Effects of Interspecific Competition Between Increasingly Dominant Peyssonnelid Algal Crust and Juvenile Massive Porites spp.

Megan Williams, California State University Northridge, Biology, Northridge, CA, United States and Peter Edmunds, California State University Northridge, Department of Biology, Northridge, CA, United States
In light of the increasing effects of climate change on coral reefs, understanding the roles of competitive interactions between corals and other taxa has become a priority, as reefs have shifted from spatial dominance by hermatypic corals to other taxa, most often macroalgae. Recently, encrusting macroalgae described as “peyssonnelid algal crust” (PAC) have dramatically increased in abundance on shallow reefs throughout the Caribbean, likely due to PAC’s strong capacity for rapid growth and spatial competition. The broad objective of this study aims to evaluate the role of PAC in mediating coral community maintenance and recovery on Caribbean reefs. On the shallow reefs of St. John, US Virgin Islands, PAC coverage was significantly affected by exposure and depth, with PAC covering 63.6% of the benthos at 3 m exposed sites, and only 1.6% at 9 m sheltered sites. However, independent of depth or exposure, PAC was overgrowing coral in 77.5% of the PAC-coral interactions observed. Using Moorea, French Polynesia as a model system to further investigate the effect of these competitive interactions, an experiment was designed to measure the growth of massive Porites spp. in isolation versus adjacent to PAC over 3 weeks, specifically testing the hypothesis that the growth rate of juvenile corals is affected by proximity to PAC. Tiles were deployed on a shallow common garden in the back reef to which juvenile massive Porites spp. were attached in isolation, or in contact with a similar-sized fragment of PAC. Results found that proximity to PAC does not significantly affect the growth rate of juvenile corals. Understanding how PAC competes with other organisms within a reef community is critical in predicting how a future PAC dominated reef might function. The results of this study suggest that juvenile corals may be able to maintain presence on a reef and grow despite an increase in competitive interactions with PAC.