Wigeongrass (Ruppia maritima L.):
A Literature Review
Growth and Production
Rate
Wigeongrass in southwest Canada can germinate and produce mature drupelets in about 2 months (Harrison 1982), whereas, in southern France, other annual-behaving plants take as long as 5 months to mature (Van Vierssen et al. 1984). In climates where spring and fall growth peaks occur, plants probably grow faster in the spring (Pulich 1989). I found no information about the rates at which wigeongrass stems, leaves, or rhizomes elongate in nature.
Yield
Vegetation
Healthy stands of wigeongrass usually contain about 500-1,500 stems or plants/m^2 (McMahan 1969; Corell et al. 1978a,b; Keddy 1987), but densities up to 5,376/m^2 occur (Anderson 1966). Plants in fine sediments probably achieve greater densities than those in coarse sediments (Conover and Gough 1966).
Annual Ruppia taxa may be less productive than perennials because the former usually occur in wetlands subject to high salinities, desiccation, and other stresses (Verhoeven 1980b; Brock 1982b). Conover (1958) and Evans et al. (1986) found that, in a temperate climate, perennial-like wigeongrass had a single peak of aboveground biomass in midsummer. The peak can occur in early fall at shallow sites where plants are temperature-stressed and photoinhibited (Wetzel et al. 1981). Standing crop can peak nearly a month after the period of maximum growth rate (Wetzel 1964). Orth and Moore (1988) found a strong correlation between percent cover and wigeongrass biomass in Chesapeake Bay. Distinct spring and fall growth periods are usual for wigeongrass in the southern United States, and great midsummer reductions in wigeongrass biomass or even complete die-offs are sometimes seen (Joanen and Glasgow 1965; Percival et al. 1970; Swiderek 1982). Flores-Verdugo et al. (1988) found winter and summer biomass peaks for wigeongrass in a tropical Mexican lagoon. They suggested that the scarcity of wigeongrass during the wet season (July to October) probably was the result of nutrient inflows from a river, which led to stimulated phytoplankton growth and increased turbidity.
Biomasses of Ruppia taxa from around the world are compared in Table 2. The highest Ruppia biomass yet recorded (1,748 g/m2 dry weight) occurred for the perennial R. megacarpa in fine-textured sediments in the shallowest (< 1 m) portions of a brackish (salinity about 20 g/L) southern Australian estuary receiving significant amounts of N and P from agricultural runoff. The highest R. maritima biomass (1,460 g/m^2 dry weight) was from a shallow (0.10-0.50 m), warm (32-33 degrees C in June), well-insolated and fertile Rhode Island embayment where salinities ranged from 20 to 22 g/L and bottom sediments were extremely rich in organic matter (36-58% in the upper 5 cm; Nixon and Oviatt 1973). Ruppia maritima biomass up to 1,000 g/m^2 dry weight occurred in a shallow Mexican lagoon that also contained large amounts of organic matter in the sediment (Edwards 1978). Verhoeven (1980b) believed that, under ideal circumstances, the largest standing crop possible for European Ruppia taxa was about 400 g/m^2 dry weight, and he suggested that an American taxon may be more vigorous.
Peak recorded Ruppia biomasses < 400 g/m^2 dry weight are common. The most common factors associated with these low biomasses are excessive turbidity, competition (most likely for light) by other angiosperms or algae, and excessive wave action or water depth. Filamentous algae can inhibit Ruppia production by shading and by entanglement, which causes plants to be more sensitive to wave action (Verhoeven 1980a).
Despite the negative effects of shading and entanglement, algal mats may benefit wigeongrass in some circumstances. Richardson (1980) found that partial shading by algal mats reduces epiphyte fouling on wigeongrass. He also noticed that algal mats in dried wetlands hold water which may increase the survival of wigeongrass lying underneath the mats. Other factors that frequently are suspected to cause low Ruppia biomass are high water temperatures, excessive salinity, overly coarse or soft sediments, and "eatouts" by waterfowl.
Propagules
McMillan (1985) found a maximum density of 4,110 wigeongrass drupelets per square meter in a Texas lagoon. There is no information on the total number of drupelets produced in a season by a single wigeongrass plant.
Wetlands managed for wigeongrass production can produce > 6.6 g/m^2 dry weight of drupelets (Swiderek 1982). In waterfowl exclosures, Prevost et al. (1978) found the dry weight of drupelets in sediments to be nearly twice that annually produced; this suggested that drupelets can persist into succeeding growing seasons.
Chemical and Caloric Content
Dry matter composes 10-17.47% of the fresh weight of wigeongrass (Lindstrom and Sandstrom 1938; Vicars 1976; Reed 1979). Leaves contain about 15.8% dry matter and the root system about 11.5% (Wetzel et al. 1981). Vicars (1976) gave the oven-dry weight as 71.6-81.9% of dry weight. Ash content varies from 15.9 to 42.0% of dry weight (Lindstrom and Sandstrom 1938; Reed 1979), but this measurement depends on how efficiently calcareous encrustations and other matter are removed from the plants. Plants analyzed by Reed (1979) had highest ash content (42% of dry weight) in spring. Kiorboe (1980) considered the ash-free aboveground and belowground dry weights to be 84% and 77%, respectively, of total dry weights. Verhoeven (1980b) used 25% as an average figure to convert dry weight biomass to ash-free biomass. The dry weight of fresh wigeongrass required to displace 1 mL of water averages 0.111 g (C. S. Gidden, 1965, unpublished data).
