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Wetland Type

Ruppia maritim s.l. occurs mostly in coastal bays (temporarily to permanently flooded and mesohaline to hypersaline); estuaries, fjords, lagoons, ponds, pannes, and sounds; and in bayous, creeks, ditches, flats, and rivers subject to tidal influence (Olsen 1945; Millard and Scott 1953; Thorne 1954; Ferguson Wood 1959; Kornas et al. 1960; Phillips 1960b; Hyer 1963; Joanen 1964; Joanen and Glasgow 1965; Verhoeven 1979, 1980a; Richardson 1980; Thorne-Miller et al. 1983; Ferren 1985). Verhoeven (1979) defined temporary water bodies for wigeongrass as those where physical conditions do not allow survival of vegetative plant parts during certain periods of the year.

In tidal estuaries, wigeongrass usually occurs at elevations between mean lower low water (MLLW) and mean higher low water (MHLW) (McNulty et al. 1972; Jefferson 1974). The species also mixes with true seagrasses up to at least 1.5 km offshore in large oceanic bays (eg., the Gulf of Mexico; Zimmerman and Livingston 1979). Wigeongrass is often propagated in coastal impoundments in the United States because it is attractive to waterfowl (Davis et al. 1985). For example, Tiner (1977) showed that nearly 7,500 ha of such impoundments exist in a single South Carolina estuary. Daiber (1974) cited several references showing that impoundments built to increase production of salt marsh hay have also created wigeongrass habitat in the eastern United States. Prolific stands of wigeongrass also occur in muskrat "eat-outs" and alligator holes in wetlands along the Gulf of Mexico (Bateman et al. 1988).

In noncoastal waters, wigeongrass occurs in fresh to hypersaline palustrine and lacustrine wetlands (Metcalf 1931; Moyle 1945; Stewart and Kantrud 1971, 1972; Reynolds and Reynolds 1975; McCarraher 1977; Pip 1979) as well as in mound springs and artesian bores (Jacobs and Brock 1982). Of 17 reported occurrences of wigeongrass in south-central Canada, Pip (1979) found 82% in lakes, 12% in ponds, and 6% in creeks.

Wetland Area and Fetch

Wigeongrass in Rhode Island showed greater coverage of small ponds (< 1.3 ha) than in larger water bodies where plants occurred only around shorelines and in coves (Wright et al. 1949). Extensive surveys of Louisiana wetlands showed that few submersed macrophytes of any kind grow in lakes > 2.59 km^2, probably because of excessive depth and wave-induced turbidity (Chabreck 1972). Breuer (1961 in Cornelius 1975) and McMahan (1969) found wigeongrass relatively unimportant in the huge Laguna Madre of Texas, where the plant occured only around protected areas. South along the Mexican Gulf Coast, however, a mixed bed of wigeongrass and Najas occupied about half of the 1000-km^2 Laguna Tamiahua; other extensive stands also grew in large interior lakes and Pacific Coast lagoons in that country (Saunders and Saunders 1981). Pip (1979) found 82% of Canadian wigeongrass occurrences in lakes > 10 ha. Nearly the entire bottom of a shallow (< 1 m), 94-ha saline lake in central North Dakota was a wigeongrass monotype for at least 3 years (H.A. Kantrud, personal observation).

Water Column

Depth

Wigeongrass occurrence spans a water depth of 0-4.5 m (Table 4). Kornas et al. (1960) found the highest frequency of wigeongrass at 2-4 m in a brackish bay, but the "richest" stand occurred at only 0.4 m. Joanen (1964) and Joanen and Glasgow (1965) saw the largest biomass in waters 0.6 m deep in the field but, under optimum growth conditions in the laboratory, most growth occurred at 0.4 m. Harwood (1975), however, found the density of wigeongrass to be independent of depth within estuarine waters 0.4-1.3 m deep.

The depth that wigeongrass will grow in any particular wetland seems more strongly related to particle size of bottom substrate than depth per se. No wigeongrass occurred on clays or silts at depths > 1.5 m, but plants were several times recorded on sand in waters > 2.0 m deep (Table 4). Similarly, optimum wigeongrass growth in clay-bottomed wetlands was not reported at depths > O.61 m, whereas lush growths in sandy-bottomed wetlands were noted at depths up to 4.0 m. In Chesapeake Bay, United States, depth distribution of wigeongrass was +20 to -100 cm (relative to mean low water) in the relatively clean eastern shore waters compared to +10 to -80 cm along the more turbid western shore (Orth and Moore 1988). Thus it is likely that the susceptibility of bottom substrate to wind-induced turbidity often governs the depth distribution of wigeongrass.

Depth, in addition to sediment chemistry and water level fluctuations, influences the growth habit of wigeongrass. Plants from shallow pannes exhibited a procumbent habit with distinctly forking stems and short internodal lengths (Fig. 2), but plants from deeper waters (Fig. 3) were more ascending and had longer internodes (Richardson 1980). In shallow sites, plants adjusted to the stress of high light and temperature by concentrating leaf area in the lower portion of the canopy (Wetzel et al. 1981).

Transparency

As mentioned earlier, wigeongrass requires much sunlight. Verhoeven (1979) believed wigeongrass could only develop normally in clear water and always found the species greatly reduced or absent in water turbid from suspended materials. Water stained with dissolved organic materials, especially from woody plants, can also reduce water transparency in managed wigeongrass impoundments (Heitzman 1978).

