Photoacclimation and nutrient-based model of light-saturated photosynthesis for quantifying oceanic primary production

Availability of remotely sensed phytoplankton biomass fields has greatly advanced primary production modeling efforts. However, conversion of near-surface chlorophyll concentrations to carbon fixation rates has been hindered by uncertainties in modeling light-saturated photosynthesis (P-max(b)). Here, we introduce a physiologically-based model for p(max)(b) that focuses on the effects of photoacclimation and nutrient limitation on relative changes in cellular chlorophyll and CO2 fixation capacities. This 'PhotoAcc' model describes P-max(b) as a function of light level at the bottom of the mixed layer or at the depth of interest below the mixed layer. Nutrient status is assessed from the relationship between mixed layer and nutricline depths. Temperature is assumed to have no direct influence on p(max)(b) above 5degreesC. The PhotoAcc model was parameterized using photosynthesis-irradiance observations made from extended transects across the Atlantic Ocean. Model performance was validated independently using time-series observations from the Sargasso Sea, The PhotoAcc model accounted for 70 to 80 % of the variance in light-saturated photosynthesis. Previously described temperature-dependent models did not account for a significant fraction of the variance in P-max(b) for our test data sets.