Integrating conceptually similar models of the growth of marine and terrestrial primary producers yielded an estimated global net primary production (NPP) of 104.9 petagrams of carbon per year, with roughly equal contributions from land and oceans. Approaches based on satellite indices of absorbed solar radiation indicate marked heterogeneity in NPP for both land and oceans, reflecting the influence of physical and ecological processes. The spatial and temporal distributions of ocean NPP are consistent with primary limitation by light, nutrients, and temperature. On land, water limitation imposes additional constraints. On land and ocean, progressive changes in NPP can result in altered carbon storage, although contrasts in mechanisms of carbon storage and rates of organic matter turnover result in a range of relations between carbon storage and changes in NPP.

We assembled a dataset of 14C-based productivity measurements to understand the critical variables required for accurate assessment of daily depth-integrated phytoplankton carbon fixation (PP,,) from measurements of sea surface pigment concentrations (C,,,). From this dataset, we developed a light-dependent, depth-resolved model for carbon fixation (VGPM) that partitions environmental factors affecting primary production into those that influence the relative vertical distribution of primary production (P,) and those that control the optimal assimilation efficiency of the productivity profile (P",,,). The VGPM accounted for 79% of the observed variability in P, and 86% of the variability in PP,, by using measured values of PBop,. Our results indicate that the accuracy of productivity algorithms in estimating PP,, is dependent primarily upon the ability to accurately represent variability in PBopt. We developed a temperature-dependent PBopt model that was used in conjunction with monthly climatological images of C,,,, sea surface temperature, and cloud-corrected estimates of surface irradiance to calculate a global annual phytoplankton carbon fixation (PP,,,,) rate of 43.5 Pg C yr-'. The geographical distribution of PP,,,, was distinctly different than results from previous models. Our results illustrate the importance of focusing Psopt model development on temporal and spatial, rather than the vertical, variability.Thousands of measurements of marine phytoplankton productivity have been made at discrete locations throughout the world's oceans since the introduction of the radiolabelled carbon uptake method (i.e. 14C method) in 1952 (Steemann Nielsen 1952). Although numerous, these discrete primary productivity measurements only provide information for infinitesimally small points over the oceans' surfaces. Scaling these discrete measurements to global projections by means of satellite-based estimates of chlorophyll concentration (C,,,) requires mathematical models that quantitatively relate primary productivity to chlorophyll (Bidigare et al. 1992). AcknowledgmentsWe thank Monica Chen for global primary production calculations and images of productivity distributions, David Siegel for assistance with the BATS productivity data, Richard Barber for generously contributing the EQPAC data, William Behrens for assistance with the BNL database, and Jay O'Reilly for the MARMAP contribution to the dataset. We especially thank Andre Morel for the LPCM data, the source code for his bio-optical model, and numerous helpful discussions. We also thank Peter Minnett and Kevin Tut-pie for assistance with data retrieval and analysis and Creighton Wirick, Zbigniew Kolber, and two anonymous reviewers for helpful recommendations and discussions.