Global Drivers of Phytoplankton Phenology Trends.

Phytoplankton are key contributors to global primary production and serve as sensitive indicators of changes in marine ecosystems. Changes in phytoplankton phenology, and in particular the onset, termination, and duration of their growing period, have important implications for higher trophic levels that feed on them. Several local studies have provided evidence linking phenological shifts to climate change. However, global analyses have so far relied on time-series observations of surface chlorophyll covering less than two decades, making it difficult to separate trends from variability and establish the coherence of underlying drivers at the global level. Here we use the OC-CCIv5 satellite-derived observations harmonised across satellite sensors and the NEMO-PlankTOM12 global ocean biogeochemical model to revisit recent trends in phytoplankton phenology and identify their global drivers during the 1998-2020 period. Using statistical analysis to separate and control for covariance among underlying drivers, we show that SST warming trends are clearly associated with earlier initiation, later termination, and longer duration of the phytoplankton growing season. In contrast, changes in MLD, SST variability, and chlorophyll are associated with shifts in initiation and termination in the same direction, such that both boundaries move in parallel, partly cancelling their effect on duration. The PlankTOM12 model successfully reproduces most of the observed patterns in phenology and their association with the underlying drivers, providing evidence of the mechanistic relationships, and helping to ascertain the global significance of the drivers. Our findings suggest that trends towards earlier initiation, later termination, and longer duration of phytoplankton growing periods are likely associated with climate change, and may therefore persist in the near future under continued global warming.