| Summary: | Ocean acidification is the increased absorption of atmospheric CO2 in seawater and the consequent decrease in pH. This phenomenon is occurring throughout the global oceans while land use changes and large human populations near coasts are linked to increased nutrient concentrations in seawater. Ulva spp. blooms caused by nutrient enrichment occur regularly in some parts of the world and are known as green tides. There is concern that ocean acidification may increase green tides and intensify ecological and economic damages. Ulva spp. can utilize bicarbonate (HCO3-) as an inorganic carbon source, but this comes at an energetic cost as HCO3- must be converted to CO2 before it can be used for carbon fixation. Therefore, increased utilization of pCO2 with ocean acidification may benefit Ulva spp. Ocean acidification and eutrophication will occur simultaneously in many coastal ecosystems. The goal of the following investigations was to determine the effects of ocean acidification and nutrient enrichment alone and their interaction on photosynthetic, nutrient, and growth physiology of Ulva spp. In Chapter 2, the response of Ulva australis to pHT and ammonium (NH4+) enrichment were investigated in a seven day growth experiment using a range of pHT (7.56 - 7.84) and ambient and enriched NH4+ concentrations. I measured relative growth rates (RGRs), NH4+ uptake rates and pools, photosynthetic characteristics, and tissue carbon and nitrogen content. There was no interaction of pHT and NH4+ enrichment on the physiological parameters. The RGR was not affected by pHT, but was an average of two times higher in the enriched NH4+ treatment. rETRmax, total chlorophyll, and tissue nitrogen increased with both NH4+ enrichment and decreased pHT. The C:N ratio decreased with decreasing pH and with NH4+ enrichment. Although rETRmax increased and the C:N ratio decreased under decreased pH, these metabolic changes did not translate to higher growth rates. The results show that U. australis growth and physiology is more sensitive to NH4+ ...
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