The temporal dynamics and spatial distributions of zooplankton in the tidal freshwater portion of the Hudson River were studied over a 3-yr period. We tested the hypothesis that advective transport regulates zooplankton biomass in the Hudson and in lakes, estuaries, and rivers for which we have published values. In the Hudson, zooplankton biomass was negatively correlated with discharge over the entire season (P < 0.0001) as well as during the warmer period of the year (P = 0.007) when biomass was greatest. The spatial distribution of zooplankton over 160-km transects was heterogeneous. Downstream changes in the abundance of a dominant species, Bosmina longirostris, indicate that certain areas of the river support net population growth whereas other areas are population sinks. We infer that zooplankton biomass in the Hudson is a function of the balance between reproduction determined by resources and losses due to advection. Zooplankton biomass differs among lakes, estuaries, and rivers in a manner consistent with the differences in water residence time. Biomass is highest in lakes, lower in saline estuaries and tidal rivers, and lowest in rivers. Advective losses appear to be important in explaining differences between planktonic communities in lentic and lotic environments.

Bacterial abundances, biomass, and production were measured over a 3-yr period at stations along a 15%km reach of the tidal, freshwater Hudson River. Bacterial abundances ranged from 1 to 10 x 1 O6 cells ml-' with maximal values in summer. Abundance and production averaged over all stations for the ice-free season (April through December) were 4.9 and 9.1 x lo9 cells liter-l dm', respectively, and both were significantly correlated with temperature. Neither bacterial abundance nor production showed significant spatial variability over the study reach. In contrast to the results from many autotrophic ecosystems, annual average bacterial abundances from different stations were not significantly correlated with algal standing stocks, and bacterial production was only weakly related to rates of primary production.Absolute rates of bacterial C production were greater than phytoplankton primary production, indicating that much of the bacterial secondary production in this portion of the river must be supported by nonphytoplanktonic organic C.

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...

Abstract. Measurements of suspended matter, particulate organic carbon and dissolved organic carbon were made over a three year period at stations spanning -150 km of the tidal freshwater Hudson River. Suspended matter concentrations varied from year-to-year and were not related to freshwater discharge. The increase in suspended matter with depth in vertical profiles suggests that, during medium to low flow conditions, resuspension of bottom sediments was as important a source of sediment as loadings from tributaries. Particulate organic carbon showed significant variability among stations, and both autochthonous primary production and detrital organic matter are contributing to POC standing stocks. Dissolved organic carbon represented over half of the total organic carbon in the water column and showed little variation among stations.Examining downstream changes in transport showed that there was significant production of both suspended matter and POC within the study reach during the ice-free season. Tributary loadings within the study reach do not appear to be the cause of these increases in downstream transport. Dissolved organic carbon behaved conservatively in that there was no evidence for net production or net consumption within the river.The spatial/temporal patterns and analyses of transport suggest that suspended matter and POC, but not DOC, were controlled to a significant extent by processes occurring within the river and were not simply related to loadings from outside.

ABSTRACT. Planktonic bacterial respiration (estimated from rates of O2 consumption) was the largest fraction of total planktonic respiration at a site in the tidal freshwater Hudson River, New York, USA.No simple relationship between respiration and bacterial production was found, indicat~ng considerable variability in bacterial growth efficiencies. Mean growth efficiency was 22 % but values ranged from c 10 to >S0 %,. Non-phytoplanktonic carbon is known to be important in supporting the high rates of planktonic bacterial productivity observed in the Hudson. Experimental additions of particulate detritus derived from the most common submerged macrophyte (Vall~sneria arnericana) and wetland plant (Typha angustifolia) to Hudson water did not result in increases in bacterial productivity. In contrast, additions of dissolved organic carbon (DOC) denved from these same plants consistently yielded large increases in bacterial production to rates several-fold greater than rates measured in the field. Growth efficiencies for DOC were 37 and 63 ' V" for T angustlfolia and V americana respect~vely.These measures of respiration, production and growth efficiency allow us to estimate the amount of allochthonous carbon necessary to support bacterial productivity Required carbon inputs range from 140 to 320 pg C 1-' d-' depending on the assumed conversion efficiency. These inputs a r e 3 to 6 x net carbon fixation by phytoplankton. The bacterial assim~lat~on of dissolved organic carbon results in DOC turnover times of 2 to 3 wk which are less than the residence time of water in the mid-Hudson during most of the year. The lack of local or downstream depletion of DOC indicates that there must be new sources of DOC within this region of the Hudson. Bacterial metabolism of terrestrially-derived organic carbon has the potential to alter the quantity and composition of material supplied to the ocean.