Observations of mariculture associated N2O loss: a need for system specific studies

Aquaculture’s contribution to global N2O emissions is poorly constrained and often reliant on supply chain/industrial emissions/life-cycle analyses which generalise system responses to farm-derived inputs and contain few examples of direct measurements made in situ. Among the studies that do report aquaculture associated N2O emissions the focus has been on pond culture and wetlands systems rather than open marine systems. Our study examined the effects of open system aquaculture culture on water column N2O cycling in two hydrodynamically contrasting southern hemisphere systems: the heavily stratified Macquarie Harbour, Tasmania, Australia and the semi-enclosed but well-mixed Big Glory Bay, New Zealand. Significant, but localised, N2O undersaturation was observed under the active salmon farm in the heavily stratified Macquarie Harbour during the peak feeding season, but not under fallowed salmon farms or the non-farmed areas. This was observed in a low-oxygen but not anoxic water column. Water column N2O was either in equilibrium with the atmosphere or supersaturated in all other instances. In Big Glory Bay N2O undersaturation was observed during winter and spring sampling surveys that generally persisted across the bay and resulted in removal of atmospheric N2O. The specific mechanisms of N2O loss are still uncertain but is likely driven by a combination of particle associated denitrification activity in farm waste plumes, denitrification/DNRA in sediments and on the detritus covered mussel shells and lines. Overall, this study demonstrates that industry impacts to N2O cycling can include loss dynamics which have previously been unreported. Therefore, global estimates of N2O emissions from aquaculture may be significantly overestimated.