Melting sea ice: An unsuspected domino effect

15 December 2021

Seven years ago, AWI researchers began constructing a new ocean observatory in Fram Strait, which has gathered data on the physics, chemistry and biology of the ocean round the clock – and in summer and winter alike – ever since. An analysis of the first yearly datasets yielded surprising insights into the role of sea ice for life in polar waters. Depending on where and how quickly the sea ice melts, the species makeup and the growth of plankton in the water column – and therefore the available food in the deep sea – can change.

The ice conditions in the central Fram Strait could hardly have been more different: while an extremely large quantity of Arctic sea ice, driven by the wind and currents, passed through Fram Strait on its way to the North Atlantic in the summer of 2017, there was less ice than average in the following year. A stroke of luck for marine research, since in the summer of 2016, experts from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) had installed a cutting-edge ocean observatory with an array of instruments in the water column, right in the middle of the sea-ice autobahn – at 79 degrees North and 4 degrees 20 minutes East. With its help, for the first time they would be able to observe in a marginal ice zone of the Arctic Ocean how the living conditions for marine algae and fauna are changing as the sea ice gradually dwindles.

“The Arctic is one of the hotspots of climate change. For the past several decades we have witnessed a dramatic decline in the area and volume of sea ice; at the same time, air and water temperatures have risen. But until recently, we never really knew how these changes were affecting the ocean chemistry, the light conditions, availability of food, the layering of water masses and therefore the fundamental living conditions for algae and zooplankton in the spring, summer, autumn and winter, or what role the sea ice actually plays for plankton growth in the ocean,” says AWI oceanographer Wilken-Jon von Appen, who coordinates the monitoring of physical parameters at the new observatory.

There are a number of reasons for these gaps in our knowledge: “For one thing, to date our ship-based expeditions to Fram Strait were always in the summer – in other words, at a time of year when the spring algal blooms were often already over and our monitoring data offered very few insights into the conditions in the water column in the winter and spring. For another, until our FRAM observatory was up and running, we didn’t have the ‘Arctic-proof’ equipment we would have needed to record all the key processes, and at all water depths, in high temporal resolution,” the expert explains.

A new era: The FRAM observatory offers round-the-clock monitoring

The installation of the FRAM observatory in the summer of 2016 marked the beginning of a new era for the AWI’s marine biologists and oceanographers: since then, temperature, salinity, light, current and nutrient sensors constantly monitor all essential parameters needed to determine the status quo in the ocean – 24 hours a day, 365 days a year; in various water layers; and from just below the water’s surface to the ocean floor, ca. 2,500 metres below.

To support this high-resolution monitoring, the various sensors are mounted on three vertical, kilometre-long plastic tethers, installed in a triangular configuration at a distance of 2,500 from each other. Each mooring is equipped with an air-filled buoy at its upper end, which keeps it in a vertical position. On the other end are four railway wheels, with a combined weight of ca. 1,000 kilogrammes. The wheels serve as an anchor and prevent the sensors from being swept away by ocean currents.

Unlike the AWI’s previous long-term observation systems in Fram Strait, the moorings for the new FRAM observatory also measure very close to the ocean’s surface; one of the most important devices, an automatic water sampler with sensors for measuring light, oxygen, nutrients, pH, carbon dioxide and chlorophyll, is suspended at a depth of just 30 metres. “Just like all plants, algae need light and food to grow. As such, we were of course aware that the most important processes, namely the algal blooms, would take place in the upper part of the water column. Nevertheless, we couldn’t put our instruments right at the surface because there would be too great a risk of them getting caught on a long sea-ice keel and being torn off. As such, starting at a depth of 30 metres was the best compromise,” von Appen explains.

The ice determines the layering of surface water

This calculated risk paid off. For two years, the FRAM moorings constantly recorded the physical, biological and chemical fingerprint of the ocean, including surprisingly different readings for the two extreme sea-ice years. In 2017, a particularly ice-rich year, the sea-ice cover in Fram Strait reached so far south that floes were still drifting over the FRAM observatory in spring and summer.

However, since the ice’s southward journey takes it to warmer waters, it gradually melts, releasing countless millions of litres of meltwater into Fram Strait. This meltwater contains virtually no sea salt. As such, it is lighter than the salty seawater and only mixes with the water immediately below the ocean’s surface. Consequently, a surface layer of meltwater – only a few metres thick but extremely stable – forms, one with significantly lower salinity than the layer below it. There is very little mixing between the two layers, which is why oceanographers speak of intense stratification or water-mass layering.

In contrast, in the ice-poor year of 2018 the meltwater influx was far lower. As von Appen relates: “The ice cover ended 50 kilometres north of the FRAM observatory, which meant our monitoring area was ice-free and no stable meltwater layer could form directly over the moorings. Instead, our instruments recorded a surface mixed layer extending to a depth of 50 metres.”

