Why Cyanobacterial Blooms Are Increasing in U.S. Lakes and Reservoirs

Why Cyanobacterial Blooms Are Increasing in U.S. Lakes and Reservoirs

Harmful algal blooms (HABs) are no longer rare or seasonal events. Across the United States, cyanobacterial blooms are appearing earlier in the year, lasting longer into fall, and reaching higher biomass levels than ever recorded. For municipal water utilities, this trend means increased operational costs, more frequent public advisories, and heightened regulatory pressure.

Understanding why these blooms are increasing is essential for developing long-term reservoir management strategies and anticipating risks before they impact drinking water treatment.

This article breaks down the major drivers behind the rise of cyanobacteria in U.S. freshwater systems—and what utility managers need to watch moving forward.

1. Rising Temperatures and Climate Warming

Cyanobacteria thrive in warm water. Many of the most problematic genera (Microcystis, Dolichospermum, Planktothrix) grow fastest at temperatures above 77°F (25°C).

As lakes warm:

• Bloom season begins earlier (late spring instead of midsummer)
• Stratification becomes more stable, giving buoyant cyanobacteria an advantage
• Nighttime cooling decreases, reducing vertical mixing
• Longer periods of stagnation allow biomass to accumulate

Nationwide monitoring shows surface water temperatures increasing by 1–3°C in many drinking water reservoirs over the last 30 years. This shift alone dramatically increases bloom frequency and intensity.

2. Increased Nutrient Loading From Watersheds

Phosphorus and nitrogen are the primary drivers of cyanobacterial growth. Even modest nutrient increases can cause large blooms.

Key nutrient pathways:

Agricultural runoff

• Fertilizer overspray
• Manure applications
• Tile drainage systems that rapidly transmit nutrients into rivers

Urban stormwater

• Fertilizer from lawns and golf courses
• Pet waste
• Roadway particulate wash-off
• High-velocity storm drains with no retention

Internal loading from sediments

When bottom waters become anoxic, phosphorus is released from sediments and transported into the water column—fueling mid-season blooms.

Extreme rainfall events

Modern storms deliver short, intense pulses of runoff, rapidly injecting high nutrient loads into reservoirs.

3. More Stable Stratification in Reservoirs

Stratification—the separation of warm surface water from colder, deeper layers—is becoming more persistent as temperatures rise.

Stable stratification favors cyanobacteria because:

• They can regulate their buoyancy and stay in the photic zone
• Competing algae (green algae, diatoms) often sink out of the light
• Deep waters become low-oxygen, increasing internal phosphorus release
• Mixing events become less frequent, allowing blooms to build undisturbed

Many utilities now observe that spring turnover is shorter, and fall turnover occurs weeks later than in past decades—extending the bloom window.

4. Longer Growing Seasons

Historically, bloom season began in June and ended in September. Now:

• Blooms are appearing in late April or early May
• Significant biomass persists into October and November
• Winter blooms occasionally occur in southern states

A longer growing season increases the number of bloom cycles per year and raises the probability of toxin events.

5. More Frequent Droughts and Slow-Flow Conditions

Cyanobacteria prefer slow-moving, warm, stagnant water.

When drought reduces inflow:

• Flushing rates decline
• Water residence time increases
• Nutrients accumulate instead of being exported
• Surface waters warm faster
• Circulation diminishes

Drought creates ideal bloom conditions, especially in shallow municipalities, reclaimed lakes, and irrigation reservoirs.

6. Intense Rainfall and Runoff Events

The flip side of drought—heavy storms—also contribute to HABs.

Large storms:

• Deliver nutrient-rich runoff rapidly
• Resuspend phosphorus from sediments
• Introduce organic matter that cyanobacteria use as substrate
• Cause short-term turbidity spikes followed by rapid clearing

This “feast then calm” pattern feeds cyanobacteria and then gives them ideal conditions to bloom.

7. Aging Reservoir Infrastructure

Many municipal reservoirs were designed decades ago and weren’t built to manage modern nutrient loads or climate conditions.

Aging systems often exhibit:

• Reduced circulation and destratification capacity
• Sediment accumulation leading to internal loading
• Aeration systems that no longer reach design performance
• Limited watershed buffers
• Outdated stormwater control structures

Infrastructure designed for 1960s climate and 1960s nutrient loads struggles under 2025 conditions.

8. Expansion of Cyanobacteria Into New Regions

Cyanobacteria are becoming more common in areas previously considered too cold.

Studies now document HABs in:

• Northern Michigan
• Minnesota boundary waters
• Canadian prairie reservoirs
• New England lakes
• High-altitude western reservoirs

This expansion corresponds to warming waters, nutrient enrichment, and reduced ice cover duration.

9. Increased Prevalence of Toxin-Producing Strains

Many cyanobacteria species have both toxic and non-toxic strains. Environmental stressors—like heat, nutrient spikes, chemical shocks, and competition—may select for toxin-producing strains.

Utilities in several states have reported:

• Higher microcystin levels
• More frequent anatoxin-a detection
• Occasional cylindrospermopsin events in regions where it was previously absent

This shift raises both treatment complexity and public health risk.

10. Reduced Dilution From Smaller Snowpack and Earlier Melt

In the western United States:

• Snowpack is shrinking
• Melt occurs earlier
• Peak flow happens weeks earlier

This means reservoirs warm earlier and dilute less, increasing HAB susceptibility.

What This Means for Water Utilities

Utilities should expect:

• More frequent blooms
• Higher peak biomass
• Longer bloom seasons
• Increased toxin variability
• More persistent taste and odor events
• Higher treatment costs

The upward trend of HABs is unlikely to reverse without significant watershed management, modern in-reservoir technologies, and improved monitoring systems.

Key Takeaways for Municipal Water Managers

• Cyanobacteria are increasing due to climate warming, nutrient loading, stratification, and hydrologic changes.
• Bloom seasons are expanding on both ends—starting earlier, ending later.
• Extreme weather (storms + droughts) intensifies bloom conditions.
• Aging infrastructure can’t keep up with modern nutrient and temperature dynamics.
• Utilities need multi-layered strategies that address both watershed and in-reservoir processes.

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