Seasonal dynamics of colored dissolved material in the Sargasso Sea

Abstract

Observations from the Sargasso Sea have shown that the light attenuation spectrum is a function of both phytoplankton pigments and a detrital-like component that varies independently. Here we examine the nature and dynamics of these detrital-like variations by analyzing a time-depth series of visible and ultraviolet light absorption spectra for colored (chromophoric) dissolved materials [CDOM; ag(λ)] and detrital particulates [ad(λ)] collected at the US JGOFS Bermuda Atlantic time-series study (BATS) site. At 440nm, CDOM absorption, ag(440), made up of on an average more than one-half of the total non-water absorption coefficient, while detrital particulate absorption, ad(440), was generally a minor constituent. The vertically integrated stock of CDOM (0–140m) increased from the beginning of spring until the end of summer in 1994 and 1995, although near-surface values (⩽40m) were strongly depressed near the surface at the peak of summer. This summertime reduction of the mixed layer ag(λ) is likely due to photooxidation of CDOM to optically inactive forms. Further, values of CDOM absorption were unrelated directly to indices of total water column dissolved organic carbon (DOC) stocks measured at BATS. We hypothesize that CDOM is produced as a by-product of the microbial breakdown of DOC and is destroyed due to photooxidation. Summertime CDOM dynamics at this site can be quantified with these two processes, as mixing below the summertime mixed layer can be assumed to be small. Our results imply, for the first time in the blue ocean, a link between microbial community activity and CDOM dynamics and clearly provide an explanation for the nature of non-chlorophyll light attenuation observed in the Sargasso Sea.