Posting by VIMS graduate student Kate Ruck.
The LTER cruise was designed to happen every austral summer (Dec.-Jan. in the southern hemisphere) for a couple of different reasons:
- This is the most productive time in the ocean waters of our sampling grid. There are plenty of phytoplankton (single-celled plants), which means there are plenty of animals eating the phytoplankton.
- It’s the time of year when the waters around the peninsula are the most accessible by boat, because all of the sea ice that built up during the winter has now melted.
- The weather is usually at its mildest during the summer, maximizing the number of days that we’re able to work while out at sea.
But we are in Antarctica and the weather can change very quickly. Recently, I helped to deploy a net during my shift on a relatively calm, overcast day, only to retrieve it about 30 minutes later in 30-knot winds and growing swells. The bad weather persisted for the next couple of days. The winds never got bad enough to stop our work, but they were persistently strong and the ocean swells reached about 15 ft. The boat was rolling so much that it was tossing loose equipment off of our workbenches and we could see water washing over the portholes in our labs. During this time we had to retrieve a few moorings that have been sitting in the ocean since we deployed them last year, making for a couple of exciting moments on the back deck.
A mooring is basically an instrument that’s suspended in the water column with a heavy weight on one end that sits on the sea floor and some type of buoyancy on the other end that keeps the whole set-up upright. We have two types of moorings that we use in our work; a sediment trap and four temperature/depth moorings.
The sediment trap is a big funnel-shaped contraption that sits 150 meters below the surface of the ocean and collects particles that are sinking from the surface waters toward the sea floor. The particles are collected in the funnel and then deposited in large, plastic bottles that contain a preservative. The trap is set on a timer, so the bottles will rotate and we can collect samples from very specific times during the year. The bottles are set to smaller time increments during the austral summer, when the ocean is more productive and there are more particles in the water, and for longer times increments during the winter when the ocean is less productive and there is a low particle level in the water.
The summer is so productive down here because when we’re this far south on the globe there is a longer period of daytime light then there is further north. And when you get below the Antarctic Circle (which we crossed a few days ago) you can get 24 hours of full sunlight. The high levels of light fuel phytoplankton growth, which in turn supports a large, high-energy food web. When all that life eventually dies, it sinks through the water column and is either buried on the ocean floor or acts as a food source for animals that live below the surface waters of the ocean. The sediment trap is useful because we can measure how much of that productivity in the surface waters is sinking to depth.
The temperature-depth mooring is a much simpler concept. It’s constructed from a very long piece of rope (~350 meters) with sensors attached at regular intervals. The sensors measure their depth from the surface and the temperature of the surrounding water. These measurements are taken every 15 minutes and we download all of the data when we pick the mooring up the next year. This huge amount of information is useful because there is a lot of warm water coming up onto the Antarctic continental shelf, which is melting the sea ice around the peninsula. Having the sensors at different depths in the water column, in different places around the waters of the peninsula, can help us pinpoint where exactly all that warm water is coming from.
We collected three of the temperature/depth moorings while the weather was rough. To collect a mooring, our electronics technician (ET for short) sends a radio signal that disconnects the mooring from its weight, and then the buoys attached to the mooring rise to the surface where they are spotted by a group of people on the bridge. A bunch of us collect outside on the back deck while the captain repositions the boat to get near the mooring and then one of our marine technicians (MTs) throws a grappling line to catch the mooring rope and then everyone helps to pull the mooring line on board. It’s not an easy task when the weather is rough. The captain wants to get the mooring as close to the stern of the boat as possible, which means he has to back the boat into some pretty large waves. Our poor MTs were getting wave after wave crashing on their heads as they were tossing the line, trying to hook a mooring that’s bobbing up and down in the waves. They’re strapped to a short safety line that’s set up like a dog run, allowing them to move port to starboard on the deck but keeping them about half a foot from the open ocean at the ship’s edge. Once they hook the mooring and haul it on board, they pass back the ropes and sensors to the science crew and the whole set up is hand-carried inside to be disassembled. It was a couple of exciting days to break up the routine of our usual station work.