Large beds of wigeongrass have disappeared as a result of a rapid increase in turbidity (Anderson 1970). Joanen (1964) and Joanen and Glasgow (1965) found that turbidity was most harmful to young plants and recommended that wetlands managed for wigeongrass have < 25-55 ppm turbidity. Gore (1965) found wigeongrass in waters with 17.5-42.5 ppm turbidity. According to Day (1952), a Secchi disc reading of 1 m equals about 185 ppm suspended solids. A saline wetland in Alberta, Canada, where wigeongrass was the dominant macrophyte, had relatively high water transparency (Secchi 3.0 m; extinction coefficient 0.8), low phytoplankton productivity, and a large standing crop of benthic algae (Gallup 1978). Wigeongrass biomass decreased markedly as Secchi depth decreased to < l m concurrent with a decrease in water levels (Bailey and Titman 1984). Zimmerman and Livingston (1979) found wigeongrass where turbidities reached 120 Jackson Turbidity Units (JTU), but the plant was one of the three major dominants only where turbidities were < 60 JTU. They also found the plant where color was 0-570 platinum-cobalt units (PCU), but most growth was in waters with < 370 PCU.

Tidal waters with dense wigeongrass populations, examined by Richardson (1980), were usually clear during the growing season but occasionally became turbid from climatic events or flooding. However, those with sparse growths were frequently to consistently turbid due to dissolved organic matter, organic and inorganic particulates, or living phytoplankton and zooplankton. Harwood (1975) noted that storm-induced turbidity can limit growth of wigeongrass.

A 40% reduction in light intensity gave a 50% reduction in wigeongrass standing crop during shading experiments of Congdon and McComb (1979). They suggested that, in tidal wetlands, reduced light intensity is an important factor limiting the area where wigeongrass can grow because plants die from overexposure to air at shallow sights where light is not limiting. In some areas, poor insolation due to fog, mountains, or short days can be the main cause of reduced wigeongrass production (Wetzel 1964). Short periods of high turbidity probably are not harmful to wigeongrass, as Millard and Scott (1953) found that the plant prospered in shallow, sometimes exposed sites, where Secchi disc readings sometimes fell to < 7 cm. In the production of high turbidity, Conover (1964a) considered winds that roil bottom sediments and detritus in shallow wigeongrass lagoons to be more important than living planktonic algae. Established stands of wigeongrass do not always increase as turbidity decreases. Thorne-Miller et al. (1983) reported a decline in wigeongrass following breachway construction from the ocean to a coastal lagoon. Secchi transparency increased to at least 2.3 m, allowing Zostera marina to become dominant. However, they noted that other important factors - such as increased water circulation and salinity - could have caused the wigeongrass decline.

The stimulatory effect of nutrient enrichment from sewage and agricultural runoff on phytoplankton probably is the main cause of manmade turbidity in areas (eg. Chesapeake Bay, where wigeongrass and other submersed macrophytes have declined; Carter et al. 1985). I found only a single case where industrial contaminants may have been implicated: in Florida, pulp mill wastes caused noticeable increases in turbidity and color up to 5 km from estuarine mouths, restricting wigeongrass to sites > 1.4 km offshore even though competition with true seagrasses and large marine algae was greater there (Zimmerman and Livingston 1979). However, higher levels of dissolved P, biochemical oxygen demand, and chemical oxygen demand were also found in the area where wigeongrass was absent.

To summarize, the relatively shallow waters inhabited by wigeongrass, its photosynthetic and physiological parameters, and its negative response to small increases in turbidity show that control of water transparency is of utmost importance to establish and maintain of stands.

Water Chemistry

The genus Ruppia tolerates a wider range of water salinity than any other group of submersed angiosperm (Brock 1979). Table 5 shows that Ruppia maritima s.l. occurs in waters containing 0.6-390 g/L.

Joanen and Glasgow (1965) found no differences in wigeongrass growth in Louisiana waters that ranged from 3.7 to 33.4 g/L salinity. The upper limit slightly exceeds sea strength (about 32 g/L). The plant was one of the few angiosperms able to grow in both hypo- and hyper-saline areas in Texas lagoons where waters ranged from nearly fresh to over 60 g/L (Conover 1964b). Nevertheless, wigeongrass can become restricted to peripheral areas when breachway construction between the ocean and closed lagoons allows inflows of water at full seastrength (Thorne-Miller et al. 1983). Sometimes, however, such breachways can result in increases in wigeongrass if dilution of hypersaline waters occurs (Breuer 1962). Wigeongrass in a Florida bay (maximum salinity 27.7 g/L) reached maximum abundance in July when salinity was minimum (13.2-14.7 g/L). In another year, the salinity fell to 5-10 g/L, nearly eliminating the wigeongrass, but stimulating dense growths of muskgrass (Chara spp.; Tabb et al. 1962). Saunders and Saunders (1981) recorded no wigeongrass, but abundant marine algae, in a Mexican lagoon when salinity was about 16 g/L. Wigeongrass first appeared during the year that salinity fell to 10.5 g/L and maximum development of stands occurred during a year when salinity was about 6.4 g/L. Wood (1959) and Strawn (1961) also noted the affinity of wigeongrass for low-salinity ocean water.

Wigeongrass tolerates extremely high salinities (up to 390 g/L) in lakes where MgSO₄ is the principal salt (St. John and Courtney 1924; Woronichin 1926). Verhoeven (1979) pointed out that the osmotic effect of such high MgSO₄ concentrations is equivalent to NaCl salinities half as high and cited Bourn's (1935) work that suggested NaCl was more toxic to wigeongrass than other salts at the same osmotic concentration. Millard and Scott (1953) saw beds of wigeongrass regularly die back in a South African estuary when chlorinities exceeded 38 g/L (salinity 69 g/L). Such plants, presumably behaving as perennials, survived for at least 2 months in seawater evaporated nearly to the point of crystallization (chlorinity 198 g/L; salinity 358 g/L).

