The primacy of dissolved organic matter to aquatic light variability

Abstract. Absorption and scattering by optically active constituents (OACs) modify the sunlit aquatic light environment, facilitating the derivation of biogeochemical data products at scales spanning in situ to satellite observations. Excluding solar illumination, plus geometric and atmospheric effects, variability in an optical parameter arises from changing OAC concentrations, wherein observed patterns in the spectral evolution of data products are associated with the connectivity and spatiotemporal dynamics of OACs. In open-ocean waters far from terrestrial and riverine inputs, the content and mixture of OACs principally relates to the dynamics of phytoplankton and the microbial loop – a trophic pathway describing the cycling of microbial primary producers, remineralizers (e.g., bacteria and archaea), plus dissolved organic and inorganic materials (as applicable). Historical bio-optical models for the open ocean primarily invoke chlorophyll a concentration (Ca) – a commonly used proxy for phytoplankton biomass – as the ubiquitous independent variable governing optical data products such as the normalized water-leaving radiance, LW(λ)N. Formulation of LW(λ)N as a function of Ca invokes an idealized food chain, wherein phytoplankton are the dominant control of OACs, including the colored (or chromophoric, depending on the literature) portion of the dissolved organic matter (DOM) pool, hereafter CDOM. This prescription, in which Ca maximally explains oceanic light variability (hereafter primacy), is tested herein using eigenanalysis – e.g., an empirical orthogonal function analysis, principal component analysis, or other eigendecomposition depending on the literature. Analyses using three independent bio-optical datasets assess the shapes and associations of the principal and secondary eigenfunctions of aquatic LW(λ)N observations. The analyses reveal LW(λ)N variations to be more strongly associated with changes in CDOM rather than Ca – even for purely oceanic datasets – indicating that CDOM dynamics are more variable and exhibit greater independence from Ca than formerly ascribed. Blue and green band-ratio algorithms routinely used for remote sensing of Ca are found to be maximally sensitive to variability in CDOM rather than Ca based on validation tests of ocean chlorophyll (OC) algorithm performance (e.g., R2 of 0.85 versus 0.78), plus partial correlation coefficients relating eigenfunction scalar amplitude functions to field or derived observations. Eigenanalyses applied to spectral subsets of the data indicate expansive spectral range observing improves the independence in retrieving CDOM absorption and Ca. The spectral subset comparisons indicate expanded spectral observations supported by recent domestic and international satellite missions constitute a new and unique opportunity to optically characterize surface ocean phytoplankton stocks without relying on explicit or implied empiricisms requiring CDOM and other OACs to covary with Ca. The shapes and associations of the eigenfunctions suggest a greater diversity of trophic pathways determine OAC dynamics – e.g., in addition to phytoplankton contributing CDOM via cellular lysis, excretion, and grazing – and are consistent with advancing knowledge of the microbial loop in the decades after bio-optical formulations based on Ca were proposed.