The second week of Expedition PS101 was dedicated to the discovery of the deep-sea ecosystem at Karasik Seamount. This giant seamount rises over 4000m above the Arctic basin and is teaming with life.
Already during the first dive everyone was watching the TV screens in the labs showing the first live seafloor images. Seamounts are often identified as “hotspots” of life in the ocean. Their specific morphology have an influence on the local hydrography, increasing the availability of nutrients and energy to marine life. Some seamounts are even active, emitting gasses which can enhance microbial growth. But do such processes also occur in the ice-covered, nutrient-limited central Arctic? This was the key question behind our observation and sampling efforts during the second week of Expedition PS101.
First we focused on the three peaks of the Langseth Ridge, which starts at almost 5000m depth in the Gakkel Ridge and reaches up to 560 m water depth at the peak of the Karasik seamount. We could test the new functionality incorporated into our Ocean Floor Observation System (OFOS). The system is attached to the ship via a fibre optics cable, and is slowly towed 2 m above the seafloor. The OFOS is traditionally equipped with a good quality stills camera and an HD video camera, but before the cruise we had the opportunity to upgrade it with downward and forward-looking sonar systems. Now we can use this system to survey a much larger area of seafloor: the camera records a 5 m wide stretch of seafloor immediately below the OFOS vehicle, and the high resolution sonar adds a view of seafloor bathymetry for up to 40 m on each side. The system can visualize structures of only 20 cm size, enhancing our rate of discovering unknown seafloor ecosystems substantially in these uncharted waters.
This special moment in a mission when the first seafloor images are transmitted is quite magical: you never know what to expect, except that it will be something new. At the top of Karasik Seamount, we were surprised by the incredibly dense accumulations of large sponges of up to a meter in size. Between these large, spherical sponges there are mat-forming sponges and thick deposits of sponge needles and worm tubes. The distinctive round sponges can also be seen clearly by our new sonar systems on the OFOS. We have encountered many fish of several species, an outcome not expected this far north, and we think we may have glimpsed some of the most northerly corals yet observed. We see large white star fish, blue snails, red crustaceans and white and brownish mussels between the sponges.
There are various rock fissures, boulders and sinkhole-like features on top of the Karasik Seamount. The flanks of the seamount, as well as the flanks of the two other peaks of the Langseth Ridge, are very steep and almost devoid of life. They are built from weathered basalt rocks, here and there we see one glass sponge or a solitary sea anemone. Everywhere around Langseth Ridge and into the trough of Gakkel Ridge, there are barely any sediments, with the tops of most rock outcrops only dusted with a bit of mud.
The interactions between life and the extreme environment of the Arctic deep-sea are one focus of our expedition. So this second weekly report of expedition PS101 is dedicated to life deep under the ice. Some of the sponges we find on the summit of the seamounts must be hundreds of years old, their sides are overgrown by polychaete worms and bryozoan colonies. Sponges are one of the oldest animal clades and they are abundant in all oceans – from shallow tropical reefs to the arctic deep-sea. Many accommodate a complex and characteristic community of microorganisms in a symbiotic relationship – such as the abundant Geodia sponges we find here.
Samples from this area we can retrieve by box corer sampling and with the telemetry-guided multiple corer. Besides the megafauna that we can see with the camera systems (organisms > 1 cm), we also look for the benthic macrofauna (organisms living on the seafloor >1 mm). The diverse community of bristle worms, crabs (amphipods and tanaids) and bryozoans on the Karasik Seamount is quite different to that of the surrounding Arctic basins. The sponges and their cohabitants live on a layer of detritus of old bryozoans, sponge needles and tubes of bristle worms, which covers the thin sediments, which lie on top of the basaltic rocks. We also study the smaller size classes of life, including the so-called “meiofauna”, i.e. tiny organisms reaching a body size of at the most one millimeter. They inhabit the interstitial spaces within the sponge needle layer and the muddy sediment: very small copepods and amphipods, isopods and nematodes, mites and many others have already been sorted onboard with our stereo microscope (Fig. 3). During our journey into the world of tiny life, we also look at microrganisms of micrometer size. First measurements show that their habitat, the sediment below the sponge mats, contains very little nutrition. Sponges as well as most other abundant life is filter feeding, which means they are collecting the sinking matter directly from the water column as food. It remains a mystery where all the food could have come from, sustaining the filter feeder communities, as the waters within this region of the Arctic contain very few particles. At this time of the year there is barely any nutrients for primary production. We did not find any evidence for fluid venting at Karasik Seamount, so potentially the solution is that the special hydrography around the mount is trapping particles as they pass by, allowing the inhabitants of the mound good opportunities to catch them from the water. We are thinking of further experiments to test this in the next 2 weeks.
