Fate of bacterial production in a heterotrophic ecosystem: grazing by protists and metazoans in the Hudson Estuary

Abstract

The Hudson River Estuary IS a heterotrophic ecosystem with high rates of planktonic bacterial production. We measured grazing on bacteria in monthly experiments dunng 1990 to determine: ( l ) the prlmary consumers of bacteria; (2) whether absolute rates of grazing on bacteria were high relative to other systems; and (3) whether bacterla were a sufficient carbon resource for consumers. Fluorescent minicells were used to measure the Ingestion of bacteria by heterotrophic flagellates and ciliates. Using the abundances of bacteria, rotifers and cladocerans in the river, we also estimated zooplankton clearance rates. Total grazing of the plankton community was assessed by measuring the disappearance of fluorescent minicells over 24 and 48 h. Heterotrophic flagellates were the primary consumers of bacteria with ciliates and the cladoceran Bosmina longirostris also important seasonally. Ingestion of bacteria by heterotrophic flagellates, ciliates, and B. longirostris was sufficient at most times to satisfy estimated carbon requirements of these consumers. Large bacterial size and relatively low consumer populations in the Hudson account for the apparent sufficiency of bacteria as a carbon source We conclude that grazing on bacteria in the Hudson River is similar to the levels of grazlng observed in other systems. Nevertheless, bacteria are a potentially significant resource, because the domlnant planktonic consumers are capable of ~ngesting bacteria at rates sufficient to support carbon demands. INTRODUCTIONDepending on food web structure, bacteria may be either a link in food webs supporting metazoan production or largely a sink where bacterial production is respired by microorganisms (e.g. Ducklow et al. 1986, Pomeroy & Deibel 1986, Pace et al. 1990, Wylie & Currie 1991. Most investigations of bacteria in aquatic food webs have been carried out on the plankton of lakes and marine systems where bacterial production is typically 30 % of primary production on an area1 basis (reviewed by Cole et al. 1988). Many nearshore marine and estuarine systems, however, are heterotrophic in the sense that total system respiration exceeds primary production (Smith 1991). In these systems allochthonous organic matter may be an espe-'Addressee for correspondence O Inter-Research 1992 cially significant carbon source supporting bacterial production (Coffin & Sharp 1987, Painchaud & Therriault 1989. Under these circumstances, bacteria might be a particularly significant resource for higher consumers presuming bacteria can grow on allochthonous carbon.An example of such a strongly heterotrophic ecosystem is the Hudson River Estuary. Bacteria and phytoplankton are only weakly coupled in this system, and bacterial production exceeds primary production by 2-to 3-fold (Findlay et al. 1991). Total system respiration estimates are consistent with the measurements of bacterial production and the notion that respiration is far in excess of phytoplankton primary production (Howarth et al. 1992). The Hudson River Estuary, with relative...

Trophic coupling across the St. Lawrence River estuarine transition zone

“…The downstream reaches of large rivers have been referred to as heterotrophic ecosystems in which bacterial processes dominate the carbon and energy flux (Findlay et al 1991, Vaqué et al 1992. A broad range of organic substrates for bacterial production is available in the St. Lawrence transition zone including marsh-derived detritus, upstream-derived dissolved and particulate carbon and urban waste discharge (Painchaud & Therriault 1989).…”

Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa): a mesocosm study

“…In coastal eutrophic areas, the nature and availability of inorganic and organic nutrients may play important roles in the structure of bacterial communities. On the other hand, ample evidence exists that heterotrophic protists are generally the main grazers of bacterial biomass, in both oligotrophic and eutrophic waters (Vaqué et al 1992, Storm 2000. Nevertheless, many small and medium flagellates also have the potential to graze on autotrophic picoplankton (Kuosa 1990).…”

Dissecting the microbial food web: structure and function in the absence of autotrophs

“…Bacterivory of protists was evaluated by the rate of disappearance of fluorescent particles (Pace et al 1990, Vaqué et al 1992. Duplicate 1 l samples were transferred into polycarbonate bottles with a known concentration of tracers (DTAF-stained Escherichia coli minicells, provided by E. Vázquez).…”