these large losses may have important ecological ramifications.

Third, the habitat occupied by benthic animals determined their response to the zebra mussel invasion. The zoobenthos of deep-water (>3 m deep), unvegetated, soft-bottom habitats declined sharply, while the zoobenthos of shallow-water, vegetated, soft-bottom habitats did not change (Fig. 19.4, bottom). Together, trophic group and habitat accounted for 51 percent of the variation in the response of benthic species to the zebra mussel invasion (Strayer and Smith, 2001).

It appears that loss of planktonic food, especially phytoplankton, was responsible for the large effects of the zebra mussel invasion on the Hudson's zoobenthos. Several pieces of evidence point to this conclusion: (a) benthic animals that feed on plankton declined, while those that feed on benthos did not (Fig. 19.4); (b) the body condition (body mass for a given body length) of unionid mussels declined, suggesting that they were receiving insuf-ficientfood (Strayer and Smith, 1996); and (c) population declines and body conditions of unionid mussels (which eat plankton) were uncorrelated with fouling rates by zebra mussels, suggesting that exploitative competition (rather than interference competition) was involved (Strayer and Smith, 1996). Thus, even though phytoplankton production forms only a small part of organic matter inputs to the Hudson, it appears to be of key importance in supporting higher trophic levels.

Figure 19.4 Effects of the zebra mussel invasion on the macrobenthos of the freshwater tidal Hudson River. Upper. Biomass of various parts ofthe community before and after the invasion. Middle. Effect of the zebra mussel invasion on populations of benthic animals in the freshwater tidal Hudson River according to trophic group. Each point represents the change in density ofataxon (usually a species) between 1990-92 and 1993-97 (animals other than unionids) or 199399 (unionids). The mean change for planktivores was significantly different than that for benthivores (t-test, p < 0.0001). Lower. Mean densities of all macrobenthos at deep water (black circles) and shallow water (white circles) stations before and after the zebra mussel invasion in the Hudson River. The interaction between habitat and the zebra mussel invasion is significant (p < 0.02). Based on Strayer and Smith (2001).

Table 19.3. Outputs of organic carbon from the freshwater tidal Hudson River. Because the different terms in the budget were estimated at different times and using different methods and assumptions, the overall budget is very approximate.

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