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.