Widespread changes in Southern Ocean phytoplankton blooms linked to climate drivers

Climate change is expected to elicit widespread alterations to nutrient and light supply, which interact to influence phytoplankton growth and their seasonal cycles. Using 25 years of satellite chlorophyll a data, we show that large regions of the Southern Ocean express significant multi-decadal trends in phenological indices that are typically larger (<50 days decade–1) than previously reported in modelling studies (<10 days decade–1). Although regionally dependent, there is an overall tendency for phytoplankton blooms to increase in amplitude, decline in seasonality, initiate later, terminate earlier and have shorter durations, except in the ice, which initiate earlier and have longer durations. Investigating relationships with prominent climate drivers highlights regional sensitivities and complexities of multiple interacting aspects of a changing climate. Seasonal adjustments of this magnitude at the base of the food web can de-synchronize energy transfer to higher trophic levels, threatening ecosystem services and impacting global climate by altering natural CO2 uptake. Using 25 years of satellite chlorophyll a data, the authors demonstrate significant and widespread changes in the amplitude, timing, duration and seasonality of Southern Ocean phytoplankton blooms. Such changes threaten ecosystem services and can impact global climate by altering natural CO2 uptake.