The Wadden Sea Ecosystem

This book covers the synthesis of the joint research project ELAWAT (Elastizität des Ökosystems Wattenmeer = Resilience of the Wadden Sea Ecosystem), which was part of the “Ecosystem Research Wadden Sea” in Germany. The major objective of ELAWAT was the analysis of potential stability properties and relevant processes for regeneration in this intertidal ecosystem. The synthesis of this project summarizes the results of several years of field studies on spatial and temporal patterns of several abiotic and biotic parameters and on the effects of disturbances. Based on this analysis, stability properties and mechanisms were identified. The results contribute to the protection of processes in this coastal ecosystem (Chap. 12). This book on the interdisciplinary project ELAWAT also provides a detailed case study on the ecology of a backbarrier system.

The “Ecosystem Research Wadden Sea” consisted of four large joint projects carried out since 1989 on the southern North Sea coast of Germany. The projects had two major goals which were closely related. One was to develop a protection and management strategy for the Wadden Sea to preserve the diversity of its biota and landscape. This aspect also addressed the sustainable use of the Wadden Sea, i.e. a use of resources in such a way that no long-term decline of biological diversity results and the needs and aspirations of present and future generations are maintained. This goal was the task of two applied projects within the “Ecosystem Research Wadden Sea”, one in the state of Schleswig-Holstein and the other in Lower Saxony (Stock et al. 1996; Dittmann et al. 1997; Kellermann et al. 1997). The second goal was to achieve a fundamental understanding of this ecosystem. This was the task of two research projects, namely SWAP (“Sylter Wattenmeer Austausch Prozesse” = “Exchange processes of the Sylt Rømø-bight”), which was carried out in Schleswig-Holstein (Gätje & Reise 1998), and ELAWAT (“Elastizitat des Ökosystems Wattenmeer” = “Resilience of the Wadden Sea ecosystem”; Dittmann et al. 1997), carried out in Lower Saxony.

The Wadden Sea of the southern North Sea covers an area of about 9.300 km2 and represents one of the largest intertidal regions on earth (Fig. 3.1.1). Between Den Helder and the Jade Bay (near Wilhelmshaven), the West and East Frisian Wadden Sea forms a classical barrier island chain (Backhaus 1943). The island Spiekeroog is located in the eastern part of this chain. Its backbarrier system, the main research area of ELAWAT, is connected to the North Sea via the Otzumer Balje (Fig. 3.1.2), a tidal inlet which separates Spiekeroog from the neighbouring island Langeoog in the west. Referring to mean tide high water (MThw), the tidal basin of the Otzumer Balje covers an area of about 73.5 km2 (Walther 1972). The basin is bordered by two watersheds: In the west by the Langeooger Plate, in the east by the Hohe Bank. The distance between the island and the mainland averages 6.5 km and the island has a length of approx. 10 km. The intertidal section of the Spiekeroog backbarrier system covers about 80 % of the area and is comparable to that of other East Frisian tidal basins. The North Frisian and the Danish Wadden Sea have about 70 % tidal flats on average, but the Sylt-R0m0-bight comprises only 30 % intertidal areas (Reise & Riethmiiller 1998). With an average tidal range of 2.7 m at the western end of Spiekeroog island and of 2.9 m in the front of the mainland, mesotidal conditions prevail in this backbarrier system (Niemeyer & Kaiser 1994). At standard sea level (NN) the water volume in the catchment area amounts to 0.112 km3. The volume of water that is exchanged during a tidal cycle via the Otzumer Balje is calculated to be 0.292 km3 on average (Chap. 3.3).

This chapter treats the contribution of statistical research and applied statistics to ELAWAT. In the introduction the role of applied statistics in scientific research is described. Definitions of statistical terms and expressions are given to make the subsequent statements definite. The main part of the chapter introduces statistical models and methods. They are arranged according to the ecologically relevant terms in ELAWAT: pattern, process, relation, and distinction between states of a system. Each paragraph starts with an ecological question and treats statistical models and methods useful for finding an answer. Case studies with data from ELAWAT demonstrate some of the methods for data analysis and interpretation. The final paragraph summarizes the methods applied in ELAWAT.

Temporal developments of dissolved nutrients, phytoplankton and zooplankton in the water column were investigated on different time scales from 1994 to 1996. Measurements of single tidal cycles allowed to assess the fate of some species in the Wadden Sea. Dissolved nutrients showed some regularly recurring patterns, which could be explained by the tidal mixing of water bodies with different nutrient concentrations. Seasonal cycles of the nutrients, phytoplankton and zooplankton showed a large interannual variability. Timing and extension of algal blooms influenced the occurrence of many meroplanktonic larvae. Most of the algal species in the backbarrier system of Spiekeroog were part of the phytobenthos and the phytoplankton.

