A weird winter: Early melt leads to late bloom in the Bering Sea
By MacKenzie Jewell, Haley Cynar and Aliya Jamil
Fall/Winter 2024
Editor’s note: For this Student Voices feature, we share acondensed version of an assignment three students undertook for a CEOAS class in ocean remote sensing. Students were asked to write clearly about an oceanographic process that could be investigated using remote sensing techniques.
Between January and April 2018, nearly 125,000 square kilometers of floating ice covered the surface of the Bering Sea. Although this seemingly-vast expanse of ice encompassed a region nearly the size of the state of Mississippi, it marked a record low in winter sea ice extent since satellite observations began nearly forty years prior, a record that stands today.
In most winters, sea ice stretches across more than 400,000 square kilometers of the Bering Sea surface. Melting of this ice cover in spring allows sunlight to hit these waters after many months of short winter days, leading to a phytoplankton bloom — rapid reproduction of microscopic ocean plants that form the base of the marine food web.
Timing here matters: Sea ice retreat influences the timing and composition of the blooms, determining which phytoplankton species thrive and whether they grow abundantly in the cold waters of early spring or in the sun-warmed waters of late spring or summer. Ice-driven changes in bloom timing can have cascading effects on the Bering Sea ecosystem, wreaking havoc with regional commercial fisheries and coastal populations, particularly Indigenous communities, that depend on fishing for food. And that’s what happened in 2018: Notable seabird die-offs and northward shifts in the distribution of some fish species like Pacific cod and walleye pollock coincided with sea ice loss.
Why was ice cover so low in the spring of 2018? The short answer is, ice didn’t form normally in the first place: In November of 2017, warm winds from the south blew north and kept temperatures unusually warm, preventing the usual expanses of ice from forming. The ice cover made a partial recovery in December and January as winds shifted direction, but the warm south winds returned in February and hastened the end-of-winter ice retreat. With these persistent unusual conditions, the average winter Bering Sea ice area was the lowest recorded to date, more than 50% below average across much of the region.
Satellite imagery shows that chlorophyll-a levels, a stand-in for phytoplankton concentrations, were lower than usual in most of the Bering Sea in both April and May of 2018. In colder years, fresh water from melting sea ice stabilizes the water column in these months as sunlight increases, suspending phytoplankton near the illuminated ocean surface where they can grow rapidly in an early spring bloom. In warm years with early sea ice retreat, like 2018, meltwater stabilizes the water column before there is sufficient light to support rapid phytoplankton growth. Over time, winds mix the meltwater deep below the surface, carrying phytoplankton with it. A bloom is delayed until late spring or early summer when the warming upper ocean re-stabilizes. In 2018, following relatively weak growth throughout April and May, chlorophyll increased rapidly in June as a bloom developed in warm open waters along the edge of the Bering continental shelf.
As sea ice extent and duration diminish with a warming climate, the Bering Sea could eventually transition permanently to a warm regime with 2018-like conditions. The resulting late blooms can result in mismatches between the life cycles of phytoplankton and other organisms in the marine food web. We might expect to see a more feast-or-famine food supply, sometimes resulting in limited food availability for fish, crabs and other species.
Understanding potential changes in Bering Sea food webs is critical for marine resource management: Bering Sea commercial fisheries currently yield 47% of U.S. fishery production by weight. Given the dramatic shifts in species composition (including those of commercial significance) across multiple levels of the region’s marine food web during 2018, it is reasonable to expect major economic impacts as the warm regime becomes more common.
Spring phytoplankton distributions in years with late versus early sea ice retreat. Left: If spring sea ice retreat is late, phytoplankton stay near the surface and spring blooms are early. Right: In warm years with early ice melt, a destabilized water column mixed by winds means phytoplankton drift and are mixed away from the sun, preventing an early spring bloom. Figure by M. Jewell
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