Harmful algal bloom

A harmful algal bloom (HAB) is the rapid and uncontrolled growth of algae in either freshwater or marine environments. Because some algae produce toxins, they can be harmful to humans, mammals, birds and fish when the toxins are ingested. As blooms grow, they deplete the oxygen in the water and block sunlight from reaching fish and plants.

Blooms can last from a few days to many months. After the bloom dies, the microbes which decompose the dead algae use up even more of the oxygen, which can create fish die-offs. When these zones of depleted oxygen cover a large area for an extended period of time, they are referred to as dead zones, where neither fish nor plants are able to survive.

Among the causes of HABs are high-nutrient conditions in water, mostly of nitrates and phosphorus which are emitted by agriculture and other industries. Higher water temperature and less circulation are also factors. HABs can cause significant harm to animals, the environment and economies. They have been increasing in size and frequency worldwide, a fact that many experts attribute to global climate change. The U.S. National Oceanic and Atmospheric Administration (NOAA) predicts more harmful blooms in the Pacific Ocean.

HABs from blue-green algae (cyanobacteria) can appear as a foam, scum, or mat on or just below the surface of water and can take on various colors depending on their pigments. Blue-green algae blooms in freshwater lakes or rivers may appear bright green, often with surface streaks which looks like floating paint. Similarly, red tides made up of dinoflagellates, also contain photosynthetic pigments that vary in color from green to brown to red.

Most blooms occur in fresh, marine, or brackish waters that have excessive nutrients and above normal water temperature which stimulate their rate of growth. The harmful effects from such blooms is due to the toxins they produce or from using up oxygen in the water which can lead to fish die-offs.

Not all algal blooms are harmful, however, with some only discoloring water, producing a smelly odor, or adding a bad taste to the water. Unfortunately, it is not possible to tell if a bloom is harmful from just appearances, since sampling and microscopic examination is required.

There are three main types of algae which can form into harmful algal blooms: cyanobacteria, dinoflagellates and diatoms. All three are made up of microscopic floating organisms which, like plants, can create their own food from sunlight by means of photosynthesis. That ability makes them an essential part of the food web for small fish and other organisms.

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