Harmful Algal Blooms
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Dr. Sarah Benedict was stumped. Alex Ballon, an otherwise healthy and energetic 24-year-old, said that about an hour ago he dropped a cold soda can because it felt as though it were burning hot. Tingles radiated down his arms and legs, he said, but parts of his face felt numb. He'd been vomiting for hours with knifing pains in his abdomen. Dr. Benedict closed the emergency room curtain and reviewed the case. She had ruled out all of the basics. No allergies. No exposure to chemicals at work. No changes in daily routine. Alex said he had just returned from a fishing trip in the Florida Keys with some of his buddies. That was all. On their last day he even caught an enormous grouper that had been lurking around one of the reefs the whole week they were there. Of course he ate part of it with his buddies, he said. He couldn't resist sampling the sweet victory of the catch. Aha! Dr. Benedict always did love fitting together the puzzle pieces of challenging cases. Eating the grouper was the part of Alex's story that completed the puzzle. Alex was probably suffering from ciguatera (pronounced sig-wa-TEH-ra) fish poisoning, a commonly misdiagnosed ailment caused by tiny toxic plants in tropical waters such as those that straddle the Keys. Alex's "prized" grouper had probably
ingested a type of toxic algae that made him sick. This scenario faces many doctors, especially those in coastal areas of the United States or other countries where seafood is a local favorite. The culprits are toxic single-celled floating plants called phytoplankton that are so small that up to 400 of them can fit on the width of a human hair! You need a microscope to watch these guys in action, but the effects that they can have on the marine environment, and even humans (as Alex learned the hard way), are anything but difficult to see. The toxic phytoplankton make up just a small fraction of the total phytoplankton population. In fact, there are thousands of different species of microscopic algae in the world's oceans, but less than 100 produce toxins. The whole group of phytoplankton makes up the base of the food chain in the oceans so they're like the "grass of the sea" and a lot of other marine animals depend on them for food. In a process called photosynthesis, they use light to produce carbohydrate and oxygen from carbon dioxide and water. Without phytoplankton, we wouldn't have air to breathe! Now let's talk about the toxic variety that are known more for their adverse effects rather than their role in the functioning of the ecosystem at large. The toxic phytoplankton, along with some of the nontoxic species, often grow in such large numbers that they form blooms, or population explosions of these tiny cells. Some blooms can discolor the seawater, and some are bioluminescent, producing a mystifying light that is visible in the water at night. Blooms can last from days to months - and sometimes longer than a year. They occur in waters all over the globe, and have been called red tides, brown tides or yellow tides. Because these descriptors tend to vary, it's more accurate to call these natural phenomena harmful algal blooms (HABs). Many blooms recur in some areas more than others. An example is the organism responsible for red tides in Florida called Gymnodinium breve (or G. breve for short; pronounced jim-no-DIN-ee-um brev-ee). This tiny plant is especially notorious along the Florida Gulf coast, often showing up in the late summer or early fall months. G. breve is a special type of phytoplankton called a dinoflagellate (dine-o-FLA-jell-et), which means it has two whiplike tails called flagellae to help it move and spiral up and down in the water column. During a bloom, the cells divide within a few hours to a few days, and the population doubles every time. An initial G. breve population of 700 cells would grow to about 10,000 within one week, which is enough cells to cover a one-inch line. Many blooms have millions of cells per liter of seawater, which would be enough to discolor the water and make it look "patchy." A combination of the organisms' life cycle (what a biological oceanographer studies) and the currents, winds and tides (what a physical oceanographer studies) causes harmful algal blooms to form. In other words, if the environmental conditions are right, and the cell is at the right stage of the life cycle, you can get a bloom. For example, HABs in Spain and California might be linked to a physical ocean process called upwelling (when deep ocean water comes up to the surface near the coast of a continent). In the Gulf of Maine, blooms occur where water from coastal currents and a host of rivers come together. The phytoplankton cells like these convergence zones, or fronts, because that's where nutrients tend to accoumulate. And how do blooms dissipate? Sometimes the wind and waves can push the fragile cells onshore to their deaths. Bacteria and viruses, the microscopic "hit-men," even smaller than the tiny algae can also attack a cell, exploding it from the inside out. HABs can harm marine organisms in many ways. The effects vary from species to species, and organism to organism. For example, some fish can die from ingesting certain tiny toxic plants; other HAB-forming species