Production, consumption and nutrient cycling in a laboratory mesocosm

Abstract

A mesocosm experiment was conducted in which estuarine plankton population dynamics and rates of production, consumption, and nitrogen uptake and release were compared for 13 d in 400 1 vats with and without additions of the diatom Thalassiosira weissflogii and the copepod Acartia tonsa. In the vats with copepods, both phytoplankton biomass and production were reduced by grazing whereas in the vats without copepods, nutrient limitation reduced both phytoplankton production and standing stock. Bacteria dominated both estimated heterotrophic carbon biomass and production early in the experiment. However toward the end of the experiment, the biomass of zooplankton was often greater than the combined carbon pools of phytoplankton, bacteria and heterotrophic flagellates. Highest release rates of ammonium and dissolved free amino acid nitrogen occurred in the vats with copepods. Highest bacterial growth rates also occurred in the presence of copepods suggesting that grazing activity and copepod excretion stimulated the growth of bactena. An Inverse relation between the biomass of copepods and flagellates suggests that the copepods consumed significant quantities of heterotrophic nanoflagellates. Estimated growth rates of flagellates were highest in the vats with copepods. Our results demonstrate that the presence oi copepoas can result in more eiiicient nutrient recycling, higher primary production/chlorophy11 and greater production by bacteria and heterotrophlc nanoflagellates.

Zooplankton activity fueling the microbial loop: Differential growth response of bacteria from oligotrophic and eutrophic waters

“…This finding is consistent with results from other studies on seasonal plankton structures and bacterial uptake of DOM conducted in various environments (Roman et al 1988;Vadstein et al 1989). Those investigations concluded that zooplankton grazing may enhance the flow of C or N to the dissolved and bacterial size fraction, being comparable in magnitude to phytoplankton exudation.…”

“…Bacteria are thought to be supplied directly with organic substrate from phytoplankton via exudation or through lysis of cells (Pomeroy 1974;Azam et al 1983). However, there is some indication that organic compounds released by zooplankton may also provide a substantial source for bacterial growth (Eppley et al 198 1;Roman et al 1988;Vadstein et al 1989). At least three ways of losing dissolved organic material (DOM) from zooplankton are proposed: release from fecal material, excretory release, and breakage of prey during feeding (sloppy feeding).…”

The importance of zooplankton‐protozoan trophic couplings in Lake Michigan

“…Stoecker and Egloff 1987). Some field studies showed that a natural assemblage of microzooplankton (ciliates and zooflagellates) comprised a large portion of the C ingested by estuarine copepods (Gifford and Dagg 1988) heterotrophic flagellates could be important food for copepods, particularly at lower phytoplankton densities (Roman et al 1988). In addition, a model of grazing control of phytoplankton in the open subarctic Pacific Ocean could be balanced only by assuming that macrozooplankton are omnivorous and graze on both phytoplankton and protozoa (Frost 1987).…”

Size-fractionated measurements of nitrogen uptake in aged upwelled waters: Implications for pelagic food webs

“…Evidence exists to suggest that ciliates, rotifers (e.g. Stoecker and Egloff 1987;Gifford and Dagg 1988), and heterotrophic nanoflagellates (Roman et al 1988) may at times be an important and preferred component of copepod diet.…”