ASSESSING THE IMPACTS OF MACROALGAL COMPETITION AND PARROTFISH GRAZING ON CORAL COVER IN THE MIDDLE FLORIDA KEYS

The coral reefs of the Florida Keys National Marine Sanctuary (FKNMS) are diverse ecosystems that support thousands of marine organisms. Unfortunately, the live coral cover on these reefs has dramatically decreased over the last 30 years even in the most protected areas of the sanctuary. Environmental changes have contributed to this decline in coral cover by creating optimal conditions for macroalgal growth. These macroalgae species are fast growing and out-compete slow growing corals for space and nutrients. Herbivores known to forage on macroalgae, including long-spined sea urchins and parrotfishes, have decreased in abundance. A synthesis of data from across the Caribbean has suggested that this decline in coral is due to a top-down cascade where a decrease in herbivores has led to increase in macroalgae which out-competes corals. For my thesis research, I tested this Herbivore Cascade model by conducting surveys across 34 reef sites in the FKNMS and by an herbivore-exclusion coral transplant experiment. At each reef, macroalgae and coral cover was estimated by analysis of photo quadrants and parrotfish density estimated by a diver survey. Reefs were then classified into four reef types based on their depth, structural complexity and substrate composition. Analysis of all reefs together did not find negative correlations of macroalgae vs. corals nor parrotfish vs. macroalgae. However, these relationships varied within each of the four reef types. For example, within Hawk’s channel patch reefs, I found strong evidence of macroalgae - coral competition, where percent coral cover was significantly lower on reefs where macroalgae cover was high. I also found a significant positive relationship between macroalgal cover and total parrotfish biomass on offshore shelf reefs. These patterns were also corroborated by the results of my herbivore-exclusion coral transplant experiment. Eighty-four corals of two different species were transplanted to seven different reefs and placed within either open or closed topped cages with and without macroalgae addition. Pictures of each transplanted coral were taken immediately after transplanting and then quarterly for one year. Net coral growth for both species was essentially zero and not influenced by macroalgal abundance, parrotfish herbivory, or reef type. The results of these two studies do not support the predictions of the Herbivory Cascade Model and suggest that coral survival may be influenced by different factors on different reef types depending on their community structure. By identifying the factors that limit coral growth at each reef type, I can make better conservation decisions about strategies for transplanting corals or the benefits of increased protection for herbivorous fishes