North Carolina Sea Grant
Coastwatch Currents

Coastwatch Currents

April 20, 2015 | Sara Mirabilio

By SARA MIRABILIO

Posted April 20, 2015

Pollen levels are peaking in North Carolina. The yellow coating of pollen is on everything — houses, cars, roads and even sinus cavities. Though unwelcome, at least it is a harbinger of spring’s arrival across North Carolina.

Spring, to me on the northern Outer Banks, means an end to cabin fever, and a start to good fishing and countless other activities warmer weather brings.

Thus, my mind turns to fishing when April showers curtail outdoor activity. On those days, the rain gives rise to runoff ditches flowing with bright yellow water from the pollen washing away.

Pollen in puddles

Pollen pools on a driveway on a rainy spring day. Photo by Sara Mirabilio

Strangely, the runoff also makes me think of fishing. Charter boat captains share accounts of “marine pollen” or “sulphur” when describing an occurrence of yellow coastal waters.

Like Dorothy following the Yellow Brick Road to the Emerald City, many fishing captains follow this yellow, watery path in search of sport fish — billfish, tuna and other coastal pelagic fish. Yet, the cause of these yellow-hued coastal waters is neither pollen nor sulphur, as it is commonly called. It’s sawdust, but not the kind swept away from under your workbench.

After an offshore fishing trip last August in which I gazed on the yellow swath of water that went on for as far as my eye could see, I became intrigued by what really caused this phenomenon. A NOAA scientist who studies algal blooms said that I could be describing a bloom of Trichodesmium, called commonly sea sawdust.

This marine cyanobacterium — also known as blue-green marine alga — is found worldwide in tropical and subtropical waters, and blooms seasonally off the North Carolina coast. Researchers have not documented any negative effects of Trichodesmium on marine life or people.

Yellow bloom of cyanobacterium in Pacific Ocean

A bloom of Trichodesmium erythraeum in the southwest Pacific Ocean, as viewed from the deck of the research vessel Alis. Photo courtesy NASA.

Bryan Fluech, my colleague at Florida Sea Grant, has written about these algal blooms. He explains that between May and September, which is the peak time of dust-storm activity in the African Sahara Desert, winds transport iron-rich dust across the Atlantic Ocean to the U.S.

The Trichodesmium growth rate is limited by iron, so a large pulse of iron can lead to an increased growth rate and a production of large colonies visible to the naked eye, called blooms. Blooms generally occur offshore in nutrient-poor waters, but currents and winds occasionally can push them near shore.

Reports of Trichodesmium blooms date back to the 1700s, when Captain James Cook of the British Royal Navy wrote about brown scum that made him think he was on a shoal. The algae colonies can be quite large — up to 0.4 inches — and look like sawdust floating on the water surface.

Trichodesmium blooms can take on a range of colors, depending on the life stage of the bloom. Healthy blooms typically are brown in color. But once the algae begin to decay, the bloom takes on a yellow-green appearance as pigments leach out and expose the algae’s green pigment, called chlorophyll.

In general, Trichodesmium is not a good food source for marine organisms. Only a few animals actively feed on it. So, following the water path may not lead to the “honey hole” of fish as thought.

Do you have another theory as to what causes this yellow water? Feel free to email me with your idea at saram@csi.northcarolina.edu.

Happy Fishing! Now, please pass the tissues.

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