A Preliminary Study Of The Impacts Of Scallop Culture On Phytoplankton Community

Phytoplankton is the main food to the bivalve filter-feeders and the marine primary producer, so the impacts on the phytoplankton community by scallop culture deserve studying. This thesis studies the impacts on biomass and structure of the phytoplankton community by scallop culture and the underling mechanisms through analysis of the field survey data and experiments in enclosure in the lab. The field survey data are from two year-round monthly campaigns in Sanggou Bay over 1983-1984 and 1999-2000. The experiments studieds i) Zhikong scallop (Chlamys farreri) feeding selection of three microalgae, the dinoflagellate Prorocentrum donghaiense, the diatoms Skeletonema costatum and Chaetoceros minutissimus, ii) the impacts on scallops' feeding selection b> algal biomass in the mixture of P. donghaiense and S. costatum, iii) scallops' digestion efficiency of P. donghaiense and S. costatum and iv) the differential growth of P. donghaiense and S. costatum's in scallop-lived seawater.Analysis of the field survey data shows that both density of dinoflagellates and their contribution to net phytoplankton inside scallop culture are significantly lower than those outside scallop culture in Sanggou Bay. The difference of diatom's density, however, is not significant between inside and outside scallop culture. In July and August when the biomass of scallops is high and scallops' feeding activity is active, the dominant phytoplankton species inside scallop culture are diatoms; by contrast, outside scallop culture the dominant species are three diatoms and one dinoflagellate in July, and dinoflagellates in August.Experiments on the scallop's feeding selection of algae indicates that scallop's filtration rate (measured in algal cells volume, hereafter referred as FRv) and feeding selection efficiency (SE) for P. donghaiense are higher than those for S. costatum and C. minutissimus.At the same density of algal mixture, scallop clearance rate declines but FRvvaries little when the contribution of P. donghaiense cells increase. Scallop SE for P. donghaiense is consisitantly higher than for S. costatum, regardless of the contribution of P. donghaiense (cell density ratio P. donghaiense: S. costatum = 1:8-8:1).At the same P. donghaiense to S. costatum cell density ratio, scallop clearance rate tends to decline with the increase of algal mixture cell density, scallop FRv and SE for P. donghaiense are higher than for S. costatum when algal cells' density is 0.5 × 106cell/dm3~20× 106cell/dm3, and there is a change in scallop's feeding selection of algae: (1) when algal cells' density is low (0.5 × 106cell/dm3), scallop SE for P. donghaiense is low; (2) when algal cells' density is 5×106cell/dm3~10×106cell/dm3, scallop FRv and SE for P. donghaiense increase; (3)when algal cells' density is high (≥10×106cell/dm3), the difference in scallop SE between for P. donghaiense and for S. costatum becomes little.When the scallops are fed at the same ration for three days, their clearance rate and selection efficiency varies day to day but is not significant, and scallop SE and FRv for P. donghaiense are consistently higher than those for S. costatum, regardless of the variation of P. donghaiense contribution in the algal food mixture.Scallop digestion efficiency for P. donghaiense and S. costatum is constant and similar (>99%) when the FRv is lower than 5.00 mm3/d. When the FRv increases from 5.00 up to 15.00 mm3/d, scallop digestion efficiency rate decreases down to 96.2%.The concentration of dissolved inorganic nitrogen, phosphate and silicate is higher in scallop-lived seawater than in natural seawater. When inoculated together in scallop-lived seawater, S. costatum out competes over P. donghaiens (in terms of both quantity and growth rate).These results indicate that i) scallops cast a more discernable control on dinoflagellates abundance than on diatoms; ii) such different extent of control may result from the scallops' higher feeding potential for dinoflagellates than for diatoms, and/or the lower competitive capacity of P. donghaiense in scallop-lived seawater.