A CONSUMERS GUIDE TO PHYTOPLANKTON PRIMARY PRODUCTIVITY MODELS

We describe a classification system for daily phytoplankton primary pr oductivity models based on four implicit levels of mathematical integr ation. Depth-integrated productivity models have appeared in the liter ature on average once every 2 years over the past four decades. All of these models can be related to a single formulation equating depth-in tegrated primary production (Sigma PP) to surface phytoplankton biomas s (C-surf), a photoadaptive variable (P-opt(b)), euphotic depth (Z(eu) ), an irradiance-dependent function (F), and daylength (DL), The prima ry difference between models is the description of F, yet we found tha t irradiance has a relatively minor effect on variability in Sigma PP. We also found that only a small fraction of variability in Sigma PP c an be attributed to vertical variability in phytoplankton biomass or v ariability in the light-limited slope for photosynthesis. Our results indicate that (1) differences between or within any model category hav e the potential to improve estimates of Sigma PP by <10%, so long as e quivalent parameterizations are used for C-surf and P-opt(b), and (2) differences in estimates of global annual primary production are due a lmost entirely to differences in input biomass fields and estimates of the photoadaptive variable, P-opt(b), not to fundamental differences between model constructs.