What Is a Toxic Algal Bloom?
Toxic algal blooms—also called harmful algal blooms (HABs)—are overgrowths of algae or algae-like bacteria in freshwater or marine environments that release harmful toxins or otherwise disrupt aquatic ecosystems. These blooms poison wildlife, pets, and people and are spreading more widely and frequently across the globe, fueled by human activity and climate change.
Toxic Algal Bloom Defined
A harmful algal bloom occurs when algae—microscopic, photosynthetic organisms—grow excessively in lakes, rivers, reservoirs, estuaries, or oceans. While not all blooms are toxic, some produce natural toxins that can be deadly to other organisms.
A toxic algal bloom can harm other organisms through:
Toxin production (e.g., microcystins, anatoxins, saxitoxins)
Mechanical disruption (e.g., smothering aquatic life)
Ecological imbalance (e.g., causing dead zones or shading out sunlight)
Causes of Toxic Algal Blooms
1. Nutrient Pollution
Runoff from agriculture, wastewater, and urban areas introduces large quantities of nitrogen and phosphorus into freshwater systems. These nutrients act as fertilizer, accelerating algae and cyanobacteria growth.
According to the U.S. EPA, nutrient pollution affects:
Over 33% of lakes
2. Climate Change
Rising temperatures give toxin-producing cyanobacteria a competitive advantage over nontoxic algae. Warmer water:
Speeds up cyanobacteria reproduction
Triggers longer bloom seasons
Intensifies droughts and storm-driven runoff
Cyanobacteria also thrive on carbon dioxide (CO₂), and climate-driven changes in carbon cycles fuel further bloom activity.
3. Hydrological Shifts
More frequent droughts, followed by intense rainstorms, lead to extreme runoff. These changes:
Transport more nutrients into water bodies
Create warm, stagnant waters perfect for blooms
Disrupt freshwater flow, concentrating contaminants
4. Invasive Species
Species like zebra and quagga mussels increase water clarity by filtering particles, allowing more sunlight to reach the water column and promoting algal growth.
What Are Cyanobacteria?
While often grouped with algae, cyanobacteria are photosynthetic bacteria. They can form dense, colorful blooms and produce dangerous cyanotoxins. The most common bloom-forming cyanobacteria is Microcystis, which produces microcystin, a potent hepatotoxin that affects the liver, kidneys, and reproductive systems in animals and humans.
Other toxins from cyanobacteria include:
Anatoxins – Neurotoxins affecting the nervous system
Cylindrospermopsin – Affects liver and kidneys
Saxitoxins – Can cause paralytic shellfish poisoning (PSP)
These toxins can bioaccumulate in aquatic organisms like fish and shellfish, posing risks up the food chain.
How Toxic Algal Blooms Affect Ecosystems
1. Aquatic Life
Toxins from blooms can kill fish, shellfish, birds, and mammals. In Texas, golden algae killed 157,000 fish in Lake Texoma over three days. Between 2001 and 2010, golden algae caused over 130 fish kills in the state, costing $14 million in economic losses.
Even nontoxic blooms can be deadly. When algae die and decompose, they deplete oxygen levels, creating hypoxic or dead zones where aquatic life can't survive. Over 166 dead zones have been reported in the U.S., with the largest in the northern Gulf of Mexico, spanning over 8,500 square miles—roughly the size of Massachusetts.
2. Human Health
People can be exposed to algal toxins by:
Swimming or recreating in contaminated waters
Ingesting tainted drinking water or fish
Inhaling aerosolized toxins from water spray
Toxins can trigger effects from mild skin rashes and nausea to serious liver damage and neurological issues. Pets, especially dogs, are highly susceptible. A study published in Toxins documented 63 dog deaths from HAB exposure across 13 U.S. states.
3. Drinking Water Contamination
Algal toxins are very hard to remove from drinking water. Boiling contaminated water doesn't destroy toxins and may increase their concentration. In 2014, a massive HAB in Lake Erie forced the city of Toledo, Ohio, to issue a "do not drink" order affecting 500,000 residents.
Up to 48 million Americans drink water from lakes and reservoirs susceptible to HABs.
4. Economic Losses
Toxic blooms damage:
Tourism and recreation (e.g., beach closures, canceled fishing trips)
Commercial fishing and aquaculture
Property values and municipal water treatment costs
Estimates suggest freshwater blooms cost the U.S. $4.6 billion annually.
Visual and Physical Signs of a Toxic Bloom
Toxic blooms can vary in color and texture. They may appear:
Green, blue-green, red, brown, yellow, or even pink
As scum, foam, or a slick, paint-like layer
With foul odors and murky or cloudy water
They often reduce visibility, block sunlight from reaching underwater plants, and disrupt the entire aquatic food web. For instance, in Chesapeake Bay, algal blooms killed off eelgrass, a vital underwater plant, contributing to a 70% drop in blue crab populations from the 1990s to 2007.
Current Status of a Solution
The Invasive Species Corporation (ISC) is actively researching microbial natural products to develop safe, targeted algaecides. Once a promising candidate is identified, it will undergo:
Efficacy and spectrum testing
Toxicology and environmental risk assessments
Process development and field trials
Regulatory submission and manufacturing feasibility
