searching for sediment
day 18 | July 3, 2023
We’ve officially spent over 2 weeks aboard the Sikuliaq! In that time we’ve collected data from over 60 stations, travelled hundreds of nautical miles (the unit of measure used at sea, one nautical mile = 1.15 “land” miles) and have gotten into the rhythm of our watch schedules. It’s hard to believe we’re already a third of a way through the cruise but very exciting to know we still have a whole month left to explore, sample and adventure through the Chukchi Sea!
There are 20 people in our science party, ranging from undergraduate and masters students, to current and recently graduated PhDs, to full-time researchers from Stanford and WHOI (the Woods Hole Oceanographic Institute). We are split into two different watch shifts, with ten of us on “day” watch from 3am-3pm and the other ten on “night” watch from 3pm-3am (I put the time of day in quotes because it’s hard to tell the difference between night & day when the sun never sets…) We conduct science operations 24/7 so this division of watch shifts means that there will be a group of scientists available and ready to collect and process data whenever needed. We typically aren’t working for 12 straight hours, don’t worry, we take breaks for meals, ping pong and bananagrams, but are “on-call” during those 12-hours of watch to do whatever needs to be done!
As I mentioned in the last blog post, our mission of this research cruise is to better understand the fate of phytoplankton within the Chukchi Sea. We do this by studying three systems: (1) algae within the sea ice (the same organisms as phytoplankton, they are given a different name because these algae are trapped within the ice rather than free-floating in the water), (2) phytoplankton blooms underneath sea ice floes and (3) phytoplankton blooms in the open ocean. These three systems are loosely chronological – meaning algae within sea ice tends to come earliest in the season (whenever spring light levels become long enough), and then come the under ice blooms, and lastly the open water phytoplankton blooms. We go out onto the ice to study the algae/phytoplankton both within and underneath the ice, but rely on another method to study the open water blooms – sediments!
We have two main tools that we use to study the sediments: the multicorer & sediment traps.
The multicorer samples from the ocean floor. A large rig with 7 bottles affixed around its center, it gets lowered down over the back of the ship and each of the bottles are plunged into the top layer of sediment on the seafloor. We sample from the seafloor to see how much phytoplankton has fallen through the water column and settled at the bottom, where it becomes an important food source for benthic species (benthic is an adjective describing the bottom of the ocean) such as brittle stars, sand dollars and even large marine mammals like walruses & gray whales (who feed by grazing along the sea floor).
The crew handle the lowering of the multicorer as it’s a heavy, large instrument and they have to use the electronic A-frame to lower it down. Once it is back on the surface, we take 3 of the 7 bottles (as we like to sample in triplicate in case of any inaccuracies). The bottles are typically filled half with sediment & half with liquid. This is because we sample from the very top layer of the seafloor where it interfaces with bottom ocean water.
We then siphon out the water (using the very advanced method of creating a suction via our mouths and funneling it into a Home Depot bucket). This water represents the nepheloid layer, meaning it is directly above the seafloor and tends to have a high concentration of suspended sediment that has gotten disturbed and mixed up away from the floor. We filter this water for both chlorophyll & particulate organic carbon concentrations (the two most common proxies that we use to determine phytoplankton amounts & photosynthetic activity) and then take out a couple syringe-fulls of the sediment itself, which gets diluted and processed as well.
From time to time we’ll shift through the sediment collected in both the multicorer tubes as well as from the Van Veen grab (a claw-like instrument that takes a literal scoop out of the seafloor) to look for interesting creatures – particularly worms as one of our PhD students is very interested in those slightly larger invertebrates!
And then we dump it all back overboard, returning the brittle stars, sand dollars and whatever else we might’ve accidentally scooped up back to their watery homes and clean the instrument in preparation for the next data collection point.
Using bottles identical to the multicorer, the sediment traps collect samples from within the water column rather than on the seafloor. Attached to a buoy floating at the surface, the trap consists of two bottles hung vertically on-top of one another, the first bottle is ~15 meters deep while the second is ~30 meters deep. We leave the bottles out at these depths in an attempt to capture any phytoplankton blooms in the middle of the water column. The hypothesis is that the amount of light that reaches 15–30 meters down is similar to the amount of light able to travel through sea ice & therefore the blooms that we find underneath sea ice floes might travel out to these middle depths in the open ocean once the sea ice melts.
In order to collect enough data from these sediment traps, we have to leave them sitting in the water column for at least 48 hours – meaning we attach them to floating buoys and a GPS beacon and sail away, leaving them bobbing around the ocean until we make our way back to collect them.
We also deploy some of these sediment traps during our ice stations, lowering them down through holes in the ice floe where they lay suspended (also at ~15 & 30 meters deep) within the water column for a couple of days. The ones in the ice serve the same function – collecting suspended particles from mid-layer phytoplankton blooms however this time it’s from underneath the ice rather than out in the open ocean!
Alongside most of the sediment traps, we also deploy wave buoys (the floating bright yellow orbs) to track the currents & wave action in the region. For the traps left in the sea ice — we leave the wave buoys on top of the ice where they are able to track the movement of the ice floe itself
The retrieval of the sediment traps is an exciting operation. Once we’ve tracked down the GPS-signal from the beacon, we pull up as close as possible and then begin the effort of “lassoing” in the buoy, clipping it to ropes and guiding it to the back end of the ship where we are able to raise it on board using the A-frame. A tricky process, but we’ve been practicing and getting exponentially faster with each trap.
Once the trap is back on board – we process it in practically the same exact way as we do the multicorer samples … siphoning out the water, filtering, processing and sifting through the sediments. The configuration of the traps and their deployment schedule has largely been thanks to James, one of the PhD students on the cruise, it’s the first time the Arrigo group has used this piece of equipment and it wouldn’t have been possible without him!
In other news – almost all of us have gotten a chance to go out on the sea ice (very soon to be all of us!).
We’ve had some VERY exciting animal sightings – lots of walruses, a pod of belugas far off in the distance, and even a polar bear mama and her two cubs!! (the night shift got very lucky to spot that one)
We’ve been playing lots of ping pong, collaboratively crushing the New York Times crosswords & even writing a group song. More to come very soon but for now enjoy this map of our progress so far (we’ve been circling in the same general area to collect sediment traps that we left behind and generate data from different times in the season) and don’t forget to follow @polarplankton on Instagram & TikTok for more frequent updates!
– Natalie
