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+ regene ration during August 1991 in the mesohaline Chesapeake Bay, when NH4regeneration was important in supporting phytoplankton production. We measured rates of NH4+ regeneration and uptake by organisms in <15-, < 63-, and <202-mu m size fractions as a function of the density of the copepod Acartia tonsa, as well as rates of grazing on 5-20-mu m partic les by microzooplankton and copepods over a 24-h period. The primary r egenerators of NH4+ were organisms <15 mu m; these organisms were stro ngly controlled by microzooplankton. Microzooplankton, in turn, were s trongly 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 ra pid growth of the primary regenerators.