A review of manganese in subtropical estuaries: Port Curtis - A case study

Manganese (Mn) is a naturally-occurring element found in rock, soil and water. In the aquatic environment, manganese mainly exists in two forms: Mn(II) and Mn(IV). Oxidation and reduction reactions convert these species, and may be abiotically or microbially mediated. Currently there is no appropriate measured rate constant for the calculation of manganese half-life within subtropical estuaries, though a recommended half-life of 28 days for Mn(II) has been suggested for use. Uptake of manganese by biota significantly increases with temperature and with decreasing salinity, whereas a decrease in toxicity is generally observed with increasing pH. Recorded manganese concentrations range from 2.8 mg/kg to 392 mg/kg (dry weight) for marine biota, between 0.4 and 1500 μg/L for marine waters and up to 9000 mg/kg in sediments. Due to observed Mn(II) LC/EC50s ranging from 1.5 to 70 mg/L, it is generally considered to have a low level of toxicity to marine biota. However, from a bioaccumulation, toxicity and oxidation perspective, there is a very limited amount of information available for effects of manganese on biota in marine waters worldwide, particularly in tropical and subtropical waters. From the existing data however, it appears that tropical species may be more sensitive to manganese than temperate species. Port Curtis, in Central Queensland, is the largest multi-cargo port in Queensland and the fifth largest port in Australia, also supporting substantial social/recreational and other commercial values including fishing, boating and tourism activities, being in close proximity to the Great Barrier Reef. Dissolved metals can pose a problem in industrial harbours; however concentrations in Port Curtis are currently below the Australian Water Quality Guidelines (AWQG) for most metals. There is currently no reliable trigger value for dissolved manganese, or manganese in sediments, in the marine environment of the Port Curtis Region or similar subtropical ecosystems, thus toxicity is difficult to estimate. Further research into the dynamics and fate of manganese will contribute to knowledge of metal bioavailability in general, as well as provide an understanding of the behaviour of trace metals under various natural environmental conditions, which will help allow more informed management decisions.