Intense blooms of dinoflagellates occur in some regions, especially the tropics. The frequency of such blooms appears to be increasing and They are appearing in areas where they have not been seen before. Run-off that contains inorganic or organic compounds react with toxic substances, such as copper and mercury, and reduces their toxicity to phytoplankton. The dense concentration of phytoplankton in bloom colors the surface water yelllow, green, brown, or reddish, depending on the species. Many dinoflagellates have a reddish color, so their blooms are called red tides .
Toxins produced by dinoflagellate blooms often kill large numbers of fish.
The near-surface water layer, where there is light, and photosynthesis is possible, is called the photic zone. Life is found thousands of meters deep in the ocean, far from the reach of sunlight. Special types of life are found surrounding chimney-like vents that release hot water rich with minerals. The photic zone layer is where most marine life exists. The depth of the photic zone layer is not constant because the angle of the incoming sunlight varies from day to day and seasonally.