Marine Biological Laboratory Logan Science Journalism fellow Jane Qiu lowers a LiCor light sensor over the side of a Zodiac to measure how deeply light penetrates the water near Palmer Station. Photo by Jennifer Bogo.
By Chris Neill
Journalist Jane Qiu lowers a light sensor into the water at Station “B” just off shore Palmer Station.
The device, which looks like a light bulb on an a J-shaped metal piece connected to a rope and a black electronic cable, measures the amount of light—specifically the number of photons hitting a detector that generates a small electric current, which we read from a hand-held box at the surface.
As the sensor slides down into the water, the readings decrease. And not just slightly. A meter down light is barely ten percent of what it was at the ocean surface.
This is one of the physical properties of water. It absorbs a lot of light. Light below the water surface—even in the clear waters at Palmer—is relatively dim.
The Southern Ocean’s tiny plants, the phytoplankton, need light, which they use carry out the photosynthesis that converts dissolved carbon into the sugars that drive the growth of phytoplankton cells.
When the phytoplankton are in the upper, light layers they grow. If they sink down to where light is low, they grow more slowly or actually lose mass because they respire carbon away faster than they can fix it by photosynthesis.
Phytoplankton don’t move up and down in the water much on their own. Because they rely on mixing of the ocean waters to push them into the light, the depth to which the ocean remains mixed controls phytoplankton growth in a major way.
When the ocean mixes only shallowly, phytoplankton spend more time in the light. When the ocean mixes deeply, phytoplankton spend more of their time deeper down where the grow much more slowly.
So how deep the ocean mixes near Palmer Station therefore controls how much plankton the ocean produces.
And because the Southern Ocean foodweb is a few short steps from phytoplankton to Antarctic krill to Adélie Pengins, changes to the physical mixing of the ocean ricochet quickly through all forms of life here.
Here’s where the disappearing ice enters into the story. Mixing is driven mostly by wind. The more wind the deeper the mixing—and the less phytoplankton the ocean produces.
Because ice protects the surface of the ocean from wind, longer ice-covered periods lead to a shallower ocean mixed layer. Less ice means more mixing.
Less ice also means less fresh water melts into the ocean surfaces. This fresher and less dense water tends to stay at the surface and also reduces mixing. So disappearing ice a double whammy—there’s more wind that hits and stirs the ocean surface and there’s less fresh water to create a surface layer.
Scientists at the Palmer Long-Term Ecological Research Project have been documenting these critical responses to disappearing sea ice around Palmer Station. There are now 80 fewer sea ice-covered days per year near Palmer compared with 1950.
Maria Vernet, a member of the Palmer LTER science team from Scripps Institution of Oceanography, showed that earlier ice retreat leads to a deeper mixed layer and lower plankton production. Martin Montes-Hugo, another Palmer LTER scientist from Rutgers University, used satellite images to show that in the northern part of the Antarctic Peninsula, where ice is disappearing, phytoplankton production is decreasing. Farther south, where ice is also declining but where much ice still remains, phytoplankton production is increasing because less ice cover means more light reaches the water.
Montes-Hugo showed there is a mixture of ice and open water that leads to maximum phytoplankton production. Climate changes have already pushed waters near Palmer past that point.