Changes could have huge impacts on ocean food chains, global carbon cycle
By Summit Voice
SUMMIT COUNTY — Changes in the Arctic ice pack are starting to have an effect on the timing of the annual phytoplankton bloom, a critical event in the marine food chain and in the global carbon cycle.
By studying satellite data, scientists with the Scripps Institution of Oceanography at UC San Diego, along with colleagues in Portugal and Mexico, discovered that the spring bloom is coming up to 50 days earlier in some areas.
“The spring bloom provides a major source of food for zooplankton, fish and bottom-dwelling animals,” said Mati Kahru, lead author of the study and a research oceanographer in the Integrative Oceanography Division at Scripps. “The advancement of the bloom time may have consequences for the Arctic ecosystem.”
Another study by the UK National Oceanography Centre supports some of the Scripps findings, showing how ongoing climate-driven changes to the Arctic sea-ice could have a significant impact on the blooming of phytoplankton.
“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 percent 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.
The researchers 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 percent 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.
“The bloom peak is most often located close to the ice edge,” said 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.”
“Our findings demonstrate strong biophysical linkage between bloom propagation and sea-ice melt back, which is independent of the actual direction of retreat,” said 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.
“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,” Yool said. 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.”