Analysis of Freshwater Inputs from Lake vs. Basin Drainage Canals Into a Subtropical Estuary Using a 20-year Continuous Dataset

Abstract:

Anthropogenic activities have modified the quality and quantity of freshwater reaching the St. Lucie Estuary, Florida. Highly managed and nutrient-rich inflows from the estuary’s basins and Lake Okeechobee have distressed the coastal ecosystem. The artificial connection with the lake brings in freshwater with different physicochemical composition and flow regimes than that from the watershed drainage canals. These differences in water composition and flows were analyzed using a statistical approach from the monthly collections of water quality data from monitoring stations at each of the canals and stations in the estuary. The water parameters analyzed include Ammonia (NH₃), color, dissolved oxygen (DO), nitrite + nitrate (N+N), pH, orthophosphate (OP), total phosphorus (TP), specific conductivity, total nitrogen (TN), total suspended solids (TSS), turbidity, water temperature, and water flow. Principal component analysis (PCA) and time series analyses were applied to analyze the seasonality (wet season and dry season months) using the twenty-year data set (1999 – 2019). Lake Okeechobee’s inputs are characterized by high turbidity, TN and N+N values in both wet and dry season. The watershed tributaries are characterized by high Color (PCU) and phosphorus values in both seasons. The Seasonal Mann Kendall Trend Test was applied to all parameters to assess upward or downward monotonic trends. Total phosphorus showed a weak decreasing trend (p< 0.001) and pH weak increasing trend (p< 0.001) in the north fork of the estuary. No significant trends found for any parameter in the south fork where lake water is released.