BOREAL DIATOM PONDS - A RARE WETLAND ASSOCIATED WITH NESTING WHOOPING-CRANES

This paper documents and characterizes a rare form of boreal wetland a ssociated with the nests of whooping cranes, an endangered species. Di atom ponds are found in wetlands in association with bulrush marshes. They are shallow (<50 cm deep) and vary in size from 10 to >1000 m in diameter, may evaporate down to diatom muck by late summer, are strong ly influenced by dissolution of gypsum, and are circumneutral to alkal ine and high in sulphates. Aquatic macrophytes are few. Pond waters ar e clear, and the predominant primary producers are a benthic diatom co mmunity that gives the ponds a characteristic yellow color (viewed fro m the air). As the diatom ponds dry, they change color from yellow to pink (when water table is at the surface) to cream (due to a dried dia tom and sulphate crust). Diatomaceous earth or sedimentary peat underl ies the ponds, which exist in a dynamic relationship with bulrush mars hes, wet meadows, fens, and bogs. In the U.S. wetland classification s ystem, diatom ponds fit most nearly within the palustrine, unconsolida ted bottom, aquatic bed type. In the Canadian wetland classification s ystem, the diatom ponds might fit in the marsh/shallow open water comp lex, with a new distinction at the type level. The association between nesting cranes and diatom ponds may be due to a combination of factor s such as long sight lines for detection of predators, the proximity o f bulrush (their favored nesting material), and use of the ponds for f eeding.