Three decades of marine bioregions evolution through analysis of phytoplankton phenology

Ocean biogeochemistry is undergoing significant modification in response to climate change. To better track these evolutions, we present a novel ocean monitoring indicator that follows the temporal evolution of marine bioregions of similar phytoplankton phenologies. Chlorophyll a concentration observations from remotely sensed ocean color describe seven distinct marine bioregions characterized by their unique patterns of phytoplankton biomass seasonality. The sizes and locations of these bioregions are monitored from 1998 to 2024 using a multi-sensor merged L4 ocean color product. While the bioregions' extents appear to follow interannual and decadal climate variability, their long term evolution suggests a poleward extension of less seasonal phytoplankton phenology, which is generally characteristic of the tropical and subtropical bands. This innovative approach allows the systematic monitoring of how phytoplankton adapt their seasonal cycle in response to changes in environmental conditions, hence giving valuable insights into the evolution of marine ecosystems over the past three decades and helping advance our understanding of global ocean biological responses to environmental change.