Phytoplankton and bacterioplankton biomass, production and turnover in a semi-enclosed embayment with spring tide induced upwelling

We examined the short time scale (i.e. hours to days) responses of phytoplankton and heterotrophic bacteria to a spring tide-driven upwelling, which is different from classical wind-driven systems, in southern Taiwan in a semi-enclosed embayment. Results showed that nitrate concentrations (NO ₃^-, <0.15 to 10.2 μM) correlated negatively with temperature (ca. 16 to 26°C). Estimated NO₃^- flux upwelled from >70 m was ∼60 mmol N m^-2 h^-1 with <1 % of it utilized by phytoplankton. Chlorophyll a concentrations doubled from 0.07 to 0.16 μg l^-1 in the 9 h after the upwelling to maximal concentrations of <0.30 μg l^-1. Euphotic zone integrated primary production (IPP, ca. 118 to 389 mg C m^-2 d^-1), and algal turnover rate (Pμ, ca. 0.27 to 0.76 d^-1) responded to the upwelling ∼5 h earlier than chlorophyll. The observed uncoupling between upwelled nutrients and the low chlorophyll phenomenon might be due to quick tidal mixing and shorter residence time of the water mass (<1 d) within the bay, in comparison to the algal turnover time (1.3 to 3.7 d). Euphotic zone integrated bacterial production (17 to 28 mg C m^-2 d^-1) and bacterial turnover rate (0.6 to 0.8 d^-1) were positively correlated with IPP, Pμ and depth integrated particulate organic carbon (ca. 540 to 6575 mg C m^-2) concentrations, suggesting a high possibility of 'bottom-up' (organic substrate supply) control. This was confirmed by the results of 3 enrichment experiments showing that bacterial growth was C limited. Our study provides mechanistic information regarding the magnitude of the interaction between physical, chemical and biological processes, since the time scale adopted by this study encompasses the turnover times of auto- and heterotrophic plankton.