Fluxes of copper-complexing ligands from estuarine sediments

Most studies of the organic complexation of Cu in natural waters have focused on distributions and processes in the water column, where a significant fraction of Cu-complexing ligands may be biologically produced. We present direct evidence for a flux of Cu-complexing ligands from estuarine sediments, demonstrating that sediments are a significant, yet previously unrecognized source of the ligands. Fluxes of Cu-complexing ligands from Chesapeake Bay sediments range from 300 to 1,200 nmol rnp2 d-l, exceeding fluxes of total dissolved Cu by 3->40-fold, suggesting that any Cu fluxing from the sediments is likely to be organically complexed. Our results indicate that benthic fluxes may supply from 10 to 50% of the standing stock of Cu-complexing ligands in Chesapeake Bay and suggest that such fluxes may strongly influence the biogeochemistry of Cu in shallow water environments and potentially in the ocean as a whole. An increasing number of studies in oceanic and estuarine waters have focused on the influence of speciation (i.e. the distribution of a metal among its various forms) on the biogeochemical cycling of trace metals. Dissolved trace metals in natural waters may exist as hydrated ions and in complexes with inorganic and organic ligands (Stumm and Morgan 1996). Because the various species interact differently with the geochemical and biological components of seawater, knowledge of the speciation of a trace metal is essential to understanding its biogeochemical fate and transport, as well as its toxicity and bioavailability. For example, the availability and toxicity of trace metals such as Cu to organisms have been shown to be proportional to the activities of the free ionic forms of these metals and not simply to their total concentrations (Sunda and Guillard 1976; Brand et al. 1986). In surface oceanic and estuarine waters, dissolved Cu appears to be complexed by at least two classes of organic ligands: a very strong class (commonly termed L,) that generally exists at concentrations ranging from 1 to 40 nM, with conditional stability constants (with respect to free Cu2+ ion, K’) of 1012-1014, and weaker classes (i.e. L,, L,) that exist at higher concentrations (6-150 nM) but with lower values of K’ (108-lOlo) (e.g. van den Berg 1984; van den Berg et al. 1987; Coale and Bruland 1988; Moffett et al. 1990; Sunda and Huntsman 1991; Donat et al. 1994; see Donat and Bruland 1995 for detailed review). On the basis of laboratory culture experiments and field observations, certain phytoplankton species have been postulated as sources of the strongest ligand class, L, (Moffett et al. 1990; Moffett and Brand 1994). The specific sources of the weaker ligands are not known. In general, all classes of these ligands represent various fractions of dissolved or colloidal organic matter, including phytoplankton exudates, as well as their degradation products (Anderson et al. 1984; Seritti et al. 1986; Robinson and Brown 1991; Kerner and Geisler 1995). In addition, various sulfide and polysulfide species, particularly in suboxic or anoxic water and sedimentary environments, may complex varying fractions of Cu and other metals (Boulegue et al. 1982; Luther and Tsamakis 1989). The waters of estuarine and coastal sediments are commonly enriched in dissolved organic matter (DOM) relative to bottom waters (Burdige et al. 1992; Burdige and Homstead 1994; Alperin et al. 1994), and fluxes of dissolved organic carbon (DOC) from such sediments have been observed (Burdige et al. 1992; Burdige and Homstead 1994). Because ligands that complex Cu and other metals form a small but important fraction of the total DOM pool, sediment pore waters could be a source of Cu-complexing ligands to the overlying waters. The association of Cu with organic matter in pore waters has been demonstrated previously (Elderfield 1981; Boussemart et al. 1989), although the role of this association in influencing speciation and the sediment-water exchange of Cu has not been adequately addressed. By applying sensitive, low-volume analytical methods [anodic stripping voltammetry at a hanging mercury drop electrode (ASV-HMDE) for determining Cu-complexing ligands (Donat et al. 1994) and a chemiluminescence method (Sunda and Huntsman 1991) for determining total dissolved Cu], we have determined that sediments can be a significant source of Cu-complexing ligands and total dissolved Cu (TDCu) to the overlying water column. We are currently evaluating the significance of these sources and their implications for the biogeochemical cycling of dissolved Cu in sedimentary pore waters from two contrasting sites in Chesapeake Bay, a large, productive estuary on the eastern coast of the United States (Fig. 1). Previous studies show that biogeochemical processes at the mid-Bay Sta. M are dominated by high rates of anaerobic bacterial activity and that benthic fluxes are largely controlled by diffusive transport (Burdige and Homstead 1994; Marvin 1995). Sediments at the lower Bay Sta. S are highly bioturbated, have lower overall rates of sediment carbon oxidation (as measured by X0, production), and are dominated by mixed aerobic and anaerobic decomposition processes. The magnitude and effects of these processes vary seasonally (Burdige and Homstead 1994; Marvin 1995; also, see the DOC and X0, fluxes in Table 1). Sediment cores used for the porewater analyses were collected with a stainless steel box corer and subcored using acrylic core tubes (11.4-cm diam). The cores were cut into intervals of l-3 cm using apparatus similar to that described by Shaw (1989). Pore waters were separated by centrifugation, then syringe-filtered (0.45 pm) into Teflon bottles. All implements used to collect, process, and store the samples were extensively acid-washed to prevent contamination. To prevent oxidation artifacts, all processing steps took place in N,-filled glove bags, and samples were stored refrigerated in the dark in N,-charged jars. Concentrations of TDCu in pore waters at both sites during all seasons ranged from ~0.1 to 25 nM and were usually