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Diving Deeper Shorts: Harmful Algal Blooms

Episode 17 (Mar. 1, 2012)

HOST: Today on Diving Deeper Shorts, we revisit our interview on harmful algal blooms from October 2009 with Allison Sill from NOAA's National Centers for Coastal Ocean Science.

Let's listen in.

HOST: Allison, first, what is the difference between a harmful algal bloom and a red tide?

ALLISON SILL: Well, red tides are not always going to be harmful and it's a general term used to describe blooms. Many tides aren't red, many of them are actually golden brown, yellow, you can actually have green tides as well. Scientists use the term harmful algal bloom to describe an algal bloom that can have a negative impact on the environment.

HOST: What causes harmful algal blooms?

ALLISON SILL: There are a variety of factors that can cause these harmful algal blooms some of which are natural and some of which are not natural. The cause also is dependent on the species type and what that species needs in order to actually bloom. When conditions such as salinity - which is the amount of salt in the water, temperature, nutrients - when these factors are optimal, the bloom can occur. And some species are actually good indicators of coastal eutrophication, which is known as nutrient pollution.

HOST: Allison, what are some of the impacts that we see from harmful algal blooms?

ALLISON SILL: Kate, as mentioned before an impact that is very obvious to an observer is actually the discoloration of the water. It might appear red, green, brown. There's also a great number of impacts on the marine environment. You can have low dissolved oxygen or anoxic conditions which can lead to the death of fish and you might see fish floating on the surface of the water. We also have what are called unusual mortality events and this is when there are unexpected strandings of marine mammals on the beach, typically it can happen in high numbers. This type of event requires a response from certain agencies. And the research is indicating that 26 percent of all unusual mortality events are actually related to marine toxins. There are also human health syndromes, five in particular that are caused by certain species.

That's all for today's Diving Deeper Shorts, where we highlight a few minutes of your favorite Diving Deeper episodes.

Want to learn more? Go to and select the October 2009 podcast archive to listen to the full interview on harmful algal blooms.

You can catch our next episode in two weeks.