Spectral range within global aCDOMY(440) algorithms for oceanic, coastal, and inland waters with application to airborne measurements

The optically active component of dissolved organic material in aquatic ecosystems, or colored dissolved organic matter (CDOM), is represented by the coefficient of absorption due to the dissolved aquatic constituents at 440 nm, a(CDOM)(440). Remote sensing of a(CDOM)(440) enables characterization of ecosystem processes and aids in retrieval of chlorophyll a, a proxy for phytoplankton biomass. Spectrally adjacent band-ratio domains, e.g., blue to green, have previously been applied for remote sensing of a(CDOM)(440) in coastal and oceanic waters with similar results compared to more complex semi-analytical algorithms. Estimation of a(CDOM)(440) from ratios of the most spectrally separated ocean color wavebands (end members), e.g., ultraviolet (UV) to near-infrared (NIR), termed end-member analysis (EMA), has previously been shown to increase the accuracy of global a(CDOM)(440) retrievals from in-water observations of diffuse attenuation and to enable a unified algorithmic perspective without requiring regional adjustment of internal bio-optical parameters. EMA of above-water observations is evaluated herein, with a focus on coastal and inland waters in which increasing optical complexity and likelihood of bottom reflectance challenge the oceanic algorithms developed for deep and optically simple (case-1) waters. Analysis herein of three independent, in situ, bio-optical datasets indicates significant correlation between a(CDOM)(440) and end-member band ratios (next-generation 320 and 780 nm or legacy 412 and 670 nm ratios) with a coefficient of determination, R-2, of 0.87 (log-scale) or higher based on a dataset spanning the dynamic range of global, conservative water bodies. For applicable wavelengths, EMA algorithms are shown to agree with case-1 relationships and to produce consistent log-scale uncertainties across more than three orders of magnitude in a(CDOM)(440) values (0.001-2.305 m(-1)). EMA using UV and NIR wavelengths (320 and 780 nm) is applied to low-altitude airborne observations and satisfies 25% uncertainty based on unbiased percent differences (UPDs) within each of three dissimilar match-up sites ranging in a(CDOM)(440) from 0.02-0.57 m(-1). Results demonstrate that EMA is a useful and robust approach for the remote sensing of a(CDOM)(440) in coastal and inland waters, which are generally shallower, contain more optically complex environments, and span a greater range in a(CDOM)(440) than oceanic waters.

Automated summary

The study focused on the optically active component of dissolved organic material in aquatic ecosystems, or colored dissolved organic matter (CDOM), and explored remote sensing techniques for characterizing this parameter and retrieving chlorophyll a. Results demonstrated that end-member analysis (EMA) is a useful and robust approach for CDOM remote sensing in coastal and inland waters, providing increased accuracy and addressing challenges posed by optical complexity and bottom reflectance.