Field measurements and lab analysis
Conductivity
Two in-situ HOBO conductivity loggers were deployed in each lake on under-ice buoys. Deployment of the buoys occurred prior to ice-on during the fall mixing event and retrieval of the loggers occurred soon after ice-off during the spring mixing event. In each lake, the buoy string held a logger at approximately 1-2m below the surface to capture epilimnetic changes and a logger at 1m from the lake bottom to capture hypolimnetic changes.
A handheld Thermo Scientific conductivity meter was used to measure absolute conductivity in µS cm-1 and temperature in ℃ at every sampling event. These measurements were used to validate the in-situ sensor measurements and mitigate any sensor drift. The handheld meter was calibrated prior to every use with KCl standards of 100 μS cm-1 at 25℃ and 500 μS cm-1 at 25℃.
Chloride
Depth discrete water samples from both lakes were collected as frequently as possible, quarterly at a minimum, to ensure seasonal variation in chloride concentrations were captured. Profile sampling in Mendota, which is approximately 25 m deep, occurred every 5m from 0-20m and at 23.5m. Profile sampling in Monona, which is approximately 21m deep, occurred every 4m from 0-20m.
Samples were analyzed for chloride concentration (mg L-1 ) via ion chromatography.
At each sampling event, 30-60 mL of water was collected, filtered through a 25 mm, 0.45μm pore size filter, and refrigerated for storage until laboratory analysis could occur. Ion chromatography was used to analyze each water sample for chloride (mg L-1). A Dionex 2100 ion chromatograph equipped with IonPac AS11 analytical column was calibrated with six NaCl solutions ranging from 1 mg L-1 to 500 mg L-1 and run with a 30 mM NaOH eluent solution.