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2m 3m

Depth Class

Figure 17.4. Suspended sediment concentrations collected when drifting from deep waters over vegetated shallows. (Class 1 is less than 2 m deep and occupied by dense SAV; Class 4 is >5 m and unvegetated.)

2m 3m

Depth Class

Figure 17.4. Suspended sediment concentrations collected when drifting from deep waters over vegetated shallows. (Class 1 is less than 2 m deep and occupied by dense SAV; Class 4 is >5 m and unvegetated.)

dynamics are of particular interest in a turbid system, such as the Hudson, where light limitation controls growth of phytoplankton and submersed vegetation (Malone, 1977; Moran and Limburg, 1986; Cole, Caraco, and Peierls, 1992; Harley and Findlay, 1994). Moreover, the dynamics of suspended sediment affect the distribution and trophic availability of particle-associated contaminants.

It is now obvious that suspended sediment dynamics during low-flow conditions (June-October of most years) are dictated by the likelihood of local resuspension and deposition (Findlay, Pace, and Fischer, 1997). In general, submersed vegetation is expected to act as a baffle, reducing local concentrations of suspended sediment (e.g., Posey et al., 1993). In shallow areas of the Hudson, even those with dense SAV beds, there is a marked increase in suspended sediment concentrations as water masses traverse SAVbeds (Fig. 17.4) (see also Barrett and Findlay, 1993), suggesting vegetated areas do not function uniformly as sediment traps. There was also considerable variability amongbeds in both suspended sediment concentration and dissolved oxygen, suggesting that factors such as plant density, species composition, or actual location (east, west, or mid-channel) mayinfluence the degree to which SAV functions as a sediment trap.

Macrophyte communities in the Hudson seem to have a variable impact on suspended matter, and do not conform to models from other systems. Differences in plant density, species composition, and shear stress may account for variation within the Hudson, as well as among systems. We suggest that the factor affecting the impact of SAV on near-shore sediment dynamics will be the relationship between the critical depth for wave-driven resuspension and the maximum depth of SAV colonization. In turbid systems, light limitation may preclude SAV colonization of areas shallow enough to be susceptible to critical erosion stress. In these cases, macrophyte patches cannot mitigate the occurrence of resuspension until a water mass reaches vegetated depths. In less turbid systems SAV might colonize greater depths, and abundant vegetation at depths of critical erosion stress will decrease the likelihood of resuspension (c.f. Ward et al., 1984). This interplay between these two critical depths may well explain intersystem variability in the effects of SAV on suspended sediment.

SAV and Nutrient Interactions

Most research on nutrients and submersed vegetation has focused on excess nutrient supply and detrimental effects on water clarity and epiphyte growth (e.g., Short and Burdick, 1996; Dennison et al., 1993) or potential limitation of plants by sediment porewater nutrient pools (e.g., Wigand, Stevenson, and Cornwell, 1997). In the relatively high nutrient conditions prevalent in the Hudson (see Cole and Caraco, chapter 9, this volume) both phytoplankton and epiphytes are likely light-limited and so fluctuations in nutrients will have less effect on light penetration. Moreover, SAV in the Hudson are not necessarily dependent on long-term nutrient stores in the sediment nor are they nutrient-limited. Nitrogen and phosphorus concentrations of leaf tissue exceed critical threshold limits by 100 percent or more (Wigand et al., 2001). Although SAV are reported to most often rely on the sediment for nutrient acquisition (Carignan and Kalff, 1980), it appears that SAV in the Hudson may incorporate nutrients from the overlying water as well. The accumulation of al-lochthonous particulates in grassbeds and subsequent microbial processing in the overlying water may result in a source of nutrients for V. americana. In fact, recent research in the Hudson River has shown that microbial assimilation of dissolved inorganic nitrogen (DIN) is enhanced in the river because of high loads of both terrestrial organic matter and DIN (Caraco etal., 1998). Concentration of these allochthonous particulates in the grass-beds and subsequent DIN mineralization would allow for a single particle to repeatedly transport nutrients to areas of SAV. Other data in support of V. americana usage of newly distributed and rem-ineralized organic material is that stable 15N isotope analysis of leaves (i15N = 8) collected from the field show a signal intermediate between the overlying water seston (i15N = 10-12) and sediment (i15N = 4.5) (Caraco et al., 1998). In contrast, lab-grown V. americana leaves (i15N = 6.5) have a lowers 15N signal, which is closer to sediment values (i15N = 4.5) (Wigand et al., 2001).

