Feedback Impacts Between Filter-feeding Molluscs And Their Environment And Carrying Capacity Of Mariculture

The impacts of marine filter-feeding mollusks' farming on the environment, and environmental conditions (food and temperature)' regulation on feeding behavior of the Zhikong scallop Chlamys farreri is studied in Sanggou Bay, China, through historical data, field survey, mooring continuous monitoring and in situ experiments. Then, a responsive growth model is established for the Zhikong scallop and an ecosystem model is built for assessment of the carrying capacity of the polyculture in Sanggou Bay. The main results are summarized as follows:Large-scale intensive culture of bivalves poses significant impacts on the marine ecosystems. The long-term (decadal) impacts include enrich of inorganic nitrogen in the water column, decrease of primary productivity and net-plankton density and increase of seawater transparency. Accordingly, short-term (inter- daily or hourly) changes, as revealed by simultaneous applied current, dissolved oxygen, chlorophyll florescence, turbidity, temperature, salinity and pH sensors for continuous monitoring up to 10 days, include reduction of current speeds by the culture installations, control of phytoplankton biomass over tidal variation by feeding of the suspension-cultured bivalves.Measurements with remote incubation of intact cores of sediment with overlying water from Sanggou Bay, indicate that sediment-water nutrients (ammonium and phosphate) fluxes and their contribution to the primary production in the water column in the bay under shellfish culture impacts, are relatively low compared with other shallow coastal waters, whilst the sediment oxygen demand rates are at middle-high levels. The sediment respiration in Sanggou Bay is estimated to represent mineralization of 102% (May) to 67% (August) of the primary production in the water column, and only 40% (May) to 3% (August) of the pariculate organic carbon that is imported to the sediment via bivalve biodeposit is buried in the sediment. The sediment nitrogen release contributes to 14% requirement of inorganic N for the primary production in the water column whereas net absorption of phosphate makes a negative contribution. Both sediment-water N fluxes and sediment oxygen demand(SOC), however, arepositively related with the organic carbon and nitrogen contents in the surface sediment while P flux is related to SOC and phosphate level in the overlying water.Results from the ecophysiological experiments with Zhikong scallops indicate: i) That scallops rely mainly on regulation of clearance rates to maintain their maximal food intake, and clearance rates are regulated to particle volume (more important) and chlorophyll a content of the seston, with an unimodal effect resulting in maximal clearance rate at food conditions in the natural seawater range, ii) That scallops selective feed on organics (more important) and phytoplankton through retention on the gill, psuedofaeces production and post-ingestion processing of the food, iii) That net absorption efficiency is positively related with organic content of the ingested food, iv) That the 0:N molar ratio is lowest when the scallop is starved and the ratio increases as feeding inceases. And v) that body weight and amenable temperature have positive effects on scallop feeding and respiration rates.Hereafter, a dynamic growth model, based on the concept of net energy balance, is configured for the Zhikong scallop individuals, using functions generated from the above experimental data and relavant published data. The model covers dynamic simulation of rapid and sensitive adjustments in feeding and metabolism as observed in response to the highly changeable food and temperature conditions. Notable novel elements include resolving significant adjustments in the relative processing of living chlorophyll-rich phytoplankton organics, non-phytoplankton organics and the remaining inorganic matter during both differential retention on the gill and selective pre-ingestive rejection within pseudofaeces. The model is validated with measurements of growth data of the cultured sc...