Submersed aquatic vegetation (SAV) plays a critical role in many aquatic systems, contributing to primary productivity, nutrient cycling and sediment dynamics, as well as providing important habitat for fishes and invertebrates (Rozas and Odum, 1987; Heck et al., 1995). Aquatic macrophyte communities are some of the most productive natural ecosystems and have been found to support numerous and diverse wildlife populations

(Dennisonetal., 1993; Kemp, Boynton, andTwilley, 1984). More than twenty aquatic plant species are found in the SAV beds of the Hudson River (see Table 17.1, Fig. 17.1). In almost all areas, the native Vallisneria americana (wild celery, water celery, tape grass) is the predominant plant, occurring in over 90 percent of the benthic grabs containing plants. In the Hudson River, SAV occupies shallows and shoals where the water depth is less than 3 m (Moran and Limburg, 1986; Muenscher, 1937), and although their distribution is limited, they canform dense conspicuous beds (Menzie, 1979; Harley and Findlay, 1994), and achieve rates of primary production as high as 5.0 g O2 m-2 day-1 (Garritt and Howarth, 1988).

Aside from its value as habitat and its contribution to ecosystem primary productivity, the spatial extent and temporal stability of SAV have been used as indicators of water quality. For example, the resurgence of submersed macrophytes in the tidal Potomac River has been attributed to reduced nutrient inputs from sewage and concurrent improvements in water clarity (Carter and Rybicki, 1986). On the other hand, Short and Burdick (1996) documented a replacement of SAV with macroal-gae in response to increased housing density (and nutrient loads) in Waquoit Bay, Massachusetts. The mechanism behind SAV reduction appears to be the decreased light availability due to phytoplank-ton increase under high nutrient loads. A variety of otherplants (phytoplankton, epiphytic microalgae, and benthic macroalgae) increase following nutrient additions (Short, Burdick, and Kaldy, 1995) reducing growth of submersed vascular plants. There have been extensive efforts to quantify the habitat requirements (light, nutrient concentrations) associated with successful plant growth (Dennison et al., 1993; Stevenson, Staver, and Staver, 1993) and monitoring of SAV spatial extent has become a central component in assessments of estuarine "health."

This chapter reviews what is known about the distribution, abundance, and ecological functions associated with submersed vegetation in the tidal freshwater Hudson River. There is relatively little historical information to use as a baseline for assessing change in extent of SAV or potential changes in species composition.

Table 17.1. Plant species composition of SAV

beds on the Hudson River

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