|Dynamik und Entwicklung makrozoobenthischer Weichbodengemeinschaften in der Deutschen Bucht (Nordsee) 1969-2000 = Community dynamics and development of soft bottom macrozoobenthos in the German Bight (North Sea) 1969-2000|
Schroeder, A. (2003). Dynamik und Entwicklung makrozoobenthischer Weichbodengemeinschaften in der Deutschen Bucht (Nordsee) 1969-2000 = Community dynamics and development of soft bottom macrozoobenthos in the German Bight (North Sea) 1969-2000. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremen. 190, A62 pp.
In order to examine the long-term development and the interannual variation of offshore macrozoobenthic soft-bottom communities of the German Bight, four stations have been sampled continuously over the last 35 years. Interannual variability and possible long-term trends were analysed based on spring-time samples. The vicinities of the stations were extensively sampled in 1998 to evaluate the spatial variability of the benthic communities around the stations. These data were also used to estimate the number of grabs needed for an appropriate description of the benthic communities and to investigate the sample-size-dependencies of community descriptors (species density, evenness, similarity, etc.) based on real data, which differ from theoretical predictions. A sufficient characterisation of a local community of small homogeneous areas for long-term studies may be assumed with a standard number of five replicate 0.1 m² van Veen grabs. Because the largest increase of the precision of estimates is reached by the first five grabs, this sample size is used as a practical compromise to detect the main trends. However, ten or more grabs are desirable for an analysis of single species population densities. Based on the present results, some recommendations are derived for offshore monitoring of North Sea soft-bottom makrozoobenthos. Benthic communities at the sampling stations show a large interannual variability combined with a variation on a roughly decadal scale. In accordance with large-scale system shifts reported for the North Sea, benthic community transitions occurred between roughly the 1970ies, 80ies and 90ies. The transitions between periods are not distinctly marked by strong changes but rather reflected in gradual changes of the species composition and dominance structure. The timing of changes in communities is similar between different parts of the North Sea and seems to be a result of climate and oceanographical features. However, the local community development is mainly a product of local environmental variation and biotic interactions, and both differ between areas. As each station represents a clearly distinct benthic fauna, the nature of the community changes differs. To evaluate possible climatic, oceanographic and anthropogenic influences, the benthic community development was correlated to various environmental data sets (NAO index; water temperature, wind, salinity and nutrient concentrations at Helgoland; Elbe river runoff). Most notable changes of environmental factors are the increasing tendency of the NAOI with its consequences on higher winter temperature and the increasing frequency of storms observed in the German Bight. An increasing concentration of phosphate during the 1970ies was reversed in the late 80ies, while nitrogen concentrations continued to increase at least until the mid-1990ies. IIISummary The development of the benthic communities at all stations shows clear correlations to the NAOI. The most dramatic changes of the communities followed the severe winters of 1970, 1979, 1986 and 1996, when reductions in species number and abundance were discernible at all stations. The shallower "Fine Sand" (FSd) and "Silt" (Slt) stations are characterised by larger interannual changes, and the situation following severe winters is not as clearly different from other years as it is the case at the deeper "Silty Sand" (SSd) and "White Bank" (WB) stations. An influence of eutrophication can be inferred from a high number of correlations found between nutrient concentrations and the benthic community development at the stations in the inner German Bight (SSd and Slt). These hint towards a coupling of the benthic and planktonic system. However, in combination with unfavourable hydrographic conditions, eutrophication also favours the occurrence of benthic hypoxia, leading to a reduction or elimination of macrobenthos. Only at WB an indication of effects of a distinct hypoxic event was observed on a single occasion. The relatively poor benthic community at Slt can be seen as a result of frequent hypoxic episodes. The community development especially at Slt and FSd is also correlated to the frequency of storms, which may create a physical disturbance of the sediment by wave erosion. The dominance of small opportunistic and mostly mobile worms at FSd may result from unstable sediments. No obvious effects of the former dumping of acid-iron wastes at the FSd station and of sewage sludge east of the Slt station were evident for the benthic communities. Bottom trawling has been shown to have strong impacts on benthic organisms, but, because of the lack of detailed information about the local fishing intensity at the stations, trawling effects can not be proven from the present data. The main factors affecting benthic community development are biotic interactions as well as climatic conditions, food supply and the disturbance regime. The most common forms of disturbances are extremely cold winters, hypoxia following algal blooms in stratified waters, and physical disturbance of the sediment by turbulent wave erosion during strong storms or by demersal fishing gear. Especially at the shallower stations, the communities are adapted to frequent disturbances, and a recovery following localised disturbances can be very quick. The "normal" community composition in this case reflects the general disturbance regime (in terms of type, intensity and frequency) rather than a "mature" community. A clear distinction between the effects of climate, eutrophication, pollution or bottom trawling is hardly possible, not only because various factors are predicted to produce similar effects, but also because the observed changes are a result of the synergistic effects of all factors.