Hudson estuaries lakes
Figure 19.6. Biomass of zooplankton and zoobenthos in large rivers, the freshwater tidal Hudson River, estuaries, and lakes. Boxes show25th and 75th percentiles (horizontal line is the median), whiskers show 5th and 95th percentiles, and dots show outliers. For zoobenthos, sample sizes are as follows: large rivers (10), Hudson River (2; i.e., pre- and post zebra mussel invasion), estuaries (23), and lakes (41). Zooplankton data from Pace, Findlay, and Lints (1992); zoobenthos data compiled from various sources.
that benthic biomass is especially high in estuaries (Fig. 19.6). Perhaps estuaries support higher benthic biomass than lakes because estuaries have greater inputs of physical energy (especially tidal currents), which leads to better vertical mixing and higher rates of food supply to the sediments (Nixon, 1988). The beneficial effects of physical energy may be reduced in rivers because of high temporal variance in energy supply rates, leading to scour, fill, and disturbance of the benthos. Further, food quality may be lower in rivers than in estuaries because of greater relative inputs of detrital allochthonous material of low nutritional quality. Thus, although site-to-site variation will be high, zoobenthos/zooplankton ratios might be highest in rivers, intermediate in estuaries, and lowest in lakes.
Traditionally, the ecological communities of the sediments and open water are considered separately, probably because the different habitats
are studied by different groups of scientists using different methods. Nevertheless, connections between the benthos and the overlying water of the Hudson are numerous and strong (Fig. 19.7). Benthic suspension-feeders, especially bivalves, can regulate the amount and kind of plankton (Dame, 1996; Strayer et al., 1999), as was shown most clearly by the zebra mussel invasion of the Hudson (Cole and Caraco, Chapter 9, this volume; Pace and Lonsdale, Chapter 16, this volume). The benthic animal community in turn depends on the amount and kind of plankton as a key food source. Seasonal, interannual (Johnson, Bostrom, and van de Bund, 1989), or long-term changes in the plankton can cause large changes in the zoobenthos. In the Hudson, the removal of edible suspended particles by zebra mussels led to large changes in the zoobenthos (Fig. 19.4). Ben-thic plant communities likewise depend on the amount of suspended particles, which regulate the amount of light that penetrates to the sediments. A concrete example of this link was the possible increase in rooted plants (Caraco et al., 2000; Findlay et al., Chapter 17, this volume) and associated animals (Fig. 19.4) after zebra mussels reduced plankton biomass in the Hudson. Further, rooted plants may negatively influence phytoplankton, through a complex series of interactions (Scheffer, 1998). Finally, as shown in Table 19.1, benthic prey dominates fish diets in the Hudson, so that there maybe strong reciprocal links between fish and zooben-thos in aquatic ecosystems (e.g., Strayer, 1991).
Thus, manyaquatic ecosystems, especiallyshallow, well mixed habitats like the Hudson, function more as unified systems than as the isolated boxes suggested by compartmentalized research studies and textbook diagrams.
I appreciate the dedicated assistance of Chris Anderson, Chris Borg, Karyl Brewster-Geisz, David Cohen, Chris Edelstein, David Fischer, Dean Hunter, Jeff Janota, Carolyn Klocker, Craig Jankowski, Greg Lampman, Colleen Lutz, Heather Malcom, Erik Molinaro, Alex Nixon, Elizabeth Pangia, Sarah Poppenhouse, Bill Shaw, Lane Smith, Martha Young, and Brian Zielinski, and the continued intellectual support of my colleagues Nina Caraco, Jon Cole, Stuart Findlay, and Mike Pace. Stuart Findlay offered helpful comments on the manuscript. I am grateful to the Hudson River Foundation and the National Science Foundation for their support of my work on the Hudson's zoobenthos. This is a contribution to the program of the Institute of Ecosystem Studies.
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