Abstract: We investigated the distribution of chlorophyll a of size-fractioned phytoplankton in the sea near Zhangzidao Island in January, 2016. Correlations of the concentrations of chlorophyll a and multiple environmental factors were explored to determine the main impact factors. Ourresults showed that, in the surface layer, the concentrations of chlorophyll a of total phytoplankton, microplankton, nanoplankton and picoplankton ranged 0.24-0.92, 0.15-0.58, 0.09-0.46 and0-0.03 μg·L^-1, respectively, with averaged concentrations being 0.69, 0.36, 0.33 and 0.002 μg·L^-1. Contributions of microplankton, nanoplankton, and picoplankton to total chlorophyll a were 51.72%, 48.01%, and 0.26%, respectively. In the bottom layer, picoplankton was not found. The concentrations of chlorophyll a of total phytoplankton, microplankton and nanoplankton in the bottom layer ranged 0.29-1.77, 0.12-1.45, 0.17-0.50 μg·L^-1, respectively, with mean concentrations being 0.78, 0.43, and 0.34 μg·L^-1. Contributions of chlorophyll a of microplankton and nanoplankton to total chlorophyll a were 52.97% and 47.03%, respectively. There was no significant difference in the concentrations of chlorophyll a of total phytoplankton, microplankton and nanoplankton between the surface and bottom layers (all P<0.01, pairedttest), indicating a homogeneously vertical distribution. Horizontal distributions of chlorophyll a of total phytoplankton, microplankton and nanoplankton also showed a homogenous pattern, with concentrations all gradually decreasing from the northwest to the south. Results from the redundancyanalysis showed that temperature, salinity, dissolved oxygen, particulate organic matter, NO₂^--N and NH₄⁺-N were main impact factors driving the distribution of chlorophyll a of phytoplankton.

Key words: biochar, straw returning, soil fertility, paddy soil, soil enzyme activity