Algal Blooms
A red and green algal bloom off the coast of Hawaii.
Single-celled algae are almost always present in sea water even if the water looks clear. When high concentrations of certain species of algae called dinoflagellates are present, patches of water look red because these algae contain red pigments – hence the nickname for this phenomena, a red tide.
High concentrations of algae can be various colors depending on the species and the type of pigments they contain. Blooms occurring offshore can sometimes float toward the shore, as shown here off the southwestern coast of Molokai, Hawaii. Some blooms have the potential to be harmful to ocean life and humans in the littoral zone. Algal blooms may turn sea water orange, yellow, brown, or purple. Red tides have been witnessed for centuries and have been seen all over the world.
These populations of algae are referred to as “blooms”, because the algae have multiplied rapidly to become concentrated in high numbers. In a bloom, there could be tens of millions of algal cells in a litre of sea water. Most blooms are not harmful, but some have the potential to be harmful, whether by virtue of natural biotoxins (poisons) produced by certain species of algae, or by their tendency to deplete the oxygen and cause subsequent death and decay of large numbers of sea creatures.
Harmful algal blooms cause millions of dollars in damage when they kill massive amounts of fish, foul beaches making them off-limits to swimmers, close fisheries and shellfisheries, and cause medical emergencies for people poisoned by biotoxins in the seafood they have eaten. Many scientists believe that harmful algal blooms are becoming more common, but they also point out that increased monitoring efforts (using helicopters, planes, and satellites) are detecting more occurrences of the blooms before they make people sick or contaminate areas.
How do certain microscopic algae (phytoplankton) cause harm to fish, shellfish, marine mammals, seabirds, and people? Basically, there are four ways.
First, the physical presence of so many cells may suffocate fish by clogging or irritating the gills. Second, when the densely concentrated algal cells die off, the decay process, assisted by bacteria, can deplete the water of oxygen, which in turn can lead to the death of oxygen-dependent marine creatures. Algae, being plants, require nutrients such as nitrogen and phosphorus to grow. When they have used up the nutrients, they tend to die off all at once. Such oxygen-related impacts are most visible in shallow bays, inlets, or seas.
Third, some algal species produce deadly toxins which directly kill the animals that ingest the poisons. Dinoflagellate toxins have killed mussels, clams, and fish. Toxins have caused respiratory problems and eye and skin irritation to humans along beaches where harmful algal blooms were present.
Fourth, shellfish such as mussels, clams, and oysters feed by filtering particles, including phytoplankton, from sea water. Toxins from certain dinoflagellate or diatom species accumulate in the tissues of shellfish. When people, sea mammals, or seabirds eat the shellfish, they ingest the toxins as well, becoming sick.
There are different kinds of toxins that cause different kinds of symptoms which, in humans, can be serious. In fact, some algal toxins are deadly. Some of the effects of these toxins include nausea, vomiting, abdominal cramping, diarrhea, muscle pain, paralysis, nerve damage, coma, and even death if consumed in large quantities.
Questions – Algal Blooms
1) What is an algal bloom?
2) What species of algae most commonly causes algal blooms?
3) Why does the water look red during many blooms?
4) How many algae are present per litre of seawater during a bloom?
5) What two factors can cause an algal bloom to be harmful?
6) List and explain the four ways algal blooms can cause millions of dollars
worth of damage.
7) Although algal blooms are becoming more common, another factor is helping
to limit their damage. What is it?
8) List and explain the four ways algal blooms cause harm to other organisms.
9) What symptoms can biotoxin poisoning cause in humans?
10) What is your opinion of the algal bloom issue? Write a brief paragraph
outlining your thoughts.
· Do you trust shellfish monitoring programs and inspection to keep our food supply safe?
· Do you think the government should increase helicopter flights over sea areas prone to red tides?
· Have you or anyone you know been made sick by consuming seafood?
· Do you think the risk is so small it’s not an important food safety issue?