MODELING THE NICHE SPACE OF CORAL ASSEMBLAGES ALONG THE FLORIDA REEF TRACT
McCaffrey, Kelly Renee
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Quantifying the niche space of organisms is critical for conservation and restoration purposes. Yet, with warming oceans, we cannot assume that the localities that were optimal for species in the past remain so today or will be optimal for that species in the future. Over the next century, the climate is predicted to drive sea-surface temperatures to even higher levels, consequently increasing the risk of mass coral bleaching and disease outbreaks, and potentially reducing the niche space available to corals. Yet, there is considerable temporal and spatial variation in coral bleaching, disease prevalence, and mortality. Using data collected from 2398 sites along the Florida reef tract from 2005-2015, this study examined the temporal and spatial patterns of coral bleaching and disease in relation to coral colony size, depth, temperature, and chlorophyll a concentration. The study also constructed a coral niche model for the threatened coral species Acropora cervicornis in Florida, to determine optimal environmental conditions for its survival. The niche model was developed at three spatial scales, 9 km, 4 km, and 1 km, using depth, wave exposure, irradiance, turbidity (Kd490), sea-surface temperature, and chlorophyll-a concentration as predictive covariates. The results of this study show that coral bleaching was most prevalent during the warmest years in 2014 and 2015, and disease was also most prevalent in 2010, 2014, and 2015. The majority of the coral colonies surveyed, independent of their size, were found in habitats with low chlorophyll-a concentrations, and high irradiance, and these same habitats showed the highest prevalence of coral bleaching and disease outbreaks during thermal stress events. The species distribution model indicated that the most optimal locations for Acropora cervicornis were in the upper and lower Florida reef tract. Although the geographical patterns of the model results did not vary with an increase in the resolution of the predictive variables, the highest resolution models predicted a higher latitudinal extent of Acropora cervicornis than the low-resolution (9 km) model, with favorable conditions extending up to 27-degrees north. The variable that best predicted Acropora cervicornis at 1 km was chlorophyll-a concentration, with colonies mostly supported in habitats with chlorophyll a concentration <1.25 mg m-3. At a 4-km resolution, chlorophyll a concentration had a negative relationship and irradiance had a positive relationship with A. cervicornis occurrence. The results show that well-lit habitats with low chlorophyll a concentrations favor Acropora cervicornis on the Florida reef tract. Yet, the results also suggest that directional selection in a warming ocean may favor corals able to tolerate inshore, shaded environments with high turbidity.