Changes in nitrogen and phosphorus within a decade in waters along a major canal and estuary in South Florida

The West Palm Beach‐C51 (WPB‐C51) canal connects Lake Okeechobee with Lake Worth Lagoon (LWL) in South Florida. This canal receives discharges from Lake Okeechobee and from agricultural and urbanized areas. The objectives of this research were to determine spatial and temporal differences and trends of N and P forms along the WPB‐C51 canal and LWL for 2009–2019. Data were obtained from the South Florida Water Management District. Average total P (TP) and total N (TN) concentrations at studied stations ranged from 55 to 183 μg L−1 and from 0.61 to 2.62 mg L−1, respectively. The TP and TN concentrations at LWL inflow were higher than the State of Florida's regulatory criteria (49 μg L−1 and 0.66 mg L−1, respectively). The TP and TN concentrations generally declined from Lake Okeechobee to LWL; however, agricultural drainage was highest for soluble reactive P (SRP) and all N forms. Temporal trends showed predominantly increasing trends for concentrations of P forms. Total P and TN load rates increased by 0.005 and 0.032 t yr−1, respectively, at LWL inflow. Results suggest nutrient assimilation by plants and P precipitation along the WPB‐C51 canal. Possible sources of SRP and N are fertilizers, nitrification, and organic matter mineralization. Increasing trends in P concentration are possibly due to legacy P and urbanization, and increases in TP and TN loads can be due to larger Lake Okeechobee discharges and higher nutrient concentrations. Results emphasize the need to implement strategies to minimize nutrient input into LWL to meet its water quality goals. Core ideas Concentrations of N and P forms decrease from source to discharge point. Decreases in P and N concentrations are driven by assimilation, precipitation, and denitrification. Concentrations of total P and total N at the canal's discharge point are higher than mandated by law. Total P and total N loads increased from 2009 to 2019 along the studied canal. Larger Lake Okeechobee discharges and urbanization drive increases in nutrient loads.