Material Cycle Of Marine Cage-Culture Ecosystem In Daya Bay

Distribution of carbon(C), nitrogen(N), sulfur(S), phosphorus(P), calcium(Ca), magnesium(Mg), iron(Fe), zinc(Zn), copper(Cu), molybdenum(Mo), selenium(Se), and strontium(Sr) in marine cage-culture ecosystem in Daya Bay was studied based on investigation in field and experiments in laboratory. Forage fish, cultural fish, epiorganisms on cages, seawater, microorganisms, benthos, and sediments in marine cage-culture areas were sampled in the field to measured element contents. Nitrogen and phosphorus waste outputs of four marine cage-cultured fish fed with trash fish were investigated under laboratory conditions.The elements were analyzed in forage fish, compound feed, some tissues of forage fish. The results showed that the concentrations of C, N, S, and Ca in feed were 215-483 mg/g, 52-133 mg/g, 8-80 mg/g, and 2-154 mg/go respectively. Concentration of elements in different species of forage fish and tissues in trash fish with different body weights varied. On average the levels of C, N, S, and Ca in cultural fish were 447.5mg/g, 10.8mg/g, 24.3 mg/g, and 38 .7mg/go respectively. There were sex, species, and body weight differences in element contents in tissues of cultural fish. Mussels and oysters predominated the periphyton communities on cages, and the epiorganisms in the cultural areas contained 7.8×10⁶ g, 9.0×10⁵ go 6.8×10⁴ g, and 1.3×10⁷ g of C, N, P, and Ca, respectively. Concentrations of N, P, and Ca of Seawater in cultural areas were 0.013-0.032 mg/L, 0.0048-0.0220 mg/L, and 64-180 mg/L, respectively. Induced by marine cage-culture, the N and P levels in cultural seawater were higher than those of seawater in control plots, but contents of Ca and Sr in seawater in cultural areas were higher than those of control plots. In the present study, a new method based on biomass-colony standard curve was established to measure biomass of bacteria and fungi. The biomasses of bacteria and fungi in cage-cultural seawater were 5.4×10^-4 mg/L and 6.3×10^-5 mg/L, respectively, and contents of N, P, and Ca in bacteria were 78 mg/g, 7.2 mg/g, and 41 mg/g, respectively. Contents of C, P, Ca, Cu, and Sr in sediments of cage-cultural areas were significantly higher than those of control plots. C and Ca levels in sediments in cage-cultural areas were 14.8 g·L fresh sediment^-1 and 30.6 g·L fresh sediment^-1, respectively. Mussels and oysters dominated the benthic communities in cage-cultural areas. C and Ca stored in bottom fauna of cage-cultural areas were 25.2 g·L fresh sediment^-1 and 43.7 g·L fresh sediment^-1, respectively.The results of laboratory experiments showed that waste outputs of cultured fish fed with trash fish were divided into soft tissues in uneaten feed, bones in uneaten feed, scales in uneaten feed, faeces, urine, caducous scales, and other parts. Element differences of these wastes were examined.In marine cage-culture ecosystem which always received feeds, elements were distributed in cages, seawater, and sediment, and some of elements were transported in or out the ecosystem. Some N and P transported by seawater moved out the ecosystem. There was a pump-like mechanism that Ca was pumped from seawater nearby to the cage-culture ecosystem, and then been sunk to the sediment.