Algal bloom

Bright green blooms are a result of blue-green algae, which are actually bacteria . Blooms may also consist of macroalgal, not phytoplankton, species. These blooms are recognizable by large blades of algae that may wash up onto the shoreline.

Of particular note are harmful algal blooms (HABs), which are algal bloom events involving toxic or otherwise harmful phytoplankton such as f the  Alexandrium and Karenia. Such blooms often take on a red or brown hue and are known colloquially as red tides.

An algal bloom is a rapid increase or accumulation in the population ofin an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells. Although there is no officially recognized threshold level, algae can be considered to be blooming at concentrations of hundreds to thousands of cells per milliliter, depending on the severity. Algal bloom concentrations may reach millions of cells per milliliter. Algal blooms are often green, but they can also be other colors such as yellow-brown or red, depending on the species of algae