Phytoplankton

Prediction of the impact of global climate change on marine phytoplankton is fraught with uncertainties. A range of environmental changes will influence phytoplankton, including warming, enhanced stratification, alteration of ocean currents, intensification or weakening of local nutrient upwelling, heavy precipitation, and storm events causing changes in land runoff and micronutrient availability. Further, elevated CO2 could directly reduce calcification through ocean acidification, but also stimulate photosynthesis or other biogeochemical processes such as N-fixation. Phytoplankton responses are likely to be species- or even strain-specific. Complex factor interactions exist and simulated ecophysiological laboratory experiments rarely allow for sufficient acclimation or take into account physiological plasticity and genetic strain diversity. In the absence of multi-decadal Australian datasets to assess directly impacts of climate change, we must use appropriate datasets from other locations, look to the geological record for past responses to climate, and examine the response of phytoplankton to climate forcing over shorter time scales (e.g. El Niño- Southern Oscillation). Given documented changes in Australia and other parts of the world, we can expect: (1) range expansion of warm-water species at the expense of cold-water species, which are driven polewards; (2) changes in the abundance and seasonal window of growth of selected phytoplankton species; (3) earlier timing of peak production of phytoplankton, especially in temperate regions; (4) Knock-on effects for marine food webs, notably when individual zooplankton and fish grazers are differentially impacted (“match-mismatch”) by climate change. Some harmful algal bloom phenomena (e.g. toxic dinoflagellates benefitting from land runoff and/or water column stratification, tropical benthic dinoflagellates responding to warmer water temperatures and coral reef disturbance) may become worse, while others may diminish in areas currently impacted. Greatest problems for human society will be caused by being unprepared for significant range expansions or the increase of algal biotoxin problems in currently poorly monitored areas. We thus require increased vigilance in seafood biotoxin and algal monitoring programmes and the Integrated Marine Observing System (IMOS) are contributing significantly to these efforts. A combination of laboratory and field approaches over multiple spatial and temporal scales is necessary to better predict climate impacts on phytoplankton.