Looking down at my mud encrusted, dry, tired hands and mud covered clothes I see the unmistakable marks of being part of the benthic team. No amount of soap or lotion has seemed to rid me entirely of the remnants of a day outside on the deck spent bringing up multi-core tubes full of mud from the floor of the Chukchi Sea and the hours of processing that follow. The multi-core we use to bring the mud up has been compared to a Mars Rover in appearance, and I do imagine in my head a scene similar to a rover landing on Mars when I picture the multi-core landing on the bottom of the sea to retrieve our mud. Certainly, the worms and other animals we bring up from the seafloor must feel like they have been abducted by aliens. The days can be long, hard and cold, with your hands feeling frozen after the first half hour handling the core tubes and slicing trays and the mud and seawater leaching all the way through your layers of waterproof and insulating clothing before the day is over. After a successful day of collecting and processing mud you feel it in every part of your body. I love every part of it though and there is nothing else I would rather be doing.
To some, it may just be a tube of mud, but in each tube we can see evidence of the different chemical reactions that are occurring throughout sediment as the food particles that rain down on the seafloor are gobbled up by bacteria and microscopic animals living in the sediments. Just like in soils on the forest floor, bacteria are the decomposers of the marine world, and different bacteria perform different jobs when it comes to breaking down the decaying plant and animal remains that fall to the seafloor. Some like to be close to the sediment surface, others like it deeper. This causes bacterial communities to differ quite a bit throughout the top of our sediment cores to the bottom and we can see evidence of that just in the color of the mud we bring up. These variations can be large, with one centimeter having a microbial community that is significantly different from the centimeter that follows.
Other researchers on our cruise are focused on the plants and animals living in the water column, but we are more interested in what happens to them when they land on the seafloor. Sediment communities rely almost entirely on bits and pieces of these phytoplankton and other organic material (like poop) that falls down to the seafloor. This stuff has been given the rather romantic name “marine snow”, and as it sinks to the bottom of the sea, some of it gets eaten by microbes and other animals. Whatever doesn’t get used in the water column ends up in the sediments, and provides food for seafloor organisms. These organisms are so dependent on this rain of food that small changes in the marine snowfall (either how much falls or what it’s made of) can have big effects on the sediment community. In polar regions, like the Chukchi Sea, a lot of the marine snowfall occurs during the Spring when the winter ice cover breaks up, freeing up ice algae and allowing light into the ocean, causing blooms of phytoplankton that eventually die off and sink to the depths. As the extent of ice cover and the timing of ice breakup changes, so will the availability of food for marine organisms. We already know that polar bears and walruses like sea ice, but we also think it is critically important for these smaller, less charming organisms at the base of the food chain. Our group is focused on figuring out how marine organisms that live within the sediment will be affected by predicted changes in the timing and availability of marine snow due to changing ice cover and ocean temperature.
When marine snow makes it to the seafloor and gets eaten by microbes or animals, it can cause release of carbon dioxide (CO2) from the sediment, just like when we eat food. Generally, the sediment is thought to be a sink for CO2, with the sediment community utilizing some of the marine snow in the few centimeters of the sediment that contain oxygen, and the rest being buried within anoxic layers of sediment – effectively locked away within the Earth for possibly millions of years. Scientists hypothesize that as the predicted warming of the oceans continues, the microbes within the oxygenated layers of the sediment will become hungrier and hungrier and eat more and more of the marine snow that they encounter. This would theoretically then result in less marine snow being buried within the sediment and more CO2 being returned to the water column (from the microbes eating more and thus producing more CO2). This could have big impacts such as contributing to ocean acidification, hypoxia (low oxygen, dead zones) and even potentially increasing atmospheric CO2 concentration.
As you can probably tell from my high use of words like ‘potentially’ and ‘theoretically’ we really aren’t 100% sure what will happen, though, because we have never observed such changes and each region and habitat will likely respond differently to these predicted changes. All that we have is our best predictions that are based off of all of the evidence that is currently available. To make these predictions stronger, though, we can do experiments, and that is a big part of what we are doing on the benthic team. The mud we bring up from the bottom of the Chukchi Sea is incubated at two different temperatures (0°C and 5°C) in our lab on the ship. We add a pre-measured amount of phytoplankton to half of these incubations, allowing us to simulate an influx of marine snow due to a plankton bloom on the surface of the ocean or a large amount of ice algae sinking at one time. We monitor the nutrient levels, the oxygen concentrations, and the dissolved inorganic carbon (i.e., CO2) from the start of the experiment to the end. We then sample and preserve the sediment within the incubations to further explore how the sediment community consumed food differently in the two different temperature scenarios once we are back in our home laboratories. With the information we gain from this, we will be able to have more confidence in our predictions of what may happen to the cycling of food and carbon within the Chukchi Sea as it continues warming, allowing us to be better prepared for the possible effects.
Ask anyone on the ship, though, and you might just hear that we are just making the ship messy and spreading mud around. I can’t argue with them on that, even though we try our best. During the blue nose ceremony (an initiation for crossing into the Arctic Circle for the first time on ship) I even got a new nickname… ‘Seabrook?’ they questioned, ‘More like Mudbrook!’. I will admit, it does probably suit me better.