https://ocean.si.edu/ocean-life/plants-algae/what-exactly-red-tide
Drifting throughout the ocean, invisible to the naked eye, are innumerable microscopic algae. They come in many shapes and sizes—some geometrically beautiful, like the diatoms, and others, like the dinoflagellates, swim in a distinctive whirling pattern. These tiny algae are essential components to ocean life as they fuel the food web by harnessing light energy from the sun. But when supplied with excess nutrients, they can multiply uncontrollably, becoming an unwanted mass commonly called a “red tide” that smothers nearby ocean life.
This massive growth of algae can become harmful to both the environment and humans, which is why scientists often refer to them as harmful algal blooms or HABs. When nutrients from inland areas flow down rivers and arrive in the ocean they supply a nutritious feast for algae, causing them to rapidly grow. This can happen naturally as rivers flood and bring nutrient-rich soil from forests and grasslands, but it can also happen when fertilizer and excrement from livestock travel down those same waterways, or when coastal development leads to excess erosion. Some algae species, like the dinoflagellate Karenia brevis, color the ocean surface a deep red, inspiring the name “red tide.” But not all red tides are red and not all of them even become dense enough to color the water. There are also “brown tides” which can be damaging as well.
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