ABSTRACT
Studies of selenium (Se) speciation in ocean waters (Measures and Burton, 1978; Burton et a l, 1980; Takayanagi and Wong, 1984, 1985) and in estuarine waters (Measures and Burton, 1978; Takayanagi and Cossa, 1985; Cutter, 1989; Cutter and San Diego-McGlone, 1990) have significantly improved the understanding of Se chemical dynamics in the aqueous state. Far fewer studies deal with Se cycling in estuarine sediment, suspended sediment, and plankton, resulting in a less than complete estuarine Se cycling model. Several key observations have been made that relate to the common coexistence of at least three aqueous Se species: Se(IV), Se(Vl), and organo-Se. Data indicate that conversion between Se species is limited and generally difficult to discern on a large spatial scale (e.g., that of an estuary). Most studies found that Se(lV) and Se(Vl) remain conservative over a wide range of salinity (Measures and Burton, 1978; Takayanagi and Cossa, 1985) as water moves from the freshwater upper reaches to the saline lower reaches of an estuary. There are exceptions, such as the estuary of the Scheldt River in Belgium (van der Sloot et al., 1985) and the San Francisco Estuary in California (Cutter, 1989), where anthropogenic activities result in significant Se inputs. Although the total loading of Se, or any other contaminant, affects the overall health of an estuary, it is the interaction of Se with the suspended and underlying sediments that may have the most dramatic and immediate effect on the biological system. This is because some of the most important components
of the food web are often found in the shallow sediment and tend to be filter feeders. Luoma et al. (1992) investigated Se uptake by bivalves (Macoma balthica) and found that all particulate forms of Se were assimilated to some degree, with particulate organo-Se being assimilated with 86% efficiency and particulate Se(0) with 22% efficiency. They also found that 98 to 99% of the Se found in M. balthica tissue came from particulate and not dissolved Se. Such findings further underscore the need to understand the dynamics of Se associations with sus pended and bottom sediments.
