USGS Home
Contact USGS
Search USGS

Times of maximum light and temperature may not be in phase and can pose some problems for wigeongrass. Conover (1958) noted that maximum wigeongrass biomass coincided with the slightly lower insolation rates associated with the time of highest water temperature in a Massachusetts estuary. Conversely, in Chesapeake Bay, wigeongrass occuring at sites where light exceeds photosynthetic saturation levels may be temperature-stressed and attain higher biomass later in the growing season when water temperatures are lower (Wetzel and Penhale 1981). Koch et al. (1974) found that, where algae are present, relatively low light levels can stimulate epiphytes and suppress wigeongrass growth. Wigeongrass retains some oxygen in the lacunar system for use in respiration, and an oxygen supply to the roots is essential in the anaerobic, highly reducing sediments characteristic of wigeongrass habitat. At such sites, roots can decay from lack of photosynthetically derived oxygen if the supply is reduced by cloaking epiphytes (Richardson 1980). The presence of oxygen-bearing lacunae in the roots would be especially important to allow survival of perennially-behaving populations during dormant periods. An oxygen supply to wigeongrass roots may also help mediate the absorption of phosphorus (P) in anaerobic sediments (Conover 1964a). Culture experiments of Thursby (1984b) show that wigeongrass roots often release oxygen. The resultant nitrification around the root zone probably is not an important source of nitrogen, however, as the roots seem best adapted to take up ammonia, rather than nitrates or nitrites (Thursby 1983). Instead, the primary function of the oxidized layer may be to reduce the potential for manganese (Mn) or iron (Fe) toxicity or to render harmless the H₂S or other substances found in anaerobic bottoms (Thursby 1984b).

Culture experiments show that wigeongrass leaves and roots take up ammonia and phosphate, but that root-to-shoot translocation predominates (Thursby and Harlin 1984). Uptake of nitrate was negligible when ammonia was supplied to roots. However, wigeongrass may rely mostly on inorganic nutrients. Pulich (1989) showed with culture experiments that low levels of inorganic nitrogen and phosphorus supplied to wigeongrass by way of the sediments resulted in development of a rhizome system with short shoots, extensive roots, and higher leaf production than with sediments containing high levels of organic nutrients, which produced plants with reduced root biomass, long branching shoots, and lower leaf production. In the same study (Pulich 1989), inorganic nutrients were ineffective in supporting growth of the seagrass Halodule wrightii which required organic nitrogen for vigorous growth. These plants grew in mixed beds in a polyhaline lagoon where the wigeongrass grew most vigorously during cool spring and fall months in sediments low in free H₂S, and the Halodule wrightii was most productive during warm summer months on more reduced, organic rich sediments. Therefore, there is experimental evidence that differential responses to sediment sulfate reduction are involved in competition between these two species. Thursby (1984a) lists concentrations of major nutrients, vitamins, and trace metals required for long-term culture of wigeongrass.

Wigeongrass effectively uses the HCO₃ ion as a source of carbon (C; Sand-Jensen and Gordon (1984). At seawater levels of dissolved inorganic C, photosynthesis was highest at pH 7.0-7.5, was maintained at fairly high levels at pH 7.5-9.0, but decreased rapidly to zero at about pH 10.2.

The epidermal leaf cells of wigeongrass probably are modified to absorb both cations and anions for osmoregulation (Jagels 1983; Jagels and Barnabas 1989). This evidence seems to refute Husband and Hickman's (1985) contention that saline conditions are a requirement for maximum growth. Jagels and Barnabas (1989) also stated that wigeongrass likely turns white and dies under conditions of high temperature and widely varying salinity because of the additional energy required for increased osmoregulation. Brock's (1979) hypothesis - that the amino acid proline serves in osmoregulation in wigeongrass - was confirmed by Pulich (1986), who speculated that the substance could also help salinity-stressed plants maintain NH₄ levels.

In summary, the known physiological characteristics of wigeongrass support Verhoeven's (1979) contention that the plant has little competitive strength outside it's rather well defined ecological niche. The plant adapts poorly to dimly lit waters or anaerobic sediments, but has specialized features enabling survival under varying salinities and high temperature beyond those tolerated by other submersed angiosperms. Although most of the physiological evidence comes from in vitro experiments, it seems evident that, to produce large amounts of wigeongrass, managers must provide shallow, clear waters and probably expect significantly lower production from (1) relatively small increases in turbidity or (2) lower temperatures because of excessive water depth. Problems with epiphytic algae may also occur in highly fertile waters.