Effects of intensive mariculture on the sediment environment as revealed by phytoplankton pigments in a semi-enclosed bay, South China Sea

Phytoplankton pigment concentrations in the surface sediments of a fish and oyster farm in Dapeng Cove, South China Sea, were measured monthly to investigate the effects of mariculture on the environment from December 2011 to December 2012. The highest contents of the identified phytopigments, sulphides and total organic carbon in the surface sediments were all found at the fish farm, followed by the oyster farm; the lowest contents were detected at the control site. Fucoxanthin in the surface sediments was the most abundant carotenoid and was significantly correlated with chlorophyll a (Chl a; r = 0.922, P < 0.01). This finding suggested that the deposited diatoms could be the main source of the Chl a concentration. Besides a differential sedimentation of phytoplankton (i.e. higher downflux at fish and oyster site than that at control site), we assume that the extent of pigment diminution from the fish site to control site was also controlled by the chemical conditions of the surface sediments. The fish farm provided the most optimum preservation conditions for the phytopigments and was a good sink for phytodetritus among the three sites because of its anoxic and reductive characteristics. Phytoplankton pigments could be useful biomarkers to evaluate the influence of mariculture on sediment environment.