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

We investigated the effect that zooplankton grazing had on NH4+ regeneration during August 1991 in the mesohaline Chesapeake Bay, when NH4+ regeneration was important in supporting phytoplankton production. We measured rates of NH4+ 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 NH4+ 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 NH4+ regenerators indirectly through trophic interactions with microzooplankton, relieving grazing pressure on or stimulating rapid growth of the primary regenerators.