Trophic relationships in the Bohai ecosystem: preliminary investigation from δ~(13) C analysis

The suspended matter, plankton, benthos and sediment samples were collected at four stations in the Bohai Sea during June 5 to 10, 1997 in order to study the trophic relationships in the Bohai ecosystem by using stable carbon isotope method. Zooplankton samples were separated into three size fractions: 1000μm, 500～1000μm and 200～500μm. The results of carbon isotopic analyses indicated the δ 13 C values of the biome in late spring in the Bohai ecosystem ranged between -25.67～-17.42×10 -3 , in which the planktonic community (not including necton) has a maximum δ 13 C difference of about 3.68×10 -3 , corresponding to there being 3.2 trophic levels in the ecosystem. There was enrichment of the heavier isotope of mesoplankton with increasing grain size, showing carbon isotope enrichment along trophic levels. Some overlaps of the δ 13 C values, however, occur among the various mesoplankton size classes, which, in fact, reflects the overlap of organism species. The increasing tendency of trophic level with increasing mesoplankton size is coincident with the results of Rau et al. [1] , Sholto Douglas et al [2] , and Fry and Quiones [3] . Benthic organisms generally have higher δ 13 C values than plankton in the Bohai Sea, which does not infer trophic levels of benthos being higher than those of the plankton, but reflects differences of their food sources and the isotopic composition of the bases in both benthic and planktonic food webs. Analytical results of limited samples of benthos indicated that four trophic levels occur in benthic food web in the Bohai Sea. The stable carbon isotope compositions of benthos also reflected their food sources and trophic locations.