Beds of V. americana appear to enrich rather than deplete nutrient porewater pools in the turbid mid-Hudson River with higher porewater concentrations of ammonium and phosphate inside plant beds compared to bare sediment (Wigand et al., 2001). Also, porewater N and P concentrations are maximal in summer when plant biomass is high again, suggesting that plant demand is insufficient to draw down available nutrient pools in the sediments. Porewater nitrogen pools could be enriched by the deposition of fine, organic particulates and subsequent mineralization in the sediment. In tidal rivers there may be large inputs of high-nutrient particulate material from upstream and tidal currents might redistribute these allochthonous materials across broad areas. The process whereby particulates are intercepted and trapped in some beds may be attributed to the slowing of currents in grassbeds with the rise and fall of the tide (Rybicki et al., 1997). Nutrient exchange and retention in grassbeds results from the interplay of physical forces (i.e., tides; currents), the structure of the grassbeds (i.e., canopy; understory), and biological processes (i.e., mineralization; root and leaf uptake).

Particulate mineralization could provide from 50 to 100 percent of nutrients necessary to sustain SAV in the Hudson depending upon the presence or absence of zebra mussels (Wigand et al., 2001). Particulate mineralization processes prior to the zebra mussel invasion could provide for most of the estimated plant nutrient demand. Since the invasion of zebra mussels and the subsequent reduction in phytoplankton biomass (Caraco et al., 1997), particulate mineralization could only provide about

50 percent of SAV nutrients (Wigand et al., 2001). However, in the presence of large populations of zebra mussels, the transfer of mussel feces and pseudofeces to shallow areas might provide an additional nutrient-rich and labile organic substrate which could fuel mineralization in the grassbeds (Strayer et al., 1999). In addition, soluble reactive phosphorus in the Hudson increased after the zebra mussel invasion, presumably due to lowered pressure on the dissolved phosphorus pools by the reduced phytoplankton stocks (Strayer et al., 1999). These additional nutrient sources could help support SAV growth since the zebra mussel invasion.

Although we have not measured the nutrient transfer due to the decay of the highly labile leaves of V. americana and other submersed macrophytes in the Hudson, we suspect that similar to other systems (Carpenter, 1980; Smith and Adams, 1986) a nutrient pulse following the decay of sloughed leaves in the fall could fuel pelagic phytoplankton, bacteria, and benthic animals (e.g., Cheng et al., 1993, see Strayer, chapter 19, this volume). Mass loss from blades of Vallisneria was extremely rapidin a microcosm study (~ 3%/day; Bianchi and Findlay, 1991) and would presumably be at least that rapid in the more turbulent river. Therefore, any nutrients remaining in the aboveground portion will be rapidly returned to the water column following plant senescence.

SAV as Habitat invertebrate use of sav

SAV beds may be an important habitat for invertebrates, typically containing higher densities and diversities of invertebrates than an equivalent area of unvegetated sediments (Cyr and Downing, 1988a, b; Chilton, 1990; Thorp, Jones, and Kelso, 1997). However, there have been relatively few studies focusing on the role shallow water vegetation beds play in supporting macroinvertebrate communities on the Hudson River (Feldman, 2001;Lutz and Strayer, 2000; Strayer and Smith, 2000; Findlay, Schoeberl, and Wagner, 1989; Menzie, 1980). Menzie (1980) found high densities of macroinver-tebrates within an SAV bed (M. spicatum) in Bowline Pond with almost two orders of magnitude greater biomass of chironomids found within the

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Bare Sed V. americana T. natans ms (x0.1)

Figure 17.5. Abundance of invertebrates on plants (gray portion ofbar) or in sediments under plants comparing different plant species and a river-wide mean for deeper bare sediments. Letters above bars refer to data source [a = Simpson et al., 1986, b = Strayer and Smith 2000,c = Lutz and Strayer 2000, d = Findlay et al., 1989, e = Menzie 1980 who used a smaller mesh size (0.12 mm)].

