Dynamics of phytoplankton community in relation to seasonal variation and Laminaria japonica culturing in coastal area

Seaweed aquaculture is an important industry with great social and eco-environmental values. This study assessed the water quality and phytoplankton community during the four seasons around the Taohua Island where seaweed aquaculture has been practiced for near 70 years. Sampling was conducted in Laminaria japonica culturing area. Significant seasonal effects were observed among measured environmental factors and phytoplankton community. The species number and density of phytoplankton were positively correlated with temperature (T), Secchi depth (SD) and total nitrogen (TN) but were negatively correlated with dissolve oxygen (DO). In spring (the late culturing period), the seaweed culture increased pH, DO and salinity, while slightly reduced total phosphorous (TP). The phytoplankton abundance was lower in the culturing area than in control area, whereas the Shannon-Wiener diversity were found to be higher. Our study demonstrated that seasonal environment changes (especially T, SD, TN and DO) had strong influence on the functional characteristics of the phytoplankton community and L. japonica cultivation can absorb the nutrients and enhance phytoplankton diversity, thus alleviating eutrophication and reducing the risk of algal blooms. This is of significance to remediate the coastal eutrophicated and to restore the ecosystem.