Nutrient and nitrogen isotope monitoring of an aquaculture fishery in North Queensland, Australia

A one-time monitoring event was initiated by an aquaculture fishery in North Queensland to assess if the discharges from the aquaculture fishery were increasing nutrient input into the Great Barrier Reef Marine Park. Leaf samples from Avicennia marina (grey mangrove) were used to compare the nitrogen (N) and phosphorous (P) concentrations, and δ 15N values in the receiving waters of the adjacent creeks and a set of reference sites established in a neighbouring creek. The same indicators were also measured in macroalgae from the nutrient extraction ponds. The mangrove leaves in the Mixing Zone of the receiving creek were slightly elevated in N concentrations (22 mg g -1 compared to 17.5 mg g -1 at the Reference sites) and δ 15N values (6.5 compared to 4.5 at the Reference sites) showing there was an influence from the outputs of the aquaculture fishery. However, P concentrations were the same in the Mixing Zone and Reference sites. Downstream of the Mixing Zone, the concentration of N and P, and the δ 15N values in the Receiving Waters were the same as the Reference sites, at levels considered normal in other studies. These results show that aquaculture fisheries can be managed to maintain nutrients at reference levels in mangrove foliage in estuaries of the receiving zones.