Fen development along the southern shore of Lake Ontario

Abstract

Fen development along a drowned-river-mouth tributary to Braddock Bay, Lake Ontario was studied to address its formation. Nested piezometers were installed to assess groundwater contributions and obtain water chemistry samples. Soil and geology information came from existing sources. We converted paleo lake levels from published reports to IGLD1985 and calendar years BP for use in analyzing vegetation changes over time using a combination of peat-core plant macrofossils and modern surveys. Piezometer data showed upward discharge, water at 3-m depth had pH 6.9, specific conductivity of 508 µS/cm, and alkalinity 206 mg/L as CaCO3. Hydraulic head and mineralized water chemistry decreased at shallower depths. Vegetative development began 1790 cal yr BP with sedges and brown moss when land surface was 0.135 m above lake level. Lake levels increased, and by 1590 cal yr BP, water was 0.17 m deep and sedges were joined by shoreline emergent species. Water depth then increased to 0.525 m but began decreasing as lake levels fell. Peatland species appeared around 810 cal yr BP when water depth was reduced to 0.225 m. About 585 cal yr BP, additional peatland species appeared when land surface was 0.075 m above lake level. Sphagnum became prominent 80 cal yr BP (0.81 m above lake level), representing 67 % mean cover in modern vegetation. Isolation of the surface from calcareous groundwater resulted in transition from rich fen to poor fen. These wetlands are rare in the lower Great Lakes and deserve protection of their characteristic hydrology, water chemistry, and vegetation structure.