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At the Bermuda Atlantic Time-Series Study (BATS) site, the field observations of dissolved organic carbon (DOC) dynamics indicate that seasonally produced "semilabile" DOC is resistant to rapid microbial degradation in the surface waters but available for microbial remineralizarion once it is delivered into the mesopelagic zone after convective overturn. In this study, we employed an experimental simulation of convective overturn events to determine whether the remineralization of semilabile DOC would occur in a controlled laboratory setting. Seawater culture experiments were conducted in which surface (≤10 m) and mesopelagic (250 m) 0.2-µm filtrates were inoculated with unfiltered water from ≤10 and 250 m in an assortment of combinations to simulate various mixtures of nutrients, DOC quantity and quality, and microbial assemblages. Results indicate that (1) microbial inocula from the upper euphoric zone were incapable of remineralizing the seasonally accumulated semilabile DOC (µmol C L-1 resolution) on the timescales of the incubations; (2) the utilization of semilabile DOC was greatest when the inoculum source was from 250 m and the filtrate source was from the upper 10 m; and (3) the decrease in bacterioplankton diversity, estimated with the Shannon-Wiener diversity index, was greater in treatments in which inoculum from 250 m was mixed with filtrate from 10 m than in treatments in which the surface inoculum was mixed with the surface filtrate. Our findings are that a portion of the surface semilabile DOC can be metabolized by microorganisms in a laboratory setting and that mesopelagic nutrients alone are insufficient to stimulate DOC drawdown > 1.3 µmol L-1 Transformations of microbial community structure were associated with the drawdown of surface DOC in simulated mixing events and suggest that microbial community structure is a factor in surface-layer DOC dynamics. [ABSTRACT FROM AUTHOR]

The cycling of the radiatively important gas carbonyl sulfide (OCS) was studied in surface waters of the Sargasso Sea. In August 1999, surface OCS concentrations averaged 8.6 pmol L-1, showed minor diel variations, and varied little with depth. An OCS precursor, total dissolved organic sulfur (DOS), was lowest at the surface (40 nmol L-1) and increased with depth. The photoproduction rate of OCS from in situ incubations averaged 9.6 pmol L-1, h-1, whereas dark production was 7.0 pmol L-1 h-1. Apparent quantum yields were 10-5-10-7 from 313-436 nm and varied with the water depth irradiated. In March 2000, there were strong diel variations in surface OCS (highest in late afternoon; overall average, 16.9 pmol L-1). Depth profiles in the afternoon showed surface water maxima and decreases with depth, whereas DOS had a surface maximum of 419 nmol L-1 and decreased with depth. Dark production was 4.0 pmol L-1 h-1. Modeling of the diel cycle suggested a photoproduction rate of 16.4 pmol L-1 h-1. Overall, the photochemical production of OCS strongly depended on DOS and chromophoric dissolved organic matter, whereas dark production was influenced by the presence of particles and perhaps microbial respiration, showing a direct biotic influence on OCS cycling. [ABSTRACT FROM AUTHOR]