As the basic status and function in marine food chains, phytoplankton plays an important role in dynamics of material and energy fluxes. Dongtou Island (121°10'E, 27°50'N), located at the southeast coast of Zhejiang Province, is the confluence of the Wenzhou Bay and Yueqing Bay mouths. The development of water eutrophication and frequent occurrence of harmful algae blooms (HABs) events at some sea area of Dongtou Island have been observed in recent years. However, report about the relationships between the environmental factors and the phytoplankton community at sea area of Dongtou Island is rare. To understand the response of phytoplankton to environmental changes, an annual investigation was carried out at the northeast sea area of Dongtou Island.
Seven net-phytoplankton samples were collected from the studied area once a month during August 2010 to July 2011 for understanding the monthly variation of phytoplankton and the relationship between the community structures and the environmental factors. A total of 5 phyla and 187 species were identified, including 143 diatom species, 40 dinoflagellate species and other 4 taxonomic (Chrysophyta, Cyanophyta and Chlorophyta) species. Twenty dominant species consisted of 16 diatom species, 3 dinoflagellate species and 1 Cyanophyta were observed all year round. The monthly phytoplankton community parameters (abundance, species number, Shannon-Wiener diversity and Pielou evenness) varied greatly. Canonical correspondence analysis (CCA) indicated that the DSi, DIN, N:P, DIP, Si:P, salinity and temperature were the main factors influencing the phytoplankton community. Compared to diatoms, dinoflagellate had a stronger response to the P concentration. The growth of dinoflagellate species was enhanced when the P concentration increased. The phytoplankton composition was also profoundly controlled by the Taiwan Warm Current (TWC), Fujian-Zhejiang Coastal Current (FZCC), and runoff from Oujiang River (OR). From July to September, warm-water species such as Cerataulina pelagic, Chaetoceros pseudocurvisetus and C. distans increased significantly as the TWC intruded (with the salinity of 29.8-30.1). In other months, the phytoplankton composition was mainly influenced by the FZCC and OR runoff. Consequently, all the coastal (with the salinity of 26.8-28.6), brackish and freshwater phytoplankton species such as Prorocentrum triestinum, Ditylum brightwellii, Pediastrum duplex var. echinatum and Synedra sp. increased clearly. Since 1980's, the environmental conditions at the studied area had changed obviously: concentrations of NO₃^--N dramaticly increased from 12.89 μmol/L in 1981-1982 to 52.63 μmol/L in 2010-2011; and N:P ratio also increased from 19.1:1 in 1981-1982 to 51.8:1 in 2010-2011. The structure of phytoplankton community has also experienced obvious variation since 1980. The cells abundance increased from 5.8×10⁵ cells/m³ in 1981-1982 to 39.2×10⁶ cells/m³ in 2010-2011, and dominant species (Coscinodiscus spp. was replaced by Skeletonema spp.) of phytoplankton were also changed a lot.