The ecosystem balance concurs with the budget of the components that the respiration of heterotrophic organisms exceeds the input from phy-toplankton production. The budget implies that some keyheterotrophic organisms must utilize organic matter that was produced outside of the river. For example, the estimate of zebra mussel respiration alone is greater than the net input of organic matter from phytoplankton plus periphy-ton plus macrophytes. This is also true for pelagic bacteria. Clearly, organic matter of terrestrial origin is an important subsidy for the Hudson River food web. On the other hand, this balance does not imply that phytoplankton are unimportant. In fact, phytoplankton are likely a major resource for some components of the pelagic food web that lead to the production of some fish. In systems, like the Hudson, with multiple types of organic inputs and multiple primary producers, it is often difficult to tease apart the key pathways that lead to the production of important resource species. Stable C isotopes, which offer a hopeful approach in some ecosystems, are not a viable approach in the Hudson because the signals of the phytoplankton terrestrial inputs are nearly identical (see Caraco etal., 1998). Radiocarbon (14C) appears to be a more useful tracer since it was discovered recently that much of the terrestrial input is quite old (500 to 1,500 years before present; Raymond and Bauer, 2001; Cole and Caraco, 2001), but much work remains to be done. Current research, which traces the production of unique fatty acids from either phytoplankton or bacteria into consumers, has revealed that copepods and some larval pelagic fish are tightly coupled to phytoplankton rather than to terrestrial detritus (K. Limburg and N. Caraco, unpublished data). The decline in some benthic filter-feeding invertebrates in conjunction with the phytoplankton decline brought about by the zebra mussel is also consistent with the idea that phytoplankton were a key element of their nutrition. Future work needs to seek ways to further unravel the importance of phytoplankton to the food web and to key resource species.
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