Abstract
Nutrient pollution from anthropogenic activities threatens freshwater and estuarine ecosystems, exemplified by the Peace River Watershed (PRW) in Florida, where excess nutrients fuel harmful algal blooms (HABs) in Charlotte Harbor. This study utilized the Watershed Assessment Model (WAM) to analyze nutrient loading and identify source areas within the PRW, which is crucial for effective mitigation. WAM integrated land use, soil, rainfall, and wastewater data to create unique cells. Daily flows and loads were simulated using field-scale sub-models tailored to Florida conditions, routed through the river network, then calibrated and validated against observed data. Results showed an annual discharge of 1.9 cubic kilometers of water, 2,370 tons of total nitrogen (TN), and 1,025 tons of total phosphorus (TP) into Charlotte Harbor. Spatially, annual TN and TP offsite loads ranged from near zero to 116 and 38 kg/ha, respectively. Elevated TN levels were observed in urban areas with septic systems and agricultural lands lacking best management practices (BMPs). High TP loads were associated with phosphate mining and intensive agricultural areas. Groundwater nitrate was elevated in urban areas with septic systems. The TN: TP ratio at the outlet showed nitrogen limitation, although 33% of the source area was P-limited. Nitrogen reduction was attributed to attenuation processes during transport. Therefore, controlling HABs in the harbor requires targeted TN reduction in hotspot areas. This study established a water quality baseline, identified critical source areas, and will help in devising mitigation strategies in the PRW and similar coastal watersheds.
Highlights
The WAM model simulated spatial nutrient loadings to identify source areas in the PRW, a coastal watershed in Florida.
Urban areas with septic systems showed elevated total nitrogen levels in the groundwater.
Phosphate mining lands generated elevated TP loads.
Agriculture showed elevated levels of both TN and TP, particularly in areas lacking BMPs.
While the PRW outlet (Charlotte Harbor) is primarily N-limited, 33% of the watershed source area exhibits P limitation.