Ingestion of phytoplankton by the micro- and mesozooplankton communities in a productive subtropical estuary

Grazing on phytoplankton by the micro- and mesozooplankton communitieswas measured during four cruises in a shallow (1.5 m) productive(up to 6 g C m^–2 day^–1 estuary in the northern Gulfof Mexico. Grazing-induced mortality on phytoplankton was alwayshigh and >95% of the grazing was by the microzooplanktoncommunity The grazing contribution from the mesozooplanktoncommunity, comprised primarily of Acartia tonsa, is believedto be small because populations were kept low by predation andadvective losses. A simple model is developed to describe phytoplankton-zooplanktoninteractions in this estuary. Attempts to understand the distributionand abundance of phytoplankton in estuaries must include estimatesof grazer-induced mortality on the phytoplankton.