Do natural lakes and reservoirs’ trophic dynamics behave differently and do reservoirs age

Background/Question/Methods

Water quality parameters have been monitored on Smith Mountain Lake, Virginia for 25 years and Claytor Lake for 17 years. Smith Mountain Lake is a 46 year old pumped storage reservoir of 20,000+ acres. Claytor Lake is 72 years old "run of the river" hydropower reservoir of 4500 acres. The parameters measured include total phosphorus, chlorophyll‑a, water clarity, and other water quality measures. Other biological measurements made include bacterial populations, algal populations and invasive weed populations. Other chemical and physical measurements made include depth profiles of temperature, dissolved oxygen, pH and conductivity. 

The trophic parameter measurements and samples are collected by citizen scientists around Smith Mountain Lake and Claytor Lake and the depth profile measurements, bacterial, algal and invasive weed samples are collected by Ferrum College students and faculty. The lakes are sampled from May through August each year and have been sampled and analyzed for 25 years. All analyses follow the procedures outlined in Standard Methods for Water and Wastewater 21^st edition (ARHA, 2005).

Results/Conclusions

Most of these values in both lakes indicate a mesotrophic condition and have not been indicative of a typical aging lake because the total phosphorus, and chlorophyll-a have not been increasing significantly and the Secchi depth (SD) has not been decreasing, all indicators of an aging lake. These results appear to indicate that these two reservoirs in Virginia may not be behaving the way natural lakes typically behave. In 2011 the average values for these trophic measurements in Smith Mountain Lake were: total phosphorus mean = 25.9 ppb (n=328), chlorophyll-a mean = 4.7 ppb (n=328), and Secchi depth mean = 2.1 m (n=477), Claytor Lake water quality parameters' values are higher than the same parameter values for Smith Mountain Lake however these values are still within the mesotrophic range. The aging of the two reservoirs may be influenced by both chronological age and type of lake circulation, which is dependent on the power generation characteristics.

Another measure of lake age is the evaluation of its trophic status using the Carlson Trophic State index (TSI), which is based on the three water quality parameters, measured on these two reservoirs. Values for the combined TSI for Claytor Lake range from 48 to 54, which is considered mesotrophic status. Smith Mountain Lake combined TSI values ranged from 30 to 70.