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 sys tem in northern Wisconsin shows that system dynamics are strongly infl uenced by seasonal transient effects as well as transience over the 10 year period of record. These data were collected for the Long Term Ec ological Research (LTER) program at the North Temperate Lakes site in northern Wisconsin, USA. The record included a period of relatively hi gh 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 side s of the lake during 1981-1985 when regional water levels were high an d during 1986-1988 when groundwater levels were high relative to the d eclining lake level. Mounds did not form after 1988. These observation s point to the importance of a long-term record in assessing the signi ficance of short-term effects. Short-term transience affects the groun dwater component of the lake budget because the mounds induce groundwa ter to flow toward the lake; when the mounds are not present, water fl ows away from the lake. Shifts in the groundwater regime will affect t he 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), ar e influenced more by changes in precipitation inputs than groundwater inputs.