Responses of natural phytoplankton assemblage to two emerging environmental threats (ocean acidification and warming) using controlled experiments

Abstract

A third of the carbon dioxide (CO2) released by the burning of fossil fuels since the industrial revolution has ended up in the ocean. The absorbed CO2 subsequently lowers the pH level. Concurrently, temperature of surface ocean has discernibly increased. The rate of CO2 and temperature increase is at least an order of magnitude faster than experienced by the Earth for millions of years, and the current concentration is estimated to be the highest in, at least, the past 20 million years. Therefore, the impact on the marine biology appears to be inevitable. This thesis presents the results of two mesocosm experiments during which pCO2 and temperature conditions were raised to mimic ocean acidification and warming in the coming decades. The mesocosm facility developed through this thesis work was used to examine (i) the effects of seawater pCO2 concentrations (in the range of the pre-industrial, present and future pCO2 levels) on the growth rate of natural assemblages of phytoplankton, and (ii) the combined effects of pCO2 concentration and temperature elevation on the production of particulate and dissolved organic matter and (iii) the dimethylsulfide (DMS) production of phytoplankton. The mesocosm facility consists of a floating raft, nine impermeable cylindrical enclosures, pCO2 regulation units, and bubble-mediated seawater mixers. Each enclosure is two-thirds filled with the seawater, and the headspace above is filled with air at a target pCO2 concentration. Each enclosure is capped with a transparent dome that transmits incoming radiation. To produce pCO2 levels higher than the ambient concentration, the mass flow controller in the pCO2 regulation unit delivers varying amounts of ultra-pure CO2 into the gas mixer where it is rapidly mixed with ambient air. To produce pCO2 levels lower than the ambient concentration, CO2-free air and ambient air are mixed in the gas mixer. Prior to daily seawater sampling, the produced air stream is diverted to the seawater mixer for thorough mixing with the seawater in the enclosure, while the major fraction of air stream continues to flow into the enclosure headspace. A performance of the mesocosm facility assessed attainment of target pCO2 concentrations in the headspace and enclosure seawater, and the mixing efficiency of the seawater mixer. The results indicate that the facility is suitable for carrying out in situ pCO2 perturbation experiments. The experiment testing the effect of pCO2 concentration (250, 400 and 750 μatm) on the growth rate of a phytoplankton assemblage suggests that only Skeletonema costatum showed an increase in growth rate with increasing seawater pCO2 concentration. The other experiment investigating the effects of ocean acidification and warming (a pCO2 of ~900 μatm at ambient temperature conditions, and a pCO2 of ~900 μatm at a temperature ~3°C warmer than ambient) showed that the elevated CO2 and temperature conditions disproportionately enhanced dissolved organic carbon production and its contribution to total organic matter production, while the total organic carbon production remained relatively constant under all conditions tested. In addition, two sets of CO2 and temperature conditions significantly stimulated the grazing rate and the growth rate of heterotrophic dinoflagellates (ubiquitous marine microzooplankton). The increased grazing rate resulted in considerable DMS production. The results from the mesocosm studies indicate the active biological feedbacks of marine phytoplankton on climatic change in the future. The knowledge gained from the studies can provide a comprehensive scientific understanding and the practical parameters in global carbon cycle models. It would contribute to predict the response of marine ecosystems to elevated pCO2 concentrations and enable scenarios of future impacts on ecosystem function and biogeochemical cycles to be developed.