The influence of water flow on mechanisms underlying coral-algal interactions.
Interactions between corals and algae have been implicated in influencing the potential fate of coral reef ecosystems. Recently, studies have concentrated on the mechanisms underlying coral-algal interactions as indicators of the effects algae can have on corals. Several studies have focused on algal-mediated, microbial activity, which can cause hypoxic conditions and indirectly yield coral mortality. However, the effectiveness of this competitive mechanism, and others, likely changes in naturally varying flow conditions. The present work quantified the effectiveness of chemically-mediated (i.e., microbially-mediated localized hypoxia) and physically-mediated (i.e., shading and abrasion) mechanisms of competition between corals and algae in differing flow environments. This work comprised of two separate studies, one focused on quantifying distributions and outcomes of coral-algal interactions in a field context of low and high flow, and a second study which focused on laboratory measurements of boundary layer hypoxia in the context of coral-algal interactions and microbes. For the first study, flow was measured on multiple spatial scales to determine areas on the reef where low and high flow conditions exist. Microbial densities were quantified as a proxy for microbial activity to determine the potential for microbially mediated competition in different flow environments. Coral-algal interactions were counted and outcomes of coral-algal interactions were estimated in the areas designated as low and high flow to determine the environments in which different algae can negatively affect coral. Interactions between corals and algae were observed over time to see the prolonged effects of algal contact on coral growth and cover. A goal of the second study was to quantify the changes in diffusive boundary layer thickness and oxygen conditions within the zones of interaction between algal turf and massive Porites spp. as flow increased. Additionally, microbial contribution to oxygen conditions was measured within algal turf-massive Porites spp. interactions to determine the effects of microbes on oxygen conditions. The results for both studies indicate the effectiveness of mechanisms of competition underlying coral-algal interactions is dependent on the flow environment. Algal turf outcompetes coral more frequently in low flow. Interaction zones between algal turf and massive Porites retain higher densities of microbes and create and maintain extreme oxygen concentration more frequently in low flow than in high flow. Although microbial densities are higher within the zones of interaction between algal turf and massive Porites located in low flow, oxygen concentrations are controlled by algal turf not microbes. These results suggest another potential mechanism of competition, algal driven oxygen extreme concentrations. However, coral cover and growth are not different in low versus high flow. Corals compensate for algal presence by growing more vertically and resist algal overgrowth due to other physiological, ecological and physical processes. Combined, these results highlight that outcomes of coral-algal interactions and the mechanisms underlying coral-algal interactions are context dependent, but outcomes are not indicative of coral reef community structure because a suite of other reef processes likely drive long-term trends in benthic cover.
Thesis or Dissertation
California State University