NH4+ REGENERATION AND GRAZING - INTERDEPENDENT PROCESSES IN SIZE-FRACTIONATED (NH4+)-N-15 EXPERIMENTS

We measured size-dependent NH4+ regeneration by N-15 isotope dilution on a seasonal basis in the plume of the Chesapeake Bay estuary (USA) using several different approaches. In our short-term (1 h) size-fractionation experiments, the mean rate of NH4+ regeneration by total or by < 202-mu-m plankton increased from winter to summer as a linear function of temperature. On the other hand, the mean NH4+regeneration rate by < 10-mu-m plankton was lowest in February, increased nearly 10-fold from February to April, then remained virtually unchanged through the end of summer. In all seasons during which experiments were conducted, we observed on occasion that the rate of NH4+ regeneration measured in the < 10-mu-m fraction exceeded the rate measured in the < 202-mu-m fraction by 8 to 1000 %. In other experiments in which size-fractionated plankton were maintained in large carboys for greater-than-or-equal-to 24 h, we also found that NH4+ regeneration in the < 10-mu-m fraction exceeded that in the < 64 or < 202-mu-m fractions. In the third type of experiment conducted, we artificially manipulated the density of copepods in unfiltered seawater and measured NH4+ regeneration. In 5 out of 6 such experiments, we found that as the numbers of copepods were increased to ca 20 l-1, the measured rate of NH4+ regeneration increased to a maximum level, then decreased when additional copepods were added. From all of these experiments, we hypothesize that total NH4+ regeneration in a natural food web can be significantly different from that measured in component size fractions, and that this effect is a nonlinear function of zooplankton density and/or multiple trophic interactions. These effects may, in addition, be a function of internal cycling of isotopically labelled substrate. This hypothesis implies that commonly used size-fractionation techniques may be insufficient or inappropriate for describing the true regeneration rates of different size classes in situ.