Can we really estimate trends in phytoplankton abundance and community structure over the 25 year ocean colour satellite era ?

Phytoplankton abundance and community structure are crucial to the natural carbon cycle and the sustainability of marine ecosystems. However, their long-term natural variability and their response to anthropogenic climate change remain poorly understood. Since 1997, numerous algorithms have been developed to estimate these two parameters using ocean colour remote sensing. These tools have enabled 25 years of continuous, global-scale observations, providing invaluable insights into the variability of phytoplankton. However, analysing recent trends using these datasets presents significant challenges. Temporal inconsistencies and discontinuities in satellite time series - resulting from sensor transitions or decommissioning - introduce biases that complicate trend detection. Here, we present an inter-comparison framework designed to address these limitations by analysing multiple ocean colour remote sensing products. We implemented a robust statistical approach to minimize the influence of temporal discontinuities on trend detection. Results reveal both regions with robust trends across products and areas of significant disagreement, with the latter being largely prevalent. This study provides novel insights into recent phytoplankton dynamics while highlighting current limitations in our capacity to monitor these changes. Our findings emphasize the importance of multi-product analyses for reliable trend assessment in ocean colour remote sensing.