Ongoing climate-driven changes to the Arctic sea-ice could have
a significant impact on the blooming of tiny planktonic plants
(phytoplankton) with important implications for the Arctic
ecosystem, according to new research conducted by scientists at the
UK's National Oceanography Centre (NOC).
"Ice-edge phytoplankton blooms in the Arctic Ocean provide food
for planktonic animals called zooplankton, which are in turn
exploited by animals higher up the food chain such as fish,"
explained Dr Andrew Yool, one of the team of NOC researchers.
During the Arctic spring and summer, sea-ice melts and breaks
up. Freshwater from melting ice forms a blanket over the denser,
saltier water below. This stratification of the water column, along
with seasonal sunshine, triggers the appearance of phytoplankton
blooms, which often form long but narrow (20-100 km) bands along
the receding ice-edge.
Arctic ice-edge blooms have in the past been studied largely
during research cruises. These studies have often focused on
regions such as the Barents Sea between Norway and the Svalbard
Archipelago, and the Bering Shelf bordering Alaska, where blooms
are thought to account for 50% or more of biological
production.
However, advances in modern satellite technology now offer the
opportunity to observe and monitor ice-edge blooms at high spatial
resolution over large areas and extended periods of time from
space.
"Our aim was to use satellite data to get a synoptic view of
ice-edge blooms across the whole Arctic region," said Dr Yool.
To do this, the research team used daily data from the NASA's
SeaWiFs satellite, which was launched in 1997. SeaWiFs continuously
observes ocean colour (sea-ice, cloud and fog cover permitting),
sampling the whole globe every two days. To provide an alternative
estimate of bloom occurrence, and an independent check on their
findings, the researchers also used data from the MODIS
satellite.
Ice-edge blooms are identified from the spectral signature of
the photosynthetic pigment chlorophyll, after correction for
contamination by other coloured organic matter in surface
waters.
So as better to understand the relationship between
phytoplankton blooms and seasonal changes in sea ice, the
researchers also used information on sea ice concentrations
obtained from the US National Snow and Ice Data Center (NSIDC).
Their study covered the period 1998-2007.
They found that ice-edge blooms occurred in all seasonally
ice-covered areas and from spring to late summer. They observed
ice-edge blooms in 77-89% of locations for which they had adequate
data. The blooms usually peaked within 20 days of ice retreat,
sometimes forming long belts along the ice edge (greater than 100
km).
"The bloom peak is most often located close to the ice edge,"
said Dr Yool, "We observed blooms propagating in a wave-like
fashion behind the receding ice edge over hundreds of kilometres
and over several months, while others remained stationary."
Because of the geography of the Arctic Ocean, sea ice does not
always retreat northwards. For example, in Baffin Bay and Davis
Strait, west of Greenland, ice shrunk both westward and
south-eastward from the north in spring and summer, with
phytoplankton blooms propagating along the ice edge as it
receded.
"Our findings demonstrate strong biophysical linkage between
bloom propagation and sea-ice melt back, which is independent of
the actual direction of retreat," said Dr Yool.
These findings are important because they indicate that future
change in Arctic sea- ice resulting from climate change could
significantly impact the occurrence of phytoplankton blooms as well
as the animals further up the food chain that ultimately depend
upon them, including fish.
Ice-edge phytoplankton blooms also play an important role in the
Arctic carbon cycle. Through photosynthesis, phytoplankton blooms
draw large amounts of carbon dioxide down from the atmosphere, some
of which is exported to the deep ocean.
What effects future shrinkage in sea-ice will have on the
ecology and biogeochemistry of the Arctic Ocean are still largely
unclear, as Dr Yool explained:
"It is quite possible that ongoing climate change will lead to
ice-free summers in the Arctic within the next few decades. As the
melt season becomes longer, ice-edge blooms may propagate over
larger distances, stripping out surface nutrients as they go.
However, whether the Arctic becomes more or less productive will
ultimately depend on complex factors affecting ocean stratification
and mixing, and thus the availability of nutrients in sunlit
surface waters."
Dr Yool and his colleagues hope that their findings will
contribute to a better conceptual understanding of the ecology of
the Arctic Ocean, which should help computer modellers forecast
future changes under global warming.
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