Composition and lability of photochemically released dissolved organic matter from resuspended estuarine sediments

Photochemically-released dissolved organic matter (PR-DOM) from resuspended sediments is an understudied flux of dissolved organic carbon (DOC) and nutrients with the potential to influence estuarine microbial food webs. There is currently limited knowledge on the composition, lability and biological alterations of this material once released into the water column. This study addresses the composition and fate of PR-DOM from resuspended sediments of the Cape Fear River Estuary (CFRE) in southeastern North Carolina. Six-hour irradiation released 22–44% more DOC, and PR-DOM was of a different composition and enhanced lability, relative to dark controls. Irradiation led to release of humic-like DOM, indicated by increased chromophoric and fluorescent dissolved organic matter, substantial increases in the humification index, and production of oxidized higher molecular weight compounds with higher aromaticity, relative to dark controls. However, DOM of lower molecular weight and reduced aromaticity was produced as well. This was indicated by increased spectral slope (S275–295) and decreased specific ultraviolet absorbance at 254 nm (SUVA254), relative to dark controls. This latter pool of DOM may be linked to nitrogen-(N-) and sulfur-(S-)containing compounds that were photodegraded or biologically altered during irradiation experiments. In subsequent lability incubation experiments, degradation of PR-DOM was more rapid than DOM released from dark controls, especially for marine humic-like fluorophores. Incubation of PR-DOM led to an 8-fold increase in compounds with molecular formulas that were unique to light-exposed DOM relative to unexposed DOM, the majority of which were N- and S-containing compounds. Given that coastal sediments are typically enriched in these nutrients, N- and S-containing compounds appear to influence the lability of PR-DOM from estuarine resuspended sediments. Estimated lability of photoreleased DOC from resuspension events in the CFRE is comparable to previous bioavailable DOC estimates and suggests that episodic photochemical interactions with sediments may act as a previously unrecognized source of labile DOC to bacteria and plankton in these coastal waters.