Red tides are giant bloomss of dinoflagellates, so called because these creatures are often present in sufficient numbers (thousands or millions of cells per liter) to turn the water red or brown. Coastal pollution produced by humans appears to be a causal factor in the red tides in some areas of the world, but red tides also occur in some places where it is not associated with human activities. Some red tides produce large quantities of toxins, which kill many fish and are accumulated by filter feeders, like shellfish. This accumulation of toxins is why one must be careful of eating shellfish at certain times of the year.
