New research by NOAA Fisheries suggests that the warming of the Bering Sea shelf is affecting the timing of algae blooms and the food web dependent upon them.
This is particularly true of the unprecedented and widespread warming during 2018-2019, according to the study, which was released Dec. 22.
“This study fills gaps by including data from the recent warming period 2018-2019 and more spatial information on how blooms vary across the region,” said Jens Nielsen, a biological oceanographer with NOAA’s Ecosystems and Fisheries-Oceanography Coordinated Investigations Program.
“Our work lays the foundation for learning more about the link between phytoplankton bloom types and how that influences zooplankton that are food for commercially important fish and benthic crabs,” Nielsen explained.
The Bering Sea shelf is a highly productive ecosystem that extends over 300 miles offshore from the Alaska coast. It typically experiences large phytoplankton blooms in spring. During high-ice years in the southern Bering Sea—and all but the warmest years in the northern Bering Sea—the timing of sea-ice retreat is a major driver of spring bloom formation.
Following the recent warm years of 2018–2019, the snow crab population decreased by 80%, in part due to starvation. Whether increasing open water blooms drove the rise in starvation of commercially important snow crabs needs further investigation, authors of the study said.
Here’s how it works: as light levels increase at the start of spring, ice-algae grows on the underside of the ice. As ice melts, the surface water gets fresher, creating stratification in the water which allows the phytoplankton to remain in the well-lit surface layer to grow.
The phytoplankton blooms feed zooplankton such as copepods and krill, and other fat-rich foods critical to young fish, birds and marine mammals. The uneaten phytoplankton fall to the seafloor where they are consumed by bottom-swelling invertebrates like crab and clams, which in turn are food for walruses and whales.
“This ecosystem is highly dependent on ice, and when ice retreats, large spring blooms provide food for zooplankton,” Nielsen explained. “However, when it is warm, ice melts early, and blooms in open waters (weeks after sea ice has disappeared) become more abundant.”
While the timing of the bloom has not shifted seasonally, open-water blooms have become more widespread as climate warming continues.
If the Bering Sea keeps experiencing warm ocean conditions as in 2018-2019, open-water blooms in both the southern and northern Bering Sea will become more widespread, Nielsen said.
Increasing open-water blooms may also redistribute resources away from the seafloor. Ice algae sink faster than open-water phytoplankton. Open-water blooms likely reduce the amount of food resources reaching the seafloor, something NOAA researchers are investigating further, the report said.