Management of the Wadden Sea

The Wadden Sea situated along the North Sea coasts of Denmark, the Federal Republic of Germany and The Netherlands represents one of the world's largest bar-built type of estuaries. The area is a typical sedimentation and mineralization basin, with a large influx of organic matter from the adjoining North Sea, consequently a delicate oxygen balance and a rich benthic macrofauna, poor in species, which serves as food for juveniles of some commercially important North Sea fishes and for large numbers of migrating and wintering waders and waterfowl. Past and present activities of the human society in the area include fisheries (mainly for shrimp and mussels, semi-culture), shipping, land reclamation, recreation, dredging for sand and shells, and waste discharge from industries and human communities. Until the present these activities, although sometimes conflicting, did not fundamentally affect the area and its biota (pollution excluded), but future claims, including the construction of large deep-sea harbours, drilling for natural gas and oil, large-scale land reclamation and increased industrialization etc., might gradually induce degradation. For instance, area reduction by continued land reclamation could lead to irreversible losses of specific biotopes (e. g. salt-marshes, mud-flats), which could affect the size of bird and fish populations in a much wider region. Increased pollution, which has already inflicted damage on bird and seal populations, could reduce the fauna and hence the value of the area as a natural sanctuary. In the event of a proposal for a new human activity in the area, the present standing practice in the countries concerned requires an evaluation of its safety and economic aspects and its environmental impact. However, the various plans are considered separately and there is a general need for integrated management of the area.     

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.     

Experiments on epibenthic predation in the Wadden Sea

Field experiments were designed to evaluate the role of predators in the Wadden Sea, small predators like shore crabs, shrimps and gobies, and large ones like flatfish and birds. Exclosures, maintained in aCorophium volutator bed, an eelgrass bed, in a sandy and a muddy flat, protected the infauna from such epibenthic predation. The resulting changes in the macrofauna were recorded and compared with an unaffected control area. In sandy and muddy flats of the lower intertidal zone, cages (mesh size5 mm) altered abundance and composition of the infauna almost entirely. Nearly all species achieved higher population densities than in the control area, and in addition, the number of species increased as well. In the the scarcely populated mud flat a dense suspension-feeder assemblage emerged, associated with numerous tube-building polychaetes. The sand flat, normally dominated by deposit feeders, also became occupied by a dense suspension-feeder assemblage, mainly cockles. In contrast, narrowly meshed cages had only little effect in the beds of eelgrass and ofCorophium volutator. Both are positioned in the upper intertidal zone. Although a number of species still responded with significant increases in abundance, many remained indifferent or even tended to be less abundant within cages. Cages provided with a 20-mm mesh nylon net, excluding only birds, flatfish and the biggest crabs, increased significantly the survival of large-sized infaunal members. This was only apparent in the upper intertidal zone. It was concluded, that small sized epibenthic predators are the major determinants of the dynamic species abundance pattern of the lower intertidal flats. In the eelgrass bed, the meshwork of rootlets constitutes an important spatial refuge from these predators. The complex habitat structure causes a diversified faunal assemblage. TheCorophium bed in the uppermost intertidal zone is less accessable to predators like crabs, shrimp and gobies. The monotonous appearance of this faunal assemblage is assumed to be the outcome of competitive exclusion and of occasional harsh physical conditions.     

Planktonic primary production in the German Wadden Sea

By combining weekly data of irradiance, attenuation and chlorophylla concentrations with photosynthesis (P) versus light intensity(E) curve characteristics, the annual cycle of planktonic primaryproduction in the estuarine part of the Northfrisian WaddenSea was computed for a 2 year period. Daily water column particulategross production ranged from 5 to 2200 mg C m^–2 day^–1and showed a seasonal pattern similar to chlorophyll a. Budgetcalculation yielded annual gross particulate primary productionsof 124 and 176 g C m^–2 year^–1 in 1995 and 1996,respectively. Annual amounts of phytoplankton respiration, calculatedaccording to a two-compartment model of Langdon [in Li,W.K.W.and Maestrini,S.Y. (eds), Measurement of Primary Productionfrom the Molecular to the Global Scale. International Councilfor the Exploration of the Sea, Copenhagen, 1993, pp. 20–36],and dissolved production in 1996, were both in the range of24–39 g C m^–2 year^–1. Annual total net productionwas thus very similar to particulate gross production (127 and177 g C m^–2 year^–1 in 1995 and 1996, respectively).Phytoplankton growth was low or even negative in winter. Inspring and summer, production/biomass (Pr/B) ratios varied from0.2 up to 1.7. Phytoplankton growth during the growth seasonalways surpassed average flushing time in the area, thus underliningthe potential of local phytoplankton bloom development in thispart of the Wadden Sea. The chlorophyll-specific maximum photosyntheticrate (P^B_max) ranged from 0.8 to 9.9 mg C mg^–1 Chl h^–1and was strongly correlated with water temperature (r² = 0.67).By contrast, there was no clear seasonal cycle in ^B, which rangedfrom 0.007 to 0.039 mg C mg^–1 Chl h^–1 (µmolphotons m^–2 s^–1)^–1. Its variability was muchless than P^B_max and independent of temperature. The magnitudeand part of the variability of P^B_max and ^B are presumably causedby changes in species composition, as evidenced from the rangeof these parameters found among 10 predominant diatom speciesisolated from the Wadden Sea. The ratio of average light conditionsin the water column (E_av) to the light saturation parameterE_k indicates that primary production in the Wadden Sea regionunder study is predominantly controlled by light limitationand that nutrient limitation was likely to occur for a few hoursper day only during 5 (dissolved inorganic nitrogen) to 10 (PO₄,Si) weeks in the 2 year period investigated.     

