Sediment composition influences growth of and mediates herbivory pressure on turf algae on a fringing coral reef

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Shayna Sura, Abigail Dimaano, Shannon Eminhizer, Jewell Roth, Rachel Turba and Peggy Fong, Ecology and Evolutionary Biology, UCLA, Los Angeles, CA

Background/Question/Methods
Ecosystems worldwide are impacted by human activities that expose them to multiple anthropogenic stressors simultaneously. Because multiple stressors can cause nonlinear interactions, it is important to understand their independent and combined effects. Coral reefs are subject to increasing impacts of human activities, including global climate change, but also local activities such as fishing and runoff of nutrients and sediments from terrestrial systems. Healthy coral reefs contain closely cropped turf algae. However, if growth of turf algae is left unchecked, it can transition into long algal turfs, which represents a degraded state. Human stressors of overfishing, excess nutrients, and influx of terrestrial sediment may contribute to shifts from short to long algal turf on coral reefs. But few studies investigate their interactive impacts on turf, and none examine sediment composition. We used a fully-crossed three-factor field experiment to examine algal turf growth in response to nutrient addition (-/+), sediment composition (none, marine, terrestrial), and herbivore presence (+/-) over 19 days. We then examined the relative impacts of marine and terrestrial sediment (percentages of 75:25, 50:50, 25:75) on caged turf growth over 11 days. We conducted our study on a fringing coral reef in Moorea, French Polynesia.
Results/Conclusions
We found a significant three-way interaction between sediment composition, herbivory, and time. On days 7 and 13, both herbivore presence and terrestrial sediment had negative main effects on algal turf growth. By day 19, herbivory and sediment composition significantly interacted to affect algal turf growth. Without herbivory, both marine and terrestrial sediment reduced growth of turf algae compared to no sediment treatments. However, with herbivory, marine sediment protected turf from herbivores facilitating greater turf growth compared to other treatments. Nutrients had no main or interactive effects on algal turf growth. For our second experiment, we found a significant effect of sediment composition on turf growth by day 7 and it persisted until day 11. Turf algae grew the least when exposed to sediment comprised of a greater proportion of terrestrial than marine sources. Our study highlights the importance of sediment composition in understanding its impacts on turf algae and how it interacts with other human stressors. Further, we found interactions between multiple stressors develop over time. Thus, our study indicates that along with assessing multiple stressors, it is important to evaluate their interactions over time to fully understand their impacts.