An algal bloom is a rapid increase in the population of algae within an aquatic system, occurring in both freshwater and marine environments. This phenomenon often results in visible water discoloration, which can appear green, blue-green, red, or brown depending on the specific algae type. While algae are a natural component of aquatic ecosystems, their excessive growth can lead to detrimental consequences for fish and other aquatic life.
Suffocation from Oxygen Depletion
One common way an increased algal population can kill fish is through oxygen depletion, a process often linked to eutrophication. Eutrophication occurs when water bodies become enriched with nutrients, primarily nitrogen and phosphorus, which act as fertilizers for algae. This triggers rapid algal growth. While living algae produce oxygen during daylight through photosynthesis, they also consume oxygen continuously through respiration, particularly at night.
When the algal population dies, decomposition begins. Bacteria break down the dead algal organic matter, consuming large amounts of dissolved oxygen. This consumption can lead to low oxygen levels, known as hypoxia, or even anoxia. Fish require adequate dissolved oxygen to breathe through their gills; when levels fall below 2-3 milligrams per liter, they begin to suffocate.
Harmful Algal Toxins
Beyond oxygen depletion, certain algae types can produce toxins that directly poison fish. These are known as harmful algal blooms (HABs) and are caused by diverse organisms, including cyanobacteria (blue-green algae) and dinoflagellates. For instance, cyanobacteria produce microcystins (liver toxins), while some dinoflagellates produce neurotoxins like saxitoxins and brevetoxins.
These toxins can directly affect fish by damaging their nervous systems, livers, or gills. Fish may ingest these toxins by consuming contaminated prey or by absorbing them directly from the water. The effects can range from impaired swimming and erratic behavior to paralysis and death. Not all algal blooms produce toxins, but those that do pose a direct threat to fish populations.
Physical and Environmental Stressors
Algal blooms can also harm fish through physical and environmental stressors, even without severe oxygen depletion or toxins. Dense algal cell concentrations can irritate and clog fish gills. This physical blockage hinders a fish’s ability to extract oxygen from the water, making breathing difficult regardless of dissolved oxygen levels.
Another stressor is reduced light penetration into the water column. Thick surface blooms can block sunlight from reaching submerged aquatic plants, leading to their death. Their decomposition further contributes to oxygen depletion in deeper waters. Intense algal photosynthesis during the day and respiration at night can cause daily fluctuations in water pH, creating an unstable environment that stresses fish and makes them more vulnerable to disease.