Pyrite and vivianite intervals in the bottom sediments of eutrophic Baptiste Lake, Alberta, Canada

The highly eutrophic status of Baptiste Lake, Alberta. may be related to al tered groundwater flow resulting from land-clearing activities since coloni al settlement approximately 80 yr B.P. Sections from the top 42 cm of sedim ent cores from Baptiste Lake. which represent 150 yr B.P. to the present, w ere analyzed for forms and concentrations of iron and phosphorus. The 30-42 cm interval (90-150 yr B.P.) is marked by intensive formation of vivianite ; concentrations of potentially bioavailable orthophosphate ion in some sec tions exceed 1 wt%. This probably reflects a period when inflowing groundwa ter contained high concentrations of iron and phosphorus. which precipitate d at the sediment surface under oxic conditions that likely prevailed at th e time. In contrast, the top 25 cm (80 yr B.P. to the present) show. sedime ntary formation of pyrite consistent with anoxic conditions at the sediment -water interface, the deposition of organically enriched sediments, and sev ere eutrophication. The waters of Baptiste Lake have become more eutrophic since colonial settlement.