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.
Many of the other red tide or HAB problems do not have this linkage to pollution, so other mechanisms must be invoked to explain the expansion in bloom incidence. In some cases, the explanations are entirely natural - such as when hurricanes or major storms carry blooms into new areas where species can colonize and recur year after year. Some HAB species have dormant "cyst" stages that allow them to survive during winter and then germinate to start new blooms at that location. One example of a natural expansion is the New England red tide, which currently causes shellfish toxicity from Maine to Connecticut. The geographic area impacted by these blooms of the dinoflagellate Alexandrium expanded dramatically in 1972 as a result of Hurricane Carrie.
In other cases, human activities are involved in the HAB expansion, such as the inadvertent transport of algal species--or their cysts--from one region to another in ballast water. Another factor in the increases is aquaculture development. Many countries are farming large areas of their coastal waters, often with rafts or cages that are used to grow fish, seaweed and many different kinds of shellfish. Young stock transported to these locations can introduce the HAB organisms.
Image: North Carolina State University
Sometimes these fish farming operations simply reveal the presence of toxic algae that were always present, but never noticed because there was not a fisheries resource there to poison. In other cases, aquaculture facilities such as shrimp and fish ponds have been built in areas formerly occupied by saltmarshes and coastal wetlands. This change alters the nutrient flux in coastal waters, especially given the heavy food supply that is often used to accelerate growth in the aquaculture facilities. Some have likened fish farms to small cities with respect to the pollution they generate, although this remains a controversial issue.
The final reason for the expansion is that scientists are much better at detecting known toxins and finding new ones than ever before. This is because analytical instruments and methods are vastly improved and because there is rapid and efficient communication throughout the world. The Pfiesteria outbreaks in Chesapeake Bay during the summer of 1997 are an example of this mechanism. Pfiesteria had been identified and was under study at North Carolina State University since the early 1990s. In summer 1997, fish with bleeding lesions were observed in the Chesapeake, and the link to Pfiesteria was quickly made.
It is now evident that the publicity from television, newspapers and even a popular book lead to the identification of Pfiesteria in the area. Without the publicity and media attention (which resulted in part because the outbreaks happened so close to the nation's capital), the fish kill probably would have been blamed on other factors. Indeed, 1987 reports from that same area document fish with lesions and swimming abnormalities now known to be indicative of Pfiesteria. At the time, though, the mortality was attributed to different causes.
It is clear that the expansion of the HAB problem is in part a matter of perception or increased awareness, and in part a matter of the actual growth of the problem. In other words, years ago we were not aware of the size or complexity of the HAB problem, but as we became better at detecting toxins and recognizing HAB phenomena, we defined the boundaries of the problem that had always been there. On top of this apparent expansion, there was genuine growth in the problem due to such factors as pollution and accidental species dispersal through human activities.
The fact that some of the increase is simply a result of better detection does not diminish the seriousness of the problem. It is big, and it is growing, and that trend is all the more worrisome in light of rapid population growth in the coastal zones of the world and our heavy dependence on nearshore waters for economics, nutrition and recreation. Some view the Pfiesteria outbreaks as a visible and dramatic warning of the dangers that arise from decades of abuse of estuarine and coastal waters. We need to rethink our policies and manage human activities in the coastal zone more carefully, unless of course we are willing to live with more episodes of sick, dead and rotting fish, and the illness and death of humans and marine mammals.