Global Climate Change and the Southern Ocean: How Antarctic Fishes Physiologically Respond to a Changing Environment from the Cellular to the Organismal Level

Studies have projected that future changes in sea surface temperature and pCO2 levels will impact higher latitudes to a greater extent than in temperate regions. For notothenioid fishes of the Southern Ocean, evolution in extremely stable, cold waters has resulted in several adaptations which have left these fishes poorly prepared for global climate change. I have analyzed the metabolic and cellular response of Trematomus bernacchii, Pagothenia borchgrevinki and Trematomus newnesi to a long-term, multi-stressor scenario relevant to the predicted changes in the Southern Ocean. By combining whole animal respirometry with cellular level analysis of energy allocation, osmoregulatory mechanisms and cellular damage, I aimed to determine if acclimation to increased sea surface temperature (4°C), increased seawater pCO2 levels (1000 μatm), or a combination of these two parameters result in energetic trade-offs and exacerbated cellular damage. The data suggest a synergistic relationship exists between elevated temperature and pCO2, as the combination of these variables further elevates metabolic rates and delays the acclamatory response. Overall, long-term acclimation to experimental treatments resulted in a novel discovery: despite evolving in the same environment and on the same time-scale, these three species of notothenioid differ in their physiological response to global climate change, and defend different biochemical pathways when confronted with a changing environment. While T. bernacchii, P. borchgrevinki, and T. newnesi all showed small acclamatory capacities, there appear to be energetic trade-offs associated with this acclimation, and overall, it may not be possible for energetic demands to be met over long time scales, which could result in long-term impacts to population numbers

Does Ocean Acidification Benefit Seagrasses in a Mesohaline Environment? A Mesocosm Experiment in the Northern Gulf of Mexico

Ocean acidification is thought to benefit seagrasses because of increased carbon dioxide (CO2) availability for photosynthesis. However, in order to truly assess ecological responses, effects of ocean acidification need to be investigated in a variety of coastal environments. We tested the hypothesis that ocean acidification would benefit seagrasses in the northern Gulf of Mexico, where the seagrasses Halodule wrightii and Ruppia maritima coexist in a fluctuating environment. To evaluate if benefits of ocean acidification could alter seagrass bed composition, cores of H. wrightii and R. maritima were placed alone or in combination into aquaria and maintained in an outdoor mesocosm. Half of the aquaria were exposed to either ambient (mean pH of 8.1 ± 0.04 SD on total scale) or high CO2 (mean pH 7.7 ± 0.05 SD on total scale) conditions. After 54 days of experimental exposure, the δ13C values were significantly lower in seagrass tissue in the high CO2 condition. This integration of a different carbon source (either: preferential use of CO2, gas from cylinder, or both) indicates that plants were not solely relying on stored energy reserves for growth. Yet, after 41 to 54 days, seagrass morphology, biomass, photo-physiology, metabolism, and carbon and nitrogen content in the high CO2 condition did not differ from those at ambient. There was also no indication of differences in traits between the homospecific or heterospecific beds. Findings support two plausible conclusions: (1) these seagrasses rely heavily on bicarbonate use and growth will not be stimulated by near future acidification conditions or (2) the mesohaline environment limited the beneficial impacts of increased CO2 availability

Seawater carbonate chemistry and calcification in Caribbean reef-building corals

Projected increases in ocean pCO2 levels are anticipated to affect calcifying organisms more rapidly and to a greater extent than other marine organisms. The effects of ocean acidification (OA) have been documented in numerous species of corals in laboratory studies, largely tested using flow-through exposure systems. We developed a recirculating ocean acidification exposure system that allows precise pCO2 control using a combination of off-gassing measures including aeration, water retention devices, venturi injectors, and CO2 scrubbing. We evaluated the recirculating system performance in off-gassing effectiveness and maintenance of target pCO2 levels over an 84-day experiment. The system was used to identify changes in calcification and tissue growth in response to elevated pCO2 (1000 μatm) in three reef-building corals of the Caribbean: Pseudodiploria clivosa, Montastraea cavernosa, and Orbicella faveolata. All three species displayed an overall increase in net calcification over the 84-day exposure period regardless of pCO2 level (control + 0.28–1.12 g, elevated pCO2 + 0.18–1.16 g), and the system was effective at both off-gassing acidified water to ambient pCO2 levels, and maintaining target elevated pCO2 levels over the 3-month experiment