Meltwater influences ecosystems in the Arctic Ocean
Freshwater from sea ice delays the biological carbon pump by four months


Date:
December 15, 2021
Source:
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine
Research
Summary:
In the summer months, sea ice from the Arctic drifts through Fram
Strait into the Atlantic. Thanks to meltwater, a stable layer
forms around the drifting ice atop the salty seawater, producing
significant effects on biological processes and marine organisms.



FULL STORY ==========================================================================
In the summer months, sea ice from the Arctic drifts through Fram Strait
into the Atlantic. Thanks to meltwater, a stable layer forms around the drifting ice atop the salty seawater, producing significant effects on biological processes and marine organisms. In turn, this has an effect
on when carbon from the atmosphere is absorbed and stored, as a team of researchers led by the Alfred Wegener Institute has now determined with
the aid of the FRAM ocean observation system. Their findings have just
been published in the journal Nature Communications.


========================================================================== Oceans are one of the largest carbon sinks on our planet, due in part to
the biological carbon pump: just below the water's surface, microorganisms
like algae and phytoplankton absorb carbon dioxide from the atmosphere
through photosynthesis. When these microorganisms sink to the ocean
floor, the carbon they contain can remain intact for several thousand
years. As experts from the Alfred Wegener Institute, Helmholtz Centre
for Polar and Marine Research (AWI) have now discovered, the meltwater
from sea-ice floes can delay this process by four months.

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

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

Meltwater regime in the summer of 2017 The first algal and phytoplankton
blooms appeared on 15 May, when the atmosphere began warming the ocean. In
the summer of 2017 a great deal of ice drifted through Fram Strait,
producing large quantities of meltwater. "This low-saline water lay atop
the saltwater without mixing," says von Appen. "And the stratification
between 0 and 30 metres was ten times as intense as between 30 and 55
metres." Consequently, very few nutrients made their way upwards from
the deeper water layers, while very little carbon made its way to the
seafloor.

Phytoplankton growth, which is the first step in the biological carbon
pump, took place almost exclusively in the top 30 metres. This intense stratification only collapsed in mid-August, when the atmosphere no
longer warmed the water's surface. The majority of the biomass drifted
down from the upper layer between September and November, was more
than three months old, and was too lacking in nutrients to interest
fauna at the ocean floor. In the meltwater regime, during the bloom the microorganisms were able to fix up to 25 grams of carbon per square metre.

Mixed-layer regime in the summer of 2018 The spring and summer of 2018
were another story entirely: conditions were relatively ice-free, which
meant less meltwater and less intense stratification of the seawater. A
mixed layer formed to a depth of ca. 50 metres. With the first of May came
the first diatom blooms; at the same time, the numbers of zooplankton,
and of the fish that primarily feed on them, began to rise. Thanks to
their faeces, only two to three weeks after the start of the bloom,
organic carbon reached depths of up to 1200 metres. Four to seven weeks
after the start of the bloom -- almost four months earlier than in the
summer of 2017 -- the biomass reached the seafloor. This material was
rich in nutrients, attracting five times more fish and benthic fauna
than in the meltwater summer. During the bloom, the algae were able to
fix roughly 50 grams of carbon per square metre, twice as much as in
the meltwater regime.

Despite all these differences between the two regimes, the biological
carbon pump wasn't necessarily more productive in the summer of 2018:
"We found that, in the summer of 2017, the majority of the organic carbon didn't reach the seafloor 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." Rather, the pronounced stratification in 2017 promoted 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 for deep-sea ecosystems on the ocean floor. As such, the latter
would seem to particularly benefit from the summertime conditions in the mixed-layer regime; in the meltwater regime, the intense stratification
blocks nutrient input in the summer and deep water mixing in the winter.

"In the future, the mixed-layer regime could spread over larger
regions of the Arctic," von Appen explains. "The conditions in
this regime are similar to those in lower latitudes, and the Arctic
Ocean could increasingly behave more like oceans in southern regions." ========================================================================== Story Source: Materials provided by Alfred_Wegener_Institute,_Helmholtz_Centre_for_Polar_and
Marine_Research. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Wilken-Jon von Appen, Anya M. Waite, Melanie Bergmann, Christina
Bienhold, Olaf Boebel, Astrid Bracher, Boris Cisewski, Jonas
Hagemann, Mario Hoppema, Morten H. Iversen, Christian Konrad,
Thomas Krumpen, Normen Lochthofen, Katja Metfies, Barbara Niehoff,
Eva-Maria No"thig, Autun Purser, Ian Salter, Matthias Schaber,
Daniel Scholz, Thomas Soltwedel, Sinhue Torres-Valdes, Claudia
Wekerle, Frank Wenzho"fer, Matthias Wietz, Antje Boetius. Sea-ice
derived meltwater stratification slows the biological carbon pump:
results from continuous observations.

Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-26943-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211215082027.htm

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