What Is Cyanobacteria?

What Is Cyanobacteria? A Practical Guide for Reservoir Managers

For municipal water utilities, city managers, and reservoir operators

Cyanobacterial blooms have become one of the most persistent and costly challenges facing drinking water utilities across the United States. As temperatures warm, drought cycles intensify, and nutrient inputs rise, more reservoirs are experiencing periods of rapid cyanobacteria growth and elevated toxin levels. For city managers and water operations teams, understanding what cyanobacteria are, why they bloom, and how they impact treatment is the first step toward prevention and long-term control.

This guide breaks down cyanobacteria from a utility perspective—biology, behavior, risks, and what operators need to monitor.

What Exactly Are Cyanobacteria?

Cyanobacteria, often called “blue-green algae,” are photosynthetic bacteria found in nearly every freshwater system. Despite the name, they are not algae—they are gram-negative bacteria with the ability to perform photosynthesis.

They thrive in:

• Warm, stagnant water
• High nutrient environments (especially phosphorus and nitrogen)
• Stratified reservoirs with limited circulation
• Shallow coves and wind-protected embayments

Key species relevant to U.S. drinking water reservoirs include:

• Microcystis aeruginosa
• Dolichospermum (Anabaena)
• Planktothrix (Oscillatoria)
• Aphanizomenon
• Cylindrospermopsis (Raphidiopsis)

These groups are responsible for most lake closures, HAB advisories, and taste/odor complaints.

Why They Multiply So Quickly

Cyanobacteria are adaptive, resilient, and opportunistic. They often outcompete other algae because of three biological advantages:

1. Buoyancy Control

Gas vesicles allow cyanobacteria to float or sink depending on light availability. This is why surface scums form during the day.

2. Nitrogen Fixation

Some genera (Dolichospermum, Aphanizomenon) can extract nitrogen from the atmosphere, giving them an advantage in low-nitrogen conditions.

3. Low-Light Efficiency

They can continue photosynthesis in lower light than many green algae.

When water warms above ~72°F (22°C) and nutrients are available, populations can double rapidly.

Why Cyanobacteria Matter to Water Utilities

HABs create serious operational, financial, and regulatory burdens for drinking water systems.

1. Cyanotoxins

Many cyanobacteria produce toxins harmful to the liver, nervous system, and skin:

• Microcystin (hepatotoxin; most common in U.S.)
• Anatoxin-a (neurotoxin; fast-acting)
• Cylindrospermopsin (hepatotoxin; more difficult to remove)

Even non-toxic blooms can become toxic when stressed.

2. Taste and Odor Compounds

The main culprits are:

• Geosmin
• MIB (2-methylisoborneol)

These compounds drive customer complaints and increase carbon usage.

3. Treatment Challenges

Operators often encounter:

• Quicker filter clogging
• Increased coagulant demand
• Higher sludge production
• Rising turbidity and DOC
• Elevated disinfectant demand

Chemical treatments can cause sudden cell lysis, releasing toxins into the water column.

4. Regulatory & Public Communication Burdens

HAB events often trigger:

• Increased sampling
• Public advisories
• State health department involvement
• More reporting and documentation

This adds administrative load and reputational risk.

What Causes Cyanobacteria Blooms?

Several interacting factors create ideal bloom conditions.

Nutrient Loading

Phosphorus is typically the limiting nutrient in freshwater systems. Major sources:

• Agricultural runoff
• Irrigation and lawn fertilizers
• Stormwater surges
• Internal loading from anoxic sediments
• Failing septic systems

Even small nutrient increases can trigger major blooms.

Warm Temperatures

Climate change intensifies HABs:

• Faster cell division
• Longer bloom seasons
• Warm nighttime temperatures reduce thermal mixing

Stratification

Stable, layered water allows buoyant cyanobacteria to dominate the photic zone.

Calm Weather

Low wind speeds promote surface accumulation.

Hydraulic Retention Time

Slow-moving, low-turnover reservoirs accumulate biomass faster.

How Reservoir Managers Can Identify Early-Stage HABs

Early detection prevents treatment disruption and reduces cost.

Visual Indicators

• Green streaks
• Surface scums
• Foam-like accumulations
• “Pea soup” appearance

Instrumentation

Modern utilities rely on:

• Fluorometers (phycocyanin sensors)
• Hyperspectral or multispectral imaging
• Sonde profiles (DO, temp, chlorophyll-a)
• Online toxin monitors

Sampling

A practical monitoring plan includes:

• Cell counts
• Toxin assays
• Nutrient levels
• Secchi depth
• Vertical temperature profiles

Routine monitoring reduces the risk of sudden bloom collapses.

The Economics of Cyanobacteria for Utilities

Blooms increase operational costs through:

• Greater chemical use
• Higher labor demands
• More frequent filter backwashing
• Increased equipment wear
• Emergency response management

Prevention is consistently more cost-effective than reactive treatment.

Long-Term Management Approaches

Most utilities combine watershed and in-reservoir strategies.

Watershed Management

• Reduce phosphorus inputs
• Increase vegetative buffers
• Retrofit stormwater systems
• Improve agricultural practices

In-Reservoir Tools

• Aeration and mixing
• Ultrasound (chemical-free control)
• Biological approaches
• Targeted dredging

Every reservoir needs a management plan tailored to depth, circulation, nutrient loading, and climate.

Takeaway for Water Managers

Cyanobacteria are resilient, fast-growing organisms that thrive in warm, nutrient-rich, slow-moving water. The challenge for utilities is not just eliminating blooms but preventing them, improving monitoring, and minimizing treatment disruptions caused by toxin release.

Understanding cyanobacteria biology is the cornerstone of managing HABs effectively. Future articles will explore why blooms are increasing nationwide and what utilities can do to reduce their operational impacts.

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