Abstract
Eutrophication is a widespread problem plaguing water bodies throughout the world. The need to control nutrient loading into lakes has become paramount in lake management. Failing to do so can lead to numerous undesired effects, including algae blooms, fish kills, and even complete ecological regime shifts. Water and nutrient budgets are useful for quantifying nutrient loading into a hydrosystem while also revealing problematic areas or times of excessive nutrient deliveries. Lake Trafford is a shallow 600 ha natural hydrosystem of circular shape with ambiguous boundaries, surrounded mostly by wetlands (e.g. Corkscrew swamp) and located near the city of Immokalee, Collier County, FL, USA. The lake has been subjected to decades of cultural eutrophication and still exhibits the turbid water state of eutrophic shallow lakes dominated by phytoplankton despite the completion of sediment dredging in 2011. To determine the current sources of nutrient loading and to calculate the total maximum daily loads (TMDL) for nitrogen and phosphorus, a water and nutrient budget was developed. Unlike most budgets which rely on modeling and numerous assumptions or estimates (e.g. the Florida Department of Environmental Protection established TMDL for L. Trafford), direct measurements of atmospheric (dry and wet), surface water (canals), and groundwater nutrient loading were made at high spatio-temporal definition. Using the measured components of the water and nutrient budget, estimates of diffuse runoffs from the surrounding watershed and out to the adjacent wetlands were made, along with estimates of internal loading of nutrients and biological uptake. Data was collected in biweekly sampling events between October 2015 and May of 2016. Groundwater seepage meters were used to asses groundwater discharge and recharge paired with pore water wells for groundwater sampling. Surface water nutrient loading from five drainage canals was monitored with Sontek IQ units and ISCO 3700 automatic water samplers. Meteorological data were collected at a weather station built on the lake, and were used to determine direct precipitation and evaporation rates. A homemade wet/dry deposition sampler was also employed to collect composite samples of atmospheric deposition on the lake. Results showed that the vast majority of water entering Lake Trafford was through the five drainage canals which delivered 34% (35066 m3 d-1) of the daily water influx, while direct precipitation (24%; 13324 m3 d-1) and groundwater discharge (14%; 7736 m3 d-1) contributed less water overall. Outflow was dominated by sheet flow moving out to Corkscrew Swamp which accounted for 65% (-44696 m3 d-1) of all effluxes, while evaporation (34%; -23544 m3 d-1) and groundwater recharge (1%; -371.6 m3 d-1) accounted for less. Sheet flow was calculated as a net value and was negative on average (-44696 m3 d-1) indicating that more sheet flow was leaving Lake Trafford rather than entering. Nitrogen loading into Lake Trafford was highest from groundwater discharge (48%; 109.5 kg TN d-1), while loading of phosphorus was dominated by surface water discharge from the five drainage canals (51%; 8.66 kg TP d-1). The residual component of the budget was assumed to be controlled by processes within in the lake (e.g. internal loading, sedimentation, biological uptake), and was found to be a negative value on average (-87.8 kg TN d-1; -12.5 kg TP d-1) indicating that in-lake nutrient dynamics were dominated by sequestration processes, such as biological uptake. The total daily load was found to be 227 kg d- 1 and 20.9 kg d-1 for TN and TP, respectively. These values are higher than those found in the lake’s adopted TMDL report of 190.5 kg d-1 for TN and 18.6 kg d-1 for TP, which was estimated during a seven year modeling period using land-use based runoff modeling. Dry season rainfall was higher than average (525 mm October 2015 – May 2016; 330 mm historic average October –May) and influenced the results. The final water and nutrient budgets provided insight into the hydrological dynamics of Lake Trafford, although future work should be conducted to better understand its hydrogeology as well as the nutrient loading into its drainage canals; particularly the eastern most canal which is an ideal area for remediation.