An algal bloom is a rapid increase in the population of algae in a body of water. Algal blooms are a natural part of the environment, but they have become more frequent and intense. They are often recognized by a distinct discoloration of the water, a direct result of the pigments within the billions of algal cells.
The Triggers Behind Algal Blooms
The primary driver of most algal blooms is eutrophication, the enrichment of water with nutrients. Excess nitrogen and phosphorus act as fertilizers, stimulating explosive algal growth. These nutrients enter waterways from sources like agricultural runoff, which carries fertilizers and animal waste from farms into streams and rivers.
Urban and suburban stormwater also plays a part, washing excess fertilizers from lawns and gardens into local water bodies. Discharges from wastewater treatment plants, even after processing, can still contain elevated levels of these nutrients. Human activities have dramatically increased the amount of nitrogen and phosphorus entering aquatic systems compared to pre-industrial levels.
Physical and environmental factors also create the perfect conditions for a bloom to occur. Warmer water temperatures, which are becoming more common, accelerate algal growth rates. Slow-moving or stagnant bodies of water, such as lakes, ponds, and calm coastal bays, allow algae to accumulate without being dispersed. Abundant sunlight provides the necessary energy for photosynthesis, further fueling the rapid multiplication of algae.
Identifying Different Types of Blooms
Algal blooms often look like a thick scum or mat floating on the surface, resembling spilled green paint or pea soup. The water itself can become discolored, turning shades of green, blue-green, brown, or even red. This coloration depends on the specific type of organism that is blooming.
The organisms responsible for these events vary between freshwater and marine environments. In lakes and rivers, the most common culprits are cyanobacteria, often called blue-green algae. Despite the name, cyanobacteria are technically a type of bacteria that perform photosynthesis, and they are responsible for the majority of freshwater blooms.
In saltwater environments like oceans and estuaries, blooms are frequently caused by single-celled organisms called dinoflagellates. These blooms are famously known as “red tides” because they can turn coastal waters a rusty red or brown color. Both cyanobacteria and dinoflagellates are types of phytoplankton, microscopic organisms that form the base of the aquatic food web.
The Dangers of Harmful Algal Blooms
While many algal blooms are harmless, some are designated as Harmful Algal Blooms (HABs) because they produce powerful toxins. Exposure to these toxins can pose significant risks to human health, depending on the route of contact.
Direct skin contact with contaminated water can lead to rashes and irritation. Inhaling toxins that have become aerosolized in water spray can cause respiratory issues, including coughing and wheezing. The most severe effects often come from ingestion, either by swallowing contaminated water or by consuming fish or shellfish that have accumulated toxins from the bloom. This can lead to a range of symptoms from gastrointestinal distress to serious neurological problems, such as memory loss and paralysis.
The ecological consequences of HABs can be severe. As the massive population of algae in a bloom dies off, it sinks to the bottom and decomposes. This decomposition process consumes large amounts of dissolved oxygen in the water, creating hypoxic, or low-oxygen, areas known as “dead zones.” These blooms also harm ecosystems by blocking sunlight, which can kill underwater plants that provide habitat for other organisms.
Human Interaction and Safety
Assume any visible algal bloom is toxic and avoid all direct contact. This means refraining from swimming, wading, kayaking, or any water sports in areas where the water is discolored or has surface scum. It is especially important to keep pets and livestock away from the water’s edge, as they are highly susceptible to poisoning. Dogs are at particular risk because they are more likely to drink the water or ingest toxins by licking their fur after swimming.
A common misconception is that boiling contaminated water will make it safe. Boiling does not remove or neutralize these toxins; in fact, it can increase their concentration as the water evaporates. Therefore, water from a bloom should never be used for drinking or cooking, even if it has been boiled or passed through a standard water filter.
Before heading to a lake or beach, check for local advisories. State or local health departments and environmental agencies often post warnings about known HABs on their websites. If you encounter a bloom that has not been reported, notify the proper authorities, such as your state environmental agency. Reporting suspected blooms helps officials monitor the situation and protect public health.