SHORT-TERM ENVIRONMENTAL VARIABILITY AND PHYTOPLANKTON ABUNDANCE IN A SHALLOW TIDAL ESTUARY .2. SPRING AND FALL

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-degrees-C), salinities (23 to 35 parts per thousand) and chl a biomass (22 mug l-1) were high, and dissolved inorganic nitrogen (DIN) low (0.13 muM). Growth-limiting N was supplied as NH4+ by biological remineralization, rising (0.2 to 3.0 muM) on 11 of 14 nights, then declining each following day to below detection. Higher ebb-tide chl a biomass produced a tidal signal. Diel 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 parts per thousand) and temperatures variable (12 to 20-degrees-C). Chl a biomass was low (4 mug l-1) and DIN high (2.0 muM). Chl a varied at the tidal and interday periods. Interday changes were correlated with water temperature changes, in turn coupled to the passage of atmospheric fronts at 3 to 4 d intervals. In both spring and fall periodic ecological processes operating on time scales equivalent to phytoplankton cell division times were found to be important in controlling chl a biomass changes.