Ruppia maritima s.l. also grows in a wide range of salinities in the prairie pothole region of interior North America. Stewart and Kantrud (1972) found wigeongrass in wetlands ranging from 0.35 to > 100 g/L and list the maritima variety as abundant at 15 to > 100 g/L. Metcalf (1931) found wigeongrass fruiting abundantly in prairie wetlands with salinities up to 36 g/L. Millar (1976) listed 15-> 45 g/L as the normal salinity of waters supporting wigeongrass in prairie Canada. In this region and the prairies of the northern United States, the occidentalis variety, sometimes called "western wigeongrass," is found in deeper waters (up to 2 m) with salinities up to about 18 g/L (Stewart and Kantrud 1971; Larson 1979; Anderson and Jones 1976). This inland variety, probably perennial from quiescent rhizomes, is also found across the northern part of the contiguous United States of America, southern Canada, and Alaska (Pip 1978; Larson 1979; Brayshaw 1985). Although it occurs in waters with as little as 60 mg/L total dissolved solids (TDS), the plant mostly inhabits waters with higher than average salinity (Pip 1979). Husband and Hickman (1989) suggested that the effects of salinity on the colonization of new sites, rather than on the performance of the plant within sites, may be the most important factor determining the distributional limits of this species in Alberta wetlands.

In southern Australia, perennial Ruppia taxa occupied deeper, permanent waters with salinities 12-50 g/L; annual types inhabited shallow, less permanent wetlands with salinities up to 230 g/L (Brock 1982a, 1982b). In the ephemeral lakes of western Australia, Geddes et al. (1981) found wigeongrass growing in waters of 3.7-78.3 g/L TDS, but at 81.7-142.0 g/L, only drupelets occurred.

Early in the growing season, wigeongrass with annual growth habit appears to have an affinity for areas with low salinity (Richardson 1980). Salinity may control fruit size and shape, and fruit produced in early summer can have a thicker coat than that from the same plant in early fall (Mayer and Low 1970; McMillan 1974).

Ruppia maritima s.l. has often been cultured to determine the effects of salinity. Best growth occurred at 4.7-22.6 g/L (Joanen 1964; Joanen and Glasgow 1965). Plants flower at 1.8-28 g/L (Bourn 1935; Mayer and Low 1970; McMillan 1974; McRoy and McMillan 1977; Verhoeven 1979) and grow at up to 70 g/L (McMillan and Moseley 1967). Mayer and Low (1970) found that 6-week-old plants tolerated higher salinity (27 g/L) than 8- and 12-week-old plants (21 g/L). Thursby (1984a) grew wigeongrass in liquid media and found the most growth at 10 g/L in both natural and artificial seawater; drupelets germinated in about 6 weeks at this salinity. Drupelets were not produced in seawater concentrated to 52.5 g/L by Bourn (1935). Ortu (1969) noted that drupelets immersed in a solution of 52 g/L NaCl would not germinate across the temperature range 10-30 degrees C. At the other extreme, wigeongrass can be grown and maintained indefinitely in tapwater (Setchell 1924; Mayer and Low 1970; McMillan 1974).

Wigeongrass tolerates salinity increases caused by normal intrusions of ocean water into coastal rivers or bays (Phillips 1960a; Stevenson and Confer 1978). Intrusions of ocean water may actually rejuvenate wigeongrass habitat by mechanically scouring away soft bottom sediments and unwanted vegetative mats in managed coastal impoundments (Baldwin 1968). Godfrey and Godfrey (1974) opined that wigeongrass habitat in North Carolina is constantly changed by coastal salt marshes that build up and are lost when inlets to the ocean open and close and when storm tides move sand that creates shallow sites for colonization. Eleuterius (1987), however, believed that waters at or near full sea strength, persisting for 2 or more years, inhibited wigeongrass growth in wetlands along the Mississippi coast, and that intrusions of sea water into bays, bayous, and rivers during during Hurricane Camille in 1969 further reduced populations. Frequent openings of a man-made spillway later allowed great volumes of fresh water to enter these wetlands, creating brackish conditions and a spectacular growth of wigeongrass that persisted for 17 years.

Nevertheless, rapid salinity fluctuations can be deadly according to Verhoeven (1979), who stated that all Ruppia taxa in the Netherlands die when chlorinity rises more than 10 g/L (about 18 g/L salinity) in a few weeks. Early experiments by Graves (1908) showed that wigeongrass leaves died from plasmolysis in 4-5 minutes when placed in a 30 g/L NaCl solution. Van Vierssen (1982a), in the Netherlands, observed that the best stands of R. maritima s.s. occurred where salinity was < 22.6 g/L and fluctuated less than 18 g/L in a single year. Richardson (1980) noticed no ill effects on wigeongrass in a New Hampshire tidal marsh when salinities plummeted at least 14 g/L in 24 hours. South African wigeongrass survived maximum salinity increases or decreases of 0.2 g/L/h even though plants died down when salinities were high (Millard and Scott 1953). McKay (1934) found wigeongrass completing its normal drupelet production in a MgSO4-dominated lake where salinity increased 44 g/L (16-60 g/L) in the nine weeks after flowering, and saw little difference in drupelet production when salinity varied about 244 g/L (16-260 g/L) between years.

Wigeongrass occurs in natural waters of pH 6.0 (Joanen and Glasgow 1965) to 10.4 (Verhoeven 1979) (Table 6). Pip (1978, 1979, 1984) noticed the affinity of wigeongrass for wetlands of higher pH (7.7-9.4) and the deficiency of the species in the granitic Precambrian Shield region of south-central Canada where waters are usually soft and slightly acidic. Outdoor experiments by Neely (1958, 1962), who was trying to grow wigeongrass by reducing acidity caused by the oxidation of iron polysulfides ("cat-clays") on pond bottoms, showed that no plants grew until waters reached pH 5.0. He recommended pH 7.0-8.0 for successful wigeongrass propagation.

