| Summary: | 碩士 === 國立臺灣大學 === 漁業科學研究所 === 107 === Phytoplankton are major primary producers in marine ecosystems and can directly impact higher trophic levels. The abundance and distribution of phytoplankton are known to be related to water temperature, nutrient, light, and zooplankton grazing. Therefore, the phytoplankton dynamic is often linked to seasonal variations and mixed-layer depth (MLD). This study compiled and analyzed in situ fluorescence and chlorophyll-a (Chl-a) concentration data at the SouthEast Asia Time-series Study (SEATS, 116°E and 18°N) station in the northern South China Sea, to understand the phytoplankton dynamics in different time scales, diurnal, seasonal, and decadal scales. In situ Chl-a concentrations and water temperature were collected for 30 cruises throughout 1999 and 2019 at the SEATS station, and the euphotic depth integrated Chl-a concentrations were analyzed and compared with sea surface temperature (SST) and MLD. In the diurnal scale, the results showed the Chl-a concentrations increased during the day, decreased during the night time, and the variations could be as high as two-fold. In the seasonal scale, the Chl-a concentrations, SST and MLD showed two distinct patterns: the cold seasons (December and January) with low SST, deep MLD, and high Chl-a concentrations; the warm seasons (March to November) with high SST, shallow MLD, and low Chl-a concentrations. Seasonal Chl-a concentrations, SST and MLD also showed correlations. In the decadal scale, the 20-year data showed an increase in SST (cold season: 0.05 ℃/ year; warm season: 0.06 ℃/ year), a decrease in MLD (cold season: -0.80 m/ year; warm season: -0.52 m/ year), and a decrease in depth integrated Chl-a concentrations (cold season: -0.73 mg m-2/ year; warm season: -0.49 mg m-2/ year) at the SEATS station. The results suggest that, in short-term study, it is important to consider the diel fluctuations when conducting field sampling for phytoplankton and primary productivity studies; in the 20-year analysis, the increase of SST and the shallowing of MLD led to the decrease in Chl-a concentrations, and which may impact the ecology and resources in the northern South China Sea.
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