The ecology of macrozoobenthos and sea ducks in the Pomeranian Bay
Creators: name: Kube, Jan
Publisher: name: Leibniz-Institut für Ostseeforschung Warnemünde
Year: 1996

description: Meereswissenschaftliche Berichte No 18 1996 - Marine Science Reports No 18 1996
description: 1. The Pomeranian Bay (southem Baltic Sea), a large shallow offshore bay, is the transition zone between the Oder Estuary and the Baltic Proper. Sandy bottoms, between 0-20 m water depth, cover an area of 8.800 km² The rivers Oder and Peene transport about 20 km³*a^-1 freshwater into the bay. In the last decades the area has been influenced by increasing anthropogenic discharges. The study area has undergone significant eutrophication. Riverine nitrogen loads were five times higher in the 1990s than in the 1960s. Phytoplankton and primary production have roughly doubled during the last 20 years. Accumulation of organic matter was observed in sediments locally. A joint Polish and German research project (TRUMP) was started in 1993 to evaluate the impact of the riverine material on the ecosystem of the Pomeranian Bay. Investigations focus on the distribution, transport, and modification of biogenic and anthropogenic inputs from the Oder River. Benthic macrofauna studies form a major part of these investigations.

2. This phD thesis aimed at 1) describing the recent distribution of macrozoobenthos communities and sea ducks in the Pomeranian Bay in relation to the prevailing abiotic and biotic factors, 2) developing and applying efficient methods for the analysis of low diverse sublittoral benthic communities, 3) to analyse short-term and long-term changes in the macrozoobenthos in relation to fluctuations and long-term trends in the environmental conditions.

3. Quantitative samples of benthic macrofauna were taken at 34 stations (6 to 30 m water depth) in the Pomeranian Bay in the period April 1993 to April 1995. Two different sampling methods were applied: 1) Van Veen grab (23kg/70 kg, 0.1 m², 1.0 mm mesh size) and 2) modified 'Reineck' box corer (190 kg, 0.0225 m², 0.5 mm mesh size). Methodological comparisons between both methods were performed. Species distribution patterns, species numbers, abundances, and biomasses were analysed. Univariate and multivariate statistics were used to compare the precision of results of both procedures in terms of species richness, abundance, biomass and size-frequency distribution. Conclusions are made about the applicability and effiency of the two methods. Five replicate samples of method two are demonstrated to be the most efficient method for routine monitoring purposes in respect to data precision and laboratory sorting-time. Recommendations are five for the treatment of data on mobile epifauna species, which can not be sampled precisely with less than ten replicate grab samples. The identification of oligochaetes to species level seems to be renouncable for multivariate statistical analyses of community structures.

4. Fourty-five macrofauna species were identified. Mya arenaria, Macoma balthica and Marenzelleria viridis are the most important species with respect to biomass. Mean total biomass values decreased from about 100 g AFDW m^-2 in the southwest of the bay to only about 10 g AFDW m^-2 in the North. Multivariate analyses suggest distinct assemblages within the shallow bay and at the slope to the adjacent deeper zones. Loose structuring was found for communities of the shallow parts. They are all dominated by filter-feeders. Surface deposit-feeders are dominant at the northern boundary of the study area adjacent to the Arkona Basin. The small range of sediment variation could not explain distribution pattern of species with a large tolerance for sediment parameters. Physical disturbance and available food supply are proposed to be important in structuring the benthic community.

5. The population structure of Mya arenaria has been investigated during a 1.5-year period in 1993/94 to follow changes in the size and age structure of the clam populations in different parts of the study area. Large spatial differences in the population structure were found between the sheltered Southwest of the bay and the shallow and exposed Oder Bank in the centre. The clam stock of the Oder Bank was formed by two different clam types. A slow growing cohort was assumed to be autochtonous on the Oder Bank. A fast growing one was assumed to have been introduced from the surrounding area. Their contribution to the total density varied seasonally and was probably dependent on the intensity of bedload transport events. Erosion was supposed to be of minor importance in the Southwest of the Pomeranian Bay. High mortality rates during the first two years of life were assumed to be caused by predation. Mortality rates of older cohorts remained stable until old age. Variations in the density of cohorts were related to interannual differences in the reproductive success. A mild winter presumably lowers the reproductive success in the subsequent summer.

6. Spatial variations in individual growth rates of Macoma balthica and Mya arenaria were investigated in the Pomeranian Bay in 1993/94. Compared to full marine environments, growth rates of bivalves were considerably lower in this brackish area (salinity about 8 ‰). Growth of M. balthica correlated significantly with phytoplankton concentration. Growth of M. arenaria was assumed to be negatively effected by intensive physical disturbances. It is supposed that reduced salinities affect the growth of M. arenaria more than that of M. balthica.

7. The distribution and abundance of Marenzelleria viridis, a North American spionid polychaete which was first recorded in the Pomeranian Bay in the late 1980s, was studied in the southwestern part of the Baltic Sea in 1993/94 in relation to environmental factors. All available macrozoobenthos samples from German Baltic waters were used to construct a general distribution map. Highest abundances and biomasses were found in semi-enclosed lagoons (39.000 ind. m^-2 and 70 g ash free dry weigth m^-2). The western horizontal distribution border and the vertical distribution range were following the 15 ‰ isohaline. Neither a horizontal nor a vertical limit was found to the East. Dense settlement was restricted to sediments with an organic content of less than 5 % and a silt content of
less than 10 %. Simultaneous population studies were carried out in the Pomeranian Bay, the Oder Estuary and the Darss-Zingst Bodden from April 1993 to April 1994. Three different age groups were identified throughout the year. Settlement of larvae took place in aututnn. Successful larval settlement was restricted to areas with a salinity above 5 ‰ and a winter phytoplankton concentration above 5 g Chl a m^-3. Benthic stages were found to be highly motile. Adults occurred up to 50 km away from recruitment areas.

8. Long-term changes in the macrofauna of the Pomeranian Bay were studied by comparing survey data from the 1950s, 1980s and 1990s. Biomass of filter-feeding bivalves increased significantly. Spatial distribution patterns of the investigated species have changed. Strong decreases in species richness were caused by oxygen depletion at stations deeper than 15 m. Saduria entomon, Monoporeia affinis and Pontoporeia femorata vanished entirely between 1981 and 1993. Although a causal relationship between simultaneous increases of nutrient levels and macrobenthic biomass cannot be verified, eutrophication is proposed to be the major process affecting changes in macrofauna assemblages. In addition, changes in hydrography and climate increased frequency and severity of oxygen depletion events in the Pomeranian Bay since the mid 1980s.

9. Between January 1992 and April 1995 the distribution and abundance of long-tailed ducks, Clangula hyemalis, were studied, and the extent to which these can be explained by benthic food supply and disturbance by an international shipping lane. Long-tailed ducks arrive in the study area in late fall and leave in early May. Maximum numbers of 800.000 birds were recorded in winter. About the half of these birds was still present in March-April. The spatial distribution of long-tailed ducks is best explained by the harvestable biomass of prevalent prey species. Bird densities were highest in areas providing a harvestable biomass &gt; 10 g AFDW m^-2. Long-tailed ducks were more or less evenly distributed in the study area during winter. In spring, they seemed to be able to narrow the selection of feeding habitats to the most profitable patches and avoided areas providing a harvestable biomass &lt; 5 g AFDW m^-2 and disturbance by ship traffic.