Seasonal Relationships between the Plankton Community and Hydrographic Conditions in a Shallow Oligotrophic Bay, Gulf of Thailand

Abstract

The variability in a plankton community was studied twice a week from September 2006 to August 2007 in a shallow bay (Manao Bay, Gulf of Thailand). Phytoplankton was collected by filtering with a 20 µm plankton net and zooplankton was collected by vertical hauling with a plankton net of 200 µm mesh size. Hydrographic conditions were measured in situ, while water samples were analyzed for nutrient content in the laboratory. Fifty-six genera of phytoplankton and 37 groups of zooplankton were recorded. The mean abundance of phytoplankton was 4,930 ± 2,801 unit l^-1 while that of zooplankton was 3,380 ± 1,264 individuals m^-3 (ind. m^-3). The dominant groups of phytoplankton were centric diatoms and pennate diatoms, while copepods and meroplankton were the major contributors of zooplankton. The plankton community did not differ in species composition and abundance between the two monsoon seasons and there were no specific noticeable characteristics. Hydrographic conditions showed slight fluctuations and nutrients had low values. Additionally, low average primary production (1.30 ± 0.67 mg m^-3) was influenced by water temperature, rainfall, salinity, silicate content and dissolved inorganic nitrogen. This bay was characterized as a tropical oligotrophic coastal water.