Wigeongrass tolerates an extremely wide range of carbonate alkalinity (Table 6). McCarraher (1972, 1977) found wigeongrass in highly saline (> 40 g/L) lakes in the Nebraska Sandhills; these lakes had total alkalinities up to 34.7 g/L and CO₃ and HCO₃ concentrations of up to 25.4 g/L and 9.3 g/L, respectively. Moyle (1945) believed wigeongrass would not get sufficient nutrients in Minnesota waters containing < 150 mg/L total alkalinity. In south-central Canada, Pip (1978, 1979) also noticed the affinity of wigeongrass for waters with higher than average total alkalinity (86-800 mg/L). However, wetlands with as little as 30 mg/L total alkalinity can support wigeongrass (Chamberlain 1960).

Major nutrients (N, P, K) are readily taken up from the water column by wigeongrass (Setchell 1946; Thursby and Harlin 1984) and extensive beds of the plant have, in at least one case, been created by fertilization with N and P (Davis 1978). However, excessive amounts of the major nutrients can cause phytoplankton blooms and epiphytic growths that can attenuate photosynthetically active radiation (PAR; Twilley et al. 1985). Plants in such environments may suffer early senescence and reduced energy supplies to propagative structures. Plants grown in algae-free culture can prosper under much lower light intensities than when algae are present (Thursby 1984a). All the major nutrients are likely to be found in excessive amounts in highly eutrophic or polluted waters. Although wigeongrass has occasionally been recorded from such waters (Neel et al. 1973; Lein et al. 1974; Nilssen 1975; Zimmerman and Livingston 1979), it seems likely that the poor light conditions usually found in polluted waters would quickly eliminate the plant, considering its high light requirements. Perhaps that is why so little is known about maximum levels of nutrients - or the commonly associated increases in biochemical oxygen demand and chemical oxygen demand - that wigeongrass can tolerate.

For N, the minimum leaf tissue content considered indicative of optimum growth conditions for wigeongrass is 2.5-3.0% (Thursby 1984a). Pip (1978, 1979) showed the affinity of wigeongrass for waters with higher than average values of N (0.9-6.8 mg/L) in interior Canada. I could find no records for the plant in waters with less than 0.6 mg/L total N (Table 6). Attempts by Harlin and Thorne-Miller (1981) to measure the effects of NO3 and NH3 additions on wigeongrass in situ were thwarted by growths of green algae.

Phosphorus concentrations of at least 0.3% in wigeongrass leaf tissue indicate optimum growth conditions (Thursby 1984a. Conover (1961) found densest stands of wigeongrass in a coastal Rhode Island wetland where bottom waters were rich in P. Harlin and Thorne-Miller (1981) found, also at a Rhode Island site, that P fertilization in situ stimulated wigeongrass biomass and resulted in longer leaves. Holmes (1972) suggested that P limits wigeongrass growth even in wetlands where 15 micrograms/L are available during the nongrowing season. Robarts (1976) saw PO₄-P levels fall to zero in wigeongrass-inhabited waters when diatom populations were high, even though up to 18 micrograms/L was available at other times. Total P fell to 0.1 mg/L by August in a Minnesota lake supporting wigeongrass (Neel et al. 1973). Phosphorous seemed more important than N in controlling the growth of Ruppia megacarpa in an Australian estuary (Lukatelich et al. 1987).

Known effects of water-column K on wigeongrass are limited to the findings of Setchell (1946), who found that plants could be cultured for years in tapwater if KNO₃ were added.

Table 6 shows the ranges in concentration of many other elements in natural waters inhabited by wigeongrass. Much other information on the tolerance of wigeongrass for these and other uncommon elements is available from irrigation drainwater evaporation ponds in California where high concentrations of Se and B have accumulated (Saiki and Lowe 1987; Schuler 1987; Schroeder et al. 1988).

Little work has been done on the effects of non-nutrients or micronutrients on wigeongrass. Setchell (1946) found that wigeongrass could be cultured without sediment in distilled water if MgSO₄ was added. Moyle (1945) established a lower limit of 50 mg/L SO₄ for Minnesota wigeongrass, but the plant was found in Florida waters where no sulfates were detected (Chamberlain 1960). Van Vierssen (1982b) indicated that wigeongrass mostly grew in waters where molar Ca/Mg and K/Mg ratios were low.

In summary, R. maritima s.l., despite its otherwise rather narrow ecological niche, occupies wetlands having a greater range of salinity than is tolerated by any other submersed angiosperm. Optimum salinity for wigeongrass growth in Cl-dominated wetlands is about 5-20 g/L, but somewhat lower salinities earlier in the growing season may enhance rapid germination and drupelet production. Salinities for best growth in inland, SO₄-dominated waters are about twice as high as in Cl-dominated waters. The effects of salinity fluctuations on wigeongrass are unclear. Wigeongrass does poorly in fresh, soft, or even slightly acidic waters. Nutrients are readily absorbed from the water column and can stimulate growth, but in eutrophic waters growth is often severely limited by phytoplankton and epiphytes.

Temperature

Growth of wigeongrass may be more strongly influenced by water temperature than other important environmental variables. For example, in a temperate estuary, time of maximum wigeongrass biomass coincided with period of peak summer temperature rather than with period of maximum insolation (Conover 1958), and growth of the plant in Texas lagoons was positively correlated with cool spring temperatures rather than with low salinities (Pulich 1985). Shallow water forms of wigeongrass must be resistant to cold as well as drought (Verhoeven 1980a).

