OXYGEN DEPLETION IN LAKE ST. CROIX: MODERN AND PALEOLIMNOLOGICAL INSIGHTS

B. Moraska Lafrancois, National Park Service
L. Triplett, University of Minnesota
J. Sieracki, National Park Service
B. Karns, National Park Service
D. Francis, University of Massachusetts-Amherst
C. Stewart, University of Massachusetts-Amherst

Adequate concentrations of dissolved oxygen are required for the growth and survival of aquatic organisms. Rivers are generally well-oxygenated due to thorough mixing and high rates of oxygen uptake from the atmosphere. Lakes, on the other hand, may exhibit a range of dissolved oxygen concentrations depending on depth and season (Wetzel 2001). During the summer months many temperate lakes become thermally stratified, and the water column becomes separated into a warmer, well-mixed epilimnion in which algal production and contact with the atmosphere ensure adequate dissolved oxygen concentrations, and a cooler hypolimnion in which decomposition of organic materials can lead to oxygen depletion. Eutrophic, highly productive lakes are most prone to hypolimnetic oxygen depletion. In these lakes high algal productivity in the epilimnion feeds rapid decomposition and respiration in the hypolimnion, leading to prolonged episodes of anoxia.

Lake St. Croix is a natural impoundment of the St. Croix River, spanning the lowermost 23 miles (37 km) of the River to its confluence with the Mississippi. Within this reach of the St. Croix, the river valley widens, the channel deepens, current velocity slows, and suspended materials begin to settle out. These characteristics imply that lake-like processes, such as high phytoplankton production, thermal stratification, and oxygen loss may occur in Lake St. Croix, and previous paleolimnological work has demonstrated a shift toward more eutrophic conditions in Lake St. Croix since the mid-1900s. Diatom production and phosphorus loads have increased and diatom species composition has shifted toward nutrient-loving planktonic taxa (Triplett et al. 2003).

Here we explore whether or not these changes in nutrient loads and algal production may induce oxygen loss in riverine Lake St. Croix, using monitoring and paleolimnological studies. In August 2005 and 2006, dissolved oxygen and temperature profiles were measured at seven to nine sites in Lake St. Croix. Thermal stratification was noted on both dates, at depths of 11-13 m in August 2005 and depths of 8-10 m in August of 2006. In both years, dissolved oxygen profiles followed thermal profiles in a clinograde pattern typical of eutrophic lakes. In general, dissolved oxygen concentrations of <3 mg/L prevailed below a depth of 12 m in August 2005 and below a depth of 8 m in August 2006, suggesting that nontrivial portions of Lake St. Croix were unfit for aquatic life on the measurement dates. Monthly monitoring throughout the 2007 season will determine how prolonged such hypoxia may be in Lake St. Croix. Additionally, a paleolimnological study, initiated in summer 2006, is using fossil chironomids to investigate if hypoxic conditions have occurred historically in Lake St. Croix or if they have been induced by more recent eutrophication.

References

Triplett, L., M. Edlund, and D. Engstrom. 2003. A whole-basin reconstruction of sediment and phosphorus loading to Lake St. Croix. Final Project Report to the Metropolitan Council Environmental Services, St. Croix Watershed Research Station, Marine on St. Croix, MN.

Wetzel, R., editor. 2001. Limnology: Lake and River Ecosystems, Third Edition. Academic Press, San Diego.