What causes algae blooms?

Algae blooms can make lakes and oceans look like abstract paintings, flooding the surface of the water with large swaths of green, yellow, brown, red, or even a nearly neon orange. What causes this phenomenon? Algae blooms are the result of a rapid increase in the population of algae in a body of water. Algae are mostly aquatic photosynthetic organisms in the kingdom Protista, and they range in size from unicellular species to giant kelps. Blooms are most commonly caused by phytoplankton, a kind of microscopic algae that drift with water currents. Blooms can last anywhere from a few days to a few weeks. Regularly occurring seasonal blooms are usually the result of optimal temperature, water, and nutrient conditions. Warmer waters are key, and seasonal blooms are common in the late summer and early fall months in the United States. Shallow and slow-moving waters, such as those present during drought, also foster algae reproduction. Some blooms are beneficial to the ocean food web because phytoplankton reside at the bottom of the marine food chain, making an abundance of them helpful for nearly all other ocean life. While many blooms are harmless, some are dangerous to people, pets, and wildlife. Harmful algal blooms, or HABs, can become so dense that they block sunlight needed by other aquatic photosynthetic organisms, clog the gills of fish, and deplete the water oxygen levels. This often kills large numbers of fish and other aquatic animals. The toxins released by some blooms can also sicken nearby birds and humans. One kind of HAB is a “red tide.” In oceans across the world, seasonally warmer waters boost the reproduction of certain dinoflagellates, causing shallow waters close to the shore to appear deep red because of the dense algae layer. Human activity has caused an increase in harmless and harmful blooms beyond seasonal norms. In fresh water, blooms are often the result of nutrient pollution, which occurs when runoff from farmlands and cities drains into bodies of water. Sewage and fertilizers in runoff are rich in nutrients such as phosphorus and nitrogen, which, combined with adequate sunlight, promote algae growth. Climate change has also contributed to more frequent, longer-lasting, and more widespread blooms. Longer periods of drought coupled with heavier precipitation can cause increased runoff, heightening nutrient pollution. There’s no way to tell if an algae bloom is toxic from sight alone, so it’s best to avoid the beach if the water looks a little too colorful for comfort.