Impact of agricultural land use on nitrate levels in Lake Manatee, Florida

ABSTRACT:

Algal blooms in the Lake Manatee reservoir necessitate treatment of the drinking water for taste degradation, creating an economic burden. This study was conducted to assess the extent to which agricultural activities in the Lake Manatee watershed may contribute to nutrient loading of the reservoir. Water quality data for Lake Manatee were collected from eight strategically selected sites within the lake from 1983 to 1993, and were correlated with historical agricultural activity (vegetable and citrus production) in the watershed to determine if evidence for agricultural NO₃-N loading was apparent. The watershed was divided into sub basins related to sampling locations in an attempt to evaluate the effects of specific agricultural activities on NO₃-N levels in the lake. This allowed the separation of sampling points representing sub basin watershed areas where I) no agricultural activities occurred, 2) exclusively vegetable or citrus production occurred, or 3) mixed production occurred. Data were analyzed using trend analysis along with a technique to deseasonalize the data for more valid overall interpretation. Results showed that the average lake NO₃-N concentration during the period was only 0.18 mg L' (0.18 ppm) and that seven of the eight sampling sites showed average annual NO₃-N increases of 5-10% throughout the study period, depending on sampling location (p < 0.10). However, no strong correlation with agricultural activity in the watershed was demonstrated. This conclusion was supported by data collected in a watershed sub basin with no agricultural activity, which showed a comparable NO₃-N concentration increase during the study period.