The effects of Zhikong scallop (Chlamys farreri) on the microbial food web in a phosphorus-deficient mariculture system in Sanggou Bay, China

Abstract

Sanggou Bay is a phosphorus (P)-deficient and nitrogen-sufficient mariculture area, and the culture of the Zhikong scallop, Chlamys farreri, is important there. We established a 7-day enclosure experiment in situ to simulate the effects of the Zhikong scallop on the microbial food web in the bay. The results showed that scallop cultivation increased the concentration of PO43-(from 0.04 mu M at the initial to 0.26 mu M at the end of the enclosure experiment) and the abundance of picoplankton (heterotrophic bacteria, Synechococcus and picoeukaryotes), total nanoflagellates and ciliates. Following the enclosure experiment, the 6-day P-enrichment (similar to 0.3 mu M) and 3-hour feeding experiments were conducted in field to study why scallop cultivation strengthened the microbial food web. The results showed that P-enrichment stimulated the growth of major microbial components when their predators were removed or suffered grazing pressure. The scallops did not efficiently retain picoplankton. The retention efficiency of the Zhikong scallop dropped with decreasing prey size, and the retention efficiencies for ciliates, 5-10 mu m nanoflagellates and <5 mu m nanoflagellates were 83.22%, 63.27% and 22.62%, respectively. It was suggested that P release by scallops may increase the abundance of main assemblages in the microbial food web, but the influence of the nutrient release into natural water may be partially diminished by trophic cascades within the microbial food web. However, the different retention efficiencies of the scallop on the different components of the microbial food web may weaken the strength of trophic cascades.