Environment: Global map of phytoplankton blooms shows intensification in the 21st century

Coastal phytoplankton blooms increased in both their size and their frequency between 2003 and 2020, a study in Nature reports. This map of bloom patterns on a global scale provides a valuable resource for assessing these events, which could inform environmental policy making.

Phytoplankton blooms are accumulations of microscopic algae that can develop in the surface layer of both marine and freshwater ecosystems. Changes in nutrient levels due to human activity are expected to increase their global frequency. These blooms can provide food and nutrients for other organisms, but they are also associated with negative effects such as toxin production that can accumulate in food webs and oxygen depletion leading to oxygen-free ‘dead-zones’ in previously balanced ecosystems. Previous studies have struggled to fully characterize bloom trends due to inconsistent sampling and the diversity of specific ecosystems in which they appear.

Lian Feng and colleagues generated a comprehensive map of bloom distribution and trends this century by assessing 760,000 images acquired from NASA’s Aqua satellite between 2003 and 2020. They report that the total bloom-affected area in 2020 was 31.47 million km2 (8.6% of the global ocean area). This was an increase of 3.97 million km2 (13.2%) from 2003. The global median frequency also showed an increasing rate of 59.2% over the observed period. The authors also observed a significant correlation between changes in sea surface temperature and ocean circulation and mean bloom frequency, with warmer temperatures coinciding with bloom occurrence in certain regions.

The detailed information about the extent and frequency of coastal phytoplankton blooms help researchers to understand how they form and dissipate. Such information could be used to assess bloom risks and benefits and could aid the development of strategies to minimize the occurrence or consequences of harmful blooms, the authors conclude.