production of the macrofauna in the intertidal zone has been observed, which is attributed to eutrophication. The hypothesis is postulated that the introduction of mussel culture between 1950 and 1960 has resulted in an increased food competition in the area, leading to a decreased stock of the macrofauna in the intertidal. Eutrophication from about 1970 onwards has improved the food conditions and as a result both the macrofauna in the intertidal and the mussel in the sublittoral area would have increased in biomass, allowing higher maximum yields of the mussel culture. The importance of monitoring programs is stressed to follow these trends in the near future and to check the above hypothesis in areas where it is decided to introduce or intensify mussel culture. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988) Publication No. 30 of the project “Ecological Research of the North Sea and Wadden Sea” (EON)     

A historic perspective on Wadden Sea eutrophication

In this paper, a reconstruction of the pre-industrial trophic status of the Wadden Sea is presented. A conceptual model is outlined that links the organic matter and nutrient dynamics in the Wadden Sea with riverine nutrient input. Fundamental processes in this model are: a nutrient-limited offshore primary production and the subsequent import of primary produced organic matter from the North Sea into the Wadden Sea. Two approaches have been followed to estimate the production and remineralisation levels under pre-industrial conditions. The first approach is based on present-day relationships between the seasonal cycle of NH₄ and NO₂ in the western Dutch Wadden Sea and suggests, on average, sixfold lower production and remineralisation rates under pre-industrial conditions (range: four to eight times). The second approach is based on present carbon budgets extrapolated to pre-industrial budgets on the basis of present relationships between winter nutrient concentrations, annual primary production and annual organic matter turnover rates, and suggests a fivefold lower organic matter turnover under pre-industrial conditions (annual primary production: ~55 g C m^–2 year^–1, annual remineralisation: ~77 g C m^–2 year^–1). Better pre-industrial light conditions in the Wadden Sea may have allowed a more efficient use of nutrients, a higher annual primary production of about 86 g C m^–2 year^–1 and annual remineralisation rates of about 108 g C m^–2 year^–1.     

General Patterns in Invasion Ecology Tested in the Dutch Wadden Sea: The Case of a Brackish-Marine Polychaetous Worm

The success of invasive aquatic species is determined by a variety of attributes such as wide environmental tolerance, high genetic variability, short generation time, early sexual maturity, high reproductive capacity, and a broad diet. Usually, introduced species, after some time lag since inoculation, show an exponential population increase and expansion. Maintenance of the immigrant species at a high population level will be dependent on interspecific competition with native species and availability of habitat and food. Eventually, the immigrant population may decline, for instance due to increased predation pressure, parasite infestation or loss of genetic vigour. These characteristic patterns in invasive species are reviewed for the case of the North American spionid polychaete Marenzelleria cf. wireni in the Dutch Wadden Sea. This species was first recorded in estuaries and coastal waters of the European continent in the Ems estuary (eastern Dutch Wadden Sea) in 1983. In the western part of the Dutch Wadden Sea the first specimens were found in 1989. The Ems estuary population showed the typical lag-phase, explosive increase, stabilisation, and eventual decline. In the western part of the Dutch Wadden Sea the latter two phases have not yet developed. The strong development and stabilisation of the population in the Ems estuary may have been caused by the availability of a yet not utilised food source. The species' final decline remains largely unexplained.     

Mercury pollution in the Ems estuary

From approximately 1960 to 1975 the Ems estuary received several tons of mercury per year from a chlor-alkali plant, a pesticide factory and some minor sources. The discharge has been reduced drastically from 1976 onwards. In 1975 and 1976 measurements were made on the distribution of mercury in the sediment. The horizontal distribution revealed a strong local enrichment of the sediment near the point of discharge. The vertical distribution was found to be in accordance with the local deposition rates. In the water phase no significant change in mercury content from 1975 to 1978/79 could be demonstrated. In 1978/79 a difference between Ems estuary and Dutch Wadden Sea was not significant. In 1978 mercury contents of eelpoutZoarces viviparus in the Ems estuary were about twice as high as in the Wadden Sea. In the Ems estuary a decrease of these contents was found between 1974/75 and 1978. A similar decline in the Wadden Sea may be related to a decreased mercury discharge by the River Rhine.     

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.     

On the dynamics of the stocks of blue mussels (Mytilus edulis L.) in the Danish Wadden Sea

As biological basis for the monitoring programme for the commercially exploited stock(s) of mussels (Mytilus edulis L.) in the Danish Wadden Sea, samples of mussels have been collected regularly since 1986, both from sub-tidal and inter-tidal mussel beds. These samples are the basis for the estimation of total biomass. They also provide data on size frequency distributions, which have been analysed for cohort identification resulting in length at age data, which again have been used for estimating parameters (L and K) for the von Bertalanffy growth equation (VBGE) as well as mortality parameters. By applying these in the Beverton & Holt model, estimates of average biomass and annual production (P) of the mussels have been obtained together with possible fisheries yields from the beds. The growth and mortality parameters and the figures for annual production and P/B are compared with figures from other investigations. These analyses have been the basis for annual assessments of the mussel stocks, which again are used in the current management of mussel fishery in the Danish Wadden Sea.     

