The effect of multiple stressors on the Florida Keys coral reef ecosystem: A landscape hypothesis and a physiological test

Abstract

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...

The future of coral reefs

“…Similarly, the collapse of reef ecosystems along the north coast of Jamaica seems to have resulted from the synergistic interaction of overfishing and disease (37). Unfortunately, our understanding of how multiple stressors interact remains limited (94,95).…”

MODIS-derived spatiotemporal water clarity patterns in optically shallow Florida Keys waters: A new approach to remove bottom contamination

“…The Middle Keys are considered a transition zone between these two current regimes (Lee, 2012;Lee & Williams, 1999;. General circulation patterns also show large scale inundation of the FRT with Florida Bay waters occurring through a few channels between islands of the Florida Keys, especially Moser Channel, Long Key Channel, Channel #2 and Channel #5 (Smith, 1994;Porter et al, 1999;Fig. 2).…”

Ecological Shifts along the Florida Reef Tract: The Past as a Key to the Future

“…The inimical-waters explanation of Ginsburg and Shinn (1964) may also be rooted in pulses of high-temperature water flowing through the tidal passes (Porter, Lewis, and Porter 1999). Shallow, lagoonal waters equilibrate rapidly with the atmosphere relative to the more deeply mixed oceanic waters that front the reef (Neumann and Macintyre 1985).…”