EFFECT OF INCREASING PHOTON IRRADIANCE ON THE GROWTH OF VALLISNERIA-AMERICANA IN THE TIDAL POTOMAC RIVER

Following declines in submersed macrophyte populations in tidal ecosys tems, revegetation of areas devoid of macrophytes may be sudden and ra pid or may not occur for years, Declines of submersed macrophyte popul ations in the Chesapeake Bay and the tidal Potomac River have been att ributed to insufficient light in the water column; however, the role o f light in promoting revegetation has never been unequivocally documen ted, Photon irradiance was artificially increased for Vallisneria amer icana transplants in two unvegetated embayments in the otherwise veget ated freshwater tidal Potomac River: Pohick Bay and Belmont Bay. Pohic k Bay had high nutrient concentrations and frequent algal blooms, Belm ont Bay was broader and shallower than Pohick Bay with turbidity resul ting from wind-driven resuspension of sediment, The total number of pl ants of V. americana in the lighted cages was 7.5 times higher than th at in the unlighted cages at Pohick Bay and 11 times higher than that in the unlighted control cages in Belmont Bay, The biomass in the ligh ted cages was 11-fold higher in Belmont Bay and 38-fold higher in Pohi ck Bay than that in the control cages, Plants were less numerous and m ore robust in lighted cages in Pohick Bay than in Belmont Bay.