Stable isotopes demonstrate the effectiveness of a tidally-staged sewage release system

Highlights: • Elevated δ¹⁵N values of algae and limpets were localized to the diffusers. • Algae and limpet δ¹⁵N values returned to baseline levels within ~250 m from outfall. • Incorporation of sewage-N into the benthic component of the food web. • Sewage-N not dominant in pelagic food web- barnacle δ¹⁵N did not significantly vary. • δ¹³C values in turf algae helped define estuarine-marine mixing area. - Abstract: Nutrient loading from sewage wastewater discharge contributes to the eutrophication of coastal waters. Wastewater from the Gold Coast, Australia is discharged into the Gold Coast Seaway (GCS) for 13.5 h d⁻¹ primarily on the ebbing tide to disperse wastewater seawards. Nitrogen stable isotopes were used to assess how effectively the tidally staged release system dispersed wastewater out of the GCS and identified pathways by which sewage-N was incorporated into food webs. Turf algae, limpets and barnacles were sampled at the GCS, at two coastal sites and at the mouth of a control estuary that lacked point-source discharge. In the GCS δ¹⁵N values of algae and limpets returned to coastal baseline levels within 250 m of the diffusers. In contrast, δ¹⁵N of filter-feeding barnacles did not significantly vary indicating wastewater-N does not dominate the pelagic food web. Nitrogen stable isotopes clearly demonstrated that the tidally-staged wastewater release system effectively disperses wastewater offshore.