Xiangshan Bay (121°25'-120°00'E, 29°05'-29°46'N), located at the East China Sea, is a long semi-enclosed bay with slow rate of water exchange. As one of the most important marine economic bases in Zhejiang Province, it has confronted a series of ecological challenges due to the coastal power plants (Ninghai and Wushashan Power Plant) running, excessive aquiculture, industrial development, tidal flat reclamation, and sea-crossing bridge construction. These large-scale human activities certainly lead to hydrodynamic alteration, temperature elevation, and eutrophication aggravation in this bay. The phytoplankton, as the basic and essential part of food chain, is very sensitive to such environmental stresses. However, present data about the combined effects of multiple marine exploitations on phytoplankton in the semi-enclosed bay is scarce. In this context, one cruise of phytoplankton and physical-chemical investigation was conducted in Xiangshan Bay in spring, 2010.
Based on 28 net-phytoplankton samples, a total of 6 phyla, 55 genera, and 158 species were found, including 135 diatom species (occupied by 85.4% of the total species number and 99.0% of the total abundance), 18 dinoflagellate species and 5 other taxonomic species belong to Chrysophyceae, Cyanophyceae, Chlorophyceae and Cryptophyceae. The phytoplankton abundances (35.29×10⁴ Cells/m³ on average) represented a strong patchy distribution, varying in the range from 3.41×10⁴ Cells/m³ to 142.35×10⁴ Cells/m³, and the high values occurred in the areas around Hengshan Island to Baishishan Island and also around Wushashan Power Plant. Coscinodiscus jonesianus (284.97×10³ Cells/m³) was the absolute dominant species with its cell number accounted for 80.8% of the total abundance, while other dominant species such as Navicula corymbosa, Skeletonema costatum and Coscinodiscus concinnus also had high cell densities (11.93, 6.92 and 4.06×10³ Cells/m³, respectively). Canonical correspondence analysis (CCA) showed that transparency, suspended solids, temperature, N/P and silicate were the main variables which affected phytoplankton community in turn. The ample solar light, moderate temperature increment of the thermal discharge, abundant nutrition and suited N/P (close to the Redfield ratio 16) induced a phytoplankton (especially the diatom) bloom at the inner bay. Due to the catchment input, water exchange and marine exploitation, the patchiness and gradient of physical-chemical parameters were obviously observed, and such complicated environment supplied different habitats for phytoplankton and resulted in spatial heterogeneity. Both the Shannon-Weiner diversity and Pielou evenness index at the mouth and outside were evidently higher than the midst and bottom of Xiangshan Bay. Thus, there was a great discrepancy among the community composition at embayment bottom, midst, mouth and outside, according to the cluster, multidimensional scaling, and similarity analysis results. Besides, comparing with the historical data, we found that the microalgal abundance increment and community structure alteration were tightly related to the various marine exploitations in this bay.