Effects of salinity changes on the photodegradation and ultraviolet–visible absorbance of terrestrial dissolved organic matter

We performed laboratory studies to determine the effects of salinity on the photodegradation of dissolved organic matter (DOM) from the Great Dismal Swamp, Virginia, an important source of terrestrial DOM to the lower Chesapeake Bay. Samples were created by mixing Great Dismal Swamp water (ionic strength < 0 mol L21) with modified artificial seawater solutions of differing salinities while keeping the final dissolved organic carbon (DOC) concentration constant. These samples were then irradiated for 24 h in a light box providing ultraviolet (UV) light similar to that of natural sunlight. Light absorbance and DOC concentrations decreased after photoexposure, whereas dissolved inorganic carbon (DIC) concentrations increased. Variations in salinity affected both DIC production and UV absorption, with the higher salinity samples showing lower DIC production and less photobleaching. Addition of an iron chelator eliminated the relationship between photochemistry and salinity by reducing both photobleaching and DIC production at low salinities. As terrigenous DOM transits through an estuary, its photochemical reactivity and optical properties may change significantly as a function of salinity, probably as a result of changes in DOM conformation or changes in iron– DOM photochemistry, or both. Understanding the cycling of aquatic dissolved organic matter (DOM) is important because DOM is a dynamic component in the global carbon cycle, acts as a nutrient source (either through direct uptake or as a substrate for remineralization), and provides ion exchange capacity within aquatic systems. As an absorber of visible and ultraviolet electromagnetic radiation, DOM can also significantly reduce both photosynthesis and photoinhibition of biota in surface waters. Photochemical processes resulting from this light absorption result in significant carbon remineralization (Miller and Zepp 1995) and transformation of organic matter (Mopper and Kieber 2002 and references therein). Photochemical alteration of DOM has been shown to affect the microbial bioavailability of DOM (e.g., Kieber et al. 1989), and coupled photochemical/microbial degradation is believed to be a major sink for terrestrial, microbially refractory DOM carried by many rivers (Kieber et al. 1990; Mopper and Kieber 2000). This coupled degradation mechanism could largely explain the low abundance of identifiable riverdelivered terrestrial DOM within oceanic DOM (Hedges et al. 1997, and references therein). 1 To whom correspondence should be addressed. Present address: Large Lakes Observatory and Chemistry Department, University of Minnesota—Duluth, 2205 East 5th Street, Research Laboratory Building, Duluth, Minnesota 55812 ([email protected]. edu). 2 Present address: Large Lakes Observatory, University of Minnesota-Duluth, Duluth, Minnesota 55812.