Towards a global, remotely sensed monitoring of lake phytoplankton phenology

Changing environmental conditions caused by climate change, eutrophication, and other anthropogenic factors affect the timing, duration, and surface extent of lake algae blooms across the globe. It remains, however, challenging to quantify the relative impacts of different environmental changes on the timing and characteristics of lake algae blooms, and to detect phenological trends over time, as these blooms vary considerably from year to year. Global data sets that may allow us to study algae-bloom properties along a wide range of environmental conditions and years are needed to address these challenges. For this study, we developed such a data set using satellite remote sensing. We analyze the phytoplankton phenology of 2025 lakes across a wide range of climate zones over a period of approximately 20 years. More specifically, we used daily lake chlorophyll estimates derived from MERIS and OLCI data to extract phenology metrics (e.g. the onset and decline of peaks in chlorophyll concentration) for individual pixels within each of the 2025 lakes. Through a newly developed method, we determined the timing of blooms, i.e. clusters of peaks in different pixels occurring within the same lake during the same period of the year, and, subsequently, studied the change in the timing, duration, and size of those blooms across years. This will, ultimately, help us to get a better overview of the extent to which lake algae blooms have changed across the globe, to attribute those changes to anthropogenic drivers, and to develop effective environmental policies to combat those changes where needed.