Arsenic and selenium species in the oxic and anoxic waters of the Oslofjord, Norway

The depth distribution of As(III) and total As, as well as Se(IV) and Se(VI) were determined on a seasonal basis in the Inner Oslofjord in order to elucidate the conditions that control their levels and variation in these waters. The Inner Oslofjord exhibits an advanced eutrophication state and is made up of several basins of various redox conditions. These are Vestfjord (generally oxic), Bunnefjord (intermittently anoxic) and Drammenfjord (permanently anoxic). Total As concentrations are generally higher in the deep water below the pycnocline (up to 25 n compared with 10–15 n in the surface layer) and show a distribution which relates to phosphate levels. As(III), on the other hand, is present at low concentrations (< 2 n ) except in the surface layer during spring, summer and autumn (about 5 n ) and in the anoxic layer of Bunnefjord (up to 15 n ). Clearly, the total As distribution is controlled by the biochemical cycle, while that of As(III) is controlled by 1. prevailing anoxic conditions, and 2. the release of assimilated reduced As, and its subsequent slow oxidation in the predominantly oxidizing surface waters. Total selenium follows a similar distribution to that of total As, with slight enrichment below the pycnocline. However, unlike As, there is a relative depletion of Se in the deep water of the permanently anoxic Drammenfjord. Selenium(IV), on the other hand, is more relatively abundant than As(III), constituting about 25% of the total Se in Vestfjord, 30% in the suboxic Bunnefjord and over 45% in anoxic Drammenfjord. Some enrichment of Se(IV) in the surface layer during the spring and summer suggests that the release of biologically assimilated Se as Se(-II) and its subsequent oxidation to Se(IV) is a significant mechanism. The persistence of Se(IV) under oxic conditions supports the reported estimate of the slow oxidation kinetics of Se(IV) to Se(VI) in seawater. The stoichiometric ratio of As to P in the biological assimilation was between 2.37 × 10−3 and 2.2 × 10−3 and that for selenium 5.8 × 10−5.