Spatial heterogeneity of phytoplankton community shaped by a combination of anthropogenic and natural forcings in a long narrow bay in the East China Sea

Phytoplankton communities in semi-enclosed bays vary on spatial scales under natural and anthropogenic forcings (e.g., water exchange, sediment resuspension, and eutrophication). However, their relative importance in shaping phytoplankton spatial heterogeneity remains poorly quantified. The long, narrow, semi-enclosed Xiangshan Bay (XSB) has been subject to extensive human activities, including eutrophication and thermal discharge from power plants, which profoundly influence the phytoplankton community, particularly in cold seasons. We hypothesize that phytoplankton spatial heterogeneity in this bay is largely structured by anthropogenic forcings. Here, high spatial resolution datasets of phytoplankton communities were obtained during four seasons in 2015. Results show that diatoms and cryptophytes dominate the phytoplankton community, followed by dinoflagellates. Colonial planktonic and single-celled benthic/meroplanktonic diatoms and cryptophytes thrive in branched bays and in the upper main bay, which is characterized by high transparency, abundant nutrients, and relatively stable water. Large-celled planktonic and colonial benthic/meroplanktonic diatoms and allochthonous species (e.g., Triehodesmiurn thiebantii) are dominant in the turbulent, turbid, high-salinity lower and outer sections. Phytoplankton abundances were significantly higher in branched bays and in the upper section than in other regions. Thermal discharge stimulated the winter bloom of Skeletonema and Chaetoceros curvisetus in the inner bay. A nonmetric multidimensional scaling and analysis of similarity indicated highly significant spatial heterogeneity of community composition throughout different regions. Canonical correspondence analysis and Spearman's rank correlation demonstrated the significance of light penetration, nutrients, salinity, and temperature in structuring phytoplankton spatial distribution. However, weak top-down control on phytoplankton by mesozooplankton was also observed. Variance partitioning analysis revealed that phytoplankton community variations were highly explained by the interaction of anthropogenic (eutrophication and thermal discharge) and natural (water exchange and sediment resuspension) factors in all seasons, although anthropogenic variables explained more variation than that of natural variables in cold seasons. This study demonstrated that a combination of anthropogenic and natural forces contributed to phytoplankton spatial heterogeneity in the XSB.