Physical and plant community changes at a Lake Michigan coastal marsh related to a two-meter increase in lake level

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College of Forest Resources and Environmental Science


Located in Oconto County, Wisconsin, Oconto Marsh #2 is a Great Lakes Coastal Wetland Monitoring Program study site that borders the shoreline of Lake Michigan. Plant communities were characterized at Oconto Marsh #2 along three transects in 2011, 2016, 2017 and 2021, a period when Lake Michigan water levels increased by two meters. Transects were placed to intersect with three vegetation zones: submergent, emergent, and wet meadow. Here, we report on physical landscape changes and the vegetation composition changes that occurred from 2011 to 2021. From satellite imagery interpretation, we show approximately 61,000 m2 of what was emergent and wet meadow vegetation in 2011, transitioned into a submerged aquatic community in 2021. High energy wave action penetrating farther landward, a consequence of higher water levels, is likely most responsible for causing these changes. Plant species richness was lowest in 2011 (32 species) and ranged from 52 to 56 taxa in later years. Using multivariate ordination and PERMANOVA, we show plant composition in 2011 was different from 2016, 2017, and 2021. While invasive Phragmites australis was treated with herbicide in 2014, disturbance from progressively increasing water levels has facilitated considerable changes in plant composition and wetland zone extents since monitoring began. Despite successful treatment of P. australis, encounters with more non-native species while sampling farther landward in later years has caused site-wide declines in multiple metrics of floristic quality. Of critical importance, in 2021, we discovered invasive Hydrocharis morus-ranae at the site, the first documentation in the state of Wisconsin.

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Wetlands Ecology and Management