1,2,3 Red Light

Red tide on Salish Sea - photo by Joseph Blackburn

For anyone looking out over the ocean this past month and wondering if there had been some kind of a mass slaughter taking place they wouldn't have been alone in their thinking.  A massive red tide that quite literally turns large patches of the ocean into the colour red seems to have been lingering all over Georgia Strait & Howe Sound, otherwise known as the Salish Sea, for most of July and it can probably be connected to the nice warm weather we've also been experiencing.  But rather than the ocean hemorraging blood it's actually an algae bloom caused by a species of marine plankton known as dinoflagellates.

These blooms happen when the reddish brown algae starts growing and accumulating too quickly in the water, and their sheer numbers produce a discolouration of the water. These algae blooms occur around the world for different reasons including; chemical run-off from agricultural activities, iron rich dust blowing off coastal deserts, and seasonal up-wellings of ocean currents which is what happens in our waters every summer, and in each case it means there are lots of nutrients for the algae to feast on. While on the one hand the presence of so many organisms can deplete the oxygen levels to the extent other creatures have to leave the area or die, these marine plankton are also an important source of food in the marine ecosystem. 

There are many different kinds of dinoflagellets but in our part of the world they are mainly either blue or red. The blue ones are harmless and produce the bio-luminescence you can see in the summer nights whenever the water is stirred up. (see link to previous blog post for more information http://www.swimswithseals.com/2013/10/strangers-in-night.html)

The red ones on the other hand produce a paralytic neuro-toxin known as saxitoxin that is inadvertently ingested by shellfish such as mussels, clams, oysters, and scallops.  Like all bivalve molluscs, they are filter feeders and, as they filter the ocean water for plankton and algae to eat, (in the case of oysters up to 5 litres per hour) the toxin is absorbed in the digestive process and remains in their bodies for several weeks. 

Typical bi-valve

While the saxitoxin doesn't harm the shellfish, it can cause paralytic shellfish poisoning in humans if they eat shellfish that have this toxin still in their system. The toxin is water-insoluble, heat and acid-stable, and ordinary cooking methods do not eliminate it.  Some shellfish can store this toxin for several weeks after a harmful algal bloom passes, but others, such as butter clams, have been known to store the toxin for up to two years. 

Paralytic shellfish poisoning is one of four types of shellfish poisoning and is the most deadly. In mild cases of PSP there may be tingling or numbness around lips (spreading to face and neck), headache, nausea, vomiting and diarrhoea . In more extreme cases there could be muscular paralysis and respiratory difficulty, potentially resulting in death due to respiratory paralysis two to twenty four hours after ingestion. When you see red tide in the ocean think 1, 2, 3 red light before eating any shellfish.

Red tide in English Bay - photo by Junie Quiroga