Water temperature, of course, affects phenology. In western Europe, Verhoeven (1979) found that drupelet germination and rhizome budding began after winter during the first 10 days when mean daily minima and maxima water temperatures exceeded 10 degrees and 15 degrees C, respectively, and that reproductive processes began only in 10-day periods when temperatures attained 15-19 degrees C. In Chesapeake Bay, wigeongrass tends to form monotypic stands in shallow intertidal and shallow subtidal areas where summer water temperatures and transparencies are high; peak biomass occurs later in the growing season after waters cool (Wetzel et al. 1981). In Rhode Island, wigeongrass actively grows from late April to late October; growth lags attrition in fall when water temperatures fall to 12 degrees C (Conover 1964a). In North Carolina, production ceases in October when water temperatures fall below 18 degrees C (Reed 1979). In the southern United States, midsummer die-offs of wigeongrass are common in impoundments and likely occur because of direct and indirect effects of high summer temperatures and increased salinity (Swiderek 1982). Prevost (1987) stressed the need for water circulation during warm summer to early fall in these wetlands to help flush out cloaking filamentous algae. It is likely that growth periods of these algae are associated with high water temperatures. Richardson (1980) suggested that flowering, fruiting, and drupelet production periods are lengthened by temperature stratification caused by dense algal mats and vegetation.

The distribution of wigeongrass can also be affected by temperature. Anderson (1969) saw Potamogeton perfoliatus (thorowort pondweed) replace wigeongrass near an area of thermal effluent discharge where water temperatures sometimes reached 35 degrees C. He suspected that this temperature allowed survival, but not growth, of wigeongrass rhizomes.

In North America, the overall water temperature range at which annual-like wigeongrass completes its life cycle is about 10-33 degrees C. Drupelets germinate at about 10-20 degrees C (optimum 15-20 degrees; Setchell 1924; Richardson 1980). Optimum germination temperatures for drupelets from Ruppia taxa from other parts of the world can differ by as much as 20 degrees C (Seeliger et al. 1984; Van Vierssen et al. 1984; Koch and Seeliger 1988). In Italy, Ortu (1969) found that the latency or dormant period of wigeongrass drupelets decreased with increased temperature but that low temperatures probably increased the germination rate of those held at relatively low salinities. Koch and Seeliger (1988) showed that drying of wigeongrass drupelets collected from an ephemeral habitat in Brazil increased germination, but high temperatures and low salinities induced germination in drupelets collected from a nearby, more stable habitat.

Seedlings develop at about 15-25 degrees C (optimum 15-20 degrees C; Setchell 1924; Joanen 1964; Richardson 1980). Vegetation grows at 12-33 degrees C (Conover 1964a; Joanen 1964; Nixon and Oviatt 1973; Orth et al. 1979; Richardson 1980; Harlin and Thorne-Miller 1981). Optimum growth temperatures in Rhode Island are 12-18 degrees C, whereas those in North Carolina are 18-22 degrees (Reed 1979). Phillips (1960a) found wigeongrass abundant in a Florida river when temperatures ranged from 18-29 degrees C.

Flowering, pollination, and drupelet production proceed at water temperatures of about 18-32 degrees C (Setchell 1924; Phillips 1960a; Conover 1964a; Joanen 1964; Richardson 1980). Setchell (1924 reported that optimum reproductive temperatures are 20-25 degrees C and that anthesis is slow and eventually ceases after prolonged periods above 25 degrees C.

It is likely that water temperatures exceeding 30 degrees C are harmful or lethal to the development of wigeongrass in most north temperate wetlands (Vicars 1976; Verhoeven 1979). Nevertheless, Edwards (1978) measured water temperatures up to 36 degrees C in a Mexican lagoon dominated by wigeongrass. A perennial wigeongrass in Florida withstands 39.4 degrees C, but flowering and growth are inhibited in temperatures > 30 degrees C (Phillips 1960a). Laboratory tests of Anderson (1966) showed that wigeongrass cells died when exposed to 40 degrees C for 30 min and all cortical aerenchyma perished in l5 min at 45 degrees C.

Water Movement

Wigeongrass prospers in still or protected waters and sometimes in rather strong currents but not in areas with excessive turbulence (Transeau 1913; Johnson and York 1915; McAtee 1939; Day 1952; Wood 1959; Orth et al. 1979; Verhoeven 1979). Wave action in small wetlands restricted wigeongrass to areas deeper than 10 cm (Davis 1978). In large open wetlands, wave action limits the growth of wigeongrass either through mechanical injury or - in wetlands with easily-suspendible bottom sediments or large amounts of vegetative debris - through increases in turbidity (Smith 1951; Joanen 1964; Joanen and Glasgow 1965; Swiderek 1982). Vicars (1976) suggested reduced wave action as one of the factors causing relatively stable wigeongrass biomass during a nearly twofold increase in plant density in a North Carolina estuary. Wave action injures surface branches of wigeongrass, leaving broken tips incapable of survival (McCann 1945). Sometimes only sterile plants are found at exposed sites (Luther (1951, cited in Verhoeven 1979). Wigeongrass is rarely seen along wave-exposed shorelines of Chesapeake Bay unless associated with Zostera marina; monospecific beds of wigeongrass are mostly found in areas protected from wave action (Orth and Moore 1988). Algal felts or mulch from previous years growth helps protect wigeongrass seedlings from wave damage or associated turbidity (Gore 1965; Richardson 1980).

Wind-induced turbidity can limit wigeongrass productivity (Harwood 1975) and sometimes be more important than planktonic algae in that respect (Conover 1964a). Williams (1979) and Gerbeaux and Ward (1986) attributed the lack of regeneration of Ruppia for many years after a storm to a combination of the removal of fine sediments and increased phytoplankton blooms. The latter probably was the main cause, however, considering the high light requirements of Ruppia and that its habitat may be rejuvenated by occasional removal of soft sediments (Baldwin 1968).

