Phytoplankton are the primary producer of the marine ecosystem, which utilize light and inorganic nutrients to synthesize organic matter. Phytoplankton are the basic link of the traditional food chain in the ocean. Bacteria are the primary component of the microbial loop, which subsist on dissolved organic compounds released by living plankton and decomposed from dead organisms. Sanya Bay is a typical tropical bay of China, located in the northern part of South China Sea in the south coast of Hainan Island. Based on the two cruises in Sanya Bay in October 2004 (autumn) and January 2005 (winter), the distribution patterns of phytoplankton and bacteria biomass were studied. The relationships between their biomass and environmental factors, such as DIN, phosphorus, DO and BOD5, were also analyzed.
Phytoplankton biomass (PB) was determined from chlorophyll a concentration, and bacterial biomass (BB) was measured by DAPI (4’6-diamidino-2-phenylindole) dying and SEM method. The results showed that the chlorophyll a was (1.40±0.78)mg/m3 in autumn and (2.25±3.62)mg/m³ in winter in Sanya Bay. Bacterial abundance was (9.87±5.90)×108 and (6.58±2.43)×108 cells/L in autumn and winter, respectively. The mean phytoplankton biomass (PB) was (70.36±38.91)mg C/m³ in autumn and (112.57±181.38)mg C/m³ in winter. The mean bacterial biomass (BB) was (19.73±11.81)mg C/m3 and (13.15±4.86)mg C/m³ in autumn winter, respectively. The distribution of PB and BB at the surface and bottom layers in autumn and winter almost demonstrated the same pattern, which showed that the highest biomass appeared in the estuary of the Sanya River, and their abundance gradually declined offshore. The distribution feature mainly resulted from the terrestrial input from the Sanya River and invasion by the offshore oceanic sea water. Temperature was one of the main factors which leaded to higher BB in autumn than in winter. Dissolved inorganic nitrogen was another important factor to control the PB and BB distribution at the surface in Sanya Bay.
There was no correlation between other environmental factors and the PB or BB except for the surface DIN in autumn. While in winter, DIN, Phosphorus, DO and BOD5 all had highly significant correlations with the surface PB and BB (p<0.01), and only DIN and BOD5 had significant correlation with bottom PB and BB (p<0.01). The correlation was significant between phosphorus, DO and bottom BB (p<0.05) in winter. As to PB and BB, the correlation coefficient was 0.870, correlation was significant for the surface layer (p<0.01), while the correlation coefficient was 0.582 for the bottom layer (p<0.05) in autumn. In winter, PB and BB all had significant correlation with environmental factors (p<0.01) both for the surface and bottom layers. Primary production was an important factor in controlling the distribution of bacteria.