A PHOSPHORUS BUDGET FOR LAKE AUBURN, MAINE

Lake Auburn, located in Auburn, Maine, has been the primary source of drinking water for both Lewiston and Auburn since 1875. The lake has a mean surface area of 9.14 square kilometers, mean depth of 12.2 meters, maximum depth of 36.6 meters, and an approximate volume of 111,000 meters cubed (Auburn Water District and Lewiston Water Division, 2002). Recent declines in water quality have resulted in algal blooms, low dissolved oxygen, and a fish ill in autumn 2012. Moderately high concentrations of total phosphorus (TP) and chlorophyll a, high turbidity, and low Secchi depths have been measured in the lake by the Auburn Water District and Lewiston Water Division (AWD/LWD) during the months of July-September over the last four years. Variability in phosphorus concentrations has increased over this time with high values occurring in the fall in association with bottom-water anoxia but there may also be evidence of increased loading from the watershed in storm events. The purpose of this project is to better constrain what is known about the cycling of phosphorus in Lake Auburn.

From June to November of 2013, stream discharge and total phosphorus concentrations were determined weekly for 8 tributaries. Sediment traps were set in the deepest part of the lake in early October and recovered in November. In addition to measures of temperature, dissolved oxygen, turbidity, and Secchi depth the AWD/LWD collected lake and stream water for analyses of chlorophyll a, bacteria, volatile organic chemicals, metals, organic carbon, and nutrients (including total phosphorus, orthophosphate, nitrate, and ammonium).

Limited transparency high turbidity, and low dissolved oxygen indicated poor water quality during the month of August, 2013. From June to November, chlorophyll a at the surface of the lake steadily increased compared to total phosphorus values, which tended to fluctuate based on sample location around the lake. Four of the tributaries measured contributed high amounts of phosphorus compared to the other five tributaries that contributed minimal amounts. Sediment trap samples will be analyzed and used to better constrain what is known about phosphorus cycling in the water column. The phosphorus fluxes determined for the tributaries and sediment traps will be used to create a box model of the phosphorus budget for Lake Auburn.