Re: What causes red tide to affect coastlines everywhere?

Hi Dale-

Red tides are caused by a group of organisms known as dinoflagellates, which are a type of phytoplankton. Red tides occur whenever dinoflagellate abundance in the water is so high that they can change the colour of the water- scientists call this a phenomenon a "bloom" of dinoflagellates. The reason that the colour of the water is changed is simply because many dinoflagellate species contain lots of red pigment- other types of phytoplankton can cause brown, green, or even purple tides! For a bit more on phytoplankton, go to a previous answer posted here on MADSCI: Re: What living things does an ocean food chain begin with?.

There are lots of reasons why red tides occur, both natural and man-made; both basically require the addition of nutrients. Natural red tides can occur when nutrient levels are increased by the movement of deep oceanic water (which is rich in nutrients) to the surface, followed by a warming of the water, which traps it in the surface. Less natural, of course, is the addition of nutrients by man in the form of pollution (for more on pollution, here's another MADSCI post: Re: Ocean Pollution ). So, when those dinoflagellates are trapped together in surface waters with all those nutrients, you can't blame them for starting to grow like crazy! Of course, the large number of people who live along the ocean, and the resulting huge amounts of nutrients we're pumping in the ocean in the form of pollution have made red tides much more frequent in recent years.

Rob Campbell, MAD Scientist