Limnological Attributes of the Oak Lake Basin: A Summary of Monitoring Data Over the Past 14 years

Troelstrup, N.H. Jr.

Oak Lake is an intermittently exposed wetland located in the northeastern corner of Brookings County, SD. This lake has been the subject of long-term study since 1971 and is arguably one of the most studied lakes in South Dakota. Oak Lake has been designated the beneficial uses of warm water marginal fish life propagation, immersion and limited contact recreation and fish and wildlife propagation, recreation and stock watering. Current monitoring efforts by Oak Lake Field Station staff are directed to evaluate support of these beneficial uses, evaluate long-term changes in lake condition and provide baseline data to support lake research. Surface water enters the lake through three primary stream inlets and ten major spring inlets. These inlet sources account for approximately 40% of average outlet discharge. Thus, significant groundwater inputs must also enter the lake. On the average, lake elevation drops 0.6m during the growing season and a long-term decline in lake elevation has occurred over the period 1982 to 2007. The growing season euphotic zone is approximately 75% of maximum depth and the basin only weakly and temporarily stratifies on the calmest summer days. A long fetch, shallow basin and moderate/frequent winds mix the water column to 50-75% of mean depth throughout the ice-free season. The lake is enriched with high concentrations of total nitrogen and phosphorus, but primary production is limited by low light due to wind-induced entrainment of bottom sediments and light limitation. Productivity-based impairments are the only water quality standards exceeded with any significant frequency. Future field station research will quantify the lake water budget and generate estimates of lake primary production. (Presentation pdf)