The impact of trophic interactions on rates of nitrogen regeneration and grazing in Chesapeake Bay

Abstract

We investigated the effect that zooplankton grazing had on NH 4 + regeneration during August 1991 in the mesohaline Chesapeake Bay, when NH 4 + regeneration was important in supporting phytoplankton production. We measured rates of NH 4 + regeneration and uptake by organisms in <15-, <63-, and <202-μm size fractions as a function of the density of the copepod Acartia tonsa, as well as rates of grazing on 5-20-μm particles by microzooplankton and copepods over a 24-h period. The primary regenerators of NH 4 + were organisms <15 μm; these organisms were strongly controlled by microzooplankton. Microzooplankton, in turn, were strongly controlled by the copepods. Thus, copepods appeared to affect the primary NH 4 + regenerators indirectly through trophic interactions with microzooplankton, relieving grazing pressure on or stimulating rapid growth of the primary regenerators.