Relationships of Aquatic Macrophytes to Sediment and Groundwater Characteristics in McDill Pond -- Implications for Lake Management

Abstract

McDill Pond is a 261 acre impoundment located in Stevens Point, Wisconsin. Since the late 1960's, abundant macrophyte growth has prompted the use of control methods. Despite control methods, surveys have shown substantial increases in aquatic plant growth over time. This study was conducted in 1996 and 1997 as part of a DNR lake planning grant to investigate the relationships of aquatic plants to sediment and groundwater characteristics. Improved understanding of the influence of environmental factors on aquatic plant growth and distribution can provide for more effective aquatic plant management. Study results will be incorporated into a comprehensive lake management plan for McDill Pond. Thirty sampling sites were established on McDill Pond. Aquatic plant and sediment data were collected in June, July, and August of 1996. Seepage meter and mini-piezometers were used to collect pore water chemistry. Interstitial water chemistry was collected at all sites in 1997 in an attempt to characterize the pore water in the aquatic plants' rooting zones. Aquatic plant samples were collected and analyzed for biomass, species distribution, and plant nutrition, and sediment was collected and analyzed for chemistry, organic matter, and texture. Seepage meter, mini-piezometer, and interstitial samples were analyzed for chemistry. Elodea canadensis (Elodea) was significantly positively correlated with sediment potassium levels, and increasing sediment ammonia levels occurred with increasing plant biomass for all four dominant species in the lake. In terms of sediment texture, Ceratophyllum demersum (Coontail) was significantly positively correlated with finer grained sediments, and negatively correlated with coarser grained sediments. Significant physical and chemical differences were noted between groundwater inflow and non-groundwater inflow sites. Groundwater inflow sites showed oxidized conditions and lower nutrient levels, suggesting that redox conditions control nutrient availability. Interstitial nutrient results also suggest the release of phosphorus and nitrogen under reduced conditions and the unavailability of those same nutrients under oxidized conditions.