Gross energy of wigeongrass is 3.2-3.6 Kcal/g dry weight (Nixon and Oviatt 1973; Paulus 1982). Mean annual caloric content of leaves and rhizomes measured by Walsh and Grow (1972) was 4.44 and 4.25 Kcal/g ash-free dry weight, respectively.
Protein content of wigeongrass varies from 5.2 to 21.9% of dry weight (Christensen 1938; Lindstrom and Sandstrom 1938; Paulus 1982; Swiderek 1982). The annual mean protein content of leaves and rhizomes is 23.2% and 20.0% of the ash-free dry weight, respectively (Walsh and Grow 1972). Grontved (1958) calculated that the standing crop of Ruppia in a Danish fjord contained about 25 g/m^2 albumen.
Lipid content in dry matter of South Carolina wigeongrass was 1.5% (Swiderek 1982). Attaway et al. (1970) found that lipids compose 2.5% of the dry weight of wigeongrass, nonsaponifiable material 1.0%, and hydrocarbons 0.073%. All these figures are much higher than those found in four seagrasses collected nearby. They postulated that the absence of short-chain hydrocarbons (C15-C22) in Ruppia may chemically distinguish the family Ruppiaceae from the Zannichelliaceae and Hydrocharitaceae and strengthen Hutchinson's (1959) taxonomic treatment of the seagrasses. A later analysis of the sterols in a sample of wigeongrass that contained 2.2% dry weight lipids revealed the species to be peculiar (when compared to four seagrasses) in its relatively high content of campesterol (Attaway et al. 1971). Analyses of Rhode Island wigeongrass by Jeffries (1972) showed that it contains a variety of C16-18 fatty acids. Parker (1964) measured ratios of stable C isotopes in Redfish Bay, Texas, wigeongrass to test their potential usefulness in determining food chain patterns.
Annual mean carbohydrate content of wigeongrass leaves and rhizomes is 27.0% and 63.6% of the ash-free dry weight, respectively (Walsh and Grow 1972). The more soluble carbohydrates composed 35.1% of the dry matter in samples analyzed by Swiderek (1982). Cellulose and starch contents are 16.4% and 3.04% of oven-dry weight, respectively (Lindstrom and Sandstrom 1938). Crude fiber content is 16.5-16.9 of dry weight (Paulus 1982; Swiderek 1982).
Wigeongrass can remove large amounts of N and inorganic P from the water column (Twilley et al. 1981). Thursby (1984a) considered dry leaf tissue content of 2.5-3.0% N and 0.25-0.35% P to indicate the minimum amounts required for optimal wigeongrass growth. Vicars (1976) showed that wigeongrass from shallow sites had lower N (1.77 mg atomic N/g ash-free dry weight) and P (0.19 mg atomic P/g ash-free dry weight) content than plants gathered from deeper sites. McKay (1934) compared the cell liquid of wigeongrass to the highly concentrated MgSO4 waters where the plants grew and found lower concentrations of magnesium (Mg), sulfate, and sodium (Na) in the plant tissue than in the water. The opposite was true for the major nutrients (N,P, and potassium [K]) as well as for calcium (Ca) and chlorine (Cl). Verhoeven (1979) found that the K and Mg content of R. maritima s.s. did not relate to the environmental concentrations of these elements but that the Ca and Na concentrations did. In addition, he found the concentrations of these elements well within the ranges listed by Hutchinson (1975) for freshwater macrophytes.
Nutrient (N, P, K) concentrations in aboveground versus belowground portions of wigeongrass have been compared in several studies (Walsh and Grow 1972; Wetzel et al. 1981; Getsinger et al. 1982; Van Vierssen 1982b; Pulich 1989). With the possible exception of K, these nutrients are generally in greater concentrations in aboveground parts. Pulich (1989) noted that, in fall, P content of roots slightly exceeds that of leaves. His experiments suggested that leaves are the major nutrient sink in wigeongrass and that water-column nutrients are used through the leaves.
Little information is available for the minor nutrients. Walsh and Grow (1972) found greater Mn concentrations in aboveground than belowground material, in contrast to the results of Van Vierssen (1982b). Negligible differences in aboveground versus belowground concentrations of Na, Ca, Mg, or Fe were noted by Van Vierssen (1982b).
The elemental composition of wigeongrass is shown in Table 3. Information is also available on concentrations of other elements found in wigeongrass vegetation growing in irrigation drainwater evaporation ponds in California (Schuler 1987; Schroeder et al. 1988). Schuler (1987) also lists selenium (Se) and boron (B) concentrations in wigeongrass drupelets from this area. In sufficient quantities, these two elements are potentially toxic to waterfowl.
In summary, available information on the chemical composition of wigeongrass provides few clues about why the plant is such an important waterfowl food. Protein, carbohydrate, and lipid content of aboveground parts are slightly higher than in sago pondweed, which is also heavily used by feeding waterfowl, but mostly for its carbohydrate-rich turions (Kantrud 1990). Several other common submersed macrophytes have higher contents of these nutrients in aboveground parts than wigeongrass (Paullin 1973), but they rank much lower as waterfowl food. Perhaps the delicate texture of wigeongrass leaves and stems adds to the plants palatability. Very little is known about the chemical composition of wigeongrass drupelets.
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