Authors: R.A. Smith
Author Affiliation: LCWC
Journal: LCWC Report; February 2021
Variation of Great Lakes water levels in the four-to-five foot range have been recorded since the mid-1800s. No lake level shifts of almost six feet over a short eight-year period have been recorded during that time. The Les Cheneaux Watershed Council (LCWC) took advantage of this unprecedented event to monitor plankton, phosphorus and temperature changes during the recent rapid Lake Huron (LH) rise. Each year, data and water specimens were collected monthly by LCWC from May through Sep and samples were subsequently analyzed at the Univ MI Biological Station, Douglas Lake. Water level increase within the Les Cheneaux Islands from an historic low in 2013 through the summer of 2020 to the historic high is shown in the figure below. LH increased almost six feet, water temperature dropped, total phosphorus concentrations decreased almost sixfold and plankton density decreased fivefold. Although annual variation occurred, trendlines for all three variables monitored had a downward slope during the study. Temperature decline attributed to the inflowing, cooler LH water appears to have decreased plankton metabolism and, therefore, overall plankton density. A dilution effect from the inflow of low-nutrient LH water would be expected to limit available phosphorus as a plankton energy source. However any phosphorus dilution was apparently offset by higher water levels suspending organics from the shorelines, which then became available as a plankton food source. Plankton growth deficits due to phosphorus availability were not obvious. These same variables will continue to be monitored as Lake Huron waters recede.