Mainly due to road de-icing salts, Lake Michigan is slowly getting saltier. It's not a problem yet, but it could become one.
Tributary chloride loading into Lake Michigan
"Our findings highlight the complexity in managing the Lake Michigan basin for chloride pollution. While large watersheds are the largest contributor of chloride to the lake, their concentrations are far below EPA toxicity thresholds (230 mg L−1) and their riverine habitat is more protected than smaller urban watersheds where winter concentrations routinely surpass 230 mg L−1. This creates a conflict between management priorities of protecting lake vs. tributary habitat. In addition, success of management actions in watersheds with high contribution of chloride-laden groundwater may not immediately be apparent due to decadal time lags between groundwater fluxes responding to surface changes (Green et al. 2021). Our chloride inventory of Lake Michigan tributaries suggests that the chloride concentration in the lake is in disequilibrium with chloride loading from its basin, and will continue to slowly rise in the coming decades. The rise from ~ 15 to ~ 24 mg L−1 may be ecologically significant, as research has shown negative impacts to biotic communities well below water quality guidelines (Arnott et al. 2020). Lake Ontario currently has a chloride concentration ~ 23–24 mg L−1, and could be used as a reference for future conditions in Lake Michigan."
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