EARLY DIAGENETIC PRODUCTION AND SEDIMENT-WATER EXCHANGE OF FLUORESCENT DISSOLVED ORGANIC-MATTER IN THE COASTAL ENVIRONMENT

Fluorescence at wavelengths characteristic of humic substances (excita tion 350 nm, emission 450 nm) have been used in this study to approxim ate concentrations of fluorescent dissolved organic material (FDOM). I n situ regulated and unregulated benthic chambers, sediment cores, and laboratory tank incubations were used to study early diagenesis of FD OM in coastal marine sediments of the Gullmar Fjord, western Sweden. I n the regulated in situ chambers, pH and oxygen were kept at relativel y stable levels, while in the unregulated in situ chambers, pH and oxy gen were left to decrease as a result of biological activity. FDOM por ewater distributions and correlation between FDOM and parameters indic ating mineralization showed that FDOM was formed in the sediment and s hould flux across the sediment-water interface. A substantial flux of FDOM was also observed during winter and spring conditions and during anoxic conditions fall. However, no flux was observed during oxic cond itions fall. Modeling indicated that oxygen penetration depth was deep er during winter than during fall, i.e., the oxygen penetration depth increased during fall towards winter values. We suggest that as FeOOH was formed when oxygen penetration depths increased, FDOM was sorbed t o newly formed FeOOH, inhibiting FDOM flux over the sediment-water int erface. In addition, at onset of anoxic conditions in the sediment sur face layer in fall incubations, FDOM flux from sediment to overlying w ater increased substantially. Increases in anoxic FDOM fluxes were acc ompanied by increases in Fe and phosphate fluxes. We suggest that redu ctively dissolved FeOOH released sorbed FDOM. FDOM released from FeOOH by anoxic conditions was not resorbed when oxic conditions were resum ed. This could be an effect of higher pH in overlying water as compare d with porewater, inhibiting FeOOH sorption of FDOM.