Abstract

Chlorophyll underpins ocean productivity yet simulating chlorophyll across biomes, seasons and depths remains challenging for earth system models. Inconsistencies are often attributed to misrepresentation of the myriad growth and loss processes governing phytoplankton biomass but may also arise from unresolved or misspecified photoacclimation or photoadaptation responses. A series of global ocean ecosystem simulations were enlisted to assess the impacts of alternative photoacclimation and photoadaptation assumptions on simulated chlorophyll, primary productivity and carbon export. Photoacclimation alternatives implicitly modulated the premium placed on light harvesting versus photodamage avoidance and other cellular functions, while photoadaptation experiments probed the impact of adding low- and high-light adapted phytoplankton ecotypes. Alternatives generated large chlorophyll responses that addressed prior model biases in ways that simple changes in growth and grazing could not. Simulations with photoadaptation, surface-skewed photoacclimation in deep mixed layers, and acclimation to light levels over mixing depths consistent with photoacclimation time scales in stratified waters were best able to match observed patterns. While chlorophyll was highly sensitive to alternative photoacclimation assumptions, primary production and carbon export were not because chlorophyll changes under near-saturating light at the ocean's surface yielded only modest phytoplankton growth changes that were counteracted by self-shading at depth. Improved photoacclimation and photoadaption constraints and reduced regional uncertainties in satellite-based ocean color estimates are needed to reduce ambiguities in the drivers of chlorophyll change and their biogeochemical implications.