Coastal sediments record decades of cultural eutrophication in Tampa Bay, Florida, USA

Abstract

Phosphorus (P) sustainability is a complex problem – it is a limited resource critical for agricultural productivity, but fertilizer production generates extensive phosphogypsum waste and can impair downstream water quality. Industrial, urban, and agricultural activities contribute to cultural eutrophication, thereby degrading both coastal and inland ecosystems and storing legacy nutrients in sediments. This study investigated the long-term effects of phosphogypsum wastewater discharges on legacy nutrient accumulation, an unintended impact of the fertilizer industry that is often overlooked and understudied. Sediment cores were collected to reconstruct the depositional history of two sites in Tampa Bay, Florida, USA that experienced past wastewater releases; the most recent in 2021. Sediments had high concentrations of stored or legacy nutrients (total P: 0.11 – 15.01 mg g⁻¹; total nitrogen: 0.04 – 0.37 %) particularly during discharge timeframes, as assessed by short-lived radioisotopes, and were predominantly in bioavailable forms, as assessed by bulk pools and ³¹P nuclear magnetic resonance spectroscopy. These values are comparable to hypereutrophic lakes impacted by agriculture and urbanization. Sediment accumulation rates were elevated relative to other Florida estuaries (Bishop Harbor: 13,092 – 46,706 g m⁻² yr⁻¹; Piney Point Creek: 3,064 – 23,990 g m⁻² yr⁻¹), which can alter biogeochemical cycling and the fate of nutrient loading. Phosphorus accumulation rates and other proxies had downcore peaks corresponding to discharge events from 2001 to 2004, 2011, and 2021 with P accumulation rates ranging from 0.5 – 559 g m⁻² yr⁻¹. These findings indicate that estuarine nutrient budgets need to incorporate stored sedimentary nutrient pools and internal benthic fluxes and highlight the need for a more sustainable P supply chain.