The effects of water flow and sedimentation on interactions between massive Porites and algal turf.
Interactions between scleractinian corals and benthic algae can be an important process structuring coral reef communities. The effects of algae on corals and vice versa are not fixed and may be influenced by abiotic factors such as sedimentation, a process often underlying reef degradation. In addition, the rate of sedimentation as well as the effects of trapped sediments may be mediated by water flow, which may influence coral-algal interaction dynamics. The first goal of this study was to test the hypothesis that gradients in sedimentation and flow on the fringing reefs and back reefs of the north shore of Moorea, French Polynesia, correlate with gradients in the frequency of massive Porites-turf interactions, as well as the outcomes of these interactions. In the back reef, Porites-algal turf interaction frequency increased by 73% as sedimentation decreased and flow increased. Along fringing reefs, interaction frequency increased by 97% as sedimentation increased. Results indicated that flow and sedimentation do not interact to influence the frequency or outcome of interactions, but rather exert significant independent effects. The second goal of this research was to test the separate and joint effects of flow and sedimentation on coral-algal interactions in a controlled laboratory setting. Results indicated that despite extensive damage to coral tissue across treatments, Porites growth as measured by the change in dry weight did not differ across treatments. This led to the hypothesis that massive Porites may allocate resources preferentially to the growth of tissues not in direct contact with algal turf, thus accounting for the maintenance of growth rates across treatments. The third goal, therefore, was to test this hypothesis in the field along gradients of flow and sedimentation, and sedimentation alone, for 1 year. Coral growth was quantified as linear extension in three directions: vertical, and horizontal away from and towards the zone of interaction with algal turf. Results indicated that as sedimentation increased, both vertical extension and horizontal extension adjacent to the interacting algal turf decreased by approximately 80%. In contrast, growth of tissue away from the algal turf interaction doubled as sedimentation rate increased. Overall, these data suggest that massive Porites may be able to compensate for the reduction in growth along the zone of interaction by increasing growth rates of tissues away from the interacting turf. The fourth and final goal was to identify a possible mechanism underlying the effects of sediment trapping by algal turf on corals. The hypothesis that spatial variation in sedimentation at the scale of a coral-algal interaction might be an additional important mechanism by which sediments influence coral-algal interactions was tested. For this study, the relative effects of sediment accumulation along interaction boundaries, on the surface of coral and algal tissue not in direct competition, and all possible location combinations was quantified. Results indicate that the accumulation of sediments along the coral-algal interface reduces the area of coral tissue damaged by 56% when sediments also are present on the non-interacting algal turf surface. Knowledge of how fine-scale patterns of sediment deposition may affect the competitive outcome of coral-algal interactions has implications for the study of coral-algal dynamics as well as increasing understanding of the relative importance of sediment trapping by algal turf. Together, these results suggest that flow and sedimentation exert significant, individual, direct and indirect effects on interactions between massive Porites and algal turf.