Influence of aquaculture on characteristics of phytoplankton community in Liusha Bay

We studied the characteristics of phytoplankton community in Liusha Bay for three quarters from August, 2012 to January, 2013. A total of 80 phytoplankton species were recorded, with 69 species of diatom belonging to 34 genera, followed by dinoflagellates represented by 9 taxa (5 genera) and blue-green algaes by 2 taxa (2 genera). The most common dominant species were Skeletonema costatum, Thalassionema nitzschioides and Coscinodiscus centralis. The range of phytoplankton density was 0.001×10⁴~1.495×10⁴ cells · L^-1 with maximum density in summer, followed by autumn and winter. In general, the phytoplankton density decreased from outer bay to inner bay. Compared with the control (non-cultured area), the diversity indices in the cultured areas were higher, especially in the fish cultured area. In summer and autumn, phytoplankton density was higher in scallop culture area, while lower in fish cage and non-cultured area. However, phytoplankton density in pearl oyster cultured area was higher in summer (1.099×10⁴ cells ·L^-1) and low in autumn (0.021×10⁴ cells · L^-1). The results indicate that both phytoplankton taxa and density declined steeply in recent years. The phytoplankton density increased and decreased by scallop culture and fish cage culture, respectively. Dense fish cages blocked the current exchange, diminishing the transportation of nutrients across the bay and the phytoplankton density in pearl oyster cultured area of Liusha Bay.