Phytoplankton are characterized by tremendous diversity that critically affects ocean ecosystem functioning. The EXPORTS “Phytoplankton & microbes” program element seeks to address the role that phytoplankton diversity plays in determining (1) net primary production (NPP), (2) standing stock of phytoplankton carbon in the euphotic zone, (3) export efficiency of phytoplankton carbon out of the euphotic zone, and (4) flux transmission of this material below the euphotic zone into the twilight zone. The EXPORTS “Optics” program element seeks to quantify the optical measurements that link ecosystem function to remotely sensed products via carbon-based optical proxies. In this proposal, we contribute to these program elements by focusing on phytoplankton diversity and optical properties/proxies of key phytoplankton community structure elements (i.e., concentration, composition and size), and how those factors influence export from and transformation below the euphotic zone. The ability to differentiate phytoplankton based upon their functional differences is crucial to quantify how different groups impact carbon stocks and fluxes. We intend to measure phytoplankton diversity and optical properties by constructing a continuous underway flow-through system for the survey ship described in the EXPORTS Science Plan. Our system will consist of imaging-in-flow cytometry, hyperspectral absorption and attenuation (scattering by difference), multispectral backscattering, and multi-excitation chlorophyll fluorescence. Water samples from CTD casts to twilight depths will also be measured in discrete mode to quantify the transformation and change in diversity of algal cells (and their associated optical properties) as they sink through the water column. In particular, we will focus on diatoms as they have been identified as the dominant phytoplankton functional type (PFT) responsible for carbon export due to gravitational settling as single cells (or chains), aggregates, or cell fragments. We hypothesize that the diatom fraction of the plankton community will be the largest predictor of the variability in carbon stock, export and flux. Further, we hypothesize that diatom size structure will provide the second order predictor of variability in carbon stock, export and flux. We will obtain direct measures of phytoplankton community composition and size structure during the two proposed cruises, but what occurs outside of those locations and time intervals is critical to resolving the seasonal and spatial variability that can only be sampled in situ on autonomous platforms or remotely from satellites. Thus, the second focus of this proposal is to quantify the linkages between phytoplankton diversity and functionality (i.e., PFTs) and (1) the optical properties that can be measured in situ and (2) the types of data products that will ultimately be obtained from next-generation ocean color satellite missions. We will use our coincident phytoplankton community structure observations and associated optical properties to construct simple optical proxies for phytoplankton concentration, composition, and size. These proxies will be critical to characterize the links among phytoplankton carbon stock, NPP, carbon export from the euphotic zone and flux transmission to the twilight zone, as well as how those linkages vary through space and time. The proxies will also link phytoplankton composition and size structure to hyperspectral phytoplankton spectral absorption and particle backscattering, respectively.
We hypothesize that robust optical proxies for pigment-based taxonomy and size structure will reduce uncertainty in modeling phytoplankton community structure. Autonomous platforms deployed with simple optical sensors would therefore provide validation for satellite data products for the surface waters and provide critical depth-dependent information to reduce uncertainty in carbon stock, flux, and transmission estimates.