sav distribution and bed characteristics
A first attempt to map the extent of SAV in the Hudson was initiated in 1995. A Geographical Information System (GIS) coverage derived from aerialphotography was used to assess several characteristics of SAV beds along the reach of the Hudson from Hyde Park to Castleton. The submersed beds, predominantly Vallisneria americana and the floating leafed exotic Trapa natans were mapped as separate patch types. The extent of SAV varies markedly along the river with as much as 17 percent of the river area occupied by plant beds in the Cementon-Saugerties reach (Fig. 17.2). For this stretch of the Hudson, the percent cover by SAV averages 13 percent. For the stretch from Troy to Piermont the percent cover is about 6.5 percent of the river surface area for Vallisneria and 2 percent for Trapa.
Biomass of Vallisneria in the Hudson River ranges as high as 550 g dry mass m-2 with signifi cant variability among sites. Mean standing stock based on about fifty Ponar grabs was 142 g dry mass m-2 with a standard deviation of 152. The median area of Vallisneria beds (3,072 m2) was substantially larger than the median area of Trapa (1,300 m2) and there are roughly three times as many Vallisneria beds as patches of Trapa. Consequently, the area covered by the native water celery is about three times the area covered by water chestnut.
The limit to distribution of Vallisneria is almost certainly light availability and given the relatively turbid nature oftheHudson River, maximum depths are generally less than 3 m. Vallisneria is the predominant species of submersed plant, possibly because of its ability to maintain photosynthesis at low light levels. Harleyand Findlay (1994) found that Vallisneria produced more oxygen per gram of leaf material at light levels below about 10 percent of full sun than either Myriophyllum or Potamogeton. When comparing Myriophyllum spicatum (L) and V. americana (Michx.), Titus andAdams (1979) found that V. americana is a much more efficient fixer of carbon at low light intensities. In another study, V. americana was the most shade-adapted of five submersed macrophytes (Meyer et al., 1943). This light-harvesting ability together with significant belowground biomass of vegetative and storage structures may contribute to Vallisneria's ability to persist and thrive under relatively poor light conditions and in habitats subject to considerable wave and current stress. One species able to outcompete V. americana in the shallows of the river is Trapa natans. This exotic species concentrates its biomass at the surface in a dense canopy of floating rosettes, which allows for increased light availability for the leaves and dense shade beneath the canopy. It seems likely (although untested) that areas currently occupied by T. natans were previously occupied by V. americana.
Primary production. The ability of any plant in the turbid Hudson to fix carbon and produce oxygen will be largely related to light availability (see chapter by Cole and Caraco). Water masses traversing SAV beds are known to be significantly super-saturated with oxygen (Garritt and Howarth,
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