Succession of dominant phytoplankton species in spring 2002 at Dapeng'Ao Cove, Daya Bay

Dapeng'Ao, a small cove at the southwest of Daya Bay in Guangdong Province of China is an area covering about 14 km 2 at an average water depth of 5 m with well developed mariculture of fish and shellfish. From April 28 to May 27 2002, a 30 day continuous observation was carried out in the cove to investigate the succession of dominant phytoplankton species. Water samples were taken from the surface and bottom layers of 3 sites in Dapeng'Ao cove at about 9 a.m. daily. Physico chemical factors were measured in situ (e.g. water temperature, dissolved oxygen) or analyzed in the laboratory of Marine Biology Research Station at Daya Bay within an hour after sampling (e.g. pH, salinity, ammonia, nitrite, nitrate, phosphate, silicate, chlorophyll a). Phytoplankton was fixed with Lugol's solution in situ , and dominant species were identified and their cell numbers were enumerated. In this continuous observation, it found that diatoms dominated (about 98.9% of total phytoplankton cells) in the waters, Skeletonema costatum, Nitzschia delicatissima, and Chaetoceros sp. were the dominant species. Skel. costatum was the leading species from April 28 to May 22 whileas Nitz. delicatissima took the leading role after May 22, resulting in a phytoplankton succession. Using Principal Component Analysis (PCA) method, the most important five components account for 78% of data information. Multiple regression method was used to elucidate the relationship among those dominant species and their contribution to the principal components (represented by the physico chemical factors), based upon the correlation coefficients between the dominant phytoplankton species and the main principal components ( p 0 01), and to develop a succession model for the dominant phytoplankton species in Dapeng'Ao cove in spring. The model shows that Skel. costatum has competitive relationship with Nitz. delicatissima and the most important physico chemical factors that are closely related to the growth of diatom dominated phytoplankton community are temperature, dissolved oxygen, ammonia, nitrite, phosphate and silicate; salinity, nitrate, and the ratio of nitrate to phosphate also influenced the growth of Skel. costatum. During the period of investigation, the most significant environment change was rainfall, which resulted in a decrease of salinity and an increase of nitrogenous nutrients in water column, and such an environment change promotes the growth of Skel. costatum. Along with phytoplankton growth, the nutrients (N, P, and Si) were exhausted gradually and could not further maintain the population growth of Skel. costatum and its density declined, which relieved the competition pressure for Nitz. delicatissima and it grew rapidly thereafter. The environment change (rainfall) occurring during the period of phytoplankton succession illustrated clearly how dominant species changed from Skel. costatum to Nitz. delicatissima in the succession process.