SEASONAL DYNAMICS OF COLORED DISSOLVED MATERIAL IN THE SARGASSO SEA

Observations from the Sargasso Sea have shown that the light attenuati on spectrum is a function of both phytoplankton pigments and a detrita l-like component that varies independently. Here we examine the nature and dynamics of these detrital-like variations by analyzing a time-de pth series of visible and ultraviolet light absorption spectra for col ored (chromophoric) dissolved materials [CDOM; a(g) (lambda)] and detr ital particulates [a(d) (lambda)] collected at the US JGOFS Bermuda At lantic time-series study (BATS) site. At 440 nm, CDOM absorption, a(g) (440), made up of on an average more than one-half of the total non-w ater absorption coefficient, while detrital particulate absorption, a( d) (440), was generally a minor constituent. The vertically integrated stock of CDOM (0-140 m) increased from the beginning of spring until the end of summer in 1994 and 1995, although near-surface values (less than or equal to 40 m) were strongly depressed near the surface at th e peak of summer. This summertime reduction of the mixed layer a(g) (l ambda) is likely due to photooxidation of CDOM to optically inactive f orms. Further, values of CDOM absorption were unrelated directly to in dices of total water column dissolved organic carbon (DOC) stocks meas ured 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 tw o 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 cle arly provide an explanation for the nature of non-chlorophyll light at tenuation observed in the Sargasso Sea. (C) 1998 Elsevier Science Ltd. All rights reserved.