Summary
Prediction of the impact of global climate change on marine phytoplankton is fraught with uncertainties. Increasing temperature, enhanced surface stratification, alteration of ocean currents, intensification or weakening of local nutrient upwelling, stimulation of photosynthesis by elevated CO2, reduced calcification through ocean acidification, heavy precipitation, and storm events causing changes in land runoff and micronutrient availability, may all produce contradictory species- or even strain-specific responses. 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 Australian datasets spanning 30 consecutive years, 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. The 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, thus calling for increased vigilance in seafood biotoxin and algal monitoring programmes. A combination of laboratory and field approaches over multiple spatial and temporal scales is necessary to better predict climate impacts on phytoplankton.