Increased Sediments, but not Nutrients, may Facilitate Dominance of Halimeda Opuntia Through Interactions with Light on Fringing Reefs in the South Pacific

Shalanda Grier1, Symone Gyles1, Joanna Marrufo2, Shayna Sura3, Paul H Barber3 and Peggy Fong3, (1)Hampton University, Hampton, VA, United States, (2)University of California Berkeley, Berkeley, CA, United States, (3)University of California, Los Angeles, Ecology and Evolutionary Biology, Los Angeles, CA, United States
Abstract:
Phase-shifts from coral to algal communities are occurring worldwide on tropical reef systems, making it important to understand the ecological processes that may promote and maintain algal dominance. Two anthropogenic stressors, increased sedimentation and nutrient inputs, may interact with light availability to facilitate algal dominance and may also support a diverse microbial community on the algal’s thallus. We conducted paired 3 factor fully-crossed field and mesocosm experiments varying light (+/- shade), nutrients (+/- fertilizer), and sediments (+/-) to determine their effects on growth of a common calcifying green alga, Halimeda opuntia, and its microbial epiphytes. The field study was on a shallow back reef habitat comprised of dead coral heads now dominated by algae, while the mesocosm experiment was in a flow through water table. In both experiments, there was a significant interaction between light and sediment, while nutrients had no effect on growth. However, in the mesocosm experiment, sediments had a strong positive effect on growth in the light but not in the shade, suggesting sediments may have provided protective shade in this high light environment. In contrast, in the field, sediments had a negative effect in ambient light, while growth was overall lower and more variable in the shade, suggesting that shading by sediments was negative in this environment. Further, metagenomic analysis of the microbial community in the field experiment revealed an increase in the relative abundance of Cardiobacteriaceae in shaded treatments. Our results suggest that anthropogenic increases in sediments interacting with light may allow Halimeda opuntia to dominate shallow reef zones that were previously dominated by coral and may contribute to changes in the algal microbiome. Thus, our work suggests that future conservation efforts need to encompass limiting sediment fluxes to fringing reef systems.

* first 3 authors contributed equally

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