From Symptoms To Solutions: Measuring What Matters To Transform Reservoir Risk Management

By Dave Shackleton

A new methodology for measuring “what truly matters” empowers reservoir managers to make informed, proactive decisions that break the cycle of lake degradation and secure long-term water quality and ecosystem health.

In 2020, alarmed by the growing prevalence of hypoxia and harmful algal blooms (HABs), Congress and the Senate tasked the Government Accountability Office (GAO) with a second investigation. The 2022 GAO report, Water Quality: Agencies Should Take More Actions to Manage Risks from Harmful Algal Blooms and Hypoxia, delivered a stark assessment of existing policies and the threats to national water security.

The GAO report exposed a critical flaw: federal efforts focus primarily on monitoring and addressing symptoms after HABs occur. This reactive approach, while necessary for crisis response, is reinforced by monitoring parameters that overlook the systemic drivers of eutrophication and hypoxia.

In response, the Reservoir Risk Assessment and Tracking System (RRATS) was developed, forming the foundation for the Reservoir Risk Index (RRI). This system provides a comprehensive framework for assessing reservoir health, integrating forward-looking monitoring and actionable insights to help managers prioritize interventions, allocate resources, and mitigate risks early.

Unlike the traditional Trophic State Index, which measures symptoms and encourages reactive symptomatic treatments, RRATS and RRI mark a paradigm shift by measuring root cause parameters to inform proactive, professional risk management strategies.

The Limitations Of The Trophic State Index

During the 1970s, growing concerns about eutrophication led Robert E. Carlson to develop the Trophic State Index (TSI) to quantify nutrient-driven productivity in lakes and reservoirs (Carlson, 1977, Limnology and Oceanography).

Although the TSI standardized eutrophication assessment, it has significant limitations and redundancies upon closer examination.

The TSI is based on three parameters: Total Phosphorus (TP), which includes both dissolved and organic phosphorus; Chlorophyll-α, a pigment indicating algae levels; and Secchi Disk Transparency, a measure of water clarity that diminishes as algae increase. While these metrics appear distinct, elevated TP drives algae growth, increasing Chlorophyll-α and reducing water clarity. The TSI essentially measures the same phenomenon in three different ways, which reduces the depth of insight provided.

By focusing solely on these indicators, the TSI overlooks key drivers like hypoxia and sediment nutrient recycling, which sustain nutrient availability and promote harmful cyanobacteria. These critical dynamics remain unaddressed in TSI assessments.

In the 1990s, Wayne Carmichael’s research (Scientific American, 1994) emphasized cyanobacteria’s role in toxic HABs. Unlike other algae, cyanobacteria use the pigment phycocyanin for photosynthesis. The TSI fails to distinguish cyanobacteria from general algae, again limiting its value.

Misguided Management Tactics

For the past five decades, the TSI narrow emphasis on symptoms has led to the adoption of expedient but misguided management practices. This approach often involves reactive, short-term actions that may temporarily improve TSI scores but ultimately worsen the underlying causes and hasten the development of HABs.

While algaecides and phosphate precipitants can reduce visible algal blooms or bind phosphorus temporarily to improve TSI scores, they compound long-term problems and elevate risk. Dead algae and precipitated phosphorus accumulate in sediments, fueling nutrient recycling under hypoxic conditions and reinforcing the cycle of degradation — factors identified in the GAO report as the two primary drivers of lake degradation.

Hypoxia, or low oxygen levels, is driven from the benthic margin where water meets sediment as organic sediment decomposes. In these oxygen-depleted zones, anaerobic microorganisms thrive that recycle phosphorus and nitrogen from sediment into the water column. These nutrients are then available almost exclusively to cyanobacteria, which can regulate their buoyancy to dive down and access these benthic nutrient supplies. In contrast, beneficial algae, which rely on passive flotation near the surface, cannot compete for these resources. Dominance by cyanobacteria not only exacerbates water quality issues but also shifts the ecological balance, reducing biodiversity and destabilizing aquatic ecosystems.

