Influence of Phagotrophic Protistan Bacterivory in Determining the Fate of Dissolved Organic Matter (DOM) in a Wetland Microbial Food Web

Abstract

A field study over an annual cycle in a shallow, wetland pond dominated by Nymphaea odorata was combined with laboratory experiments to determine the fate of DOM from aquatic macrophyte primary productivity in the microbial food web. DOM concentrations, bacterial abundance and productivity, and protistan abundance were more than an order of magnitude higher during warm months (water temperatures >20°C) than during cool months (<20°C). Out-of-phase oscillations between microbial populations suggested protistan bacterivore grazing pressure affected bacterial abundance during the warm months. Microcosm experiments designed to mimic warm-month conditions using wetland pond microbial communities showed similar rates of macrophyte leachate uptake by both grazed and ungrazed bacteria. However, in the presence of heterotrophic nanoflagellates, bacterial biomass accumulation was less than one-half of that in treatments without bacterivores, and calculated carbon mineralization was fivefold greater in the treatment with bacterivores. No difference in amounts of NH₄-N between treatments was detected, nor were decreased rates of soluble reactive phosphorus (SRP) different between experimental treatments. Experimental results indicated uptake of DOM from N. odorata by aquatic bacteria was rapid and could support the microbial food web in this system. However, grazing of bacterioplankton by nanoflagellates under experimental conditions did not result in significant increases in protistan biomass. The majority of bacterial productivity was respired in the presence of grazers rather than being incorporated into grazer biomass.