Bender, Michael: Princeton University (Project Lead)
Project Funding:
2013 - 2015
NRA: 2012 NASA: Ocean Biology and Biogeochemistry
Funded by NASA
Abstract:
A range of algorithms exists for determining net primary production (NPP) from chlorophyll and other properties accessed by satellites. However, there are no successful algorithms for calculating carbon export production, the sinking flux of organic carbon out of the euphotic zone. This is an important limitation, as export production has a profound influence on the ocean carbon cycle, the distribution of other bioactive elements, and the atmospheric CO2 burden. We propose to access carbon export by developing an algorithm for calculating net community production (NCP), which is the rate of photosynthesis in excess of respiration. We propose to focus on the mixed layer of the Southern Ocean. Mixed layers are deep in the Southern Ocean, capturing most photosynthetically active radiation. Also, restricting our work to the mixed layer enables high resolution studies of NCP and other fundamental physical and biogeochemical properties along long cruise tracks. The Southern Ocean itself is of great interest because it plays a major role in the global carbon cycle, and because our extensive existing observations of NCP enable an algorithm for calculating this property to be extensively tested and refined. In the Southern Ocean as elsewhere, the main fate of NCP is carbon export. Thus, rates of NCP give a first-order estimate of the flux of carbon out of the mixed layer.
NPP and NCP in the Southern Ocean vary primarily with iron sufficiency and average flux of mixed layer irradiance. Iron sufficiency can be accessed by measuring variable fluorescence (from ships) or fluorescence quantum yield (from space), while mixed layer irradiance can be computed from sea surface PAR, mixed layer depth, and the diffuse attenuation coefficient. We can thus measure and map the response of NCP to iron and light both on oceanographic cruises and by remote sensing (with MLD from Argo floats). Fluorescence quantum yield is a property recently advanced as reflecting iron sufficiency in the Southern Ocean, similar to variable fluorescence. Our strategy is to make collocated measurements of these properties on cruises of the South African icebreaker Agulhas, and derive an algorithm for NCP based on the observed relationships. We will then use this algorithm, with Argo float data and remotely sensed properties, to calculate NCP throughout the Southern Ocean, and use the results to understand the roles of iron limitation, light limitation, and other influences on the variations of NCP and carbon export in the Southern Ocean.
We propose a 2-part research program. One part involves seagoing measurements, on Agulhas cruises in February, 2014 and 2015, of variable fluorescence and net community production in the Atlantic sector of the Southern Ocean. This work will extend South African measurements of inherent optical properties, iron concentration, continuous mixed layer depth (by underway CTD), nutrients and carbon system properties. We will use variable fluorescence values measured on this cruise, along with data from the literature, to compare with climatological values of fluorescence quantum yield. The comparison will inform us about the fidelity with which fluorescence quantum yield records iron limitation. The broader data will give collocated values of NCP, iron sufficiency, and MLD that will serve as the core data set for deriving an algorithm that calculates NCP as a function of these and related properties.
The second part involves deriving an algorithm for calculating NCP, starting with empirical correlations as well as NPP algorithms modified to account for iron limitation. We will refine this algorithm by challenging it with our extensive data for Southern Ocean NCP, and then use it to construct climatological maps of NCP from satellite retrievals and Argo float data.
Finally, we will use our NCP maps to advance our understanding of the controls on NCP and carbon export in the Southern Ocean.
More details may be found in the following project profile(s):
