pdf WQ13 Impact of Lake Huron level decrease on Les Cheneaux Island water quality variables from 2020 through 2022.

Authors: R.A. Smith

Author Affiliation: LCWC

Journal: LCWC Report; February 10, 2023



A study published in 2021 tracked the changes in nutrients, plankton and temperatures in Les Cheneaux Islands (LCI) waters during a seven-year period from 2013 through 2020 in which L Huron (LH) increased to a record high level and at a record rate (Ref. 1). The LH level has decreased during both 2021 and 2022 since the record high was documented in 2020 by the Army Corps of Engineers (USACE). This report quantifies changes in the same three variables during the period of decline that were monitored during the time of rapid rise.

Seasonal water temperatures within LCI channels increased from 2020 through 2022 by 1.5°-2.5° C (3.2°-5.3° F) during which time LH level declined by 66 cm (26 inches).   With lower depths in the LCI channels one could expect water column nutrient concentrations to increase. As a result of warmer water and increased nutrients one could also expect plankton densities to increase. This was not the case. Rather, plankton density decreased by about 20% from 2020; the year of record high LH level.

It is not obvious why the plankton density dropped during a time of seasonal water temperature and nutrient increases. Conversely, a plankton concentration increase could be expected under these circumstances. One possible explanation is that micronutrients provided in the LH water were reduced in concentration due to the lower LH level. Another possibility is that the three year observation is not sufficient to accurately track trends of the complex water chemistry dynamics between LH and the LCI channels.  Monitoring of these variables continues.