NH4+ regeneration and grazing: interdependent processes n size-fractionated 15NH4+ experiments

Abstract

We measured size-dependent NH,' 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,' regeneration by total or by c 202 pm plankton increased from winter to summer a s a linear function of temperature. On the other hand, the mean NH,' regeneration rate by c 10 pm plankton was lowest in February, increased nearly 10-fold from February to April, then remalned virtually unchanged through the end of summer. In all seasons during which experiments were conducted, we observed on occasion that the rate of NH,' regeneration measured in the c l0 pm fraction exceeded the rate measured in the <202 pm fraction by 8 to 1000 %. In other experiments in which size-fractionated plankton were maintained in large carboys for 2 24 h, we also found that NH,' regeneration in the c10 pm fraction exceeded that in the <64 or c202 pm fractions. In the third type of experiment conducted, we artificially manipulated the density of copepods in unfiltered seawater and measured NH,' regeneration. In 5 out of 6 such experiments, we found that a s the numbers of copepods were increased to ca 20 I-', the measured rate of NH,' regeneration increased to a maximum level, then decreased when additional copepods were added. From all of these experiments, we hypothesize that total NH,' 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, b e 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.

Net nitrogen uptake and DON release in surface waters: importance of trophic interactions implied from size fractionation experiments

“…We infer from these results and simple measurements of community structure on the basis of size fractionation that the size and species composition of the community are important variables in determining the rate of DON release. Because of the lack of complete correlation between trophic structure and organism size (Glibert et al 1992), it is not possible to interpret these results completely on the basis of the chl a and PN information collected with these experiments. In order to test some of the hypotheses described above as explanations for the observed patterns of PN and DON production, and of the effect of different size fractions in increasing or decreasing rates, it may be necessary to focus more specifically on the actual species composition of the community.…”

“…These correlations are compelling evidence that DON release results from trophic dynamic interactions such as grazing and excretion, rather than from an experimental artifact. Glibert et al (1992) used size fractionation experiments to investigate the dependence of NH 4 + regeneration on grazing. They reported that NH 4 + regeneration rates in small fractions sometimes greatly exceeded the rates when the larger fraction, presumably containing the grazers, was present.…”

Inorganic and organic nitrogen cycling in Chesapeake Bay: autotrophic versus heterotrophic processes and relationships to carbon flux

“…Generally, microzooplankton are responsible for the bulk of NH,+ regenerated in a range of systems, including summer in Chesapeake Bay (e.g. Harrison 1978, Glibert et al 1992. However, the low C:N ratio of released DON we measured suggests that this material would be a substrate that would promote bacterial regeneration of NH4+ (Goldman et al 1987, Jargensen et al 1993.…”

Contrasting patterns of dissolved organic nitrogen release by two size fractions of estuarine plankton during a period of rapid NH/ consumption and N02-production

“…By 30 h, however, the ratios of DON release to NH,+ uptake had increased to 0.31 and 0.53 in the <202 pm and 1.2 pm plankton respectively. Rates of NH,+ regeneration have been shown to be higher in size-fractionated plankton samples, compared to whole plankton assemblages; the extent of this effect is in part a function of the density of the largest grazers (Glibert et al 1992).…”