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In ploting the depths of V. americana against secchi disk transparencies (Fig. 4), Davis and Brinson (1980) found it has the capacity to maintain a relatively high biomass in turbid systems. Davis and Carey (1981) suggested that this species may even spread with perturbations that increase turbulence. Vallisneria americana increased in lower Currituck Sound, North Carolina, in 1978 during conditions of increased turbidity (Davis and Brinson 1980). In contrast, tropical storm Agnes, which struck the East Coast in 1972, was cited as a factor in the decrease of V. americana in different portions of Chesapeake Bay (Bayley et al. 1978; Kerwin et al. 1976). The main effects were increased turbidity and uprooting of plants. Bourn (1932) also believed that the turbidity levels existing in Back Bay and Currituck Sound during the late 1920's and early 1930's prohibited growth of submerged macrophytes.

Vallisneria americana may be disadvantaged in deep turbid water because of its limited elongation potential resulting in inability to concentrate photoreceptive biomass at or near the water surface in low light environments (Barko et al. 1984). Vallisneria americana compensates for possibly disadvantageous morphological features (in comparison with a plant such as Myriophyllum) by a greater physiological adaptability to low light regimes (Titus and Adams 1979b). This species was the most shade-adapted of five macrophytes studied by Meyer et al. (1943). In Trout Lake, Wisconsin, V. americana was found growing in 4.5 m of water where the light intensity was 4.5% of that at the surface (Spence and Chrystal 1970a, 1970b). Vallisneria americana is light adaptable; it acclimates rapidly to increasing light and efficiently uses low light (Titus and Adams 1979b). Plants emerging from winter buds under laboratory conditions have the ability to elongate to a mean length of 44.0 cm in total darkness (C. E. Korschgen, unpublished data).

Hunt (1963) found 30-150 cm to be the optimum depth for growth of V. americana in the Detroit River, Michigan, although the plant occurred as deep as 335 cm. Vallisneria americana exhibited a constant decrease in the rate of apparent photosynthesis with an increase in depth of immersion, but it maintained an appreciable rate of photosynthesis (25% of that at the surface) to a depth of 10 m, where light intensity was 0.5% of that at the surface (Meyer et al. 1943). Carter and Rybicki (1985) observed that the water depth in which most V. americana was growing was above or slightly below the 10% photic zone as determined from spring measurements. The deepest spring biomass occurred where the photic zone was about 5% of the surface measurements.

Hydrostatic pressure, rather than light availability, probably controls maximum depth distribution of aquatic macrophytes, according to Davis and Brinson (1980). They concluded that 6 to 7 m is the maximum depth for V. americana. The plant is commonly found at depths of 1-5 m and densities of 100-1,000 rosettes/m² in Lake George, New York; densities at maximum depth (7 m) were 16-400 rosettes/m² (Sheldon and Boylen 1977).

Vallisneria americana grows in substrates ranging from gravel to hard clay, but grows best in silty sand (Hunt 1963). The plant seemed to grow as well in mud as in sand in Lake Mendota, Wisconsin (Denniston 1921); however, V. americana was restricted to sandy-textured sediment of University Bay, Lake Mendota in 1966 (Lied and Cottam 1969). Hunt (1963) believed that only an impervious substrate or a soft, shifting one prevented establishment of V. americana. In Lake Onalaska, Wisconsin, Navigation Pool 7 of the Upper Mississippi River, V. americana beds are distributed over substrates of all textures and degrees of organic matter (G. A. Jackson and C. E. Korschgen, unpublished data). .Vallisneria americana grows in organic, peatlike substrates at Rice Lake National Wildlife Refuge in central Minnesota (C. E. Korschgen, unpublished data). In the tidal Potomac River, the majority of winter buds were between 10 and 20 cm deep in silty clay and between 5 and 15 cm in sand (Rybicki and Carter 1986). Laboratory experiments by Carter et al. (1985) showed that emergence of winter buds was affected by the depth of the substrate. Most winter buds emerged from the substrate when buried by 15 cm of sediment. Only 25% emerged when covered with 20 cm of sediment, and none emerged when covered with 25 to 55 cm of sediment.

Barko et al. (1982, 1984) found that the growth of V. americana was severely restricted at water temperatures less than 20° C. Vallisneria americana grew at water temperatures of 19 to 31.5°C in the Detroit River (Hunt 1963) and 22.7 to 26.3°C in Lake Erie (Meyer et al. 1943). In laboratory tests (Wilkinson 1963) V. americana grew best within a water temperature range of 33 to 36°C; below 19°C arrested growth occurred and above 50°C plants became limp and disintegrated. Vallisneria americana has not been found to overwinter in green form in Lake Mendota, Wisconsin (Lind and Cottam 1969).