How deep the surface water extends, and how well-mixed it is, are absolutely vital for marine life; if it’s well-mixed, it means there are sufficient nutrients like nitrate and phosphate, offering marine algae ideal conditions for growth. Moreover: the thicker the surface layer is, the more of these nutrients are available for algal blooms. “On the other hand, if a thin but stable meltwater layer forms at the surface, its nutrient content is quite limited. And once the nutrients are gone, they can’t be replenished all summer long. The pronounced water-mass layering prevents nutrient-rich deep water from below mixing with the surface water,” says AWI marine biologist and plankton expert Dr Eva-Maria Nöthig.

In contrast, in the summer of 2018 the deep-sea buffet was filled with nourishing particles early on. In the ice-free, well-mixed and 50-metre-deep surface layer, various diatom species began blooming early in the year. Within a matter of days, their explosive growth lured algae-eating zooplankton and fish to the upper water column. The two groups consumed a respectable percentage of the bloom and subsequently excreted faecal particles that were much heavier than the water and promptly sank – without being recycled in the surface layer. This difference to the particle transport in the previous year is a critical one, as the material that made its way to the deep sea in the early summer of 2018 was less than six weeks old and still contained so much nourishing chlorophyll that it draped the seafloor like a bright green veil, offering the organisms living there a veritable feast.

The AWI experts were also able to prove that the denizens of the deep sea gladly took advantage of this copious bounty: oxygen readings taken by their new deep-sea robot confirm that the activity rate on the ocean floor spiked substantially after the nutrient-rich “snow” had fallen. As such, the local organisms must have really gorged themselves.

Meltwater zones: In Fram Strait today, in the Central Arctic tomorrow?

Sparse food in one year, followed by a virtual surplus in the next – it raises the question: what do these two, radically different observations mean for marine life in the marginal ice zone of the Arctic Ocean?

“Our data indicates that in regions where sea ice melts rapidly, it’s more likely that a stable meltwater layer forms at the surface, one in which the biological development is very different from what we’re used to seeing in the Arctic marginal ice zone – and which directly affects the available food, down to the deep sea,” claims Eva-Maria Nöthig.

Moreover, given the current sea-ice development in Fram Strait, it can be assumed that the phenomena of meltwater layering and foam algae blooms could become more prevalent in the Arctic in the future, as the ice cover grows thinner and the ice concentration declines.

Declining ice concentration means the ice cover gradually breaks up, and more and more patches of open water form between the floes. Such porous ice cover is characterised by far more marginal ice zones than a solid ice sheet, offering optimal conditions for the formation of a meltwater layer. Another factor is the dwindling sea-ice thickness: thin floes melt faster than thick ones, increasing local meltwater input.

“Due to its natural ice conditions and currents, today we’re already seeing in Fram Strait those processes that, as climate change progresses, could soon become more common in the Central Arctic. And that’s precisely what makes it such a valuable model region for climate and marine research, and makes the data gathered by our FRAM observatory so exciting,” says Nöthig.

She and her colleagues now face the challenge of analysing the FRAM observational data for the years 2018 to 2021. In this regard, Wilken-Jon von Appen will primarily focus on the temperature and salinity data. “When we began gathering our FRAM data, we didn’t know how thin the layering in the upper water column could be. In response, in subsequent years we equipped our moorings with more sensors. Now even the buoys are fitted with temperature sensors, so we’re confident that we’ll soon have a better grasp of the water-mass layering processes at the ocean’s surface, and the role of sea ice in them,” he says.

The FRAM observatory – A quantum leap in technical resources

For nearly a quarter of a century, experts from the Alfred Wegener Institute have maintained time series in Fram Strait to investigate the impacts of climate change on ocean circulation and marine flora and fauna. By consolidating and building on previous moorings and the deep-sea observatory “Hausgarten” to create the new FRAM observatory, since the autumn of 2014 they have expanded researchers’ technical resources several-fold. Using funding from the Helmholtz Association, stationary monitoring units like moorings have been equipped with new and significantly more instruments, allowing physical, chemical and biological parameters to now be recorded in far higher spatial and temporal resolution, all year round. These resources are complemented by mobile components like deep-sea robots, ice buoys and autonomously navigating underwater robots. Taken together, they allow continuous, autonomous monitoring in zones (e.g. on, in or directly below the ice, or on the abyssal plain) and times of year that were previously impossible. You can find additional information on the FRAM observatory at the AWI website.

Dr Wilken-Jon von Appen is a physical oceanographer at the Alfred Wegener Institute and, with the support of 25 colleagues, has analysed the first two year-long datasets from the FRAM moorings in the central Fram Strait. His detailed analysis has now been published as a scientific paper in the journal Nature Communications. The original title is as follows:
von Appen et al., 2021. Sea-ice derived meltwater stratification slows the biological carbon pump: results from continuous observations; DOI: https://doi.org/10.1038/s41467-021-26943-z

   
Dr. Eva-Maria Nöthig conducts research as a marine biologist at the Alfred Wegener Institute and has coordinated the AWI’s long-term investigations into plankton in the Central Arctic and in Fram Strait for nearly 30 years, and since 2009 with the PEBCAO-Gruppe. She and her team have proven e.g. that, as the West Spitsbergen Current grows warmer, North Atlantic amphipods are migrating to Fram Strait.



Text: Sina Löschke (www.schneehohl.net)

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