EFFECTS OF NITROGEN AND PHOSPHORUS ON THE GROWTH RATES OF A TOXIN AND BLOOM-PRODUCING ALGAE IN A LAKE

Algae are critical components of aquatic ecosystems that power their food webs and biogeochemical cycling. The effects of increased nutrient concentrations on the growth rates of Microcystis aeruginosa (Kützing) was studied in situ in Lake Sinclair, Baldwin County, Georgia, U.S. Water from the lake is used to cool the turbines of a coal burning Power plant, for hydropower, and for recreation. About a billion gallons of water per day is extracted from the lake and pumped back creating a constant churning of water with potential environmental impacts. Increased nutrients such as nitrogen and phosphorus in water bodies are known to promote harmful algae blooms, but can differ in the quiet coves of a lake versus the deeper well mixed sites. M. aeruginosa is a common cyanobacterium species that can produce a toxin known as microcystin. Algal toxins are produced when a population is at or above carrying capacity, so understanding what slows growth rates of this taxon is of great interest to hydrologist and water quality managers. There is a growing need to understand the variables that impact the growth of M. aeruginosa and conditions that promote harmful algal blooms. Original designs of sub-surface microcosm systems were developed for the analysis on M. aeruginosa growth in variable nutrient concentrations. The systems were tested in the lake for 15 days during the spring and summer of 2014. The nutrient concentrations analyzed were 25, 50, and 100 mg/L phosphorus and 150, 500, and 1,000 mg/L nitrogen. The nitrogen concentrations showed the highest initial growth rates during the first five days, but the culture population ceased to grow thereafter. All of the phosphorus concentrations promoted increased growth rates during the experiment. The highest growth rate was observed in the 100 mg/L (P) samples. M. aeruginosa is limited by phosphorus due to its continual growth under reduced phosphorus concentrations throughout the experiment. In low turbidity areas of the lake, the phosphorus-limited trait observed for M. aeruginosa could be used as an indicator for potential bloom promoting reservoirs and be used to develop an early warning system.