Mid-Holocene hemlock decline and diatom communities in van Nostrand Lake, Ontario, Canada

Terrestrial ecosystem disturbances inferred from the fossil hemlock pollen decline (ca. 4,800 BP) and recovery (ca. 3,800 BP) affected van Nostrand Lake, including the diatom communities. Ecological models suggest the lake responded by eutrophying, reflecting higher nutrient influx resulting from increased erosion. A decline in lake productivity followed as the forest vegetation recovered and erosion slowed. Lastly, as the forest switched from early and middle successional species to mature species, lake productivity increased as erosion increased nutrient inflow, especially phosphorus. However, this eutrophication response was delayed or buffered, perhaps due to wetland development surrounding the lake. The lake never fully returned to its initial, pre-hemlock decline state, but oscillated between more eutrophic and more mesotrophic or oligotrophic stages, possibly reflecting other disturbances in the catchment and climatic change.