I am not a specialist of the field, but I found this paper interesting:
Boyce, D., Lewis, M. & Worm, B. Global phytoplankton decline over the past century. Nature 466, 591–596 (2010). https://doi.org/10.1038/nature09268
Authors wrote: "We observe declines in eight out of ten ocean regions, and estimate a global rate of decline of ∼1% of the global median per year." (since 1899).