The Effect of Multiple Stressors on the Florida Keys Coral Reef Ecosystem: A Landscape Hypothesis and a Physiological Test

Changes in land use and water management practices in south Florida have altered the quality and quantity of freshwater flowing into Florida Bay. By the 1980s, reduced inflow and drought led to an extensive hypersaline phase in the bay. This phase had a drastic effect on benthic communities within the bay and possibly also on coral communities within the bay and the Florida Keys National Marine Sanctuary. Physical oceanographic measurements demonstrate the presence of warm, hypersaline, and turbid water on coral reefs offshore from the Florida Keys, especially near passes which conduct water from Florida Bay to the Atlantic Ocean. To examine the effect of Florida Bay water intmsions on coral reefs, we tested for significant effects of two stressors, elevated temperature and salinity, on coral production, respiration, and survival. Elevated temperatures produce significant reductions in photosynthesis, respiration, and net P: R ratios after 6 h of exposure, and elevated salinities produce similar results after 30 h. Exposure to both elevated temperature and salinity produces a highly significant (P > 0.01), but short-term, mitigative interactive effect. The combination of the two stressors was less stressful (for the response variables measured) than the sum of the stressors acting independently. After 36 h of exposure, however, the mitigating effect disappeared and corals exposed to the combined stresses did not survive. A three-dimensional response surface, which predicts P: R ratios as a function of varying salinity and temperature, is used to construct a testable hypothesis to explain recent declines in coral cover on some reefs within Florida Bay and the Florida Keys. We chose salinity and temperature to test a multiple stressor model because they are relatively easy to manipulate. However, any enviromentally realistic model must include other potential stressors, such as turbidity, elevated nutrients, and environmental contaminants. Reef-building corals harbor endosymbiotic photosynthetic als have usually been confined to single factors such as temdinoflagellates (Symbiodiniurn spp.) in their tissues (Trench perature (Mayer 1914; Edmondson 1928; Clausen and Roth and Blank 1987; Rowan and Powers 1991). In water <1