Bare Sed V. americana T. natans ms (x0.1)

Figure 17.5. Abundance of invertebrates on plants (gray portion ofbar) or in sediments under plants comparing different plant species and a river-wide mean for deeper bare sediments. Letters above bars refer to data source [a = Simpson et al., 1986, b = Strayer and Smith 2000,c = Lutz and Strayer 2000, d = Findlay et al., 1989, e = Menzie 1980 who used a smaller mesh size (0.12 mm)].

SAVbed than in unvegetated deeper waters. Findlay et al. (1989) also found high densities of invertebrates in T. natans in Tivoli South Bay. In the freshwater tidal portion of the estuary, Strayer and Smith (2001) found a large difference in the invertebrate community between vegetated, shallow water habitat and unvegetated deeper water sites with many taxa being strongly positively correlated with the aquatic vegetation. In the chapter onfreshwater benthos, Strayer proposes that presence/absence of SAVis one of three major environmental controls (salinity, vegetation, grain size) on the composition of the macroinvertebrate community.

The invertebrate community of plant beds differs qualitatively from that of bare sediments as well. In particular, the invertebrate community of plant beds often is especially rich in the large or active animals (for example, amphipods, mayflies, caddisflies) that contribute disproportionately to fish diets. Beds of Myriophyllum sp. and T. natans are known to support dense communities of invertebrates, especially chironomids (Menzie, 1980; Findlay et al., 1989, Feldman, 2001)(Fig. 17.5). Strayer and Smith (2001) also found much greater numbers of chironomids within SAV beds than on unvegetated substrate. Thus, macrophyte beds maybe important sources of fish food, because of the richness of the invertebrate community and the relative abundance of large, attractive prey. V. americana beds were sampled as part of a broad survey of the Hudson River zoobenthos (Strayer,

Smith, and Hunter, 1998 and unpublished) and in a smaller scale study of invertebrate distribution (Lutz and Strayer, 2000). The invertebrate fauna directly associated with the plants (epiphytic) is a large proportion of the total number of animals occurring in vegetated habitats and the numbers of individuals in sediments of vegetated areas is frequently higher than in sediments of unvegetated sites (Lutz and Strayer, 2000).

The importance of these habitats for invertebrates appears to have changed over time. Before zebra mussels arrived in the Hudson, invertebrate density was about the same in these V. americana beds as on open, unvegetated sediments. After the zebra mussel invasion, which drastically reduced phytoplankton biomass and increased water transparency (see Cole and Caraco, chapter 9, this volume), invertebrate density rose in V. americana beds and fell in unvegetated habitats, suggesting that benthicprimaryproductionmay have become increasingly important to the Hudson River food web (Strayeretal., 1998).

fish use of sav

Fish may acquire plant-associated animals from macrophyte beds either by moving into these beds and feeding on the resident animals or by remaining outside macrophyte beds and feeding on animals that are carried away from the plants by currents. This latter pathway may be significant because it allows fish to remain in deepwater habitats and could distribute animals (and ultimately, macrophyte carbon) producedinmacrophytebeds throughout the river. Although movement of ben-thic animals through the water column (drift) is very well known in small streams (e.g., Allan, 1995) where it may form a large part of the diet of stream-dwelling fish, it has scarcely been investigated in large rivers or estuaries like the Hudson. Although we often see benthic animals in our plankton samples, showing that drift does occur in the Hudson, there are no previous estimates of the size or source of this drift. The few studies that have been done in large rivers (Berner, 1951; Eckblad, Volden, andWeilgart, 1984) showthatdrift ofbenthic animals maybe substantial (100-1,000 macroinvertebrates/m3), and thus has the potential to form a large part of riverine fish diets. Menzie (1980) noted that Grabe and Schmidt found high d c b e

Table 17.2. Fish species composition in Vallisneria americana, Trapa natans and Myriophyllum spicatum

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