As with benthic life on top of the seamount, the zooplankton drifting above in the water column are also extremely abundant. Zooplankton includes the passively drifting, mostly small 0.2-200 mm animals that are studied on our cruise with two different gears: the Multinet and LOKI (Lightframe On-sight Key species Investigations). The Multinet is equipped with many nets (150 µm mesh size) to sample different depth layers within the ocean during a vertical haul from the seafloor to the surface. The zooplankton recorder LOKI is used to study the small-scale distribution of zooplankton by taking pictures of the organisms and simultaneously recording depth, water temperature, salinity, fluorescence and oxygen concentration.
The zooplankton scientists send their nets as deep as possible, to catch the almost unknown fauna living close to the seabed, not only at the top of the seamount but also in the deep Gakkel Ridge and surrounding basins. First results show that planktonic life between 2 and 4.8 km water depth is diverse and much richer in terms of biomass and abundance than ever observed on previous expeditions to the central Arctic (1995-2015). Is this abundance typical for the Gakkel Ridge area due to advection of Atlantic inflow, or is it an effect of water circulation in vicinity of seamounts? Do these pelagic organisms serve as food for sponges on the seafloor? What do they eat, when Arctic primary productivity is so low? These are the questions to be answered during this expedition.
From the vertical distribution and reduced swimming activity of large Calanus copepods, which contribute >70% to the plankton biomass, we conclude that the zooplankton community is entering its overwintering state (Fig. 5). Calanus copepods have stopped feeding and are currently descending to depths between 200 and 1000 m where they will hibernate for nine months, using the huge lipid reserves stored within their bodies during the summer. Some members of the science team have also adopted this strategy, as a consequence of having such an excellent ships cook on board.
So far the plankton team have identified within our samples over 120 different zooplankton species, which represent more than half of the known diversity for the deep Arctic basins. Individuals of all the species identified are photographed and stored for later genetic analyses to build a molecular library of arctic biodiversity.
But besides quantifying biodiversity of life under ice, there are other important questions for which we collect data and observe processes in the field. How the Arctic marine communities will react to rapid climate change and how the ongoing sea ice decline will affect the life of plankton and benthos are major scientific and societal challenges, to which this expedition will contribute data. Likewise there are basic geological problems to solve. The origin, activity and effects of the Langseth Ridge and the surrounding sections of Gakkel Ridge are poorly understood. Are the seamounts caused by volcanism or by tectonic processes and is this area active in terms of heat flux, fluid and gas emission? How important are the seafloor structures for hydrography and for the Arctic foodweb. We are looking forward to the next weeks of field work to get some further insights into this intriguing region.
It is only one week ago that we started the first dives here, and during this week we have deployed almost all our instruments into the water, to the seafloor or onto the ice, depending on their scientific aims. Weather is not great – it is either quite foggy, or it snows, and most of each day is twilight. Temperatures are between -1 and 10°C. But the ice conditions favor our research plans, as most of the ice is thin and relatively new, ice through which Polarstern can maneuver and position very well. Every day we are amazed how we can do targeted seafloor work – just now we have retrieved a multicorer sample from a specific sediment field of only 100m diameter – in full ice, strong winds and at over 3000 m water depth. What a great ship and crew !
Last week was very exciting and a great start of the expedition. We have successfully retrieved the second modular FRAM observatory, have deployed sea ice buoys for international Arctic research, and could complete several sea ice stations while doing deep-sea work. Also we carried out the first test dive of the new NUI Underice H-ROV. The little submersible robot became unexpectedly autonomous right at the start of the test and it did take some effort to keep it from taking a long dip under the ice. We understand – it is really beautiful down below.
Currently we begin to study the deep Gakkel Ridge in the vicinity of hydrothermal venting. This ridge is called an “ultraslow spreading zone” but it seems to be much more active than anticipated.
All participants are in good health and are sending greetings to families, friends and colleagues.
Antje Boetius
