Large-scale Patterns of Meiofaunal Abundance in the Bohai Sea

The Bohai Sea is a marginal sea with an area of approximately 77 000 km 2. It has restricted water exchange, being enclosed by the Liaodong and Shandong peninsulas and connected to the Huanghai (Yellow Sea) by the Bohai Strait. In recent years the Bohai Sea has been subject to intensive offshore exploration for, and production of, natural gas and petroleum reserves. It is considered to be highly polluted and the state of the environment in the area is of great concern to the Chinese government and agencies. According to SEPA (the Chinese State Environmental Protection Administration) data excessive amounts of waste water (including industrial effluent) and untreated sewage, and overfishing have severely depleted marine resources in the Bohai Sea. As a result of these factors red tides have become a frequent occurrence. Although considerable effort is invested in measurement and monitoring environmental variables, most published studies deal with physical and geological aspects of the area, and published information about the ecology of the Bohai Sea is limited. Ecological studies of the biota inhabiting sediments in the Bohai sea have examined the composition of macrofauna and freeliving marine nematodes. Other studies, mainly taxonomic works, have also dealt with meiofaunal taxa. Meiofauna are considered to be energetically important in benthic food webs, and play a role in the recycling of nutrients, both directly and also indirectly, for example bacterial colonies on sand grains may be maintained in an active growth phase as a result of grazing by bacterivorous nematodes. Meiofauna have been shown to be useful as potential indicators of anthropogenic perturbation in aquatic ecosystems. Studies on the ecosystem in the Bohai Sea are important for protecting fisheries resource and aquaculture. A national and international oceanographic and ecosystem study of the Bohai Sea has been launched which includes sampling for benthos, including meiofauna, from a large scale grid of stations. This paper studies relationships between benthic environmental factors and patterns in abundance and distribution of major meiofaunal taxa in the Bohai Sea. Samples were collected from three cruises in June 1997, A grid of 22 stations, giving a broad geographic coverage of the Bohai Sea and Bohai Strait was selected. Five of these stations were sampled in June 1997. 20 stations (of which 18 were the same) were sampled on cruises durng September/October 1998 and April/May 1999. Undisturbed sediments were brought on deck using a modified 0.1 m 2 Gray O'Hara box corer, and subsamples were taken using a sawn off syringe to a depth of 50mm, taking care to avoid core compression in the process, and fixed in formalin. In the laboratory samples were washed on a 0.048mm sieve to remove formalin and most of the finer sediment fraction. Meiofauna was then extracted using Ludox by centrifugation or by flotation. Each sample was washed into a lined petri dish and meiofaunal organisms other than nematodes were picked out and counted under a binocular microscope. Analyses of a range of measures, including the percentage content of gravel, sand, silt and clay, median particle diameter ( MdΦ ), quartile deviation( QDΦ ) and quartile skewness ( SKΦ ), were used to indicate variation in sedimentary grain size and the distribution of sediment type. The sediments at stations in Lai Zhou Bay were silty clay or clayey silt, and at stations in middle and eastern parts of the Bohai Sea, and in the Bohai strait, were coarse silt or fine sand. Analyses showed that many sediment measures were highly intercorrelated within years, and that consistent patterns of variation existed between years. There were, however, some clear changes in the contents of clay and sand between the later two cruises, especially in Laizhou Bay, the middle east part and the Bohai strait. The abundance of meiofauna and nematodes varied greatly. The average numbers of meiofauna were 2274±1039 ind./10cm 2 in 1997, 869±509 ind./10cm 2 in 1998 an