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Each approach estimates the difference between annual GPP and annual R and is a minimum estimate of the amount of terrestrially derived organic Cthat must be respired within the river. The approaches and their limitations are explained in the references. See also Findlay et al., 1998. The diel O2 from Howarth et al., 1996 includes some data in the post-zebra mussel period. It is not included in the mean.

Each approach estimates the difference between annual GPP and annual R and is a minimum estimate of the amount of terrestrially derived organic Cthat must be respired within the river. The approaches and their limitations are explained in the references. See also Findlay et al., 1998. The diel O2 from Howarth et al., 1996 includes some data in the post-zebra mussel period. It is not included in the mean.

tidal-freshwater river is not the dominant source of organic matter. Further, respiration of phytoplank-ton themselves greatly reduces the amount of organic C of phytoplankton origin that is available to consumers (Table 9.1). In the post-zebra mus-selperiodprimaryproductionfrom aquatic macro-phytes is comparable to that of phytoplankton, especially if we consider the net input (GPP-plant respiration). In both periods, loading of dissolved and particulate organic matter from the watershed is by far the dominant input, and advective losses downstream the dominant output. The net balance shows that the respiration of the known components in the river exceeds primary production, and this difference is more pronounced during the zebra mussel period. Thus, during the pre-1992 period, respiration exceeded GPP by about 43 g C m-2 y-1 and by about three times this amount (153 g C m-2 y-1) post 1992. The R in excess of GPP must be supported by the respiration of some of the vast amount of organic material loaded from the watershed.

If it is true that the sum of the components of heterotrophic respiration exceed net primary production, we should be able to measure this net heterotrophy at the ecosystem scale. For example, if R exceeds GPP, the system will be under-saturated in dissolved O2, and O2 will invade from the atmosphere. Similarly the system will be supersaturated in CO2 and CO2 will evade to the atmosphere. Both these conditions are true for the

Hudson (Raymond, Caraco, and Cole, 1997; Caraco et al., 2000; Cole and Caraco, 2001). Estimates of these net gas balances are shown in Table 9.2 and show at least broad agreement to the balance of the known components. While there is a good deal of uncertainty in each individual estimate, all of them indicate that the tidal, freshwater Hudson is net heterotrophic and magnitude is not extremely far from that based on the sum of the components that we outlined in Table 9.1.

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