Phytoplankton biomass modifies the penetration of light and impacts the physical properties of the upper ocean. We quantify these impacts and the feedbacks on phytoplankton biomass for the global ocean using an Ocean General Circulation Model coupled to an ocean biogeochemistry model. Phytoplankton biomass amplifies the seasonal cycle of temperature, mixed layer depth and ice cover by roughly 10%. At mid and high latitudes, surface temperature warms by 0.1–1.5°C in spring/summer and cools by 0.1–0.3°C in fall/winter. In the tropics, phytoplankton biomass indirectly cools the ocean surface by 0.3°C due to enhanced upwelling. The mixed layer stratifies by 4–30 m everywhere except at high latitudes. At high latitudes, the sea-ice cover is reduced by up to 6% in summer and increased by 2% in winter, leading to further feedbacks on vertical mixing and heat fluxes. Physical changes drive a positive feedback increasing phytoplankton biomass by 4–12% and further amplifies the initial physical perturbations.