Net community production drivers identified in the South Atlantic Ocean

Net Community Production, or NCP, is a measure of the balance between phytoplankton photosynthesis and the respiration of the whole plankton community. A positive value for NCP indicates an uptake of CO₂ by the plankton community, whereas a negative value occurs where CO₂ is actually released. This new satellite NCP timeseries indicated that wind speed and mesoscale features drive regional NCP variability in the South Atlantic Ocean. This has key implications for the biologically driven exchange of CO₂, carbon transfer into the deep ocean and carbon available for transfer up the food web in the region.

In situ observations of NCP, coincident with observations of phytoplankton primary production, chlorophyll a and optical measurements, have been made on AMT cruises. These measurements have allowed AMT scientists to develop multiple relationships between NCP and phytoplankton primary production (Serret et al. 2009; Tilstone et al. 2015). These relationships can be applied to satellite observations of phytoplankton primary production to provide spatially and temporally resolved regional NCP estimates. However, the accuracy of these satellite estimates must be assessed using in situ NCP observations.

Ford et al. (2021) performed a comprehensive accuracy assessment on a suite of satellite observations for NCP, phytoplankton primary production and chlorophyll a in the South Atlantic Ocean, identifying the most accurate estimates for each parameter. They assessed this accuracy by considering both the satellite and in situ observation uncertainties. The most accurate combination of these satellite estimates were applied to produce the first 16 year timeseries of NCP for the South Atlantic Ocean.

This satellite based timeseries was used to explore the environmental and climate drivers of NCP, whilst accounting for the uncertainties in satellite NCP. Wind speeds were identified as a key driver of NCP within the South Atlantic subtropical gyre, where elevated wind speeds increased NCP. Similarly, elevated wind speeds were shown to enhance NCP within the coastal regions of the Benguela upwelling system. Offshore of the upwelling system, a switch in drivers from wind speeds to sea level height anomalies were observed consistent with mesoscale features, such as filaments and eddies, driving NCP offshore into the South Atlantic gyre. These results indicated that wind speed and mesoscale features control regional NCP variability.

Author: Dan Ford (PML)

Net Community Production in the South Atlantic.