Presenter: Rachel Brewton (Harbor Branch Oceanographic Institute, Florida Atlantic University)

Description:
Over the last 30 years, water quality in Southwest Florida’s Caloosahatchee River and Estuary (CRE) has degraded, while the occurrence of harmful algal blooms (HABs) has increased. In this region, Karenia brevis (red tide) blooms originating offshore, advect toward Florida’s Gulf Coast, and blue-green algae blooms (Microcystis spp.) originating in Lake Okeechobee flow into the CRE. These HABs could be enhanced by anthropogenic nitrogen (N) and phosphorus (P) from adjacent watersheds, which climate change could exacerbate. North Fort Myers, FL, is a heavily developed, low-lying city on the CRE serviced by septic systems with ongoing water quality concerns, such as bacterial contamination and nutrient pollution. To identify sources of pollution within North Fort Myers and determine connections with rainfall and downstream HABs, this study examined septic system- groundwater- surface water interactions through the analysis of water table depth, nutrients (N and P), stable isotopes of groundwater (δ¹⁵N-NH₄ and δ¹⁵N-NO₃) and surface water particulate organic matter (POM; δ¹⁵N and δ¹³C), POM elemental composition (C:N:P), and chemical tracers (sucralose, pharmaceuticals, herbicides, and pesticides). Nutrient and POM samples were also collected HAB events. Most water table depth measurements were too high to support proper septic system functioning (>1.07 m) throughout the study area. High concentrations of NH₄⁺ and NO_x were found in groundwater and surface waters. δ¹⁵N values of groundwater and POM were frequently enriched (>3 ‰) at study sites and in downstream HABs. Additionally, the human wastewater tracers sucralose, carbamazepine, and acetaminophen were detected in groundwater and surface waters. These data suggest that groundwater and surface water in North Fort Myers are contaminated by septic system leachate, which is negatively affecting water quality and contributing to the maintenance and intensification of downstream HABs.