Williams 9317733 The current paradigm explaining how coral reefs maintain high biomass of organisms and extremely high rates of gross primary productivity is that tight recycling of nutrients and organic matter occurs within the reef resulting in zones of net autotrophy alternating with zones of net heterotrophy. Autotrophic upstream communities are thought to support downstream heterotrophic assemblages with the overall balance resulting in ecosystem P/R ratios near unity. According to this paradigm, coral reefs are not coupled significantly to the surrounding oligotrophic ocean. Recent studies suggest that coral reefs may be much more dependent on hydrodynamic processes than currently believed. Although nutrient concentrations of tropical waters are very low, an enormous volume of water is advected across the reef and could result in a large flux of nutrients to benthic primary producers. The major upstream autotrophic zone is the reef flat where algal turf assemblages are responsible for the majority of primary productivity. Previous work has demonstrated that rates of primary productivity and nitrogen fixation of algal turf are affected significantly by water flow speed. Furthermore, flow measurements on one reef suggest that algal canopy height significantly alters the local hydrodynamic regime and as a result, metabolic processes of algal turfs may be diffusion-limited for a significant proportion of time. This project will test the hypothesis that rates of primary productivity and nitrogen fixation of coral reef algal turfs are diffusion-limited. Measurements of the flow environment on a reef flat will be made and used to estimate the degree to which algal turfs varying in canopy height are diffusion-limited. The project will then move on to test hypotheses about the specific factors that result in diffusion-limitation. The results of this project should fill a gap on empirical measurements of water flow in coral reef environments and how water flow affects algal metabolism. The results of this research may lead to a significant paradigm shift in understanding how coral reefs function. Demonstration that reefs are open ecosystems that are strongly coupled to the surrounding ocean environment would have important implications for predictions of the effects of global climate change on these unique ecosystems. ***
