Denitrification, anammox and nitrate reduction in the sediments of the Southern Great Barrier Reef Lagoon

We provide the first reported estimates of anammox activity in tropical continental shelf sediments (southern section of the Great Barrier Reef lagoon; GBRL). The measured contribution of anammox to total N₂ production was up to 70% but restricted to only 1 of the 4 (2 inshore and 2 offshore) sites assayed. Sediment characteristics (contents of total organic carbon [TOC] and manganese [Mn], C:N ratio) at this site appeared to favour anammox activity and the estimated maximum rate was 4.9 µmol m^-2 h^-1. Anammox bacteria may be a significant contributor to N₂ production along the coastal zone of the GBRL. The availability of labile (low C:N) TOC seemed to drive denitrification to completion in the offshore sediments. However, rates of NO₃^- reduction to NH₄⁺ at the offshore sites were comparable to or higher than denitrification rates. It was unclear whether dissimilatory or assimilatory processes were responsible for the observed reduction of NO₃^- to NH₄⁺ at the offshore sites. At the 2 inshore sites, NO₃^- reduction to NH₄⁺ was a larger sink for NO₃^- than denitrification. Anammox does exist in the tropical continental shelf sediments of the GBRL and should be studied further to determine its role in larger scale N cycling. The roles of assimilatory and dissimilatory nitrate reduction to ammonium also need to be assessed within the GBRL.