Algae

Red tides are events of great abundance (or "blooms") of red, brown, or yellow-colored dinoflagellates of various species. These algae synthesize biochemicals, such as saxitoxin and domoic acid, which are extremely poisonous and can kill a wide range of marine animals, as well as humans who eat shellfish containing the toxins. The toxic syndromes of humans associated with dinoflagellate toxins are known as paralytic, diarrhetic, and amnesic shellfish poisoning.

Scientists cannot yet predict the environmental conditions that cause red tides to develop, although it seems that they are related to the availability and ratio of nutrients to temperature. Red tides are natural phenomena, but some scientists believe that human interference may have increased the frequency of these phenomena in some regions.

The dinoflagellates involved with toxic dinoflagellate blooms are commonly of the genera Alexandrium, Dinophysis, Nitzchia, or Ptychodiscus. The algal toxins can be accumulated by filter-feeding shellfish such as clams and oysters. If these are eaten while they contain red-tide toxins, they can poison humans or animals in the local ecosystem. Even creatures the size of large whales can die from eating fish containing large concentrations of dinoflagellate toxins.

In addition, freshwater algae can cause problems when they are overly abundant. Algal blooms can cause foul tastes in the water stored in reservoirs, which may be required by nearby towns or cities as drinking water. This can be a significant problem in naturally productive, shallow-water lakes of the prairies in North America.

Eutrophication is another major problem that is associated with algal blooms in lakes that are receiving large inputs of nutrients through sewage disposal or the runoff of agricultural fertilizers. Eutrophication can result in severe degradation of the aquatic ecosystem when large quantities of algal biomass sink to deeper waters and consume most of the oxygen during their decomposition. The anoxic (deficiency in oxygen) conditions that develop are lethal to the animals that live in the sediment and deep waters, including most species of fish. Because the primary limiting nutrient in fresh waters is usually phosphate, inputs of this nutrient can be specifically controlled by sewage treatment, and by the banning of detergents containing phosphorus. This has been done in many areas in North America, and eutrophication is now less an environmental problem than it used to be.