This open peatland community occurs along the shorelines of the Great Lakes, near the mouths of estuarine streams, and in association with sandspit landforms. Great Lakes shore fen is locally common along the southwestern shore of Lake Superior because the basin is slowly subsiding due to differential isostatic rebound from the last episode of Pleistocene glaciation. This has created conditions along the Wisconsin shore that favor the development of drowned river mouths, sandspits, and extensive peatland complexes. Shore fens are generally in direct contact with clear, cold, circumneutral (pH ~7) waters of low nutrient status.
A characteristic floating sedge mat is dominated by wire-leaved graminoid plants, including woolly sedge (Carex lasiocarpa), twig-rush (Cladium mariscoides), sweet gale (Myrica gale), and buckbean (Menyanthes trifoliata). Other common herbs in the floristically diverse coastal fens of the Lake Superior region include marsh horsetail (Equisetum palustre), marsh bellflower (Campanula aparinoides), intermediate bladderwort (Utricularia intermedia), lesser bladderwort (Utricularia minor), water bulrush (Schoenoplectus subterminalis), elliptic spike-rush (Eleocharis elliptica), narrow-leaved willow-herb (Epilobium leptophyllum), water-parsnip (Sium suave), and bog willow (Salix pedicellaris). Sooty beak-rush (Rhynchospora fusca) and the rare coast sedge (Carex exilis) are locally common in some coastal fens on the Apostle Islands. The floating sedge mat is often bordered on the downslope side by a lagoon that supports marsh vegetation composed of varying mixtures of submergent, floating-leaved, and emergent species. Toward higher ground and in the shallower portions of the peatland basins, the mat is grounded. Sphagnum mosses become increasingly important and accumulate as peat, and there are significant changes in fen composition. These sphagnum-based, herbaceous peatland communities are classified as poor fens.
