Short-term environmental variability and phytoplankton abundance in a shallow tidal estuary II. Spring and fall

Abstract

Fixed-point sampling of a shallow tidal estuary was performed hourly for 14 d periods to characterize phytoplankton biomass variability at 2 to 96 h periodicities via spectral analysis. In all, 28 parameters were monitored, encompassing meteorology, hydrology, water chemistry and phytoplankton production physiology. The spring (May 1982) and fall (November 1982) periods of rapidly changing water temperatures are compared. In spring, the estuary was in transition from riverine to lagoonal condition. Temperatures rose steadily (22 to 28"C), salinities (23 to 35%) and chl a biomass (22 pg I-') were high, and dissolved inorganic nitrogen (DIN) low (0.13 PM). Growth-limiting N was supplied as NH,' by biological remineralization, rising (0.2 to 3.0 PM) on 11 of 14 nights, then declining each following day to below detection. Higher ebb-tide chl a biomass produced a tidal signal. Die1 tidal height variations and daily wind stress changes accounted for about half the daily 2-fold oscillation in chl a, the remainder was driven by daytime phytoplankton growth followed by nighttime grazing losses. In fall, the estuary was more riverine (20 to 32%) and temperatures variable (12 to 20C). Chl a biomass was low (4 pg 1-'1 and DIN high (2.0 PM). Chl a varied at the tidal and interday periods.