Information on marine biomass decline from recent ipcc report: "Global models also project a loss in marine biomass (the total weight of all animal and plant life in the ocean) of around -6% (±4%) under SSP1-2.6 by 2080-99, relative to 1995-2014. Under SSP5-8.5, this rises to a -16% (±9%) decline. In both cases, there is “significant regional variation” in both the magnitude of the change and the associated uncertainties, the report says." phytoplankton in particular is projected to decline by ~10% in worst-case emissions scenario.

https://www.carbonbrief.org/in-depth-qa-the-ipccs-sixth-assessment-on-how-climate-change-impacts-the-world/

https://www.nature.com/articles/s41558-021-01173-9/figures/3