The impact of suspended oyster farming on nitrogen cycling and nitrous oxide production in a sub-tropical Australian estuary

In this study we quantified nitrate (NO₃⁻) reduction (denitrification, anammox and DNRA) and N₂O production in sediments and epibiont communities associated with Sydney Rock Oyster (Saccostrea glomerata) farming. In sediments beneath an active suspended oyster farm, DNRA accounted for 98% of NO₃⁻ reduction with rates of up to 169 ± 45 μmol N m⁻² h⁻¹. Much of this DNRA was fuelled by NO₃⁻ derived from nitrification. Reference sediments had significantly lower DNRA rates of 83.8 ± 28.2 μmol N m⁻² h⁻¹, however this constituted 96% of the sites total NO₃⁻reduction. Fatty acid analysis showed that sediment organic matter was more labile in the oyster impacted sediments, facilitating subtle shifts in sediment oxygen demand which increased the Fe²⁺ availability with respect to the reference sediments. The difference in DNRA rate between the sites was attributed to autotrophic oxidation of soluble Fe²⁺ in sediments underlying the oyster cultures. DNRA was absent in the oyster shell epibiont communities and rates of anammox and denitrification were lower than in the sediments. Production of NH₄⁺ from the oysters and their associated epibionts was larger than DNRA and reached a rate of 206.2 μmol N m⁻² h⁻¹. Nitrous oxide production rates were generally low compared to other aquaculture systems and the net flux of N₂O for the combined oyster cultivation system (i.e. sediments plus epibionts) was negative, i.e. there was N₂O consumption in the sediments beneath the oysters. Overall, subtropical suspended oyster farming systems favour inorganic N retention over N loss.