While there is significant variability in water temperature, nutrient availability and plankton production in reservoirs, these bodies of water are nonetheless "hot spots" for plankton nitrogen fixation. These hot spots allow some algae to have significant blooms, which may contribute to taste and odor problems if the reservoir is a source for drinking water. Understanding what causes these hot spots could help municipalities better understand how to control algae outbreaks and potentially improve the water quality for their citizens.
However, exactly what physical factor causes these nitrogen fixation "hot spots" has not been well understood until now, with the completion of a nearly two-year study by Baylor University biologists.
Used by bacteria, algae and other organisms, nitrogen fixation is the natural process by which nitrogen in the atmosphere is converted into organic nitrogen. According to the scientific literature, the process is essential for life because nitrogen fixation is required to biosynthesize the basic building blocks of life.
The new Baylor study found that nitrogen fixation does not happen uniformly on the surface of lakes. In fact, the Baylor study found that three specific conditions need to be in place for nitrogen fixation to succeed:
Water temperature needs to be above 72 degrees
The ratio of nitrogen to phosphorus needs to be low
The amount of available nitrogen in the water needs to low
The results appeared in the journal Lake and Reservoir Management.
"This study helps us understand when and how nitrogen fixation happens and gives a complete data set that lets us evaluate all of these factors at the same time so we can understand their relative importance," said Dr. Robert Doyle, professor and chair of the department of biology at Baylor, who also is the director of Baylor's Center for Reservoir and Aquatic Systems Research. "This study shows that for a hot spot of nitrogen fixation to happen, these three environmental conditions need to be in place."
The Baylor study looked at five different spots in a Texas reservoir. The researchers then tested the water temperature, pH level, nutrient content and dissolved oxygen content and compared them to nitrogen fixation rates. The results show that nitrogen fixation rates were highest during the months of June through October when the water temperature was above 72 degrees. Likewise, nitrogen fixation rates were low at the deeper test-spots around the lake which had less light.
The results also show that the amount of nitrogen in the water needs to be low and the ratio to nitrogen to phosphorus needs to be below 15 to 1. The amount of available nitrogen in the atmosphere also needs to be low, relative to other nutrients in the water.