In the past twenty years, heterotrophic microorganisms have become widely recognized as integral parts of aquatic ecosystems that play important roles in food webs, nutrient transformations, and organic carbon budgets. Planktonic bacteria can serve as effective links in food webs from dissolved organic matter to larger organisms, although the degree of efficiency varies greatly among aquatic systems (del Giorgio and Cole, 2000).

Microbes growing on particulate and dissolved carbon can also act as significant sinks for inorganic nutrients derived from the water column (Caraco et al., 1998). In large river systems where primary production is low relative to external loadings (see chapters by Howarth, and Cole and Caraco) one would predict that the microbial loop (Pomeroy, 1974) wouldbe significant relative to the traditional macro-grazing food web. Dissolved organic carbon is the largest term in the organic carbon budget for the Hudson and links between this input, heterotrophic bacteria, and higher organisms in the food web could represent a significant energy basis for the system. In the tidal freshwater Hudson River we have documented moderately high abundances and rates of secondary production for the planktonic bacteria and estimated their contribution to organic matter metabolism. This chapter provides an overview of their abundance, spatial and temporal distributions, and metabolic processes. The greatest amount of information is available for the reach of the Hudson River from northern Haverstraw Bay (River km-RKM 64) to Castleton (RKM 228). Sampling there is restricted to the icefree season (usuallyApril through December). This chapter deals with the free-living, planktonic het-erotrophic bacteria in the main channel. The mi-crobial ecology of tidal marshes is covered by the chapter by Kiviat et al. Issues of human pathogens or sewage-derived indicator microorganisms are covered in the chapter by Brosnan et al.

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