During the growth period, or bloom , each single algae cell may replicate itself one million times in two to three weeks . During the reproductive riot of the bloom , warm, shallow seawater tends to become discoloured by the sheer concentration of algae seeking the sunlight. Since red is the most common pigment, the phenomenon has come to be called Red Tide . Some of the toxins these species produce are seriously toxic. Often, the algae themselves are unaffected, as are the filter feeders, especially shellfish , for whom micro-algae are the principal diet.
Algal blooms can also cause other problems besides toxicity.
When an algae bloom dies back in a shallow bay, oxygen-depleting bacteria may thrive on the dead algae and so kill fish and other marine life. Sometimes floating algae cut eelgrass beds off from sunlight, robbing shellfish of habitat. On fish farms, concentrations of algae can simply clog the gills of the fish, causing respiratory failure. Almost all incidents involve the accidental ingestion of mussels, clams, oysters or other shellfish affected by toxic algae. Since there is no visual method for determining whether particular shellfish are affected, Canada has been carrying out extensive laboratory monitoring of shellfish toxicity since 1943.