A recent drop in outflow and surface temperatures changed the water quality profile of Island Park reservoir. It is likely that these changes have reduced the cyanobacteria bloom we experienced earlier this month, which peaked around July 10. This is partially written as a response to Mike Lawson’s fishing report, where he mentioned he wasn’t sure why the river conditions had suddenly improved.
The brief version: Reservoir outflow decreased by approximately 150 cfs (cubic feet per sec) between July 12 and 14, from this year’s high of 1,600 cfs to 1,450 cfs, and this slowed the movement of water through the reservoir just enough to allow thermal stratification to set back up. The return of stratification, combined with shorter day lengths, prompted a relatively sudden decrease in water temperature by 1.5 ˚C (approximately 2.7 ˚F) at the depth of the cyanobacteria bloom. We saw evidence of the temperature decrease by comparing our reservoir profile data taken on July 1 and July 8 (before the bloom) to July 15 (after the bloom). The temperature of the water surrounding the bloom dropped below the range that promotes rapid growth, resulting in a die off. It is also likely that the bloom exhausted its own food supply, but we won’t have data to confirm this for a couple more weeks. Regardless of whether it was a temperature or a nutrient supply issue, or both, cyanobacteria have such a short lifespan (usually around 24 hours) that a bloom will quickly diminish in size once conditions become unfavorable.
The full story: A recent drop in outflow and surface temperatures changed the water quality profile of Island Park reservoir. Island Park reservoir usually functions very much like a lake in the middle of a river; quickly moving river water hits the storage reservoir and slows down, allowing cooler water to sink to the bottom while the warmer upper layers continue to retain heat from the sun. This is known as thermal stratification. In Island Park reservoir, periods of very high outflow to fulfill irrigation demand increases the ratio of outflow to inflow, which speeds up the movement of water through the reservoir. This allows more mixing of the layers of water through the column and disrupts stratification. This disruption of stratification was observed from late June through July 8 while outflow was at its maximum of 1,600 cfs. Then outflow decreased by approximately 150 cfs between July 12 and July 14 and this slowed the movement of water through the reservoir just enough to allow thermal stratification to start to reestablish itself. This resulted in a 1.5 ˚C (approximately 2.7 ˚F) decrease in water temperature in the top 20 feet of water, which is where we observed the cyanobacteria bloom (see Figures 1 and 2 below).
It is likely that decreased water temperature has reduced the cyanobacteria bloom we experienced earlier this month. There are numerous species of cyanobacteria, a few are found in Island Park reservoir (and the Henry’s Fork) at low “background” levels, and each species has its own specific habitat requirements that allow it to thrive. A few examples of important habitat requirements for cyanobacteria are respective ranges of sunlight intensity, nutrients, and temperature. Water quality parameters, such as temperature or nutrient concentration, can change dramatically with depth in lakes and reservoirs. For instance, the top 10 feet of water may be conducive to growth of a specific species but the lower 5 feet are unfavorable. This is why we saw that the cyanobacteria bloom was thriving only at a certain range of depth in the water column. Once environmental factors align to create the perfect niche for rapid cyanobacterial growth, a typically low population of cyanobacteria will explode and produce what is known as a ‘bloom’ in the reservoir. Individually these organisms live for a short period, often around 24 hours, so their populations respond very quickly once their environment becomes unfavorable. Therefore, once thermal stratification set back up inside Island Park reservoir, the cyanobacteria lost their favorable conditions and quickly died out. The downstream effect of this rapid die off inside the reservoir was that the observed discoloration in the river downstream of Island Park dam disappeared nearly as quickly as it appeared.
It is also likely that the bloom exhausted its own food supply. It is possible that the production of nutrients in the reservoir—from decomposition of deposited material at the bottom of the reservoir—is a much slower process than the rate at which this modestly-sized cyanobacteria bloom was consuming them. Along with the reservoir profiles we capture each week, we also take nutrient samples from the top and bottom layers of the reservoir every other week. We have to outsource the nutrient analysis to a lab and it takes approximately 3 weeks to get results back. Hence, in a couple more weeks we will be able to compare the nutrient concentration from Friday July 8 (the Friday before the bloom appeared) to nutrient concentrations on Friday July 22, after the bloom had dwindled. If the cyanobacteria bloom exhausted its food supply, we would expect to see a relative decrease in nutrient concentrations from the week before to the week following the bloom. I'll report on the results in a forthcoming blog. Stay tuned…
Figures below are referenced in the text above.
Figure 1 caption: Reservoir profiles taken from inside of Island Park reservoir near the siphon intake for the hydroelectric power plant. The y-axis is reservoir elevation in feet, so the top of each panel represents the surface of the reservoir and, as you move down the y-axis, the data show how each parameter changes through the water column toward reservoir bottom. Temperature in degrees Celsius is shown in the top panels of each figure and cyanobacteria content is shown in the bottom panels. The units for cyanobacteria relate to relative changes in biomss. Left-hand profiles were taken July 8, before the peak of the cyanobacteria bloom on July 10, and right-hand profiles were taken July 15, after the peak of the bloom.
Figure 2 caption: Reservoir profiles taken from inside of Island Park reservoir near the intake for the U.S. Bureau of Reclamation (USBR) Gates outlet structure. The y-axis is reservoir elevation in feet, so the top of each panel represents the surface of the reservoir and, as you move down the y-axis, the data show how each parameter changes through the water column toward reservoir bottom. Temperature in degrees Celsius is shown in the top panels of each figure and cyanobacteria content is shown in the bottom panels. The units for cyanobacteria relate to relative changes in biomss. Left-hand profiles were taken July 8, before the peak of the cyanobacteria bloom on July 10, and right-hand profiles were taken July 15, after the peak of the bloom.