Widespread sewage pollution of the Indian River Lagoon system, Florida (USA) resolved by spatial analyses of macroalgal biogeochemistry

Highlights: • Macroalgae biochemistry used to resolve IRL estuary nitrogen sources. • Widespread sewage nitrogen pollution indicated by nitrogen isotopes. • Resource managers should mitigate IRL sewage nitrogen sources. - Abstract: The Indian River Lagoon (IRL) system, a poorly flushed 240 km long estuary in east-central Florida (USA), previously received 200 MLD of point source municipal wastewater that was largely mitigated by the mid-1990's. Since then, non-point source loads, including septic tank effluent, have become more important. Seventy sites were sampled for bloom-forming macroalgae and analyzed for δ¹⁵N, % nitrogen, % phosphorus, carbon:nitrogen, carbon:phosphorus, and nitrogen:phosphorus ratios. Data were fitted to geospatial models showing elevated δ¹⁵N values (>+5‰), matching human wastewater in most of the IRL system, with elevated enrichment (δ¹⁵N ≥ +7‰ to +10‰) in urbanized portions of the central IRL and Banana River Lagoon. Results suggest increased mobilization of OSDS NH₄⁺ during the wetter 2014 season. Resource managers must improve municipal wastewater treatment infrastructure and commence significant septic-to-sewer conversion to mitigate nitrogen over-enrichment, water quality decline and habitat loss as mandated in the Tampa and Sarasota Bays and the Florida Keys.