Derivation of dissolved organic carbon concentrations from SeaWiFS data

Abstract

Coloured dissolved organic matter (CDOM), which represents a fraction of dissolved organic carbon (DOC), contributes strongly to the absorption of blue light in Baltic waters. This makes the Baltic an ideal test site for deriving DOC concentrations from ocean colour imagery. Two hundred and five high‐resolution (1.3 km) Sea‐viewing Wide Field of view Sensor (SeaWiFS) images from 1998 were processed using SeaDAS and combined into monthly composites of reflectance. A simple reflectance ratio algorithm, generated from in situ data, was used to derive CDOM absorption at 440 nm (a g440) and an algorithm from the literature was applied to derive DOC concentrations, with a constant CDOM spectral slope coefficient of 0.019 nm−1 to extrapolate to the required DOC‐algorithm wavelength. Monthly composites were chosen to improve the radiometric signal‐to‐noise ratio, given limitation of spatial and temporal coverage caused by clouds, ice and low solar illumination. Surface DOC concentrations ranged between 4 mg l−1 and 5 mg l−1, with elevated values in the cloudiest months (March and October). An increase in surface DOC lagged one month behind the annual cyanobacterial bloom, which peaked in July. The Gulf of Bothnia was found to have relatively high DOC concentrations in all months and comparisons with water‐leaving radiance (555 nm, associated with particulate scattering) maps showed no consistent correlation between DOC and suspended particulates.