Figure 17.2. Percent cover by the native submersed Vallisneria and exotic Trapa. X-axis labels refer to USGS topographic quadrangles arranged from the south end of the study reach near Hyde Park to the north end near Castleton (Hyde Park, Kingston East, Saugerties, Cementon, Hudson South, Hudson North, Ravena, and Delmar). Water-chestnut was not present in the Delmar quadrangle.
1988; Carter, Rybicki, and Hammerschlag, 1991; Barrett and Findlay, 1993) suggesting these plants may be significant in river-scale primary productivity. For instance, continuous measurements of dissolved oxygen in several SAV beds shows that these sites are super-saturated with oxygen approximately 30 percent of the time and O2 concentrations can be as high as 150 percent of saturation (Fig. 17.3). Recent modelling efforts based on photosynthesis-irradiance relationships for both phytoplankton and submersed plants show that oxygen production by SAVis a large component of midsummer oxygen budgets (see Cole and Caraco, chapter 9, this volume). The relative importance of submersed macrophytes as primary producers has increased dramatically since the zebra mussel invasion (1992). Prior to 1992, SAVNet Primary Production (NPP) and Gross Primary Production (GPP) were roughly 10 percent of phytoplankton NPP and GPP. Since the zebra mussels (see chapter by Strayer) have dramatically depressed phytoplankton, SAV NPP and GPP are currently about half phytoplankton values.
sav contribution to food webs and metabolism
The stable carbon signatures of several important consumers in the mid-Hudson are intermediate between carbon signatures of SAV and phytoplankton, suggesting a mixed diet (Caraco et al., 1998). Whether this has always been the case or is a recent shift due to zebra mussel depletion of the phytoplankton food resource (Caraco et al., 1997) remains an open question. The reduction in phytoplankton primary production together with the modest increase in water clarity (Caraco et al., 1997) suggests that both the relative and absolute importance of SAV are greater now than in the recent past. Benthic invertebrates in the Hudson River appear to have been insulated from the loss of phytoplankton and benthos density in shallow vegetated habitats has increased in the past decade (see chapter 19, this volume). Perhaps the availability of organic matter from submersed plants has acted to buffer the large decline in phytoplankton allowing many shallow water benthic invertebrates to maintain or increase their population.
effects of sav on suspended sediments
As the largest sessile organisms in these communities, macrophytes can have quite an impact upon their physical environment. One of their most important effects is the reduction of water velocity through a bed (Carpenter and Lodge, 1986; Losee and Wetzel, 1988; Rybicki et al., 1997) commonly resulting in increased rates of sedimentation and decreased resuspension of fine grain sediments (Kenworthy, Zieman, and Thayer, 1982; Kemp et al., 1984; Ward, Kemp, and Boynton, 1984; Posey, Wigand, and Stevenson, 1993). Riverine beds of aquatic vegetation have been shown to act as a sieve, retaining suspended parti-culates and hastening the decomposition of trapped allochthonous organic matter (Kenworthy et al., 1982; Fisher and Carpenter, 1976; Fonseca and Fisher, 1986; Rybicki et al., 1997). Sediment
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