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. Under 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 manipulations in sediment sulfide concentration and light regimes was examined 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 and lowered using FeSO4. Photosynthesis vs, irradiance (PI) relationships were determined experimentally at ten light levels throughout the 21 day experiment. Photoadaptation was detected in response to the previous 4 day light history of the plants, as maximum photosynthesis (P-max) 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 photosynthesis equals respiration, and decreases in the initial slope of the PI curve. The effects of eutrophication through reduced light and increased sediment sulfide on P-max were additive. Elevated sediment sulfide levels may contribute tb seagrass loss in stressed areas as the potential for utilization of available light is reduced.