Sediment nutrient recycling compounds and sustains the problem by acting as a persistent internal source of phosphorus and nitrogen. Decades of nutrient accumulation in sediment layers create a "time bomb", which, in hypoxic conditions, releases stored nutrients back into the water column that fuel explosive toxic cyanobacteria blooms, further accelerating eutrophication and increasing the frequency and intensity of HAB events.

The consequences of these root causes are profound. Hypoxia and nutrient recycling directly facilitate the dominance of cyanobacteria, whose blooms can release harmful toxins into the water posing significant risks to public health, impacting drinking water supplies, and disrupting aquatic ecosystems. Only by addressing these fundamental drivers of the self-reinforcing cycle of degradation that leads to HABs can the cycle be broken and responsible risk management of water quality be enabled.

The Reservoir Risk Assessment And Tracking System

Understanding and targeting these root causes is critical for transitioning from reactive crisis management to proactive, preventative measures. RRATS was specifically designed to address these challenges, providing a comprehensive framework for monitoring and mitigating the key drivers of reservoir degradation.

RRATS is designed to monitor and quantify the critical factors contributing to eutrophication and HABs. It measures the volume of hypoxic water to determine the extent of oxygen-depleted zones within the reservoir. It also tracks the surface area of the hypoxic sediment, which evaluates the spatial extent of the sediment-water interface under low-oxygen conditions favorable to anaerobic sediment nutrient recycling.

Additionally, RRATS includes phytoplankton monitoring, which observes biodiversity to identify changes in the phytoplankton community, particularly the balance between beneficial algae and cyanobacteria, and measures the overall phytoplankton biovolume.

Data gathered in RRATS provides specific insight for the development of tailored actions to be taken to reverse eutrophication, eliminate hypoxia and prevent toxic HABs.

A Risk Scorecard

The Reservoir Risk Index simplifies the data collected by RRATS into a straightforward and objective scorecard that indicates the risk status of a reservoir. This index enables the comparison of reservoirs objectively, irrespective of their size, depth, or geographic location. It tracks changes in risk over time, allowing managers to evaluate the success of preventive interventions and helps prioritize resource allocation to the reservoirs with the highest risk, ensuring interventions are both timely and effective.

By consolidating RRATS data into an objective score, the RRI not only aligns with the methodologies of professional risk management but also breaks the cycle of misguided interventions that TSI inadvertently perpetuated.

Tracking Performance Of Interventions

RRATS and RRI can be utilized to assess and monitor the success of prevention-focused strategies aimed at minimizing the risks of hypoxia and HABs. These strategies include enhancing oxygen levels, implementing biological augmentation, managing nutrients, and controlling sediment.

Technologies such as Rapid Acting Dissolved Oxygen Restoration (RADOR) systems have been shown to reduce stratification, maintain oxygen balance, and improve water quality, thereby preventing hypoxia and HABs by eliminating the conditions that cyanobacteria exploit.

Another step in restoring balance to a lake is the use of enzymes to break down organic muck. When enhanced by natural enzymes, aerobic bacteria and microorganisms consume the organic muck and nutrients, while aquatic insects and zooplankton feed on the bacteria, increasing the natural food source for fish.

Introducing critical micronutrients has also been demonstrated to promote the growth of beneficial phytoplankton and zooplankton, re-establishing the foundation of the food web. This renewed competition helps beneficial algae outcompete toxic cyanobacteria, restoring balance and preventing HABs while improving the food web’s overall nutrition.

Empowering Change

RRATS and RRI are not just abstract ideas but practical tools proven effective in real-world applications, revolutionizing the assessment and management of reservoir risks.

By prioritizing monitoring root causes rather than second-order symptoms, RRATS and RRI empower reservoir managers, policymakers, and stakeholders with new insights to better manage the risks that eutrophication and HABs create. These advancements offer a dependable approach to preserving water quality, safeguarding public health, and ensuring the long-term sustainability of essential water resources.

For more information on implementing RRATS and using the RRI to guide your reservoir management practices, visit www.clean-flo.com, email contact@clean-flo.com, or call 1-800-328-6656.

Dave Shackleton is president of Clean-Flo International, a U.S.-based leader in biological water management solutions for managing water quality in biological environments such as wastewater treatment, rivers, lakes, and reservoirs.