Southern Ocean estimates of phytoplankton primary production through coupled observations and experiments

Abstract

Phytoplankton primary production (PP. in the Southern Ocean (SO. is a key contributor to global atmospheric CO2 drawdown, responsible for 30-40% of global anthropogenic carbon uptake. Addressing the potential extent and magnitude of PP in the SO, under current and future climate change scenarios, is constrained ultimately by chronic under sampling, thereby limiting our understanding of the direction, magnitude and rates of change. Active chlorophyll fluorescence measurements can provide rapid, non-intrusive measurements of phytoplankton PP at high temporal and spatial resolution, addressing the space-time gaps, in comparison to in vitro photosynthetic-irradiance assays. Despite the strength of active fluorescence measurements to better resolve the nature and drivers of PP and the significant progress in fluorescence-based productivity algorithms, they are not routinely applied to determine PP in part due to the conversion from fluorescence units to ecologically relevant rates of carbon fixation. Earth systems models often require the photosynthetic currency of fixed carbon, to improve upon current PP estimates and to constrain the model limits. To this end, a series of active chlorophyll fluorescence measurements were performed on 2 cruises during winter and summer, with co-located photosynthetic-irradiance assays to derive a regionally and seasonally constrained conversion factor from fluorescence to carbon. Significant differences were found both spatially and temporally in the rates of PP, when measured using either fluorescence or in vitro assays. Multivariate analyses of the PP estimates and in situ biogeochemical parameters revealed novel empirical relationships that can be used to improve upon existing fluorescence-based production algorithms and biogeochemical coupling to earth systems models. - Abstract as displayed in the - Abstract booklet. The presentation on the day may differ from the - Abstract.