Remotely Sensed Estimates and Controls of Large-Scale Oceanic Net Community Production

Oceanic net community production (NCP), defined as photosynthesis in excess of respiration, lowers the CO2 concentration at the ocean surface and in the process regulates atmospheric CO2 levels on seasonal to glacial-interglacial time scales. The magnitude of oceanic NCP, and the regulating factors are however poorly constrained. This dissertation aims to derive estimates of the large-scale distribution of NCP and to explore the mechanisms driving this variability, at regional scales (Western Antarctic Peninsula; Chapter 2), basin scales (Southern Ocean, Chapter 3), and global scale (world oceans, Chapter 4).

In Chapter 2, we use remotely sensed properties and in-situ observations of O2/Ar-NCP from 2008 to 2014 to explore the interannual variability in NCP at the Western Antarctic Peninsula. We find that annual NCP in the shelf and coastal regions is up to eight times higher than in offshore regions, with hotspots observed around canyons. The interannual variability in annual NCP observed in the region is likely controlled by the iron supply from subsurface or horizontal advection.

In Chapter 3, we use remotely sensed properties to investigate the impact of mixed-layer dynamics on NCP in the Southern Ocean. We find that, as expected, NCP is largely controlled by light availability on seasonal time scales. On intra-seasonal time scales, a deepening of mixed layer increases NCP which we attribute to increased nutrient availability. On interannual time scales, NCP correlates with a host of parameters (i.e., stratification, wind kinetic energy, and mixed layer depth), but not to mixed layer depth (MLD). Although we do not observe a secular trend in NCP for the entire Southern Ocean, NCP increases (decreases) in the Atlantic (Pacific) sector over the 1997-2014 period. Overall, our results show that the driving mechanisms behind the NCP distribution vary as a function of the temporal and spatial scales under study.

In Chapter 4, we derive two global satellite NCP algorithms using O2/Ar measurements and the machine-learning methods of genetic programming and support vector machine. Our new algorithms are comparable to other algorithms in their prediction accuracy and magnitude of global biological carbon fluxes at the ocean surface, but predict a more spatially uniform NCP distribution for the world’s oceans.

In Chapter 5, we develop a mechanistic model of the carbon export potential from the mixed layer as a function of light availability. We show that the model is remarkably consistent with in-situ observations of O2/Ar-derived NCP and export production estimates from 234Th and sediment traps. Our model suggests that carbon export production in the Southern Ocean is likely co-limited by light and nutrient availability.

We end with a discussion of future projects in the concluding chapter 6.