Plankton blooms are made up of tiny microscopic organisms known as phytoplankton. These organisms are effectively tiny plants; they make their own food through the process of photosynthesis. Phytoplankton are crucial to the world ecosystem, as they are the producers of a large amount of the world's oxygen (oxygen is a byproduct of photosynthesis). Individual phytoplankton are invisible to the naked eye; however, a bloom can appear green or red because of the chlorophyll in the phytoplankton.
Several different causes must converge for a plankton bloom to occur. For so many plankton to come together at once, there must be a high amount of nutrients that are needed for photosynthesis; thus, there must be high levels of carbon dioxide and water for a bloom to begin. Additionally, blooms are more likely to occur in the springtime because the warmer temperatures and sunlight (which is also needed in high levels for a bloom to occur) may trap nutrients on a small area of the water's surface, which subsequently drives phytoplankton there.
There are some concerns with the safety of water in which there is a plankton bloom. This is especially true for a subgroup of plankton, known as cyanobacteria, which operates more like bacteria than plants, and thus has the possibility to cause infections and disease in human beings. Additionally, red blooms are considered toxic to human beings; these are caused by a subset of plankton known as dinoflagellates. Even normal, green/blue-green plankton blooms pose some risk to humans, as the water becomes more difficult to swim in, and thus there is a risk of drowning.
Due to their seasonal causes, there are several famous examples of plankton blooms that recur on a near annual basis. For example, the Gulf of Maine is host to annual blooms in both the spring and fall. Scientists have also spent over a decade studying a phytoplankton bloom in the Arctic Ocean, with the conclusion being that it is growing on an annual basis and the blooms coming earlier every year. Additionally, the Red Sea in Egypt takes its name from the dinoflagellate blooms that occur in it.