Assessing the phytoplankton-heterotrophic bacteria link in the eutrophic Delaware Estuary.

摘要

The microbial loop concept suggests heterotrophic bacteria play a central role in mediating the flux of organic matter in marine systems. Based on the contribution of phytoplankton and bacteria standing stocks to the particulate organic carbon pool in estuaries, it has been suggested that the microbial loop is of diminished importance in these environments. I employed dual-labeling with 13C/15N tracers as an alternative to more routine estimates of primary production using 14C. Extending these techniques in size-fractionation experiments, I was able to examine the importance of the estuarine microbial loop by quantifying the instantaneous transfer of autochthonous production to heterotrophic bacteria. The results suggest that the instantaneous transfer of autochthonous C and N to bacteria varies spatially in the Delaware Estuary. Carbon and nitrogen transfer rates are low and similar (ca. 3% of phytoplankton uptake) in the upper estuary. In the lower estuary, C and N transfer increase and become unlinked, with C transfer averaging at most 10% and N transfer rates twice that of carbon transfer. Inorganic nitrogen supports 80--100% of bacterial N demand in the upper estuary while autochthonous dissolved organic nitrogen may support >85% of bacterial N uptake in the lower estuary. Nitrate uptake by bacteria was generally low throughout the estuary, even though ambient NO 3- concentrations can be high in Delaware Estuary (ca. 80--100 muM). I examined the influence of nitrogen species (either NH4+ or NO3-) on phytoplankton growth, DOM release, and bacterial use. Phytoplankton grown on NO3- showed ca. 2-fold higher bicarbonate uptake compared to phytoplankton supplied with NH4+; nitrogen uptake rates were similar for both inorganic N substrates. The increased C uptake seen in NO3--supported growth was accompanied by increases in DOC (ca. 25% of primary production). Bacteria assimilated a fraction of the DOC produced, equivalent to ca. 5% of primary production. There was no evidence of autochthonous DOM uptake by bacteria in the NH4+ treatment. These results suggest that the species of inorganic nitrogen that is available for phytoplankton growth may influence estuarine trophic structure and the importance of the estuarine microbial loop.