Photo courtesy of Michael Sullivan/Skidaway Institute of Oceanography
Skidaway Institute’s Catherine Edwards examines the glider Ramses, which her team is preparing to deploy this winter to collect data in Long Bay.
Catherine Edwards and Jim Nelson of Skidaway Institute of Oceanography are spending this winter chasing a mystery off the coast of the Carolinas from Cape Romain to Cape Fear. It’s here that nutrients like phosphorous and nitrogen well up each year from the deeper water and set off a bloom of microscopic sea plants called phytoplankton. Those plants in turn feed the fish on your plate, such as gag and scamp grouper. “I really think this process is priming the fishery,” Edwards said. “This part of South Carolina is a good fishery, and we know it has high concentrations of spawning sites. It’s a huge dose of nutrients. For the four months when the baby fish come, it’s bananas.”
Just how this swath of ocean gets a mega-dose of fertilizer every winter isn’t clear. “It’s not uncommon for cold, nutrient-rich water to upwell,” Edwards said. “But there’s something specific to Long Bay that made it that way. We want to find out the physics of that. Why Long Bay? Why winter?” Edwards, a physicist, said the study intends to find out with its multidisciplinary approach that includes robotics, biology, engineering and physics. “It ties into the big picture stuff everybody cares about,” she said. “I care about it. I like grouper.”
The scientists are seeking their answers with a variety of tools, the most intriguing of which are two underwater gliding robots named Pelagia and Ramses, the latter after the mascot of the University of North Carolina where collaborator Harvey Seim works.
They will gather data about the foundation of this enormous food web just at the edge of the continental shelf. The torpedo-shaped vehicles are equipped with sensors and recorders for measures such as temperature, salinity, oxygen and dissolved organic matter.
The gliders lack a propeller and are instead driven by changes in buoyancy. To dive, the glider pulls in a half cup of sea water and moves its internal battery forward. To rise again, it spits out the water and slides its battery backward. As it dives and comes back up at an angle, it makes forward movement. At the surface, it phones home from its satellite phone. “It says ‘Hi, my name is Ramses. Here is my position, and here’s my data,’” Edwards said.
The gliders will be controlled from shore with a system co-developed by Fumin Zhang at Georgia Tech Savannah. Two graduate students there, Klimka Szwaykowska and Dongsik Chang, are developing instructions for keeping the gliders on track with weather and tides data. “It’s never been done in these currents over these time periods,” Szwaykowska said.
Attack of the remora
Despite the high-tech aspects of the projects, there are innumerable low-tech problems to contend with. Example A: Fish.
In field tests this fall Ramses surfaced regularly for several days then stopped communicating for 16 hours. To the relief of its nervous owners, the $180,000 glider eventually ejected its weight, an emergency option that allows it to surface when the usual water-spewing method doesn’t work. The scientist aren’t certain what weighed it down, but they got one big clue when they retrieved the glider. “We saw a remora swimming near it,” Edwards said.
Remoras are fish that typically attach to sharks and feed off the predator’s leftovers. Now the Ramses team is working on ways to make its surface less attractive to these hitchhikers. Edwards rolls her eyes a bit when the remora problem come up. “Everybody wants to hear about the remora,” she said, a little wistfully. But there’s so much more science on the project that fascinates her.
“Two of the guiding questions are why this feature is so persistent over the winter, and what are the dynamics that sustain this bloom,” said Edwards. The mystery of it appeals, as does the combination of many scientific disciplines.
“Is it changing or is it one continuous bloom?” she asked. “How do the wind and tides and Gulf Stream interact to get this upwelling? “Something really big is going on,” she said.
Armed with a better understanding of the physical processes that “fertilize” the outer shelf and how phytoplankton take advantage of that natural Miracle Gro, the research team will be able to answer larger questions about how biology and physics interact in Long Bay.
Funded by a $1.6 million grant from the National Science Foundation, the project will run for three years.