Marine Biologist Don Anderson is a senior scientist at the Woods Hole Oceanographic Institution and Director of the U.S. National Office for Marine Biotoxins and Harmful Algal Blooms. His research has concentrated on the physiological ecology of toxic marine and freshwater algae, especially the dinoflagellate Alexandrium. Here is his answer.
Image: Woods Hole Oceanographic Institution
Red tide is a term used to describe many different phenomena associated with the growth and accumulation of single-celled algae in the ocean (and occasionally in fresh water). These growths are marked either by the intense discoloration of the water from the pigments in the algae or by the harm that the "blooms" of these tiny plants can cause. Scientists now prefer the term "harmful algal bloom" or HAB over "red tide" because the water is not always discolored when blooms cause damage, and conversely, the situation is often harmless even when the water is quite red.
This question correctly notes that there has been an increase in the occurrence of toxic and harmful algae blooms over the last several decades. The evidence comes from increasingly frequent scenes of dead and rotting fish on beaches; shellfish harvesting quarantines; dead whales, manatees and other marine animals; and a number of other highly visible outcomes.
The latest species to join this list of organisms causing HABs is Pfiesteria piscicida, unknown to science seven years ago. Since its discovery in a North Carolina estuary, Pfiesteria and closely related organisms have been reported in Florida, Virginia and Maryland estuaries, and they no doubt occur in many other adjacent states.
Pfiesteria causes harm to humans and to a variety of marine animals, especially fish. Strictly speaking, Pfiesteria is not an alga but belongs to a group of single-celled organisms called dinoflagellates. It has been included under the umbrella term HAB because it shares many ecological, toxicological and genetic characteristics with other HAB species.
Image: North Carolina State University
Scientists are in agreement that there are more harmful algal species, more toxins, more fisheries resources impacted, more areas affected and higher economic losses than ever before. The general public and politicians are quick to blame pollution for these increases, as the HAB species are plants that thrive on the nutrients contained in sewage and other pollutants.
In reality, however, only some of the HAB outbreaks can be linked to pollution. The best linkage is probably with Pfiesteria, which is most frequently encountered in polluted waters. In North Carolina, these are areas downstream from hog farming operations, and in Maryland, downstream from large chicken farms. It should be stressed that the absolute linkage between Pfiesteria and pollution has yet to be established, but the evidence seems quite strong.
It remains unknown why pollution may stimulate blooms of Pfiesteria more than others, but the answer may relate to the fact that this species can take up certain forms of organic nutrients, such as dissolved amino acids, that are found in pollution. Also, Pfiesteria can consume algae that have taken up the simpler, inorganic nutrients it is unable to utilize directly, such as nitrate. In this way, Pfiesteria is both directly and indirectly able to utilize pollutant nutrients.