Because of its shallow and rather weak root system, wigeongrass usually grows better in lagoons and bays where current flow is less than in channels, main basins, and tidal rivers (Ferguson Wood 1959; Reed 1979; Congdon and McComb 1981). Kerwin et al. (1976) speculated that the flushing action of river water following tropical storm Agnes could have been a factor in decreased wigeongrass in Chesapeake Bay.

In some cases wigeongrass can be extremely robust in areas of considerable current flow. Saunders and Saunders (1981) found some of the most luxuriant and productive stands of wigeongrass in Mexican lagoons where currents swept flocculent silts out to sea. They also suggested that habitat for wigeongrass and other choice submersed plants eaten by wintering waterfowl improves in subtropical lagoons when hurricanes scour away soft silts and flush out beds of floating pest plants. Conover and Gough (1966) and Richardson (1980) attributed the robustness of wigeongrass in areas of current flow to a better supply of nutrients and dissolved gases to leaf surfaces and the near absence of epiphytes. Wigeongrass beds fertilized in situ with P grow well in currents up to 4 cm/s (Harlin and Thorne-Miller 1981). Davis (1978) saw wigeongrass flourish and produce drupelets in areas with high rates of water flow but did not verify sexual reproduction.

Philip (1936) considered wigeongrass to have many features that adapt it to fluctuating water levels. The species occurs, sometimes in great abundance, in bays, lagoons, or channels with tides up to 1 m (Scott et al. 1952; Nixon and Oviatt 1973; Larrick and Chabreck 1978; Getsinger et al. 1982). It is common in - or sometimes almost restricted to - intertidal zones exposed to air up to 4 h daily (Johnson and York 1915). Keddy (1987) found wigeongrass at sites exposed up to 6.96 h at each low tide. Where exposure times are greater, such as in drained pannes or dessicated inland wetlands, wigeongrass quickly disappears (Bourn and Cottam 1950; Chapman 1960; Bolen 1964; Congdon and McComb 1979). In British Columbia, Bigley and Harrison (1983) observed that exposure of wigeongrass beds to air in tidal areas results in fewer shoots, less drupelet production, and earlier flowering. Nevertheless, length of the life cycle remains the same in plants found lower in the intertidal zone. McCann (1945) believed that wigeongrass would die quickly if exposed to direct sunlight.

Stable water provides good growing conditions for wigeongrass in managed wetlands; however, water circulation and incremental water-level increases may be required (Singleton 1951; Beter 1957; Prevost 1987). The plant withstands prescribed drawdowns for wildlife management purposes, but excessive or irregular water level fluctuations that expose bottom soils for long durations eliminate existing stands or cause great difficulty in establishing new stands (Joanen 1964; Joanen and Glasgow 1966). When tidal and seasonal water inundation was restored to Florida impoundments, wigeongrass was replaced by annual and perennial glassworts (Salicornia spp.) and black mangrove (Avicenna germinans; Gilmore 1987).

Water level fluctuations can affect wigeongrass indirectly by influencing water chemistry. Kimble and Ensminger (1959) reported that abnormal high tides during a hurricane probably distributed wigeongrass into interior marshes where the influx of saline water and slow runoff created favorable conditions for growth. Conversely, water level increases between growing seasons in subsaline prairie wetlands often result in the replacement of wigeongrass by luxuriant growths of the less salinity-tolerant sago pondweed and muskgrass (H.A. Kantrud, personal observation).

I found no references on the effects of ice action on wigeongrass. I noted little change in the distribution of wigeongrass in highly saline North Dakota wetlands as long as the area inundated remained similar between growing seasons. These wetlands freeze to the bottom every winter, and their wigeongrass populations behave as annuals, producing many drupelets. Although there is no evidence, wind-driven ice or "ice lift" of bottom sediments (Martin and Uhler 1939) possibly could be a factor in the distribution of wigeongrass in deeper waters where the plant likely would grow as a perennial and depend on overwintering rhizomes for reproduction.

Bottom Substrate

Texture

The influence of light, temperature, exposure, and salinity on wigeongrass was so large that Luther (1951 in Verhoeven 1979) and Verhoeven (1979) considered substrate preferences to be of secondary importance. Nevertheless, it is probable that long diffusion distances and low rates of diffusion and exchange of nutrients are important factors limiting growth of submersed macrophytes in coarse bottom substrates (Barko and Smart 1986).

McAtee (1939) stated that wigeongrass grew in bottom sediments ranging in texture from sands to mucks. In fact, wigeongrass can easily be grown without sediment (Setchell 1924; Seeliger et al. 1984; Thursby 1984a; Thursby and Harlin 1984), and plants can lose all roots in highly reduced organic soils and grow on the water surface (Conover 1964a). Despite these observations, I present information here that may be useful to wetland managers regarding on possible interactions between wigeongrass and substrate.

Bottom substrate texture is related to physical and chemical conditions, so it is difficult to prove that texture per se is important in the distribution of submersed hydrophytes. For example, Higginson (1965) related the distribution of Ruppia spiralis, Halophila ovalis, and Zostera capricorni in some nutrient-rich, coastal Australian lakes to sediment nutrients, organic matter content, minerals, and texture; and water depth. He found the presence of pure Ruppia stands closely related to areas of greater water depth, higher sediment clay and organic matter content, and lower sediment sand content. He concluded that: (1) concentrations of nearly all nutrients and minerals were highest in sediments of greatest clay content, but there was no evidence that this increased fertility was caused by chemical rather than physical characters; (2) the zonation of the plants was the result of differences in sediment conditions; and (3) an interaction of depth and sediment type adequately explained the distribution of the plants.