The benthic recolonization of experimentally disturbed sandflat sites by diatoms, nematodes and small macrofauna was studied with several treatments, differing in intensity and timing of disturbance. Furthermore, recolonization following a natural disturbance, the ice winter 1995/96, was studied and compared with the results of the experiments.

Diatoms were the fastest organisms recolonizing the disturbed sites. Within a few days, their species number, species composition and individual abundances were comparable to control situations. This colonization pattern depended on the diatoms available in the ambient sediment at the time of disturbance. The mode of recolonization was passive via sediment transport. The ice winter had little effect on the diatoms and abundances were comparable to preceding years. Yet, three species, which had not been recorded in the Wadden Sea before, occurred in the tidal flats in 1996.

Nematodes reached ambient densities in the experimentally disturbed plots within a few weeks or months, with a slower recolonization the longer the disturbance had lasted. After the ice winter, nematode abundances increased steadily and reached even higher values in summer 1996 than in the years before.

Smaller sized macrofauna recolonized disturbed experimental plots in spring faster than plots disturbed in autumn. The recolonization was also faster, the shorter the disturbance had lasted. The first stages of recolonization reflected the ambient situation of the species composition and abundance of benthic organisms. After several months, species numbers, abundance and community composition in the disturbed plots could not be distinguished from the reference plots. The first macrofaunal species colonizing the experimentally disturbed plots were the polychaete Pygospio elegans, Ostracoda and the amphipod Urothoe poseidonis. These were also dominant in the benthic community in 1996 after the ice winter. Juvenile polychaetes colonized the experimental sites to a higher degree than the ambient sediment. The course of the recolonization was mainly dependent on the availability of settling organisms, i.e. larvae as well as postlarvae. The study period was not long enough to record all successional stages.

Investigations in the framework of ELAWAT showed that cold winters with ice formation lead to mass mortalities of cold sensitive species in all parts of the Wadden Sea ecosystem and to an increased mobility of organisms. While in the phytoplankton cold resistant species became dominant due to their increased growth, changes in dominance in the benthos were due to mortality of coldsensitive species. Inspite of the massive die-off of large individuals of the macrofauna, no strong increase in TOC (total organic carbon) was measured in the sediment.

In the following regeneration, the seasonal development in the water column differed markedly from the preceding years, which could be explained by extraordinarily calm weather, low water temperatures and competition between algal species. In the zooplankton the meroplankton showed severe changes in the temporal sequence of appearance and density of species when compared to 1994 and 1995.

During a “black area event” (end of May/beginning of June 1996) a short term reduction in depth of the oxidized sediment layer was measured together with a small increase in TOC and bacterial activity. Besides this event, regeneration of the benthic community by successful recruitment of several species was undisturbed. Due to the fact that the “black area event” and recruitment (larvae were already in the water column, settlement occurred after redevelopment of an oxidized layer) were temporally disconnected, a fast recovery of even strongly affected areas was possible.

Among the macrozoobenthos the trophic groups of the suspension-feeders and epibenthic predators were most negatively affected by the ice winter. The disappearance of suspension-feeding species such as Mytilus edulis, Cerastoderma edule or Lanice conchilega was partly outbalanced by very good recruitment of single species (M. edulis, My a arenaria). Predation pressure by epibenthic predators was low and thus not a limiting factor for the recruitment of prey species.

Most parts of the macrobenthic association recovered fast from the effects of the ice winter (< 1 year), with the exception of populations of the more longliving cold sensitive species which presumably take 3-4 years to recover completely. Comparing the ice winter effects observed in the backbarrier system of Spiekeroog (East Frisian Wadden Sea) and Sylt (North Frisian Wadden Sea) the duration of the ice cover was longer in the Sylt area, but had in principle the same effects on the pelagic and benthic system (modification of the species composition towards cold adapted/cold tolerant species). The largest differences between the areas were observed in the temporal development of the pelagic system (dominance of different species, different temporal course in the meroplankton). There are indications that additional differences occurred subtidally. As the source populations for the recolonization of the intertidal by cold sensitive species lie subtidally, future studies on recruitment after ice winters should include both the intertidal and subtidal benthic systems.