are nontoxic to fish but kill other marine animals. Some types of non-toxic algae called diatoms have tiny spines made of silica on the outside of the cell that look pretty neat under a laboratory microscope, but which sreak havoc on a fish's gills in the water and make it difficult for the fish to breathe. HABs also affect marine mammals such as manatees and dolphins. Manatees, which breathe air at the sea surface, can suffer from lung problems and even die from inhaling airborne toxins released by HABs. Dolphins might accumulate the toxin in their organs after eating fish contaminated by the toxic algae in a process called bioaccumulation. And finally, when the algal cells in a bloom begin to die and decompose, they can rob the water of its life-sustaining dissolved oxygen (a condition called anoxia), which causes a variety of sea life to basically suffocate. As Alex Ballon learned the hard way, humans can get sick from these HABs too. HABs can cause five types of diseases in humans: Paralytic Shellfish Poisoning (PSP); Diarrhetic Shellfish Poisoning (DSP); Amnesic Shellfish Poisoning (ASP); Neurotoxic Shellfish Poisoning (NSP); and Ciguatera Fish Poisoning (CFP), the one with which Alex became familiar. Common symptoms of all five of these sicknesses include numbness, dizziness and nausea. What often happens is that people eat large fish that have bioaccumulated the toxins. The grouper Alex ate grew big and fat from years of "toxic snacks" of smaller reef fish. Toxic algae can also be stripped from the water by filter-feeding animals like clams or oysters, and people who eat these shellfish (cooked or raw) can become ill. You cannot detect many of these toxins in fish and shellfish and cooking the shellfish doesn't neutralize the toxins, so it's tough to avoid getting sick. Sometimes the HAB toxins are released into the air when waves break up the cells near the beach, which may cause beachgoers to experience asthma-like symptoms such as coughing and eye irritation. HABs have been documented back to the days of the earliest Indian settlers who noticed that every once in a while patches of water were discolored - it looked like a splotchy sea. However, the number of known HAB species has increased dramatically in the United States in the past three decades. The reasons remain unknown, but possibilities include the following:
The bottom line is that a lot of
questions remain unanswered. But don't hate the feisty HAB
species just because they seem to be responsible for so many nasty
things. Like every organism, they serve a functional purpose
in nature. Even if human influences such as pollution might
make them worse, HABs are natural events. You might think
of them as the forest fires of the sea; they temporarily shake
things up in the ocean environment, but a certain overall "balance"
in the ecosystem is restored down the road. Marine Mammals In the last two decades there have been several unusual marine mammal mortality events that were most likely caused by different types of toxic phytoplankton:
The year was 1903 and Professor Ostenfeld was baffled. Dense plankton blooms of the diatom Odontella were suddenly appearing in the North Sea. This species was well known from the waters of the Indo-Pacific, but had never been reported in European waters before. It was not possible for currents to carry it to this region, and since it was such a large diatom (another type of phytoplankton) it probably had not been previously overlooked. So where did it come from? Ostenfeld concluded that this species had been introduced via the water or sediment in ships' ballast tanks. Almost 90 years later, Gustav Hallegraeff confirmed this theory by culturing a related species found in ballast water brought from Japan to Australia. When a ship is carrying cargo, the large empty spaces (ballast tanks) need to be filled with water to stabilize the ship while out at sea. When the ship reaches its destination and the cargo is removed, the ballast water is emptied and equilibrium is restored. Resting spores (one stage in the phytoplankton life cycle) are well suited to survive the long dark voyage in ballast tanks, while planktonic stages do not fare as well. One ballast tank can contain more than 300 million toxic spores! One notable and recent example of contamination by ballast water transport occurred in the late 1980s when a non-indigenous toxic dinoflagellate species was introduced into Tasmanian waters. Unfortunately, it contaminated sensitive aquaculture areas and many commercial shellfish farms suffered financially. Australia was again affected when the first outbreaks of paralytic shellfish poisoning appeared in this area. The responsible dinoflagellate had most likely been introduced in the last ten to twenty years from ballast water from Korea and Japan. This organism is now well established in southern Tasmania, and has been the cause of many subsequent shellfish bed closures. As a result, the International Maritime Organization has introduced guidelines for ballast water-handling procedures. Practices such as reballasting at sea, ballasting in deep water, disposal of ballast tank sediments away from sensitive marine areas, and possibly treating the ballast water with heat or chemicals are suggested to reduce the risk of harmful introductions. |