Physiological responses of transplants of the freshwater angiosperm Vallisneria americana along a salinity gradient in the Caloosahatchee Estuary (Southwestern Florida)

Fluctuations in freshwater input may affect the physiology and survival of submerged aquatic vegetation (SAV) occurring in oligoaline to mesohaline es tuarine regions. Controls on the distribution of the freshwater angiosperm Vallisneria americana, were investigated by transplanting ramets. Pots (3.8 -1) containing ramets were distributed among four sites (upstream site [lea st saline], donor site, near downstream site, and far downstream site [most saline]) in the Caloosahatchee Estuary (Southwest Florida) during wet (May -August) and dry (October-February) seasons. During 2-4 mo of each season, physiological indicators were monitored, including photosynthesis, glutamin e synthetase activity, and protein content in shoots, and carbohydrates and total nitrogen and carbon in shoot and subterranean tissues. Where the phy sical environment (light or salinity) was suboptimal, all physiological ind ices, except photosynthetic rate, showed similar stress responses, which ra nged from a slow decline to a rapid drop in physiological function. Levels of soluble carbohydrates decreased in response to unfavorable conditions mo re rapidly than did insoluble carbohydrates. Shoot protein of V. americana declined prior to transplant death, suggesting that measuring protein conte nt may provide a rapid assessment of physiological health. V. americana tra nsplants at the low-salinity upstream site died during both wet and dry sea son experiments, likely in response to light limitation and/or partial buri al by sediments. At the far downstream site, death occurred within 2-4 wk, and was attributable to elevated salinities (> ca. 15 parts per thousand). Comparison of physiological responses with salinity and light regimes at th e donor and near downstream sites suggest that light may ameliorate salinit y stress. This study demonstrates that V: americana, nominally classed as a freshwater macrophyte, is capable of a remarkable degree of halotolerance.