Grid-based ecological models are a valuable tool in terrestrial ecology which have considerably improved the understanding of many terrestrial systems. Therefore an attempt is made here to apply the grid-based approach to marine benthic soft bottom communities. The concept of patch-dynamics and the theory of mosaic-cycles are the theoretical background which motivated this attempt. Both these concepts are based on the notion of a spatially structured landscape whose spatial and temporal dynamics is ecologically significant. Using two grid-based models we try to find out whether in the intertidal of the Wadden Sea the spatial and temporal dynamics of dominant macrozoobenthic species meet the criteria of the patch-dynamics concepts and the mosaic-cycle theory. The first model takes into account the processes settlement, succession, disturbance events (storms, ice winters) and the dispersal of mussels (Mytilus edulis) into adjacent areas. This first model demonstrates that the frequency of disturbance events and dispersal of M. edulis are the major factors which determine the spatial distribution of the model species. However, the first model has four main methodological deficiencies: (1) the results of the model indicate that the spatial scale to which the model may apply must be rather small (<100xl00m²). (2) For the same reason the model does not reproduce large-scale zonation patterns of benthic communities. (3) Abiotic factors apart from disturbance events which are known to be decisive in the Wadden Sea, e.g. topography, are not taken into account. (4) Interactions between neighbouring spatial units (“cells”) are all but neglected in the model. Thus, based on the topography of a selected part of the Wadden Sea (sandflat “Swinnplate” and surroundings), an advanced second model is developed (TOPOGRID) which only considers the two species Lanice conchilega and M. edulis. This model demonstrates elements of a future model of macrozoobenthos in the intertidal in the Wadden Sea which would be supported by much more expert knowledge than TOPOGRID, more field studies which are designed parallel to the model, and oceanographic models of the current regime. With respect to the theoretical concept which was to be tested with the two models presented here, we conclude that neither the concept of patch-dynamics nor the theory of mosaic cycles are applicable to the Wadden Sea in their original, full meaning.

With stability properties as the conceptual framework to study processes in the Wadden Sea ecosystem, attention was focused on qualitative aspects. The relationships between the approach of ELAWAT and historical developments of ecosystem research and ecological concepts are briefly described. While the realization of this approach was restricted by the dynamic nature of the system under study and the limited duration of the project, a potential for future research is seen in the consideration of the single criteria of the ecological checklist used. Furthermore, quantitative and qualitative approaches should be combined in the future to allow an integrated analysis of dynamic ecosystems like the Wadden Sea.

ELAWAT (“Resilience of the Wadden Sea ecosystem”) was one of the larger joint projects ever undertaken in German marine sciences. As this project is ending, we have compiled our experiences with that joint research approach in this chapter, as this could be helpful in respect to future projects and research policies. In order to learn from this experience, we try to be frank in stating deficits, mistakes and matters that were overlooked. These were mainly caused by the severe structural, financial and personnel problems found in the German University system at the moment, so that we comment on that situation as well. This chapter contains only those experiences and recommendations drawn from ELAWAT; in other parts of the “Ecosystem Research Wadden Sea” there were different experiences. In addition, we have listed several research questions which remained either unanswered or which arose during the “Ecosystem Research Wadden Sea”, and we also recommend areas for future research emphasis.

The protection of processes has been proclaimed both on a national level in the National Park declarations for the Wadden Sea and on an international level by the 6th Trilateral Governmental Conference. This approach means that all natural processes and dynamic developments are allowed to happen in an ecosystem in an unrestricted way. However, the realization in the Wadden Sea National Parks is hampered as the current zonation concept does not protect sufficiently large and coherent areas, and because the integration of human activities in a sustainable way (as required by the “Man and the Biosphere” programme) has not yet been achieved. Furthermore, the public acceptance for the protection of processes has to grow. Based on the results of ELAWAT it is recommended to take the relevance of source populations, dispersal and habitat heterogeneity into account for coastal zone management, as they are essential for regeneration processes after disturbances. The protection of entire tidal basins could ensure the capacity for resilience in this ecosystem.

What are the processes and mechanisms relevat for stability properties in the Wadden Sea ecosystem? as part of the “Ecosystem Research Wadden Sea”, the interdisciplinary project ELAWAT (“Resilience of the Wadden Sea Ecosystem”) was carried out from 1993 to 1997 to increase te understanding of processes essential for regeneration and ecological relations within ecosystem.temporal and spatial distribution pattern of abiotic and biotic components were investigated as well as the regeneration following natural and expirementally induced disturbances.The main study was the backbarrier system of the island of Spiekeroog in the East frisian Wadden Sea and two tidal flat areas therein, the Gröninger Plate and Swinnplate,were studied in detail.This book covers a synthesis of the results of this project and provides a case study on the ecology of an intertidal backbarrier system.