Is it Ash? Is it Oil? No! It’s Trichodesmium!

During recent coastal water quality and habitat surveys, we’ve come across slicks of what turned out to be blue-green alga, also known as cyanobacteria, in this case, an alga called Trichodesmium.  A biologist at the Division of Aquatic Resources confirmed that these slicks or “scuzzes” are known to periodically occur around Maui but the abundance this year appears unusual.

Trichodesmium scuzz by Olowalu, Maui, Hawaii. September 2023.

Trichodesmium scuzz by Olowalu, Maui, Hawaii. September 2023.

A sample collected off Kihei on September, 2023 proved to belong to the genus Trichodesmium. Trichodesmium, also called “sea sawdust” due to its appearance, plays a pivotal role in marine ecosystems. These microscopic organisms are widely distributed throughout tropical and subtropical oceans, forming distinct colonies that can be visible to the naked eye. Trichodesmium is actually ecologically important, as it is a primary producer in the marine food web and also contributes to nutrient cycling and nitrogen fixation, benefiting marine life and the planet as a whole.

A little bit of ecology

Trichodesmium is crucial to the health of marine ecosystems. These cyanobacteria are photosynthetic, using sunlight to convert carbon dioxide and nitrogen gas into organic matter – a process known as nitrogen fixation. This ability to fix atmospheric nitrogen makes Trichodesmium a critical source of bioavailable nitrogen in nutrient-poor ocean regions, such as Hawai’i and much of the tropical Pacific. As a result, it provides essential nutrients to a variety of marine organisms, including phytoplankton, zooplankton, and filter-feeding invertebrates. By supporting the base of the marine food chain, Trichodesmium indirectly sustains the health of larger marine species, all the way up to fish and marine mammals.

Trichodesmium from a 2023 scuzz in Kaanapali, Maui, Hawaii. Photo: Trey Garnes.

Trichodesmium from a 2023 scuzz in Kaanapali, Maui, Hawaii. Photo: Trey Garnes.

Blooms and Causes

Trichodesmium blooms are periodic and can cover vast stretches of ocean surface. These blooms occur due to a combination of factors, including warm water temperatures, nutrient availability, and calm sea conditions. The warm waters of tropical and subtropical regions create ideal conditions for Trichodesmium growth, while the presence of nutrients like iron and phosphorus can stimulate bloom formation. There is some speculation that ash from the recent Maui wildfires may have dropped enough nutrients into our coastal waters to trigger the blooms we are now seeing close to shore.

Calm seas and low wind speeds can further allow Trichodesmium colonies to remain at the ocean’s surface, forming visible mats that can stretch for kilometers, which is what we are currently seeing around Maui. These blooms can have both positive and negative effects on marine ecosystems, promoting productivity but also potentially leading to oxygen depletion and harmful algal blooms under certain conditions.

Health Concerns

While Trichodesmium is an essential component of marine ecosystems, it is not suitable for human consumption. Ingesting or touching Trichodesmium colonies can cause skin irritation and, in some cases, lead to respiratory problems due to the release of certain toxins and irritants. It is advised to avoid direct contact with these cyanobacteria. Moreover, the ecological consequences of large-scale Trichodesmium blooms, such as oxygen depletion and changes in water chemistry, can have negative effects on marine life and local fisheries.



Fun fact

There are several species of Trichodesmium. Some make little urchin-like balls made up of individual strands others look more like bundles of rods on close inspection.  All species have spaces (vacuoles) that hold gas and as a Trichodesmium rolls around underwater, the flash of light off these bubbles can give the water a sparkly appearance.

Report your sightings!
If you see a Trichodesmium scuzz washing up on the beach or while you are out on the water, please share your observation.  Date, time and location are important details. A photograph or video of the bloom would be appreciated.

Please share your sightings with us by email at [email protected].


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