Water temperature effected a greater range of response in V. americana leaf length than did light. However, at water temperatures of 20°C or more, overall V. americana biomass production and shoot density generally increased with increasing light, particularly between low and middle light levels (Barko et al. 1982). Total chlorophyll in V. americana increased with decreasing irradiance irrespective of temperature, but total chlorophyll content increased with rising temperature (from 12 to 32°C at 4 Celsius degree increments) at all light levels (Barko and Filbin 1983).

Vallisneria americana is common in both lotic and lentic water bodies. Rooted plants that occur in moving water have tough, flexible stems or leaves, a creeping growth habit, frequent adventitious roots, and vegetative reproduction (Hynes 1970). The plant's greater allocation of biomass to below-sediment parts may allow the species to maintain itself in shallow water having relatively great wave action (Titus and Adams 1979a). McAtee (1939) recommended planting V. americana in quiet to slight-current waters. Some current is usually necessary for V. americana growth (Moyle and Hotchkiss 1945).

Aquatic macrophytes have wide tolerances to water chemistry regimes (Pip 1979; Hellquist 1980), and V. americana is no exception. The plant may tolerate a broader range of water chemistry characteristics than those discussed here; the published information is influenced by the lakes available for study. Only a few field and laboratory studies have been conducted to determine the exact tolerance limits of V. americana.

Alkalinity

Bourn (1934), citing studies conducted in North Dakota and Nebraska, concluded that V. americana does not grow in saline or alkaline lakes typical of western watersheds. Other reports list alkalinity (mg/L) in water bodies where V. americana is found (Table 2).

pH

Titus and Stone (1982) found that the dissolved inorganic carbon (used in photosynthesis) uptake rates by V. americana declined with increasing pH. The uptake rate declined by 61% from pH 7 to 8, but changed only slightly from pH 8 to 9. Vallisneria plant weight, number of rosettes per plant, and number of buds per plant were found to decrease with declining pH (Hoover 1984; Grise 1983). Vallisneria americana is rarely reported growing in lakes with pH values below 6. Grise et al. (1986) found that at pH 5 iron and aluminum toxicity may limit plant growth. In laboratory experiments in which plants were grown at pH 5 the leaf tips browned and senesced prematurely and winter buds were small and less able to support growth the following spring. Other references to pH in water bodies where V. americana are found are in Table 2.

Salinity

Although V. americana is considered a freshwater plant, it will grow in water that has elevated salt concentrations (Hunt 1963). Salt content of water consists principally of chloride sulfate, sodium, magnesium, and calcium (Todd 1970). Vallisneria americana has survived in salt concentrations as great as 20% (7,000 ppm) of seawater and thrived at 12% (4,200 ppm; Bourn 1934). Distribution of submerged aquatic vegetation in Chesapeake Bay tends to be determined by a salinity regime. The plant is found in areas where salinity is 3,000 to 5,000 ppm (Steenis 1970). Haller et al. (1974) determined that water with more than 6,660 ppm salinity is toxic to V. americana. Haramis and Carter (1983) found V. americana to be the only plant to persist in the transition zone of the Potomac River where the salinity was 500 to l0,000 ppm; it grew at l0,000 ppm but died and decayed at 13,500 ppm (Carter et al. 1985). Vallisneria americana did not grow in Potomac River water, which had a chlorine content equivalent to 6,000 to 8,000 ppm of sodium chloride (Martin and Uhler 1939).

Sincock (unpublished report) concluded that V. americana plants in Currituck Sound were capable of tolerating higher salinities when grown in a silt substrate rather than sand. This difference is possibly due to high cation exchange in silt soils that protect the root structure. Sand substrates are not found to have the same buffering capacity.

Other

Water samples were analyzed monthly from Pools 7 and 8 navigation channel area of the Upper Mississippi River for 10 years (1972-81; Dawson et al. 1984). These data (Table 3) illustrate a water chemistry regime in which V. americana has been very prolific for more than 25 years (C. E. Korschgen, unpublished data). In water chemistry analyses (Crowder et al. 1977) of Lake Opinicon, Ontario (Table 4), V. americana showed the highest percent frequency and second highest total abundance of all aquatic macrophytes sampled in the lake.

The toxicity of most chemical compounds to specific aquatic macrophytes under different environmental conditions has yet to be thoroughly investigated, but ch1orine, chromates, cyanides, heavy metals, phenols, and aromatic solvents are probably toxic to all macrophytes, even in low concentrations (Sculthorpe 1967). Stevenson and Confer (1978) discussed the uptake of heavy metals and effects of petrochemicals.

