Fresh water from sea ice delays the biological carbon pump by four months

During the summer months, Arctic sea ice drifts through Fram Strait into the Atlantic. Thanks to the melt water, a stable layer forms around the drifting ice above the salty sea water, producing significant effects on biological processes and marine organisms. This, in turn, has an effect on when carbon from the atmosphere is taken up and stored, as a team of researchers led by the Alfred Wegener Institute have now determined using the Ocean Observing System. FRAM. Their findings have just been published in the journal Communication Nature.

The oceans are one of the greatest carbon sinks on our planet, thanks in part to the biological carbon pump: just below the surface of the water, microorganisms like algae and phytoplankton absorb carbon dioxide of the atmosphere by photosynthesis. When these microorganisms sink to the bottom of the ocean, the carbon they contain can remain intact for several thousand years. As experts from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) have discovered, melting sea ice can delay this process by up to four months.

From summer 2016 to summer 2018, the ocean observation system FRAM (Frontiers in Arctic Marine Monitoring) continuously collected data in the Fram Strait (between Greenland and Svalbard). Dense groups of moorings were installed at two sites in the strait to monitor as many aspects of the coupled physico-biological processes in the water as possible. Physical, biogeochemical and acoustic sensors distributed in the water column and on the ocean floor, as well as devices that collected water and sediment samples for later analysis in the laboratory, were used. “For the first time, for two whole years, we were able to comprehensively monitor not only the seasonal developments of microalgae and phytoplankton, but also the entire physical, chemical and biological system in which these developments take place,” explains Dr. Wilken-Jon von Appen, climate researcher at AWI and first author of the study.

During this period, the export of sea ice has reached two extremes: in the summer of 2017, an extraordinarily large amount of ice was transported out of the Arctic through the Fram Strait. This produced a large amount of low-salt meltwater and pronounced water stratification. In contrast, unusually little ice was transported out of the Arctic in the summer of 2018, meaning there was very little meltwater and therefore no pronounced salinity-based stratification. The processes involved in the biological carbon pump evolved so differently during these two extremes that experts call them two different regimes: the meltwater regime (summer 2017) and the mixed layer regime (summer 2018).

Meltwater regime in the summer of 2017

The first algal and phytoplankton blooms appeared on May 15, when the atmosphere began to warm the ocean. In the summer of 2017, a large amount of ice drifted into Fram Strait, producing large amounts of meltwater. “This low-salt water sat on top of the salt water without mixing,” von Appen says. “And the stratification between 0 and 30 meters was ten times more intense than between 30 and 55 meters.” Consequently, very few nutrients moved up from deeper layers of water, while very little carbon went to the seabed. Phytoplankton growth, which is the first stage of the biological carbon pump, took place almost exclusively in the top 30 meters. This intense stratification only collapsed in mid-August, when the atmosphere no longer warmed the surface of the water. The majority of the biomass drifted from the upper layer between September and November, was over three months old, and too lacking in nutrients to be of interest to ocean floor fauna. In the meltwater regime, during flowering, microorganisms were able to fix up to 25 grams of carbon per square meter.

Mixed diaper diet in the summer of 2018

The spring and summer of 2018 were a whole different story: conditions were relatively ice-free, which meant less meltwater and less intense seawater stratification. A mixed layer formed at a depth approx. 50 meters. With the first of May came the first flowers of diatoms; at the same time, the number of zooplankton and the fish that primarily feed on them began to increase. Thanks to their droppings, only two to three weeks after the start of flowering, the organic carbon reached depths of up to 1200 meters. Four to seven weeks after the start of flowering, almost four months earlier than in the summer of 2017, the biomass reached the seabed. This material was rich in nutrients, attracting five times more fish and benthic fauna than during the summer of the melt. During flowering, the algae were able to fix about 50 grams of carbon per square meter, twice as much as in the meltwater regime.

Despite all these differences between the two regimes, the biological carbon pump was not necessarily more productive in the summer of 2018: “We found that in the summer of 2017, the majority of the organic carbon did not reach the seabed until after September,” says von Appen. “If you look at the period between early May and late November, the carbon export in the mixed layer regime was only a third higher than in the meltwater regime.” On the contrary, the pronounced stratification in 2017 favored longer-term growth over several months, since carbon and nutrients were trapped in the upper layers. In contrast, the ice-free situation in 2018 produced a brief, intense bloom and rapid export, providing food and carbon to deep-sea ecosystems. Thus, the latter would seem to particularly benefit from the summer conditions in the regime of mixed layers; in the meltwater regime, intense stratification blocks the supply of nutrients in summer and the mixing of deep waters in winter.

“In the future, the mixed-layer regime may expand to larger regions of the Arctic,” says von Appen. “Conditions in this regime are similar to those at lower latitudes, and the Arctic Ocean may increasingly behave more like oceans in southern regions.”