Ammonium regeneration and grazing: Interdependent processes in size-fractionated nitrogen-15 ammonium experiments
Libert, P., M.; Miller, C., A.; Garside, C.; Roman, M., R.; Mcmanus, G., B.
Marine Ecology Progress Series 82(1): 65-74
We measured size-dependent NH-4+ regeneration by 15N 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 NH-4+ regeneration by total or by lt 202 mu-m plankton increased from winter to summer as a linear function of temperature. On the other hand, the mean NH14+ regeneration rate by lt 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 NH-4+ regeneration measured in the lt 10 mu-m fraction exceeded the rate measured in the lt 202 mu-m fraction by 8 to 1000 %. In other experiments in which size-fractionated plankton were maintained in large carboys for gtoreq 24 h, we also found that NH-4+ regeneration in the lt 10 mu-m fraction exceeded that in the lt 64 or lt 202 mu-m fractions. In the third type of experiment conducted, we artificially manipulated the density of copepods in unfiltered seawater and measured NH-4+ 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 NH-4+ regeneration increased to a maximum level, then decreased when additional copepods were added. From all of these experiments, we hypothesize that total NH-4+ 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.