Abstract
Cyanobacterial harmful algal blooms (cHABs) are increasing globally, particularly in nutrient-enriched freshwater systems. The Peace River in southwest Florida is a major tributary of the Charlotte Harbor Estuary and an essential freshwater resource increasingly affected by anthropogenic land use, including agriculture and phosphate mining. This study examines how nutrient availability, both seasonality, and as a result of land use influences algal growth potential and community composition in the Peace River. Water samples were collected monthly from four sites—two upstream and two downstream of major nutrient sources—between January and May 2025. Total nitrogen (TN) and total phosphorus (TP) concentrations were used to calculate TN:TP molar ratios, which consistently indicated nitrogen limitation across all sampling events (TN:TP < 16). Algal growth potential was assessed through laboratory incubations with three nutrient amendments (N, P, N & P, and control treatments), and community composition was evaluated using Imaging FlowCytobot (IFCB) technology and manual classification.The dominant genera were Aphanocapsa, Dolichospermum, Microcystis, and Desmodesmus, with Aphanocapsa being most abundant. Algal abundance distributions were highly skewed and leptokurtic, supporting the use of nonparametric and negative binomial statistical approaches. Seasonal analysis revealed significantly higher algal abundance during the wet season than in the dry season, particularly for Aphanocapsa. Downstream sites influenced by agriculture and mining activities showed elevated nutrient concentrations and a greater algal growth response to nitrogen addition.
These findings identify nitrogen as the primary limiting nutrient in the Peace River and indicate that anthropogenic land use strongly shapes algal dynamics and bloom potential. These results provide valuable guidance into nutrient management and bloom mitigation strategies for the Peace River watershed and contribute to broader efforts to understand cHAB drivers in subtropical freshwater systems.