Funded NASA EPSCoR R3 Projects

FY 2022

“Improving Estimates of Land-to-Ocean Carbon Flux Through Characterization of Colloidal Inherent Optical Properties”. Margaret Estapa, Scooh of Marine Sciences, University of Maine.

The retrieval of dissolved organic carbon (DOC) concentrations from ocean color remote sensing reflectance (Rrs) is a key step in the monitoring and prediction of carbon fluxes from land to ocean. However, its retrieval has traditionally been challenging in estuarine and coastal ocean systems, because at visible and near-ultraviolet wavelengths, colored dissolved organic matter (CDOM) absorption coefficients (ag) and DOC are often not well-correlated, and because sensors with high spatial and temporal resolution are required. Further complicating matters is the typical, operational partitioning of carbon and inherent optical properties (IOPs) by size, through filtration with size cutoffs ranging from 0.2-1 μm. In the transition from land to ocean, this size range encompasses an important pool of colloidal material whose optical properties are not well-characterized. The estuaries that empty into the western Gulf of Maine will serve as the site for this research, because these watersheds generate a high “dynamic range” in CDOM sources, and because climate- driven changes in precipitation are expected to drive greater export of terrestrial organic carbon to the Gulf of Maine.

This proposal has three main objectives. The first is to adapt a generalized algorithm for the retrieval of CDOM optical properties and DOC concentration from ocean color for use in coastal Maine estuaries, and with imagery from the high resolution Landsat 8/9 and Sentinel 2a/b sensors. Our second objective is to determine the influence of colloidal matter (< 0.2 μm) on the optical properties of DOC in transitional waters between the land and the ocean, and to conduct a sensitivity analysis of the DOC retrieval algorithm to natural variability in colloidal IOPs. The third objective is to estimate fluxes of DOC through coastal Maine estuaries to the Gulf of Maine using high resolution ocean color observations, as well as uncertainties in fluxes that arise from variable carbon-specific IOPs of DOC in the source waters.

We will use opportunistically-collected surface water samples from three contrasting estuaries (Penobscot, Damariscotta, and Sheepscot) to characterize sub-micron, colloidal optical properties. A flow-field flow fractionator coupled to a volume scattering sensor and offline long-path UV-visible absorption measurements will provide detailed information on the size dependence of colloidal IOPs, and how these covary with organic carbon (OC) concentration. We will then test the agreement of size-fractionated colloidal IOP and OC measurements with the assumptions of an existing, DOC algorithm developed for the northeast United States, and the sensitivity of calculated DOC fluxes to variations in the carbon-specific IOPs of source material in the region.

The application of high resolution ocean color imagery for retrieval of nearshore aquatic carbon fluxes is in its infancy. Here we would develop these methods for use in coastal Maine, which is a step towards providing new monitoring data for use in coastal carbon budgets and climate change assessments. This project will also provide new information about the IOPs of colloids smaller than 0.2 μm, which is an unresolved issue in the field of ocean color remote sensing. The collected data will also broaden our understanding of ultraviolet optical properties as PACE data come online.