Phytoplankton community structure shaped by key environmental factors in fish and shellfish farms in Daya Bay, South China

Abstract

In coastal cultural regions of Dapeng Cove (southwest of Daya Bay), the composition, abundance and community structure of phytoplankton, as well as physicochemical variables, were investigated to identify factors driving seasonal changes. During summer, local phytoplankton biomass was low, probably because of nutrient depletion. In September, sudden heavy rainfalls resulted in a substantial increase in nitrogen. As a result, a disturbance-tolerant diatom, Skeletonema costatum, increased rapidly. Organic matter associated with freshwater runoff, fishery activities and settled algal cells increased consumption of dissolved oxygen (DO), especially in fish farms. It was suggested that heavy rainfall events was probably a trigger of low oxygen level and exerted a major effect on nutrient cycling and the phytoplankton community. In winter, phytoplankton community was characterized by dinoflagellates, which are adapted to well-mixed conditions. Spring blooms composed of Pseudonitzschia delicatissima and P. pungens were triggered by warm temperature and increasing light intensity. Excessive ammonium levels in March probably contributed to bloom initiation. Dissolved inorganic nitrogen (DIN) and total phosphorus (TP) concentrations in fish farms were significantly higher than that in bivalve cultures and control sites although phytoplankton abundance in fish farms was not. We suggest that intense fish culture farming exerts more pressure on the ambient environment than shellfish farming.