Forney and Davis (1981) concluded that the concentrations of atrazine and glyphosate normally found in farm field runoff wafer will not pose any threat to V. americana. However, they did not state the concentrations found in runoff water. Correll and Wu (1982) determined that 650 g/L dissolved atrazine significandy inhibited photosynthesis in V. americana, and 120 g/L caused 100% mortality within 30 days. After 47 days V. americana had a 50% mortality at 12 g/L; the production of new plants at the end of runners and the leaf surface area increase of survivors was significantly reduced.

A 100-ppm concentration of 2,4-D killed V. americana and 10-ppm concentrations inhibited its growth (Gerking 1948). Lakes containing 0.5 and 0.25 ppm simazine suppressed V. americana growth but did not eliminate it (Norton and Ellis 1976). Other herbicides varied in their effects on V. americana (Table 5).

Vallisneria americana experienced retarded growth, loss of chlorophyll, and collapse when exposed to total available chlorine levels of 0.5 to 0.125 ppm in laboratory studies by Webster and Rawles (1976). They suggested that chlorine pollution may have been a cause for the loss of submersed aquatic plants in Chesapeake Bay in the 1970's.

Influence of Other Plants

Rapid increases of Eurasian milfoil, Myriophyllum spicatum (such as those that occurred in the late 1950's and early 1960's in Chesapeake Bay) probably reduced native species, including V. americana (Orth and Moore 1981). But when Eurasian milfoil declined in the late 1970's, native species returned to approximately their former abundances. The ability of Eurasian milfoil to extend its growing season in relation to that of V. americana (by photosynthesizing at low temperatures) may have been a factor in its replacement of V. americana in Lake Wingra, Wisconsin (Titus and Adams 1979b). Also, because V. americana distributes the major portion of its shoot biomass near the sediment surface (hence lower light regime) it was apparently incapable of successfully competing with canopy-forming species such as Hydrilla verticillata in Florida (Barko et al. 1984) and M. spicatum in Lake Wingra in Wisconsin (Titus and Adams 1979b).

However, Titus and Adams (1979a) have given two possible reasons for why M. spicatum has not completely replaced V. americana in University Bay of Lake Mendota or in Lake Wingra, Wisconsin. First, V. americana has an inherently higher productivity during summer months. Second, it has a better rooting system that allows it to exist in shallow water subject to wave wash.

Titus and Stephens (1983) studied the neighbor influences of Chara vulgaris and Potamogeton arnplifolius on seasonal growth patterns of V. americana in Chenango Lake, New York. They determined that plant dry weight was consistently, but not significantly, lower for plants with neighbors than those without neighbors; leaf numbers, stolon length, and winter bud production did not differ significantly. Vallisneria americana allocated more biomass to vertical extension in the presence of neighbors and to horizontal extension in the absence of neighbors. Titus and Stephens (1983) speculated that neighbor influences may have greater importance in more productive macrophyte communities, whereas dense macrophyte growth may have a more profound effect on the physicochemical environment.

By reducing photosynthetic light, epiphytic algae may suffficiendy light-stress submerged macrophytes enough to jeopardize growth the next year (Phillips et al. 1978).

Influence of Animals

Carter and Rybicki (1985) studied transplanted V. americana in the Potomac River and observed that grazers such as waterfowl, muskrats (Ondatra zibethicus) and red-bellied turtles (Pseudemys rubriventris) influenced the establishment of plant beds. Plants grown in full exclosures during their first year became established and were evident the following year.

Common carp (Cyprinus carpio) are implicated in the loss of submersed macrophyte beds, either by uprooting plants or by increasing turbidity or sedimentation. Carp do not seem to feed directly off the bottom; instead they uproot the vegetation and then feed on the shoots and roots that settle to the bottom. When the fish have stirred up a cloud of mud, debris, and plants, they swim through the cloud sucking up whatever is edible (Eddy and Underhill 1974). Carp introduced into Lake Wingra in the late 1800's probably caused V. americana to disappear from the lake by 1929 (Davis and Brinson 1980). One year after carp removal by rotenone in the Middle Harbor of Lake Erie, Ohio, V. americana was found in areas where it had not been present before treatment (Anderson 1950).

Grass carp (Ctenopharyngodon idella) feed on Chara, Najas, Hydrilla, Myriophyllum, and V. americana in decreasing order (Sutton and Blackburn 1973).

A great variety of invertebrates are known to attack waterfowl food plants and occasionally become highly destructive. Leaves of V. americana in Potomac River beds are occasionally riddled by certain fly larvae (Martin and Uhler 1939). Plant beds in Pool 7 have many invertebrates on and in the leaves (E. Chilton, personal communication). The effect of these invertebrates on the productivity of, or disease introductions to, V. americana plants is unknown.