IMPACTS OF ZOOPLANKTON GRAZING AND EXCRETION ON SHORT-INTERVAL FLUCTUATIONS IN CHLOROPHYLL-A AND NITROGEN CONCENTRATIONS IN A WELL-MIXED ESTUARY

Four summer field studies ranging in duration from 31 to 48 h were completed near the mouth of the well-mixed Newport River estuary, Beaufort, North Carolina U. S. A. Short-interval sampling (1-3 h) revealed a distinct diel fluctuation in chlorophyll a concentration, with an afternoon peak followed by a decline usually beginning several hours before sunset. During one study, abundances at three depths and in situ measurements of gut fullness of adult, female Acartia tonsa Dana, a common phytoplanktivorous copepod, were determined simultaneously with chlorophyll a measurements. Estimates of grazing by postnaupliar copepods greater than 75 mu m explained approximately 9% of the change in ambient chlorophyll a concentration during the large afternoon decline, 19% during the nocturnal decline. Short-interval sampling revealed that fluctuations in inorganic nitrogen concentration, unlike chlorophyll a concentration, were affected by tidal phase and runoff as indicated by NO//3** minus plus NO//2** minus concentrations.