Nutrient availability in a freshwater-to-marine continuum: Cyanobacterial blooms along the Lake Okeechobee Waterway

Water from the Lake Okeechobee watershed historically flowed south through the Everglades. Hydrologic alterations created the Lake Okeechobee Waterway, where lake water is periodically shunted east to the St. Lucie Estuary (C-44 canal) and west to the Caloosahatchee River and Estuary (C-43 canal). Within the last two decades, Microcystis blooms have developed in Lake Okeechobee and been discharged to the downstream urbanized estuaries, resulting in negative environmental and human health impacts. To better understand drivers of cyanobacterial blooms across this modified waterway, two cruises were conducted from the St. Lucie Estuary through Lake Okeechobee to the Caloosahatchee River Estuary during 2019 and 2020. Opportunistic sampling was also conducted during Microcystsis blooms. Cruise stations were sampled for environmental parameters, dissolved nutrients, chlorophyll a, cyanobacterial cell concentrations, and microcystins, as well as particulate organic matter (POM) nutrient properties. Higher ammonium (NH₄⁺), nitrate + nitrite (NO₃^-), dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), and POM stable N isotope (δ¹⁵N) values were observed in the estuaries and Kissimmee River than in Lake Okeechobee. The nitrogen to phosphorus ratio (N:P), microcystins, and Microcystis cell concentrations were higher in Lake Okeechobee than documented over past decades. During Microcystis blooms, high NH₄⁺, SRP, total dissolved nitrogen (TDN), TDP, and sucralose were observed with elevated algal δ¹⁵N. These results demonstrate the importance of local basin contributions, including those within the lake, to estuarine Microcystis blooms. This suggests that decreasing nutrient loading within the St. Lucie and Caloosahatchee estuaries would help to mitigate these urban blooms. High POM δ¹⁵N values, NO₃^- concentrations, and N:P ratios in the Kissimmee River suggest that expanding urbanization north of the lake represents an increasing human N source contributing to cyanobacterial blooms in Lake Okeechobee.