Native Parasites Adopt Introduced Bivalves of the North Sea

Introduced species may have a competitive advantage over native species due to a lack of predators or pathogens. In the North Sea region, it has been assumed that no metazoan parasites are to be found in marine introduced species. In an attempt to test this assumption, we found native parasites in the introduced bivalves Crassostrea gigas and Ensis americanus with a prevalence of 35% and 80%, respectively, dominated by the trematode Renicola roscovita. When comparing these introduced species with native bivalves from the same localities, Mytilus edulis and Cerastoderma edule, trematode intensity was always lower in the introduced species. These findings have three major implications: (1) introduced bivalves are not free of detrimental parasites which raises the question whether introduced species have an advantage over native species after invasion, (2) introduced bivalves may divert parasite burdens providing a relief for native species and (3) they may affect parasite populations by influencing the fate of infectious stages, ending either in dead end hosts, not being consumed by potential final hosts or by adding new hosts. Future studies should consider these implications to arrive at a better understanding of the interplay between native parasites and introduced hosts.     

Monitoring the Wadden Sea — an introduction

The Wadden Sea is an ecotone of far-reaching importance. Its nature is continuously changing, and ecological monitoring is needed to identify and analyse trends, and to inform the general public about accelerated change caused by human impact. Sustained ecological research with a diversified monitoring program is essential to overcome the ‘immature phase’ of short-term and single-site studies in ecological science. Long-term and wide-scale phenomena are of increasing relevance to human welfare. Ecological monitoring includes retrospective and ongoing recordings of physical, chemical and biological parameters on a regional scale. It improves our understanding of natural patterns and processes, and of the distortions caused by man. It may provide early warnings. It is a necessary precondition for translating the principle of anticipatory action into practical policy, and it also documents the effects of actions taken. Monitoring the Wadden Sea will help to protect its great variety of flora and fauna, and will also prevent it from deteriorating into a gray coastal backwater, serving for utility purposes only.     

Inoculum Density-Dependent Mortality and Colonization of the Phyllosphere by Pseudomonas syringae

Pseudomonas syringae inocula containing cell concentrations ranging from 10⁵ to 10⁹ cells per ml were applied to the primary leaves of bean plants. The plants were incubated under conditions of high temperature and illumination and low relative humidity. Bacterial mortality rates and the proportional population decline of the inoculum were lowest at the highest inoculum concentrations. Addition of a high concentration of heat-killed cells to the inoculum containing a low concentration of viable cells significantly reduced both the mortality rate and the proportional population decline of the viable cells. The mechanisms underlying this density-dependent mortality may include cooperative protective effects of extracellular factors, such as bacterial extracellular polysaccharides, and physical protection by neighboring cells. Although epiphytic populations derived from inoculum concentrations of 10⁸ or 10⁹ cells per ml tended toward 10⁶ CFU/g, the presumed carrying capacity of the leaf, populations derived from lower inoculum concentrations never achieved this carrying capacity. Assuming that epiphytic populations of P. syringae reside in discrete protected sites, our results suggest that at low inoculum concentrations, following a period of environmental stress, the number of viable cells may have dropped to zero in some sites; hence, the carrying capacity of the leaf could not be achieved.     

Red List of marine macroalgae of the Wadden Sea

This list forms part of the Report on the RED LISTS OF BIOTOPES, FLORA AND FAUNA OF THE TRILATERAL WADDEN SEA AREA. For basic information concerning, for example, function of these lists, species taken into account, structure of the lists and abbreviations used, see also the general introduction to the Red Lists.     

Nitrogen removal in coastal sediments of the German Wadden Sea

Although sediments of the German Wadden Sea are suspected to eliminate a considerable share of nitrate delivered to the SE North Sea, their denitrification rates have not been systematically assessed. We determined N₂ production rates over seasonal cycles (February 2009–April 2010) at two locations with two sediments types each, the first site (Meldorf Bight) receiving nitrate during all seasons from the Elbe river plume, and a second site on the island of Sylt, where nitrate is depleted during summer months. In sediments from the Sylt site, N₂ production ranged from 15 to 32 μmol N₂ m^?2 h^?1 in the fine sand station and from 7 to 13 μmol N₂ m^?2 h^?1 in the coarse sand station; N₂ production was not detected when nitrate was depleted in May and July of 2009. N₂ production in the Meldorf Bight sediments were consistently detected at higher rates (58–130 μmol N₂ m^?2 h^?1 in the very fine sand station and between 14 and 30 μmol N₂ m^?2 h^?1 in the medium sand station). Analysis of ancillary parameters suggests that major factors controlling N₂ production in coastal sediments of the German Wadden Sea are the nitrate concentrations in the overlying water, the ambient temperature, and the organic matter content of the sediment. Extrapolating our spot measurements to the zone of nitrate availability and sediment types, we estimate an annual nitrogen removal rate around 16 kt N year^?1 for the entire northern sector of the German Wadden Sea area. This corresponds to 14% of the annual Elbe river nitrogen load.     

Concentrations of particulate matter in the North Frisian Wadden Sea

The amount of particulate matter for different size classes was determined in the North Frisian Wadden Sea over a period of 10 months from November 1973 to August 1974. Particle volume, particulate organic carbon and phaeopigment concentrations decrease with advancing season, reaching minima in June. It is concluded that concentrations of particulate matter including chlorophyll were sufficiently high throughout the study period to support the growth and reproduction of appendicularia which abundantly occur in the Wadden Sea.