Ranching of southern bluefin tuna is the largest contributor of industrial nutrient pollution to Spencer Gulf's marine environment. The industry contributes 1,946 tonnes per annum, distributed across Boston Bay & Lincoln Offshore aquaculture zones. Kingfish aquaculture is the region's next largest nutrient polluter (734 tonnes per annum) but is distributed across a larger area which includes Port Lincoln, Arno Bay, Port Neill and Fitzgerald Bay (near Whyalla). These combined nutrient inputs are ecologically significant, as Spencer Gulf is an inverse estuary and a naturally low-nutrient environment. Wastewater treatment plants from the region's largest settlements at Port Augusta, Port Lincoln, Port Pirie and Whyalla contribute a combined total of 54 tonnes of nitrogenous nutrient to Spencer Gulf.