Plants convert nutrients, including nitrogen and phosphorous, into biomass. Nutrient uptake is just one factor that contributes to changes in nutrient levels throughout the year.
We have observed a strong seasonal cycle in nitrate concentrations in the Upper Gallatin River Watershed by monitoring nitrate quarterly. Nitrate is a form of the nutrient nitrogen that is readily available to plants and animals.
Nitrate levels are highest in the winter. During the cold, dark winter months, dormant algae and aquatic plants use less or minimal nitrate in stream water. In addition, low water levels further concentrate nitrate levels.
During high water in the spring, the concentration of nitrate decreases when snowmelt feeds rivers and streams. Nitrate levels continue to diminish during the summer growing season when algae use nutrients to grow. To account for algae consumption of nutrients, we collect both nutrient and algae biomass data during the summer. Nutrient levels begin to increase again in the fall as algae die and water levels drop.
The seasonal nitrate cycle is similar in the streams in the Big Sky area; however, the magnitude of the winter nitrate peak is consistently much higher in the West Fork than any other stream in the area due to human sources of nitrogen in the Big Sky area.