Copepod grazing and the fate of phytoplankton in the northern Gulf of Mexico

Abstract

Ingestion of phytoplankton by the copepod community was measured during spring (May 1992) and late summer (September 1991) at two sites in the northern Gulf of Mexico. During both studies, influence of the Mississippi River was more evident at the plume site, located near the mouth of the Mississippi River, than at the mid-shelf site, located farther from river discharge. with the exception of the plume site during spring, the copepod community ingested 14–62% of the daily phytoplankton production. The small transfer (4–5%) of phytoplankton to copepods in spring at the plume site is attributed to high river discharge, which stimulates phytoplankton production to very high rates before the copepod community can numerically respond sufficiently to take advantage of this bloom. This response is also slowed by cooler temperatures in the spring (22°C) compared to the late summer (29°C). In addition to copepods, larvaceans were abundant at all study sites. Indirect estimates of their nutritional demands indicate they were important grazers. The copepod community consumes a significant fraction of the phytoplankton production that is stimulated by riverine nutrient inputs in this region.