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On the surface

Story by Claire Johnson

Photos by John McCord, assistant director for engagement and outreach at the Coastal Studies Institute

Claire Johnson is a doctoral student in the College of Arts and Sciences’ earth, marine and environmental sciences department and a graduate research assistant at the Coastal Studies Institute. In this first-person account, Johnson describes a boat trip to collect sargassum seaweed to study in the lab. The work helps scientists better understand the nitrogen cycle, which is vital to the air we breathe and the food we eat.


My goal is to investigate the biological cycling of nutrients — specifically nitrogen — by the microbial community living on the surface of sargassum and to understand how specific environmental variables alter this process. To study this, we wait for the Gulf Stream to present perfect conditions: high visibility, low wind and calm seas. And then we adventure out into the open ocean to catch the sargassum that floats along its currents.

I set my alarm for 4:15 a.m. so that I can get to the lab in time to fill the sample coolers with ice and strap down the dozens of buckets in the truck bed so that we can get to the dock in Hatteras. By 7 a.m., we’re aboard the Albatross III, embarking on an hours-long journey through the choppy waters of Hatteras Inlet. Depending on the position of the Gulf Stream, it can take us anywhere from one to two hours to reach its western edge and, if we’re lucky, we’ll begin to see sargassum.

View of seaweed in the ocean, both above and below water. A blue sky with large, wide clouds is seen on the horizon.

To understand how sargassum and the community it hosts influence its immediate environment, we compare sites within the Gulf Stream with and without the algae. Much of our cruise time is spent collecting hundreds of liters of water in 50-pound buckets for filtration in the lab later in the day.

Then there’s the sargassum. If you ever encountered its green tangles out on the open ocean before, you’ll be absolutely amazed by all the creatures you find living on and amongst it — some you can see and many more that you can’t. Because we are specifically interested in microbially mediated processes, we carefully sort through each frond we collect.

A woman on the edge of a boat in the ocean holding a net full of seaweed.

Sometimes sargassum mats are overrun with beautifully colored blue, purple, green, orange and pink brine shrimp. Almost certainly, we’ll pick out some sargassum crabs, which can be as big as your hand, and juvenile triggerfish. If we’re lucky, we’ll find a sargassum fish or two. We keep only the fronds and return all our little friends back to the safety of their floating home.

By late afternoon, we complete our sample collection and experimentation and head back to the docks. A long night of sample processing lies ahead — hours of pouring, measuring, filtering, cleaning and weighing. If we don’t process these samples quickly enough, the organisms will die and our samples will be ruined. If we’re lucky, we’ll finish around midnight.

Two women on a boat in the ocean and a man in the background. One woman is placing seaweed into a container half-filled with water that is being held by another woman.

After almost 10 years and 40 cruises, we have amassed an incredible amount of data that suggests that sargassum plays a much more significant role in marine nitrogen cycling than previously thought. Understanding sources and quantities of marine nitrogen inputs is incredibly important as the nitrogen cycle is intricately linked with those of carbon and phosphorus — significant changes in one will likely affect the others.

These cycles are vital to the air we breathe and the foods we eat. Learning how they work and how changes in climate and human activity affect them over time is key for our survival.

Fish swimming in the ocean by seaweed.

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