Quantification of non-algal light attenuation in the Sargasso Sea: Implications for biogeochemistry and remote sensing

Observations of the diffuse attenuation coefficient spectrum (Kd(z,t,λ)), made as part of the U.S. Joint Global Ocean Flux Study (JGOFS), Bermuda Atlantic Time-series Study (BATS), show significant seasonal variations that do not covary with the patterns in chlorophyll a concentrations. The spectral structure of these changes indicates that the variations in optical properties are caused by detrital particulate and/or colored dissolved organic materials. Multiple regression analyses over wavelength are used to deconvolve this “colored dissolved or detrital material” (CDM) signal from the chlorophyll signal. Generally, CDM light attenuation is nearly as large as the chlorophyll-related attenuation and it exhibits a seasonal signal related to changes in mixed-layer depth. In the winter, values of CDM are roughly uniform throughout the upper 150 m of the mixed layer, whereas in the summer significantly reduced CDM values are found in a shallow mixed layer with elevated values in the seasonal pycnocline. The summer-time pattern appears to be caused by the combination of reduction of CDM within the mixed layer by photo-oxidation and its production at depth. This seasonal cycle is different than that observed for dissolved organic or particulate organic carbon at BATS. Hence, optical inferences of CDM are not directly related to biogeochemically relevant materials, such as dissolved organic carbon. Quantification of the photon flux absorbed by CDM can be used to assess the role of CDM quanta absorption in photochemical transformations. We suggest that CDM-regulated rates of photo-oxidation play a critical role in the regulation of near-surface dimethyl sulfide concentrations on a seasonal timescale. Last, the existence of a strong CDM signal requires modification of the techniques used to estimate chlorophyll from satellite ocean color sensors.