Comparative study on the influene factors of chlorophyll-a and nutrient structure between winter and summer in the central Bohai Sea

Abstract

Based on the data of chlorophyll a (Chl a), nutrients, and hydrological environment parameters of the central Bohai Sea in January(winter) and June(summer) of 2016, we studied spatial and temporal variations of Chl a, nutrient structure, analyzed the influence factors for the phytoplankton growth and the changing trend of the phytoplankton community structure over long-time scale. The results showed that nutrient structure and Chl a had significant seasonal variation characteristics (<italic>P</italic><0.01). In winter, the concentrations of dissolved inorganic nitrogen (DIN), active phosphate (PO^3-₄-P) and silicate (SiO^2-₃-Si) were (10.51±4.52)μmol·L^-1, (0.28±0.14)μmol·L^-1, and (10.63±2.09)μmol·L^-1, respectively, and NO^-₃-N (92%) was the main form of DIN, and all nutrient contents were above the threshold, meanwhile the nutrient ratios of N/P, Si/P, and Si/N were 69.30±86.80, 64.50±68.80, and 1.20±0.50, respectively, the N and Si limitations were not obvious except that some stations showed P potential limitations because of input of the Northern Yellow Sea cold flow, the sediment resuspension, and the release by the mineralization of organic matters. Affected by seawater temperature (<italic>T</italic><8℃), the phytoplankton reproduction and growth were restricted and the content of Chl a (0.99±0.25) mg·m^-3 was low. In summer, influenced by the Yellow River dilute water, the Northern Yellow Sea cold flow, and phytoplankton uptake. The concentrations of DIN, PO^3-₄-P and SiO^2-₃-Si were (4.72±4.20) μmol·L^-1, (0.04±0.04) μmol·L^-1, and (3.81±1.72) μmol·L^-1, and decreased by 55.0%, 85.7% and 64.1%, respectively, NH⁴⁺-N (48%) and NO^-₃-N (45%) became the main forms of DIN, compared to those in the winter. The nutrient ratios of N/P, Si/P and Si/N were 274.19±328.06, 202.18±182.07 and 1.39±1.00, respectively, increased significantly compared with that of winter. The nutrient limitations showed obvious P limitation, and the contents were below the threshold of DIN and Si in some stations, which would limit the growth of phytoplankton, especially the diatoms. The content of Chl a was (4.58±5.51) mg·m^-3 with the seawater temperature of (22.1±1.88)℃, 4.6 times higher than that in winter, there were some stations in the Yellow River Estuary, the content was beyond 10 mg·m^-3 and showed an outbreak risk of alage bloom.