Nutrient inputs to estuarine and coastal waters worldwide are increasing and this in turn is increasing the prevalence of eutrophication and hypoxic and anoxic episodes in these systems. Many urbanised estuaries are also subject to high levels of anthropogenic
metal contamination. Environmental O(2) levels may influence whether sediments act as sinks or sources of metals. In this study we investigated the effect of an extended O(2) depletion event (40 days) on fluxes of trace metals (and the
metalloid As) across the sediment-water interface in sediments from a highly
metal contaminated estuary in S.E. Tasmania, Australia. We collected sediments from three sites that spanned a range of contamination and measured total
metal concentration in the overlying water using sealed core incubations.
Manganese and
iron, which are known to regulate the release of other
divalent cations from sub-oxic sediments, were released from sediments at all sites as
hypoxia developed. In contrast, the release of
arsenic,
cadmium,
copper and
zinc was comparatively low, most likely due to inherent stability of these elements within the sediments, perhaps as a result of their refractory origin, their association with fine-grained sediments or their being bound in stable sulphide complexes.
Metal release was not sustained due to the powerful effect of
metal-sulphide precipitation of dissolved metals back into sediments. The limited mobilisation of sediment bound metals during
hypoxia is encouraging, nevertheless the results highlight particular problems for management in areas where
hypoxia might occur, such as the release of metals exacerbating already high loads or resulting in localised toxicity.