Seasonal diel variations of picoplankton and nanoplankton in a subtropical western Pacific coastal ecosystem

Tsai, An-Yi, Kuo-Ping Chiang, Jeng Chang, Gwo-Ching Gong

Limnol. Oceanogr., 50(4), 2005, 1221-1231 | DOI: 10.4319/lo.2005.50.4.1221

ABSTRACT: We analyzed seasonal and diel fluctuation patterns of heterotrophic bacteria, Synechococcus spp., and nanoflagellates at a coastal station at the southern edge of the East China Sea. Synechococcus spp. and nanoflagellates exhibited diel fluctuation at water temperatures above 25°C. Cell concentrations of Synechococcus spp. were significantly higher during the evening, whereas those of nanoflagellates were higher during the day. The day and night amounts of heterotrophic bacteria did not differ significantly, and we did not observe diel rhythms in these organisms below 25°C. The fractionation experiments we performed between August and October showed that growth rates of bacteria were high (0.73-1.00 µg C L-1 h-1) during the day. However, because there was an increase in nanoflagellate grazing, there was no change in the abundance of bacteria over the day. Synechococcus spp. was not actively consumed by nanoflagellates during the day, but its rate of production was exceeded by the rate of grazing by nanoflagellates during the night. This out-of-phase Synechococcus spp. growth and mortality caused by grazing created diel variations in its abundance. We also found that picoplankton contributed 24-36 µg C L-1 d-1 to the microbial loop, and Synechococcus spp. and bacteria contributed equally to this carbon flux.

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