Inferred history of a boreal pond from sediment and vegetation characteristics

Abstract

Shallow ponds associated with peatlands in boreal Alberta contain large quantities of stored carbon in their peaty sediments, and exhibit spatial changes in response to dry or wet conditions. Peatland ponds around Utikuma Lake experienced a partial to nearly complete drawdown in 2002, in response to drought. A fourfold investigation was begun to assess the natural drought and flood cycles, assess the current C stocks along the shore, determine what species were colonizing the newly exposed shorelines, and to place the magnitude of the current drought into historical perspective. Ten vegetation communities were found to have colonized the exposed sediments. Paleostratigraphy shows that the surface communities differ markedly from paleocommunities. Stratigraphy of the wetland sediments demonstrates that only two changes occurred, one a successional switch from a sedge-dominated marsh to a forested bog, the other a much more recent disturbance that caused water levels near the pond to rise. No evidence of constantly changing paleocommunities was found, suggesting that previous droughts have not left a visible paleorecord. The more recent disturbance has likely redistributed peat into the pond basins and subsequently it has broken down to detritial peat, altering its physical structure, and perhaps its rate of decomposition. Continued exposure of the peat is likely to enhance decomposition. In this study both flooding and drought may have impacted the wetland in ways likely to result in higher rates of decomposition and enhanced CO2 emissions under global warming. Key words: Peat stratigraphy, Holocene climate, thermokarst melting, detrital peat, carbon stocks