Was total primary production in the western Wadden Sea stimulated by nitrogen loading?

Borum and Sand-Jensen (1996) described empirical relationships between nitrogen (N) loadings from land and total (benthic + pelagic) primary production rates in shallow coastal marine waters. We applied these relationships to N loadings of the western Wadden Sea system, and compared the production estimates with actually observed primary production rates of autotrophic components (phytoplankton, microphytobenthos, macroalgae and seagrasses) for those years for which field data were available. During the 1980s and early 1990s, primary production values appear in good agreement with those derived from the empirical relationships. During the 1960s and early 1970s, however, these relationships substantially overestimated the total primary production in the western Wadden Sea. Based on ambient nutrient concentrations and the Redfield ratio, production in that period was considered not to be limited by N but by phosphorus (P) during most of the time. It is concluded that primary production is not invariably stimulated by N loading from land. If other factors (i.e. additional nutrient sources, N:P ratios, internal nutrient dynamics and co-limiting effects of nutrients and light) are not taken into account, management regulations that are targeted at diminishing the effects of eutrophication hold the risk of seriously under- or overestimating nutrient reductions that are thought necessary to achieve their goals. Received: 30 November 1998 / Received in revised form: 12 July 1999 / Accepted: 15 July 1999     

Rapid increase of the grey seal (<Emphasis Type="Italic">Halichoerus grypus</Emphasis>) breeding stock at Helgoland

Over the past 25 years, the grey seal (Halichoerus grypus) population in the North Sea increased on average at 5% per year, while founding a number of new colonies. Based on published data, this note compares recent trends in pup production at nine breeding sites, with focus on the German island of Helgoland. Since 1997, numbers born at Helgoland increased at 35% per year (95% CI: 29–42), reaching an all-time high of 53 in 2007. Yet, this was less than 1% of all grey seal offspring in the North Sea area. Some other small stocks had similarly high growth rates of 20–50%, which must be explained by immigration from the larger colonies, as sustained autochthonous increase cannot exceed 13% per year in the species. Further, the Helgoland breeding stock increased significantly faster than the neighbouring colonies at Amrum (13%) and in the Dutch Wadden Sea (20%), indicating higher offspring survival and/or higher relative influx of migrants. This is attributed to the fact that at Helgoland newborn seals are safe from floodwater. In the other two sites, in contrast, pups are born on sandbanks and are thus often hit by winterly storm tides, which poses a risk to their survival. Because of the favourable breeding conditions at Helgoland, this new colony is expected to soon become an important stronghold of the grey seal in the Dutch-German North Sea area.     

A review of data on production and import of organic carbon in the western Wadden Sea

Summary Annual phytoplankton primary production in the tidal channels of the western Wadden Sea cannot be estimated more precisely than 150±50 g C.m^–2a^–1, due to large spatial and short-term temporal variations. This implies that year-to-year variations and eventually long-term trends have to be very pronounced to be measurable even with a weekly sampling program.Short-term temporal variation in primary production of microphytobenthos living on the tidal flats is less pronounced, but spatial variation is large. Primary production on high tidal flats is larger than on flats lower in the tidal zone. Year-to-year variation on a tidal flat station occupied now for 12 years was large. This variation cannot be explained by year-to-year variations in nutrients, light or temperature, but probably by year-to-year variations in grazing. Macrophytobenthic primary production plays a subordinate role in the western Wadden Sea after the disappearance ofZostera fields. It is difficult to give one figure for the annual primary production of an average tidal flat due to the variations mentioned. Tentatively, microphytobenthic plus macrophytobenthic plus phytoplankton primary production on and above an average tidal flat is estimated at 150±50 g C.m^–2a^–1,i.e. the same as estimated for phytoplankton primary production in the tidal channels.The western Wadden Sea receives a considerable amount of particulate organic carbon from outside the area, estimated at 240 g C.m^–2a^–1. Formerly the North Sea was thought to be the only source. New data indicate also an import from the Ysselmeer. The import of total organic carbon, particulate plus dissolved, from the Ysselmeer exceedsin situ primary production. From this import Ysselmeer DOC is probably not used very much in the Wadden Sea. However, Ysselmeer POC, consisting for 50% on average of living phytoplankton cells, may form a suitable food source for Wadden Sea invertebrates. Cell counts ofScenedesmus sp., a freshwater alga used as a natural tracer for Ysselmeer POC in the western Wadden Sea, indicate that a large part of the Ysselmeer POC is retained here.     

