Chair: Elvira Poloczanska
Leah B. Reidenbach (1), Pamela A. Fernandez (2), Pablo P. Leal (2), Fanny Noisette (2), Catriona L. Hurd (2), Christina M. McGraw (3), Andrew T. Revill (4), Janet E. Kübler (1)
1 California State University, Northridge, Northridge, CA, 91325, United States of America
2 Institute for Marine and Antarctic Studies, Hobart, TAS, 7001, Australia
3 University of New England, School of Science and Technology, Armidale, NSW, 2351, Australia
4 Commonwealth Scientific and Industrial Research Organisation (CSIRO), Hobart TAS, 7001, Australia
Background
Ocean acidification will affect primary producers by increasing the partial pressure of carbon dioxide (pCO2) in seawater. Increasing pCO2 will increase the diffusion of CO2 across the cell membrane and may enhance the photosynthetic rates and growth rates of macroalgae. How macroalgae will be affected by ocean acidification is likely dependent on additional environmental variables including nutrient enrichment. The macroalgae Ulva spp. are intertidal species that occur worldwide and are responsive to coastal nutrient enrichment. Ocean acidification may alter the growth response of Ulva spp. to increased nutrients by altering the photosynthetic and nutrient physiology.
Methods
Two separate experiments were completed to investigate the interacting effects of ocean acidification and eutrophication on Ulva spp. In the both experiments, Ulva spp. were grown in a cross of three pCO2 levels under ambient and enriched ammonium. In one experiment, the light intensity was sub-saturating for photosynthesis and in the other, the light intensity was sufficient to saturate photosynthesis (200 and 600 μm photons m-2 s-1, respectively).
Findings
In sub-saturating and saturating light, growth rates were approximately two times greater in the enriched ammonium treatments than in the unenriched treatments (p<0.001 and p=0.004, respectively). No difference in growth rate was found across pH treatments in sub-saturating light (p=0.688); a possible difference was found in saturating light (p=0.055). No interacting effects of pCO2 and ammonium were found.
Conclusions
The growth rate of Ulva spp. may not be affected by changing pCO2 levels when light intensity is insufficient to saturate photosynthesis but is expected to increase with nutrient enrichment regardless of light availability. This knowledge increases our understanding of how marine environments may change with higher CO2. Our results did not support the hypothesis that eutrophication and ocean acidification would have synergistic effects on the growth rate of Ulva spp.