Analysis of sediment samples from the sediment-water interface of Point River Bay, northern Lake Winnipegosis, a very large lake in central Manitoba, indicates that Cribroelphidium gunteri, a coastal marine foraminifer, is living and has adapted to this northern lake environment in salinities as low as 1–2‰. In Point River Bay, summer water temperatures reach 15.6°C, marginally above the minimum 14.5°C required for reproduction by C. gunteri. This benthic foraminifer colonized saline parts of the lake during the warm Holocene Hypsithermal (6000–3500 years BP), probably by avian transport. Previous analysis of stratigraphic data suggested that C. gunteri had died out in this area as conditions became cooler. This hypothesis had been corroborated by the post-Hypsithermal retreat of the marine range of C. gunteri from the Maritimes of Atlantic Canada to the south of Cape Cod, MA. Although recent colonization of the lake cannot be precluded, marine source populations of C. gunteri are now quite distant, making the only viable colonization mode, avian transport, very difficult. The adaptation of the mid-Holocene populations of this species to the progressively colder post Hypsithermal climate and often to extremely low salinity values in Lake Winnipegosis is remarkable. The great abundance of C. gunteri in sediments of Lake Winnipegosis, in some areas making up most of the sediment, also raises potential concerns about the interpretation of supposed marine sections based exclusively on the presence of foraminifera.

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