A synoptic assessment of water quality in two Great Lakes connecting channels.

We conducted a probabilistic water quality assessment of two Great Lakes connecting channels, the St. Marys River, and the Lake Huron-Lake Erie Corridor (HEC) in 2014-2015. We compared the condition of the channels to each other and to the up- and down-river Great Lakes with data from an assessment of the Great Lakes nearshore conducted in 2015. We assessed the condition of each channel as good, fair, or poor by applying the most protective water quality thresholds for the down-channel lake. Condition in the St. Marys River rated mostly fair for total phosphorus (TP) and mostly good for chlorophyll , and area-weighted mean concentrations were intermediate to nearshore Lake Superior and Lake Huron. A large proportion of the area of the St. Marys River was in poor condition for water clarity based on Secchi depth; while nearshore Lakes Superior and Huron were mostly in good condition for water clarity. Area-weighted mean concentrations of TP and chlorophyll in the HEC were more like nearshore Lake Huron than Lake Erie. For those indicators, most of the area of the HEC was rated good. The HEC appears more degraded when Lake Huron thresholds are applied rather than Lake Erie thresholds. Appropriate thresholds for the connecting channels should align with assessment objectives and be at least as protective as thresholds for the down-channel lake. Future iterations of this assessment will allow evaluation of water quality trends in the connecting channels.