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The ecophysiological effects of CO2 enrichment on the seagrass Halophila ovalis

Wong, Stephanie (2016) The ecophysiological effects of CO2 enrichment on the seagrass Halophila ovalis. Honours thesis, Murdoch University.

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Ocean acidification is one of the biggest challenges happening in the marine environment and causes a shift in dissolved inorganic carbon (DIC) concentrations and lowers the seawater pH. This causes negative impacts on many marine organisms and thus affecting the ecosystem. However, the effect of increasing CO2 concentration dissolved in the ocean can potentially be beneficial to the growth of seagrass. This effect was examined on the seagrass Halophila ovalis (R. Br.) Hook f., the dominant seagrass species found in Swan-Canning Estuary, Western Australia. This study was done with controlled experiments, using CO2-enrichment as a simulation of ocean acidification. The seagrass was collected and cultured in the laboratory for 15 days while pH and alkalinity of seawater and photochemical efficiency (Fv /Fm) of the seagrass were monitored. Chlorophyll content, growth (shoot plastochrone interval, leaf and rhizome elongation) and biomass productivity of the plants were measured at the end of the experiment. The seagrass was collected in early and mid-winter for two experiment replicates and a strong seasonal variation was observed. A diurnal pattern was found in pH for both CO2-enriched and control aquaria, showing a buffering effect by seagrass photosynthesis. Significant differences were found in the DIC concentrations with decreasing pCO2 and increasing HCO3- concentrations in the first experiment but opposite results found in the second experiment. The healthier seagrass in the first experiment showed a decreasing photochemical efficiency over time while the seagrass in the second experiment showed an increasing photochemical efficiency potentially due to the recovery from storm stress and epiphyte load. Significantly higher biomass productivity was found in the seagrass from the CO2-enriched aquaria of the first experiment but not in the second. It was difficult to determine whether the increase in biomass productivity was caused by the addition of CO2 or the seagrass reproduction. It is suggested that more replicates and long term experiments are needed to study the relationship between seagrass productivity and seasonality along with the effect of increasing dissolved CO2 in seawater. Field experiments are also needed in the future to explore the potential of using seagrass in buffering the effect of ocean acidification which might help the broader marine community at an ecosystem level to survive the ongoing environmental changes.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): van Keulen, Mike and Moheimani, Navid
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