Climatic change and the Wadden Sea,The Netherlands

The predicted increase in atmospheric carbon dioxide and the effects of global warming will influence the Wadden Sea, The Netherlands, an area of exceptional ecological value. The direct effect of elevated atmospheric CO₂ on terrestrial coastal ecosystems is either marginal or unknown. The slight acidification of the sea which is predicted might have an impact on primary producers and juvenile animals. The effect of CO₂ fertilization on marine primary production remains to be elucidated. Profound changes will occur if sea level rises at the predicted rate of 60 cm per century, as sedimentation rates will be insufficient to maintain the salt marshes on the barrier islands. The marshes of the mainland coast will be impoverished, as high and low marshes are not expected to continue to coexist at the same locations. As sediment supply to the Wadden Sea is sufficient to compensate for sea level rise, the estuarine character of the Wadden Sea, with sand- and mudflats, is expected to remain largely unchanged.     

Impact of eutrophication on the silicate cycle of man-made basins in the Rhine delta

The impact of eutrophication on the biogeochemical cycle of silicate in the Rhine delta was analysed by 1) comparing the seasonal variation in river water, stagnant fresh water and coastal seawater and 2) observations in well-controlled experimental reservois subjected to different regimes of phosphate precipitation. The high input of dissolved sliciate from Rhine water was rapidly depleted in receiving waater systems through vigorous phytoplankton (datom) growth. In reservoirsca. 50% of the silicate input was retained over a seven years' observation period. Regeneration of silicate immobilized by diatoms was accelerated by very dense blooms of phytoplankton in reservoirs and in Lake IJssel that increased the pH value over 9.The prevailling lowratios of Si/N and Si/P in Rhine water entall the risk of massive blooms of flagellates or cyanobacteria in receiving waters. The 50% reduction in nitrogen and phosphate emission agreed upon in the Rhine Action Plan may reduce butnot eliminate this risk.     

Variability in annual recruitment success as a determinant of long-term and large-scale variation in annual production of intertidal Wadden Sea mussels (<Emphasis Type="Italic">Mytilus edulis</Emphasis>)

To understand the background of the strong variation and recent decline of stocks and production of mussels (Mytilus edulis) on tidal flats of the Wadden Sea, we analysed long-term (twice-annual for 26 years) and multi-station (15 sites) estimates of numbers, mean individual weights, biomass, and annual production on Balgzand, a 50-km² tidal-flat area in the westernmost part of the Wadden Sea (The Netherlands). Somatic production was estimated from summed growth increments of soft tissues per half-year period and expressed in ash-free dry mass (AFDM). In adults, positive values in spring/summer regularly alternated with negative values in autumn/winter, when up to ∼25% (mean: 14%) of individual weight gains in the preceding season were lost. No weight losses were observed during the first winter of the life of mussels. The 26-year mean of net somatic tissue production P amounted to 5.5 g AFDM m⁻² a⁻¹ at a mean biomass B of 3.2 g AFDM m⁻²; the ratio P/B varied strongly with age composition of the mussel population and ranged between 0.5 and 3.0 a⁻¹ (mean: 1.7). Within the restricted areas of mussel beds, mean biomass and annual production values were two orders of magnitude higher. In the Wadden Sea, mussel beds cover a typical 1% of extensive tidal flat areas. Numerical densities of recruits showed straight-line relationships with subsequent life-time year-class production. Once recruits had reached an age of ∼10 months, their numbers predicted subsequent production within narrow limits. Production per recruit averaged 0.21 g AFDM for 10-mo recruits and was not related to recruit density. Local variation in annual production varied strongly, with maximal values between mid-tide and low-tide level, where recruitment was also maximal. Production per recruit was higher at low than at high intertidal levels. Frequently failing recruitment is indicated as the main cause of declining mussel stocks in the Wadden Sea. As in other bivalve species, a declining frequency of the occurrence of cold winters appears to govern declining recruitment success and consequently declining production.     

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)     

Habitat suitability of the Wadden Sea for restoration of Zostera marina beds

A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea. Received: 26 April 1999 / Received in revised form: 15 October 1999 / Accepted: 16 October 1999     

Invasive alien plants in marine protected areas: the <Emphasis Type="Italic">Spartina anglica</Emphasis> affair in the European Wadden Sea

The common cord-grass Spartina anglica, a fertile hybrid of S. maritima and S. alterniflora, was planted in the European Wadden Sea extensively during the late 1920s and 1930s to promote sediment accretion. After establishment, it colonised as a pioneer plant in the upper tidal zone, where it occurs frequently in coherent swards at the seaward front of saltmarshes and in patches on the tidal flats. Often, a conspicuous, almost monotypic, belt of S. anglica is formed. Over the last two decades, an increase in abundance and accelerated spread of S. anglica was observed, possibly promoted by warmer spring temperatures. This alien species may benefit from global warming, and there is considerable concern about its harmful impacts on the native biocoenoses and native biodiversity of the unique Wadden Sea ecosystem, encompassing effects on hydromorphodynamics and coastal protection. For a definitive assessment, however, an adequate quantification and comparison of documented and potential effects of S. anglica is important, but currently unavailable. Consequently, no management strategy exists for the prevention or restoration of the Wadden Sea ecosystem. Thus, the development of an alien species plan on the level of the Trilateral Cooperation on the Protection of the Wadden Sea is essential.