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Wetland Fact of the Month

Calcareous Fens (Southeastern) (OPp93c)

OPp93c communities are some of the rarest plant communities in Minnesota.  These systems are open peatlands hydrologically supported by calcium-rich groundwater delivered to the surface through artesian upwellings or at the base of steep slopes with permeable calcareous overburden wherin groundwater can emerge from the bedrock.  Southern calcareous fens are almost always found at the base of north-facing river terraces and slopes that correlate with shallow aquifers. Deep peat deposits form mounds or aprons near the seepage heads.  As groundwater emerges to the surface bicarbonates precipitate as a result of the increased temperature, resulting in harsh alkaline soils that support a very specific plant community adapted to the cold, mineral-rich environment.  The downslopes of peat mounds are often covered in tufa - a grayish white crust of precipitate that can be several inches thick.  Often these communities are small in size and exist as inclusions within a larger wetland complex.  One of the largest concentrations of these communities is found within the Lower Minnesota River Valley near the confluence with the Mississippi River.


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