RELATIONSHIP OF SEDIMENT SULFIDE TO MORTALITY OF THALASSIA-TESTUDINUMIN FLORIDA BAY

Sediment porewater sulfide concentrations in Florida Bay seagrass beds affected by the catastrophic mortality of Thalassia testudinum (Turtl e-grass) were considerably higher than those of seagrass beds in the I ndian River, Charlotte Harbor, or Tampa Bay. Sulfide concentrations in apparently healthy seagrass beds were highest in fall and might have contributed to chronic hypoxic stress of Thalassia roots and rhizomes. High porewater sulfide concentrations measured in dying areas of seag rass beds suggest that sulfide produced by microbial degradation of dy ing Thalassia might exacerbate stress on adjacent, surviving seagrass. Sulfide concentrations in recent die-off areas initially were higher than in adjacent, surviving grass beds. By the end of the study, howev er, the pattern was reversed apparently due to depletion of Thalassia- derived organic matter in the sediments of die-off areas. In June 1990 , high sulfide concentrations preceded a die-off episode at one site, suggesting (1) elevated sulfide concentrations might be involved in a suite of factors that trigger die-off episodes or (2) elevated porewat er sulfide results from death and decomposition of belowground Thalass ia tissue before necrosis of shoots becomes visible. In either case, e levated porewater sulfide concentrations might be of value in predicti ng die-off. We conclude that porewater sulfide probably is not the pri mary cause, but a synergistic stressor, which has acted in concert wit h factors (such as hyperthermia, hypersalinity, and microbial pathogen s) suggested by other researchers, to cause Thalassia die-off in Flori da Bay.