Exploring Net Community Production estimates and drivers in the North Pacific and North Atlantic

Abstract

The Biological Carbon Pump (BCP) is a natural mechanism in the ocean that exports carbon in the deep ocean and is estimated to transfer between 5 and 12 Pg C from the surface to the deep ocean annually. While the underlying mechanisms of this process – primary producers create organic carbon from CO2 through photosynthesis, some of this organic carbon is recycled in the surface ocean, while some of it is exported to depth via physical or biological processes – have been identified for decades, this process remains difficult to quantify and predict. We estimate the carbon export potential from the surface ocean by estimating net community production (NCP) from continuously measured in situ O2/Ar ratios. In this dissertation, I aimed to assess the coherence of many methods of measuring NCP and determine factors, both physical and biological, that drive changes in NCP. Together, these goals allowed me to offer suggestions to improve modeling efforts to estimate the BCP from autonomous or remote sensing observations. To explore these topics, I used many different methods. In Chapter 2, I showed that measurements of NCP collected from different methods were consistent around Ocean Station Papa in the North Pacific after accounting for spatial heterogeneity. I compared estimates of NCP from shipboard O2/Ar measurements; O2, NO3-, particulate organic carbon (POC), and dissolved inorganic carbon (DIC) measurements from autonomous platforms, and shipboard incubations based on changes in Chl a and NO3-. I used a generalized additive mixed model to compare the datasets when spatial and temporal differences in the measurements were considered. In Chapter 3, I explored drivers of NCP by comparing how NCP related to various in situ biomarkers and biogeochemical rates measurements. I used moving Pearson’s correlations to assess how continuous measurements of biomarkers such as Chl a, POC, phytoplankton carbon, temperature, and community particle size distribution correlated to changes in continuous NCP. In addition, I showed that NCP was likely driven by changes in production, rather than respiration, in both the North Pacific and North Atlantic by comparing NCP with incubation-based estimates of gross primary production (GPP), net primary production (NPP), and microbial community respiration (mCR). Finally, I modeled NCP from the available biomarker data and determined that POC is a better proxy for estimating NCP than Chl a, in both locations. Finally, in Chapter 4, I examined how changes in the microbial community (from 16S and 18S amplicon sequencing) paired with changes in NCP in the North Pacific. I showed that at coarse taxonomic groupings, such as Phylum, Class, or plankton functional type, had no correlation to changes in NCP, while individual amplicon sequencing variants (ASVs) had strong correlations to changes in the surface ocean organic carbon balance. This indicates a need for increased granularity in microbial community composition estimates to effectively model NCP or carbon export from surface ocean microbial communities. Altogether, my research increases confidence in global NCP estimates from various platforms, presents potential improvements to biogeochemical modelling efforts, and suggests that respiration does not drive changes in NCP in the ocean.