Phytoplankton photosynthesis, a fundamental process that sustains the ocean's food webs and regulates climate by removing carbon dioxide from the atmosphere, is expected to be affected by global warming. However, direct approaches to measuring photosynthesis are too inefficient and costly to provide large-scale estimates and satellite-based approaches are too inaccurate to allow detection of climate-induced trends in photosynthetic production. In this study, we use a decade of data from a fleet of autonomous profiling floats to measure photosynthetic productivity from daily cycles of biomass across the temperate and polar Southern Hemisphere. These productivity estimates are within the range of different satellite estimates. We argue that the float-based approach makes the large-scale observation of climate-driven changes in marine photosynthesis feasible.

Abstract. This paper provides an overview and demonstration of emerging float-based methods for quantifying gross primary production (GPP) and net community production (NCP) using Biogeochemical-Argo (BGC-Argo) float data. Recent publications have described GPP methods that are based on the detection of diurnal oscillations in upper ocean oxygen or particulate organic carbon concentrations using single profilers or a composite of BGC-Argo floats. NCP methods rely on budget calculations to partition observed tracer variations into physical or biological processes occurring over timescales greater than one day. Presently, multi-year NCP time-series are feasible at near-weekly resolution, using consecutive or simultaneous float deployments at local scales. Results, however, are sensitive to the choice of tracer used in the budget calculations and uncertainties in the budget parametrizations employed across different NCP approaches. Decadal, basin-wide GPP calculations are currently achievable using data compiled from the entire BGC-Argo array, but finer spatial and temporal resolution requires more float deployments to construct diurnal tracer curves. A projected, global BGC-Argo array of 1000 floats should be sufficient to attain annual GPP estimates at 10-degree latitudinal resolution, if floats profile at off-integer intervals (e.g., 5.2 or 10.2 days). Addressing the current limitations of float-based methods should enable enhanced spatial and temporal coverage of marine GPP and NCP measurements, facilitating global-scale determinations of the carbon export potential, training of satellite primary production algorithms, and evaluations of biogeochemical numerical models.