Light Dependence of Selenium Uptake by Phytoplankton and Implications for Predicting Selenium Incorporation into Food Webs

The potentially toxic element selenium is first concentrated from solution to a large but highly variable degree by algae and bacteria before being passed on to consumers. The large loads of abiotic and detrital suspended particles often present in rivers and estuaries may obscure spatial and temporal patterns in Se concentrations at the base of the food web. We used radiotracers to estimate uptake of both selenite (Se(IV)) and C by intact plankton communities at two sites in the Sacramento/San Joaquin River Delta. Our goals were to determine (1) whether C and Se(IV) uptake were coupled, (2) the role of bacteria in Se(IV) uptake, and (3) the Se : C uptake ratio of newly produced organic material. Se(IV) uptake, like C uptake, was strongly related to irradiance. The shapes of both relationships were very similar except that at least 42–56% of Se(IV) uptake occurred in the dark, whereas C uptake in the dark was negligible. Of this dark Se(IV) uptake, 34–67% occurred in the 0.2–1.0-mm size fraction, indicating significant uptake by bacteria. In addition to dark uptake, total Se(IV) uptake consisted of a light-driven component that was in fixed proportion to C uptake. Our estimates of daily areal Se(IV) : C uptake ratios agreed very well with particulate Se : C measured at a site dominated by phytoplankton biomass. Estimates of bacterial Se : C were 2.4– 13 times higher than for the phytoplankton, suggesting that bacteriovores may be exposed to higher dietary Se concentrations than herbivores. Depending on its concentration, selenium (Se) can be either a serious toxic contaminant or a limiting nutrient for aquatic organisms (Brown and Shrift 1982; Harrison et al. 1988). Se toxicity is a potential problem in many ecosystems where an arid climate and high Se soil concentrations combine to concentrate Se in surface and ground waters (Lauchli 1993). Anthropogenic inputs in the form of industrial waste (Bowie et al. 1996) and atmospheric deposition of airborne Se-containing particles released from coal-burning power plants (Cutter and Cutter 1998) are also important sources to some aquatic environments. Understanding the spatial and temporal variability in the selenium concentration in aquatic organisms has been of particular concern in freshwater bodies of central California and other western states, where a combination of naturally high Se concentrations in the soil, irrigation practices, and agricultural management strategies has led to instances of catastrophic poisoning (Presser 1994; Presser et al. 1994). To understand and predict Se concentration in consumer organisms, it is necessary to know the concentration of Se in organisms that form the base of the food web (Fisher and