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variables and should not be construed as cause and effect. There was no relationship between detrital POC (total POC minus the algal component) and either bacterial variable.

Experimental manipulations are commonly used to identify associations between presumptive resources and consumers. Large ecosystem manipulations are still fairly rare but in the Hudson the zebra mussel invasion provided an opportunity to examine the food web consequences of a major new filter-feeder capable of drastically reducing phytoplankton stocks. Prior to the zebra mussel invasion, correlational and budgetary analyses suggested that planktonic bacterial secondary production was, at best, weakly connected to carbon from phytoplankton (Findlay et al., 1991). The zebra mussel invasion provided a "natural" experiment to test the linkage and, in fact, bacterial abun-danceincreasedpost-zebramusselandproduction went up slightly (although not significantly). These observations confirm that these bacteria were not reliant on carbon fixed by phytoplankton, suggesting, by default, that growth may be linked to allochthonous carbon. Small scale bottle experiments (bacterial growth bioassays) have been used to examine use of POC and DOC from a number of specific plant materials and sources, such as wetland outwellings and tributaries (Findlay et al., 1992; Findlay et al., 1998b). These assays revealed that bacteria are able to grow at roughly equivalent rates on a wide range of sources including DOC from different submersed and emergent plants, various tributaries, and DOC exported by wetlands.

In order to metabolize the organic matter derived from these compositionally distinct sources, the planktonic bacteria differentially allocate their extracellular enzymes resulting in different enzymatic "fingerprints" for bacteria growing on the various sources under experimental conditions. In the Hudson itself these "fingerprints" are not spatially isolated and enzyme patterns are fairly similar along a 150 km reach. This homogeneity in enzymes maybe the result of (1) the overwhelming dominance ofone source (the DOC load at head of tide is by far the largest single source) or (2) the longitudinal mixing in the river is sufficient to disperse all the separate "point sources" along the reach such that all inputs are available for metabolism across large areas. It appears that planktonic bacteria downriver of what we call the tidal freshwater portion (i.e., south of Newburgh) may rely to a much greater extent on phytoplankton-derived organic carbon than allochthonous sources. In a detailed transect conducted during spring high flows (Sanudo-Wilhelmy and Taylor, 1999), bacterial abundance and growth were strongly correlated with Chl a in marked contrast to the up-river pattern. Moreover, the relationship observed downriver was as strong as the cross-system correlation between planktonic bacteria and phytoplankton documented by Cole, Findlay, and Pace (1988). Even more striking is the apparent switch in the spatial pattern in bacterial abundance and growth observed in the lower river and New York Harbor. While we have documented a gradual decline in bacterial numbers and thymidine incorporation between RKM 220 and 64 (Figs. 8.2 and 8.4), Sanudo-Wilhelmy and Taylor (1999) describe manifold increases in abundance and production over the reach from roughly 90 km above Manhattan to Sandy Hook, in lower New York Bay.

These contrasting correlations and strong spatial patterns suggest a dramatic switching in carbon sources andregulation in planktonic bacteria in the more saline portions of the estuary. Rates of phyto-planktonic primary production in the mesohaline Hudson are very high relative to other regions of the River and even appear to have increased in recent years (see Chapter 10). This large increase in available autochthonous carbon could represent an important resource for heterotrophic bacteria and generate strong spatial patterns in bacterial abundance.

One logical scenario consistent with these patterns is dominance of the upriver carbon supply and metabolism by the large allochthonous load delivered at the head of tide (Findlay et al., 1998b) which apparently overwhelms all the "point sources" such as wetlands and other tributaries providing carbon in the tidal freshwater reach. Previous estimates of metabolic carbon demand (Findlay et al., 1992 and see below) imply there should be depletion of metabolizable carbon in the lower reaches and bulk DOC concentrations decline over this reach (Findlay et al., 1996). The downriver declines in abundance and growth would be consistent with a gradual winding down of an allochthonously-driven microbial loop. Perhaps in the more saline portions of the estuary, the microbial loop is revitalized by local inputs of phytoplankton-derived carbon and as nutrients are delivered to the estuary and water clarity improves moving seaward, the traditional phytoplankton-bacterioplankton trophic link assumes predominance.

The ability of bacteria in the tidal freshwater portion of the Hudson estuary to metabolize a significant portion of the allochthonous load has been suggested and verified by a number of lines of evidence. Firstly, bacteria grow in bioassay experiments receiving DOC derived from various tributaries (Findlay et al., 1998b) at rates equal or greater than in water from the mainstem Hudson. Separate and independent estimates of carbon metabolism (respiration) and net heterotrophy (Cole and Caraco, 2001; Raymond, Caraco, and Cole, 1997; Howarth et al., 1996) clearly require metabolism of a significant fraction of the al-lochthonous carbon load to drive observed patterns in dissolved oxygen and CO2. Estimates of in situ bacterial growth coupled with their relatively low growth efficiencies (Findlay et al., 1992; Roland and Cole, 1999), allow estimates of what proportion of the allochthonous load is needed to fuel growth. Median bacterial production estimated from thymidine incorporation is 153 |g C L-1 d-1, which translates to 337 gC m-2 y-1, assuming 200 days of growth per year and a mean depth of 11 m for this reach. This value is large relative to estimated allochthonous loading (650 gC m-2 y-1; Howarth et al., 1996) and large relative to estimated system respiration 100-300 gC m-2 y-1 (see the chapter by Cole and Caraco). Given the uncertainties in all components of the budget it is probably safe to state that: (1) planktonic bacteria are responsible for a major fraction of system respiration, and (2) this metabolism requires degradation of a substantial proportion of the al-lochthonous carbon load.

Degradation of allochthonous dissolved organic carbon in a range of large river-estuarine systems has been documented through a number of independent lines of evidence. The budgetary approach outlined here is supported by shifts in the apparent age of DOC in transit and marked changes in 14 C ages and S 13 C strongly suggesting turnover of DOC components rather than simple conservative transport (Raymond and Bauer, 2001a and b). Aside from budgetary and tracer approaches, planktonic bacteria have shown rapid shifts in metabolism under changing carbon supply conditions with responses in extracellular enzymes as rapid as a few hours (Cunha, Almeida, and Alcantara, 2001) or days (Pinhassi et al., 1999). The capacity to rapidly shift degradative pathways implies that the diversity of carbon compounds entering estuaries (or produced at various points along estuaries) does not represent a fundamental obstacle to metabolism during transit. Bacterial communities can change in composition during downriver transport (Leff, 2000), providing a further opportunity to adjust degradative capacity. The relative contribution of allochthonous loading versus internal sources from phytoplankton production, floodplains (O'Connell et al., 2000), or wetland export will vary among river systems based on their relative abundance. Given reasonably long transit times (tens of days or more), planktonic bacteria via a number of mechanisms can access these carbon pools, which allows significant microbial growth and alteration of the DOC delivered to the oceans.

Studies of bacteria in the Hudson River revealed a large biomass linked to terrestrial carbon sources rather than the traditional dependence on phytoplankton-derived carbon. Their high growth rates and relatively low efficiency leads to bacterial respiration being a major fraction of organic matter mineralization. The broad capacity to acquire carbon from the diversity of sources entering the tidal Hudson allows high secondary production throughout the river and even a whole-system phytoplankton removal by zebra mussels, which did not depress bacterial growth. Spatial patterns of bacterial abundance and productivity suggest a switching from depleted terrestrially-derived carbon to a reliance on autochthonous primary production in the lower reaches of the Hudson River Estuary.

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