On June 22nd, 2021, a report came in that the secchi depth of Big Floyd was about 11 feet (3.4m) and North Floyd was over 20 feet (6.1m). PRWD responded on 6/24/21 to investigate the cause. Temperature, dissolved oxygen, specific conductance, pH, clarity, phosphorous and algae growth were sampled by PRWD Staff on 6/24/21 (similar secchi depths were observed). Temperature data indicated the lake had no thermocline, with consistent temperatures throughout the water column. The week before, staff observed weak stratification on both lakes with secchi depths around 20’. Early summer heat drove lake temperatures up quickly and established a weak thermocline (see diagram below).
When temperatures dipped down to the 40’s on June 21st and 22nd, surface temperatures began to cool. As water cools it becomes denser, causing it to sink while warmer water on the bottom rises to the top. This process is called lake turnover, or mixing, and tends to release phosphorous trapped in bottom sediments into the water column causing algae blooms. Lake turnover can be caused by temperature changes, high winds, or a combination of the two. Disturbance by excessive boat traffic can sometimes contribute to lake turnover as well.
The average annual secchi depth for North Floyd is about 8’, a far cry from the 20’ observed on the 22nd and 24th. North Floyd is experiencing better than average water quality due to limited rainfall this year. The main contributor of nutrients to North Floyd is Campbell Creek, which barely flowing after several months of below average precipitation. While the drastic decrease in secchi depth for Big Floyd is shocking, 11’ is the annual average for the last 20 years. The District will continue to keep a close eye on water quality as we continue into the summer of 2021. by Adam Mortenson PRWD