Effects of ocean acidification and eutrophication on the macroalgae Ulva spp.

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+ than it is to pH and that there is no interactive effect of NH4+ enrichment and decreasing pH. In Chapter 3, Ulva lactuca was grown for 22 days under a range of pCO2 and NH4+ concentrations and a multiple linear regression was used to analyze RGRs, NH4+ and NO3- pools, in situ NH4+ and NO3- uptake rates, tissue carbon and nitrogen content, carbohydrate and protein concentrations, and photosynthesis irradiance curves (P-I curves). The results from model selection and model-averaging techniques allowed me to make predictive models across a range of relevant ocean acidification and eutrophication scenarios and measure the effect sizes of pCO2, NH4+ enrichment, and their interaction. Overall, there was no effect of pCO2 and NH4+ on RGRs after day 5. However, there was a synergistic effect of pCO2 and NH4+ enrichment on the growth rates during days 0 – 5. When pCO2 and NH4+ concentrations increased simultaneously, NO3- uptake rates increased, which may have contributed to increased growth as seen in days 0 – 5. Maximum photosynthetic rates (Pmax) decreased with increasing pCO2 and there was a positive interaction of pCO2 and NH4+ on indicating CCMs were altered under these conditions. This shows that under high light intensities, Pmax was negatively affected by pCO2 and CCMs are not altered when nutrients are limited. Ultimately, there was no longer-term effect of ocean acidification and eutrophication on Ulva lactuca growth. Nutrient enrichment is a major cause of green tide blooms around the world and Ulva australis had the ability to enhance nutrient, photosynthetic, and growth physiology with NH4+ enrichment. Conversely, Ulva lactuca collected from a eutrophic environment, did not respond to NH4+ in terms of growth. Both chapters provided evidence that ocean acidification is unlikely to affect the growth rates of Ulva spp. However, the exception was a positive interactive effect of pCO2 and NH4+ enrichment on the growth rate of U. lactuca during the first five days, suggesting ocean acidification could play a role in initiating Ulva spp. blooms in a eutrophic environment. This could be an important consideration for determining how green tides will be affected by ocean acidification in coastal areas where nutrient enrichment occurs in pulses, resulting in transiently increased nitrogen concentrations.

Reidenbach L., 2017.  Effects of ocean acidification and eutrophication on the macroalgae Ulva spp. MSc thesis, California State University, Northridge, 70 p. Thesis (restricted access).

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