Photodegradation of strong copper-complexing ligands in organic-rich estuarine waters

Exposure to solar radiation substantially decreased the strong copper-complexing capacity of samples collected from the organic-rich Cape Fear estuary, North Carolina. Samples exposed to natural sunlight for 1–2 d experienced a loss of strong Cu ligand (modeled at fixed K9CuL 5 1013.5) ranging from 15–33%, and .90% using long-term exposures under controlled conditions with a Xe arc solar simulator light (14 d summer sunlight). Pseudo firstorder rate constants of strong Cu ligand photodegradation averaged 0.28 d21 for two separate Cape Fear samples exposed to simulated solar radiation, much higher than corresponding dissolved organic carbon photooxidation rate constants (,0.01 d21). Degradation rates measured in solar simulator experiments predicted ligand concentrations in separate samples exposed to natural sunlight within 30% after rates were normalized to the ultraviolet (UV) light absorption coefficient at 300 nm. UV light is not solely responsible for photodegradation because exposure to photosynthetically active radiation also decreased strong Cu ligand levels, but with a smaller rate constant (0.06 d21). Although photodegradation appears to be a relatively minor sink within the Cape Fear estuary because of a shallow light penetration depth (,2% of water column) and short residence times (,1 week), strong Cu ligand levels in optically clearer South Atlantic Bight (SAB) surface waters are likely to be substantially reduced by photodegradation. Ligand degradation may be especially important to Cu-sensitive ecosystems where large increases in bioavailable and potentially toxic levels of free Cu21 ions are possible. Strong complexing ligands control the bioavailability and potential toxicity of free Cu21 ions in most estuarine and marine waters (Croot 2003). These ligands typically exist in excess of dissolved Cu concentrations in coastal waters (Shank et al. 2004c) and near equal levels in the open ocean (Coale and Bruland 1990; Moffett 1995). Any process with the potential to alter or destroy the binding capacity of these organic complexants has the potential to substantially influence Cu speciation and diminish the free Cu21 buffering capacity. Distributions of Cu21 ions can significantly affect the ecology of Cu-sensitive cyanobacteria populations (Mann et al. 2002). If organic ligands are photoreactive, absorption of solar radiation could be a primary pathway by which their Cu21 buffering capacities are degraded. Croot et al. (1999) showed that strong Cu ligands produced by the marine cyanobacteria Synechoccocus degraded in natural sunlight, 1 To whom correspondence should be addressed. Present address: NRC Associate—U.S. EPA, 960 College Station Road, Athens, Georgia 30605 ([email protected]).