Role of limnological processes in fate and transport of nitrogen and phosphorous loads delivered into Coeur d'Alene Lake and Lake Pend Oreille, Idaho, and Flathead Lake, Montana

Abstract

The fate and transport of nutrient loads, following their delivery into a lake, result from the integration of all hydrologic, physical, chemical, and biological processes that operate within that lake. If only empirical relations such as areal water load, hydraulic residence time, trap efficiency, and mean depth are used to estimate nutrient fate and transport without consideration of the other complex limnological processes involved, the magnitude of nutrient retention by the lake may be incorrectly predicted. The nutrient retentions empirically predicted for northern Idaho's Coeur d'Alene Lake and Lake Pend Oreille and northwestern Montana's Flathead Lake did not agree with the nutrient retentions measured for the three lakes on the basis of quantitative differences between input and output loads. The three lakes are within the 81,600-square-kilometer Northern Rockies Intermontane Basins study area, which was added in 1996 to the U.S. Geological Survey's National Water-Quality Assessment Program. The lakes were selected for evaluation of nutrient fate and transport because they are affected by nutrient enrichment, their input and output nutrient loads had been quantified, and their limnological characteristics had been extensively studied. The three lakes represent a broad range in physical limnological characteristics, which can be expected to influence the fate and transport of nutrients within the lakes. Lake volumes range from 2.8 (Coeur d'Alene Lake) to 53.9 (Lake Pend Oreille) cubic kilometers. Lake Pend Oreille is the deepest (357 meters), and Coeur d'Alene Lake is the shallowest (63.7 meters). Coeur d'Alene Lake has the shortest hydraulic residence time (lake volume divided by mean annual outflow volume), 0.50 year; the other two lakes have longer hydraulic residence times-2.2 years for Flathead Lake and 2.4 years for Lake Pend Oreille. The annual loads of nutrients delivered into the three lakes from drainage basin and atmospheric sources varied widely; total nitrogen loads ranged from 945,000 to 5,670,000 kilograms, and total phosphorus loads ranged from 43,600 to 408,000 kilograms. Lake Pend Oreille received and discharged the largest loads of both nutrients; Coeur d'Alene Lake received and discharged the smallest loads. Coeur d'Alene Lake and Lake Pend Oreille retained about 15 percent of the total nitrogen loads they received; Flathead Lake retained about one-third of the nitrogen load it received. The retention of total phosphorus was much different for Coeur d'Alene and Flathead Lakes; respectively, they retained about one-half and three-fourths of the phosphorus loads they received. Lake Pend Oreille retained less than about 17 percent of the total phosphorus load it received. If only morphometric values such as mean depth and maximum depth were considered, the lake with the largest values, Lake Pend Oreille, would be expected to retain the largest percentage of total nitrogen and phosphorus loads received. The unexpected small retention of both nutrients, particularly phosphorus, by Lake Pend Oreille indicated that limnological processes other than just physical sedimentation were affecting the fate and transport of nutrient loads delivered to that lake. Nutrient retention, or the lack thereof, was strongly related to circulation processes, in a spatial and temporal context, in the three lakes. The inflow plumes from their primary tributaries were routed primarily as overflow, especially during snowmelt runoff, when each lake received most of its annual loads of total nitrogen and phosphorus. The long, narrow shape of Coeur d'Alene Lake, along with its short hydraulic residence time, was not empirically predictive of nutrient retention. In contrast, the other two lakes were much deeper, had much larger and wider basins, and had hydraulic residence times longer than 2 years; all of which would predict substantial retention of nutrients.