PHOTOSYNTHETIC RESPONSES OF EELGRASS (ZOSTERA-MARINA L) TO LIGHT AND SEDIMENT SULFIDE IN A SHALLOW BARRIER-ISLAND LAGOON

Highly reducing sediments are prevalent in seagrass environments. Unde r anoxic conditions, hydrogen sulfide can accumulate as an end product of anaerobic respiration at levels which may be toxic to halophytes. The photosynthetic response of Zostera marina L. (eelgrass) to manipul ations in sediment sulfide concentration and light regimes was examine d in Chincoteague Bay in June 1991. Neutral density screens were used in a mesocosm experiment to decrease downwelling irradiance to 50 and 15% of insolation. Sediment sulfide levels were enriched using Na2S an d lowered using FeSO4. Photosynthesis vs, irradiance (PI) relationship s were determined experimentally at ten light levels throughout the 21 day experiment. Photoadaptation was detected in response to the previ ous 4 day light history of the plants, as maximum photosynthesis (P-ma x) decreased in response to lower daily light levels. Negative impacts of sulfide on eelgrass in this study were observed through reductions in P-max increases in the light intensity at which gross photosynthes is equals respiration, and decreases in the initial slope of the PI cu rve. The effects of eutrophication through reduced light and increased sediment sulfide on P-max were additive. Elevated sediment sulfide le vels may contribute tb seagrass loss in stressed areas as the potentia l for utilization of available light is reduced.