The goal of EXPORTS field campaign is to develop a predictive understanding of the export and fate of global ocean primary production. The downward transport of organic matter from sunlit surface ocean occurs over the entire dissolved-particulate size continuum with processes spanning from diffusion of non-sinking dissolved organic matter (DOM) to passive sinking of particulate organic matter (POM). The mass flux is typically estimated using the size distribution of particles of sizes > 1 µm. The effect of particle composition on sinking speed and downward transport of non-sinking dissolved and small particulate organic carbon are two of the priorities recently identified for future research towards a transformative understanding of the biological pump[ Burd et al. 2016]. To better understand how variation in particle size distribution impact carbon export estimates and therefore to better understand if size or composition is the main driver of the export, we propose to apply the latest technological and theoretical advancements in the field of ocean optics to quantify in situ size and density distributions of particles in the dissolved-particulate continuum from 20 nm to 20 mm. Specifically, we propose to achieve the following objectives:
Objective (i): Measure the vertical distribution of the spectral absorption coefficient (a) and volume scattering function (VSF, β), which will be augmented by additional optical observation of an imaging flow cytometer and an underwater vision profiler (UVP).
Objective (ii): Estimate the size and density distributions of particles of sizes from 20 nm – 200 µm from the VSF data, the particles size distribution from 5 - 100 µm from the flow cytometer data, the particle size distribution from 100 µm to 20 mm from the UVP data, and phytoplankton size fractions from the absorption data.
Objective (iii): Estimate the total and size-fractioned mass flux using the optically derived results of particle size and density distributions from Objective (ii). The size-fractioned flux will be contrasted between (1) dissolved vs. particulate; (2) small (< 100 µm) vs. large (> 100 µm) particles; and (3) communities dominated by different classes of phytoplankton.
Objective (iv): Apply statistical analysis to investigate (1) how the mass flux estimated based on VSF-inversion results (i.e., size and density distribution from 0.02 – 200 µm) relates to the total flux and (2) how the phytoplankton community (macro-, nano-, and pico-) that can be retrieved from the absorption measurements relates to the total flux.
Our ultimate goal is to test if the total mass/carbon flux can be adequately estimated using the biogeochemical variables that can be retrieved from the measurements of inherent optical properties of absorption and volume scattering function in the surface water. This is directly related the EXPORTS’s overarching hypothesis - carbon export from the eutrophic zone and its fate within twilight zone can be predicted knowing characteristics of the surface ocean ecosystem.