QUANTIFICATION OF NON-ALGAL LIGHT ATTENUATION IN THE SARGASSO SEA - IMPLICATIONS FOR BIOGEOCHEMISTRY AND REMOTE-SENSING

Observations of the diffuse attenuation coefficient spectrum (K-d(z,t, lambda)), made as part of the U.S. Joint Global Ocean Flux Study (JGOF S), Bermuda Atlantic Time-series Study (BATS), show significant season al variations that do not covary with the patterns in chlorophyll a co ncentrations. The spectral structure of these changes indicates that t he variations in optical properties are caused by detrital particulate and/or colored dissolved organic materials. Multiple regression analy ses over wavelength are used to deconvolve this ''colored dissolved or detrital material'' (CDM) signal from the chlorophyll signal. General ly, CDM light attenuation is nearly as large as the chlorophyll-relate d attenuation and it exhibits a seasonal signal related to changes in mixed-layer depth. In the winter, values of CDM are roughly uniform th roughout the upper 150 m of the mixed layer, whereas in the summer sig nificantly reduced CDM values are found in a shallow mixed layer with elevated values in the seasonal pycnocline. The summer-time pattern ap pears to be caused by the combination of reduction of CDM within the m ixed layer by photo-oxidation and its production at depth. This season al cycle is different than that observed for dissolved organic or part iculate organic carbon at BATS. Hence, optical inferences of CDM are n ot directly related to biogeochemically relevant materials, such as di ssolved organic carbon. Quantification of the photon flux absorbed by CDM can be used to assess the role of CDM quanta absorption in photoch emical transformations. We suggest that CDM-regulated rates of photo-o xidation play a critical role in the regulation of near-surface dimeth yl sulfide concentrations on a seasonal timescale. Last, the existence of a strong CDM signal requires modification of the techniques used t o estimate chlorophyll from satellite ocean color sensors. Copyright ( C) 1996 Elsevier Science Ltd