Macrobenthic community structure in the southern and central Bohai Sea, China

The Bohai Sea is one of the most important fishery areas in China. It is an enclosed sea receiving many waste-water outfalls from several highly developed surrounding districts. There are also oil wells located in the Sea, and for these reasons the Chinese government has been paying great attention to its environmental maintenance. Four integrated surveys and other specific investigations in the Bohai Sea have been conducted. Changes in the fishery and other biological resources that are attributable to the transformation of structure and function of the Bohai ecosystem have been detected. Nonetheless, published information is very limited. Macrobenthos are considered to be one of the most important components of marine ecosystems. To understand their role in energy flow and material cycling in the Bohai ecosystem, and to assess the health of the Bohai Sea, a quantitative study of 22 sampling stations in the southern and central Bohai Sea was carried out on three cruises in June 1997 (defined as cruise 976 in this study), September 1998 (cruise 989) and April 1999 (cruise 994). At each station three replicate samples of undisturbed sediment were taken with a modified 0.1 m~2 Gray-O'Hara box-corer. The macrofauna were sieved through 0.5 mm mesh, then preserved in 5%~7% (V/V) formalin or 75% (V/V) ethanol for further laboratory analysis. Macrobenthic community structure and its relationship to environmental factors were determined, followed by a series of studies on macrobenthic abundance and biomass, secondary production and species diversity. PRIMER and SPSS 8.0 software packages were used for data analysis. There were some variations among the indices of relative importance (IRI) of dominating macrobenthic species and small changes in community structure in different cruises. PCA indicated that distribution of the macrobenthic communities correlated with environmental gradients. Among all the environmental factors examined, water depth and nitrate concentration in the bottom water, followed by microfauna abundance, had the closest relationships with the macrobenthic community. Four macrobenthic community groups could be identified at the 50% similarity level according to CLUSTER and MDS, based on average species densities at 20 stations all of which were sampled in both 1998 and 1999. The dominant species in group I were Alvenius ojianus and Leptomya minuta; in group II Moerella jedoensis:in gropu III Philine argentata and Amphipholis kochii (which included only the shallowest B1 station in Laizhou Bay); and in group IV Thyasira tokunagai and Hiatella orientalis (comprising only the deepest station, A4, located in the Bohai strait). There was no obvious disturbance in macrobenthic community structure that could be attributed to pollution from heavy metals and organic matters in the sediments. However, except in the Bohai strait where the water exchange was comparatively better, most other sites demonstrated a disturbed macrobenthic community structures, which might have been the result of pollution or eutrophication. It is suggested that the Bohai Sea has become a typical eutrophic zone because of the relatively high concentration of NO_3-N and PO_4-P, and an elevated ratio of N to P in the bottom water. Compared with the data obtained in early 1980s, there were some changes in the population composition which were most likely attributed to severe eutrophication, increasing fishery activity and alteration of predators.