A modified stratification index method to assess reservoir water quality trends

Harmful algal blooms (HABs) have been a harm to reservoir health for decades and it is believed that as climate changes and temperatures rise, frequency and severity of HABs are likely to increase as well. Understanding the relationships between physical factors in a reservoir and bloom trends could be key for keeping rising blooms at bay. A modified stability index based on temperature dependent water density at multiple depths was adapted into a code to process high frequency reservoir monitoring data. Metrics and visual tools were developed to use this stability index to objectively analyze how stratification - including intensity, start date, and turnover point - and water quality characteristics are changing, and how they are likely to change in the coming decades. This code was then used to analyze the relationships between physical and environmental reservoir characteristics, a generated stratification index, and algal bloom behavior for several United States Army Corps of Engineers (USACE) reservoirs, with specific focus on Shenango reservoir. Surface temperature, maximum temperature difference, and depth were found to correspond with strength of stratification. The calculated stratification index showed significant strength of correlation to algae when compared to other commonly collected reservoir parameters. Bettering our understanding of when stratification is occurring within reservoirs, as well as when blooms are occurring, will lead to more informed management decisions and better HAB control. For Shenango reservoir, it was determined that a hydrodynamic management strategy that maintains stability (horizontal flushing, epilimnetic withdrawals) during peak periods, while removing epilimnetic biomass and reducing nutrient availability, would be the most practical management strategy for bloom reduction.