UCLA

Nutrient enrichment is a commonly-occurring anthropogenic stressor in aquatic environments expected to increase worldwide. Here, I focus on the impacts of nutrient enrichment on two marine communities: temperate shallow estuaries undergoing nutrient-driven eutrophication and coral reefs subject to nutrient addition that co-occurs with other stressors. Eutrophication as a result of nutrient enrichment, well known as a stressor in marine environments, is increasingly recognized for its impacts to pH. Macroalgal blooms, one form of eutrophication, increase pH (basification) while undergoing rapid growth, but also cause acidification at night and during decomposition. In chapter 1, I quantify pH during growth and decomposition of an Ulva bloom in a shallow temperate estuary. Basification (pH = 9.8) in the afternoon was coupled with acidification (6.9) in the morning. Critically, tidal inundation limited these pH excursions. I demonstrate that Ulva blooms can cause basification in shallow estuaries and lead to large diel pH oscillations. In chapter 2, I evaluated the impact of a decomposing Ulva bloom on carbonate chemistry and seagrass by conducting a flow-through mesocosm experiment. I measured pH on different temporal scales including daily cycles, weekly changes, and twice to measure full carbonate chemistry parameters. I measured dissolved oxygen (DO) and growth of Zostera marina. Decomposing algae drove acidification and reductions in DO, with greater diel variation of pH with higher algal biomass. Differences decreased throughout the experiment, with no effects on seagrass. Thus, macroalgal blooms drive acidification and co-occurring hypoxia during decomposition, but Z. marina can tolerate these extreme conditions. In chapter 3, I quantify the effects of nutrients and co-occurring stressors on a coral reef algal turf community. I conducted a 3-factor field experiment manipulating nutrients, sediment depth, and access to herbivores. All three factors and time interacted to control turf height. Herbivore removal and nutrient addition increased turf height. However, addition of sediment negated the positive effect of nutrients. My results demonstrate that without understanding complex interactions among stressors, our ability to predict responses is limited. In summary, this dissertation contributes to our understanding of local anthropogenic stressors, how they interact with each other, and how they drive further stressors.