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 intrusions 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 Ͼ 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 dinoflagellates (Symbiodinium spp.) in their tissues (Trench and Blank 1987;Rowan and Powers 1991). In water Ͻ10 m deep, healthy individuals of most coral species produce more oxygen than they consume . Montastrea annularis maintains its daily integrated net production : respiration ratio considerably above 1.0 (Achituv and Dubinsky 1990;Patterson et al. 1991; Porter et al. in prep.). Multiple stressors can lower this P : R below unity, thereby reducing the growth rate and reproductive capacity of reef-building corals. These stressors can include nutrient enrichment (both N and P), turbidity (caused by living and non-living suspended particulate matter), sedimentation, salinity, and temperature extremes.None of the lower invertebrate groups, including the cnidaria, are capable of either osmoregulation or homoiothermy (Hoegh-Guldberg and Smith 1989). Short-term effects of either salinity or temperature stress can be exhibited as changes in basal metabolic function, including effects on animal respiration and symbi...

Colonies of the hermatypic coral Pocillopora damcornis were collected from the shallow reefs of Kaneohe Bay, Hawaii, to assess the wavelength-dependent effects of ultraviolet (UV) radiation on photosynthesis. Measurements of photosynthesis and respiration were made while corals were exposed to different UV irradiances, keeping visible radiation constant, using long-band pass filters. A differential action spectrum (biological weighting function) for the inhibition of photosynthesis by UV radiation was then determined for P darnicornis The action spectrum revealed an increase in the wavelength-dependent effects of UV radiation on photosynthesis between 290 and 310 nm that is greater than those increases reported for action spectra on natural assemblages and unialgal cultures of marine microalgae. The greater effect at these wavelengths is a result of the high biologically effective doses of UV radiation experienced by these corals on shallow reefs, and the decrease in the absorbance of UV radiation by UV absorbing compounds found in the host tissues and algal symbionts between 290 and 310 nm. The irradiances of wavelengths in the region between 290 and 310 nm are those which will increase in the event of any decrease in stratosphenc ozone over equatonal regions. If the observed sensitivity of P. damicorn~s in this spectral region is common in other species, it may have important consequences for growth, reproduction, and occurrence of the bleaching phenomenon for shallow water corals.