LONG-TERM AND SHORT-TERM TRANSIENCE IN A GROUNDWATER LAKE SYSTEM IN WISCONSIN, USA

A 10 year record of water level fluctuations in a groundwater/lake system in northern Wisconsin shows that system dynamics are strongly influenced by seasonal transient effects as well as transience over the 10 year period of record. These data were collected for the Long Term Ecological Research (LTER) program at the North Temperate Lakes site in northern Wisconsin, USA. The record included a period of relatively high lake levels from 1981 to 1985, followed by declining levels in 1986-1987 and low levels in 1988-1990. Short-term transient effects in the form of seasonal groundwater mounds consistently occurred on all sides of the lake during 1981-1985 when regional water levels were high and during 1986-1988 when groundwater levels were high relative to the declining lake level. Mounds did not form after 1988. These observations point to the importance of a long-term record in assessing the significance of short-term effects. Short-term transience affects the groundwater component of the lake budget because the mounds induce groundwater to flow toward the lake; when the mounds are not present, water flows away from the lake. Shifts in the groundwater regime will affect the lake's chemical budget in the long term. However, the trends in the chemical budget that occurred within our 10 year record, an inter-val shorter than the hydraulic residence time of the lake (12.7 year), are influenced more by changes in precipitation inputs than groundwater inputs.