Loading of phosphorus and nitrogen to Lake Waco, Texas

Abstract

Cyanobacterial blooms in Lake Waco are linked to nutrient loading from the watershed. Direct measurement of nutrient levels over a period of 8 years facilitates calculation of loading of water, phosphorus (P) and nitrogen (N) to Lake Waco. The North Bosque River (NBR) provides 61% of the flow to Lake Waco, while the Middle Bosque River (MBR) contributes 15.5%. The South Bosque River (SBR) and Hog Creek (HC) each contribute about 7.5% of the total water load to the lake, while other sources contribute less. The lake receives average annual total phosphorus (TP) loading of approximately 147,000 kg and an annual total nitrogen (TN) load of about 1,450,000 kg, with considerable variability expected among years. The NBR contributes 67% of the TP and 37% of the TN. The MBR provides slightly more than 17% of the TP and 41% of the TN. Inputs from SBR are estimated at about 8% of the TP and 10% of the TN. All combined, HC, direct drainage to the lake, atmospheric deposition, ground water inseepage, human inputs associated with recreation, waterfowl inputs and internal loading contribute <8% of the TP and about 12% of the TN. The NBR is clearly the dominant source of TP to the lake, while MBR and NBR contribute roughly equal amounts of TN and much more than any other source. Inputs from wastewater treatment facilities (WWTF) represent about 4% of the TP input and <2% of the TN load to Lake Waco. Despite greater availability of the forms of P in WWTF inputs, this is a minor source for the lake. Inputs from dairy operations, including areas of active animal use and waste application fields, are estimated to contribute 34–42% of the NBR TP load and 23–28% of the overall TP load to Lake Waco. Dairy operations are estimated to contribute about half of the biologically available P load to Lake Waco and generate low N:P ratios in the lake. Despite high loading and resultant blooms, Lake Waco exhibits lower concentrations of P than would be predicted by multiple models, and the actual concentrations stimulate less algal growth than would be expected. In the absence of major inputs related to human activities, the predicted P load to Lake Waco would be less than half the current load, and very little of it would be biologically available. The initial regulatory target of a 50% reduction in P loading to NBR set by the Texas Commission on Environmental Quality is appropriate. If dairy inputs are successfully controlled, measurable improvement can be expected. Supplemental materials are available for this article. Go to the publisher's online edition of Lake and Reservoir Management to view the free supplemental file.