The effects of sediment texture may interact with salinity. In Alberta, Canada, Husband and Hickman (1989) found that frequency of occurrence of a perennial-like Ruppia depended on sediment texture in two mixosaline lakes, but not in a freshwater lake, where the plant was found primarily on coarse-textured substrates. When in freshwater, the absence of the plant from fine-textured sediments, was not correlated with the abundance of other macrophytes. Local abundance in relation to sediment texture was similar among lakes. Abundance was not significantly correlated with lake salinity, except on sandy sites. They suggested that the effects of salinity on the colonization of new sites, rather than the performance of the plant within sites, was important in determining the distributional limits of the plant.

Table 4 lists the predominant substrate texture for many stands of Ruppia maritima s.l. worldwide. Nearly all wigeongrass that grew in waters deeper than 2.0 m occurred on sand or shell bottoms, whereas all records for clay and silt bottoms were in waters < 1.5 m deep. This is probably attributable to differences in light attenuation of waters overlying sediments of varying susceptibility to resuspension by wave action.

Martin and Uhler (1939) considered wigeongrass to be more tolerant of firm sand than any other submersed plants eaten by waterfowl. Ruppia often grows well on sand in thalassic nearshore flats, bays, fjords, and estuaries and in the rivers that empty into them (Olsen 1945; Conover 1958; Kornas et al. 1960; Strawn 1961; Philipp and Brown 1965; Muus 1967; Tenore 1972; Copeland et al. 1974; Van Vierssen 1982a). Coastal wigeongrass populations studied by J. L. Sincock (1965, unpublished data) grew best on sand, followed by shell, loam, and silt; plants were less frequent on clay, muck, and peat in an area where turbidity limited growth. Shell or muddy sand support abundant wigeongrass growth in a spring-fed coastal river in Florida (Phillips 1960a). Pulich (1985) indicated that wigeongrass adapts to nutrient-poor substrates containing little organic matter but up to 72-98% sand and shell. However, Thorne-Miller et al. (1983) found better growth in fine sands containing substantial organic matter. Wigeongrass also commonly occurs on sandy bottoms in athalassic waters (Moyle 1945; Neel et al. 1973; McCarraher 1977).

Wigeongrass growth diminishes when natural sandy-organic sediments are replaced by washed sand, even when plants are submersed in the natural waters of their origin (Moyle 1945). In culture, Ruppia maritima s.s. achieves exponential growth earlier on sand than on mud even though plants growing in mud are nearly twice as heavy after 4 months of growth (Verhoeven 1979).

Silt bottoms in coastal lagoons and estuaries support wigeongrass (Koch et al. 1974; Dawe and White 1986). Silt (and marl) bottoms in mixosaline Alberta wetlands are generally high in frequency and abundance of a perennial Ruppia (Husband and Hickman 1989). Olsen (1945) and Gore (1965) opined that soft bottoms would not easily support wigeongrass because of the susceptibility of seedlings to wave action. Indeed, in a Massachusetts estuary, wigeongrass was absent on soft flocculated silts but present on nearby sands (Conover 1958). Similarly, many years of observations on wigeongrass in large lagoons in Mexico suggested that soft flocculated silts were inhospitable to the plant, whereas firm bottoms of a wide variety of other textural types supported luxurious stands (Saunders and Saunders 1981).

Clay bottoms, especially in sheltered areas, are favorable for wigeongrass (Joanen and Glasgow 1965; Pehrsson 1984). Swiderek (1982) found much higher wigeongrass production in South Carolina ponds with firm clay substrates than in ponds with soft bottoms subject to increased sedimentation and wind-induced turbidity. During the hot summer months, wigeongrass also persisted longer in the clay-bottomed ponds, and Swiderek recommended reserving these ponds strictly for propagation of wigeongrass rather than other waterfowl foods. Jensen (1940) believed Utah wigeongrass could not compete with Potamogeton pectinatus except on heavy clays with little organic content. Craner (1964) also found that Utah wigeongrass thrived on heavy clays but survived poorly when coexisting with P. pectinatus in silts and clay loams. Verhoeven (1979) associated western European wigeongrass with clay bottoms high in organic content (3-10% of dry weight). Density of wigeongrass was also highest on organic clays in Finland and clay-bottomed wetlands in the Netherlands (Van Vierssen 1982a). In Utah, organic clay bottoms and upland clay soils, when flooded artificially, quickly produced a fair crop of wigeongrass that followed an initial growth of Chara (Nelson 1954). In Australia, R. spiralis is monodominant only on bottoms high in clay content (42.3%); where amounts are less, mixed stands or other plants occur (Higginson 1965).

Wigeongrass is moderately productive on loam bottoms according to J. L. Sincock (1965, unpublished data). A higher standing crop of Ruppia maritima s.s. grew on mud than on sand; Verhoeven (1979) attributed this to the higher nutrient content of the former substrate. Ruppia cultured by McRoy and McMillan (1977) showed better survival on fine sandy loam than on river sand and plants flowered only on the loam. I found no other specific references to loam, but "muds" - especially those of a sandy or silty nature - were often mentioned as wigeongrass habitat (Klavestad 1957; Tabb et al. 1962; Eleuterius 1971; Getsinger et al. 1982).

Bottoms of fibric (peats), hemic (mucks), or sapric (sapropels or gyttja) organic materials often support wigeongrass (McAtee 1939; Stieglitz 1966; Verhoeven 1979; G.S. Gidden, 1965, unpublished data), and the plant sometimes is the most common submersed macrophyte on these bottoms (Spiller and Chabreck 1976; Van Vierssen 1982a). Some of the highest biomasses of wigeongrass occur on sediments high in organic content (up to 10 kg/m² dry weight or 57% organic matter in a 3-5 cm core) (Higginson 1965; Nixon and Oviatt 1973; Edwards 1978). In saline Saskatchewan lakes, the best beds of Ruppia occur where clay bottoms are covered by a layer of organic matter (Tones 1976). Heitzman (1978) noted luxuriant wigeongrass growth on firm organic bottoms, as long as they remained free of silt and detritus. However, Mahaffy (1987) saw better wigeongrass growth in sediments that contained < 3% organic matter.

