UC San Diego

Natural heterogeneity and global change are key environmental drivers of ecosystem structure and function in both terrestrial and marine ecosystems. At the foundation of all food webs are the primary producers, which require macronutrients and photosynthetic substrate in order to fix inorganic carbon into organic sugars and fuel energy transfer into food webs. This dissertation is an examination of the ecophysiology, or the interaction of organismal physiology with the environment, of key benthic primary producers on coral reefs. Reef-building corals and algae are the most abundant primary producers on coral reefs, and I use coarse functional groupings categorized as reef-building corals, fleshy macroalgae, calcareous macroalgae, crustose coralline algae (CCA) and turf algae assemblages. I assessed the influence of, 1) a natural gradient in inorganic nutrient availability, and, 2) simulated global change on the ecophysiology of corals and algae by functional group. The Southern Line Islands are an archipelago of islands that span the equatorial upwelling region and demonstrate predictable heterogeneity in inorganic nutrient availability. The dominant species of corals and algae demonstrated higher pigment concentrations and photosynthetic efficiency across the archipelago as a function of increasing inorganic nutrient concentrations. This suggests that natural fluxes of inorganic nutrients have an important positive influence on primary producers. I then conducted laboratory experiments on Palmyra Atoll and in Moorea, French Polynesia to test the effects of ocean acidification (OA) and warming on different functional groups of algae. Across a suite of species, OA increased the growth of fleshy macroalgae and turf algae assemblages, but decreased growth and calcification of calcareous macroalgae and CCA. Ocean acidification had a stronger effect than warming on the biomass of turf algae assemblages. Positive effects of OA on turf algae metabolism were increased by warming. These findings suggest that fleshy and calcifying algae respond differently to global change stressors. Ocean acidification has the potential to increase growth and productivity of fleshy algae, while concurrently decreasing growth and calcification of calcifying algae. Anthropogenic activities are increasingly altering the natural environment, and the results of this dissertation improve our ability to predict the response of corals and algae to increasing exposure to nutrients, OA and warming in the near-future ocean.