I saw them on the Potomac and York Rivers. Red tides are a common summertime occurrence in Chesapeake Bay. They are caused by microscopic alga-like organisms called dinoflagellates. These nutrients act just like the lawn and plant fertilezers most people use at home on their lawns and in their gardens. Some of these are harmless, others produce toxins that injure or kill invertebrates and fishes, and yet others have toxins that are not injurious to marine life but when focused through thte food chain can cause illness and death in humans.
Red tides are a natural phenomenon.
They can, however, cause considerable damage to estuarine resources. When the nutrients that cause the bloom are depleted, the dinoflagellates die. Bacteria decompose the dead algae and use up much or all of the oxygen in the water. The resulting hypoxia, or anoxia, will kill the shellfish and fish that are unable to excape the oxygen-depleted waters. Measues to control red tides are aimed at reducing nitrogen and phosghate loads entering the Chesapeake Bay.