In summary,with the possible exception of rubble or bedrock, wigeongrass can grow on all common bottom substrates found in nature. Flocculated silts probably are the least favorable bottom substrate for wigeongrass growth. Under highly reducing conditions, plants lose their root system but can sometimes live suspended in the water column in sheltered wetlands. Plants will be found in deeper water and will be less subject to wave damage where bottoms are firm or coarse-textured and less subject to particle resuspension. However, plants growing in protected areas where bottoms are usually fine-textured and rich in nutrients and organic matter will produce greater biomass.

Sedimentation and Disturbance

After erosion carries particles of upland soils into wetlands, the newly deposited sediments move by wave action to central deeper areas or are trapped by vegetation in sheltered peripheral areas. No mechanism may exist to move sediments in extremely protected areas. Colloidal particles tend to flocculate with increasing salinity, thereby increasing water clarity. It is in riverine habitats and shallow areas subject to long wind fetches that submersed macrophytes are most likely damaged by sedimentation (Vicars 1976; Bellrose et al. 1979). Plants with highly dissected leaves can easily be crushed or coated by sediment and are at a disadvantage to linear-leaved species such as wigeongrass (Schiemer and Prosser 1976; Vander Zouwen 1982). Millard and Scott (1953) saw wigeongrass prosper in portions of a South African estuary that experienced nearly constant inflows of fine silts and colloidal clays whenever surface water was present, but they did not measure silt deposition rates.

There is some evidence - but no experimental data - that wigeongrass is quite tolerant of disturbance. Chapman (1960) remarked about luxuriant beds of wigeongrass in Iraqi waters much disturbed by water buffalo (Bubalis bubalis). Breuer (1961 in Cornelius 1975) mentioned that the plant occurs around emergent deposits of dredge spoils in the Laguna Madre, Texas. Ward and Armstrong (1980) predicted that turbidity caused by dredging of a Texas lagoon would only temporarily lower wigeongrass productivity, and might, after plant recovery, increase it - but no subsequent surveys were conducted. In Nebraska, wigeongrass has been noted in excavated, flooded sandpits (Larson and Martin 1972).

Chemistry

Wigeongrass propagules occur in chloride-dominated bottom substrates that contain up to 7.2% salts, although vegetation frequently dies back at lower concentrations (Flowers 1934; Jensen 1940; Millard and Scott 1953; Bolen 1964; Gore 1965; Flowers and Evans 1966; Ungar 1968; Percival et al. 1970). Wigeongrass grows in coastal Louisiana sediments with 0.89-2.99% salinity; maximum growth occurs at 0.89-1.72% (Joanen 1964; Joanen and Glasgow 1965). It was suspected that growth by rhizomes was responsible for maintenance of large stands where sediments contained > 1.12% salts because drupelets germinated poorly.

Sediments that support wigeongrass vary in pH from 3.1 to 8.8 (Table 7). Wigeongrass prospers at the lower end of this range if water column pH does not fall below 6.5 (Wilkinson 1970). Nevertheless, acidification of bottom substrates, a result of oxidation of iron polysulfides, is a serious problem for managers of wigeongrass impoundments in the southern United States (Neely 1958, 1962; Swiderek 1982).

Better growths of wigeongrass can occur in sediments where concentrations of inorganic nitrogen and phosphorus are highest (Conover 1958). In infertile waters, sediments are an important source of nutrients for wigeongrass (Husband and Hickman 1985). In eutrophic waters, wigeongrass probably does not depend on sediments for nutrients, even though roots are active assimilation sites and root development is a direct function of the chemical environment where the plants occur (Conover 1964a; Conover and Gough 1966).

Nitrogen concentrations as high as 4.7 mg/g have been found in sediments supporting wigeongrass (Neel et al. 1973; ,a href="table7.htm">Table 7). Joanen and Glasgow (1965) measured lower levels of available sediment P in wetlands that yielded larger amounts of wigeongrass and, within individual wetlands, lower amounts of P inside wigeongrass stands than outside them. This suggested active uptake of this essential element by wigeongrass roots. Levels of sediment K in stands of wigeongrass did not change during 10 months of this study. Verhoeven (1979) found no relationship between K concentrations of three Ruppia taxa and the amount of K in either the sediment or water column.

Sediment Mg and Ca showed little or no change during 10 months in the wigeongrass ponds studied by Joanen (1964) and Joanen and Glasgow (1965). Verhoeven (1979) found that, although the Mg content of Ruppia plants show no relation to environmental Mg, the Ca and Na content of the plants relate to that of the sediments and water column. Higginson (1965) found R. spiralis associated with greater amounts of sediment N, K, Mg, Fe, and organic matter, but with lesser amounts of Ca.

It is likely that Ruppia plants favor aerobic sediments with low levels of sulfides and free H₂S (Conover and Gough 1966; Baldwin 1968; Lipkin 1977; Davis 1978; Pulich 1989). Nevertheless, wigeongrass frequently occupies reduced sediments where leaves supply oxygen to the roots. Plants without rhizome systems can grow suspended in ooze in extremely reduced sediments (Conover and Gough 1966). Such beds must occur only in very sheltered locations.

Concentrations of micronutrients and trace elements found in natural sediments supporting wigeongrass are shown in Table 7. Similar data for these and many other less common elements are available for irrigation drainwater evaporation ponds in California (Severson et al. 1987; Schroeder et al. 1988).