Tracking Arctic species in the past may unlock their future
By documenting the migration of Arctic species into the Yellow Sea during the last Ice Age, researchers have shown how changes in the East Asian Winter Monsoon affect their fate.
Arctic species are sensitive to changes in climate, as they often can only live below a certain temperature threshold. Since the Yellow Sea represents the southernmost extent at which many Arctic species can live, it is a good place to study changes in climate that lead to species migration.
The past can also help us understand what might happen under climate change in the future, by understanding the causes of migration and conditions that could push species to extinction.
A research team led by Dr He Wang and Dr Moriaki Yasuhara from the School of Biological Sciences and the Swire Institute of Marine Science of the University of Hong Kong studied the impact of East Asian winter monsoon on marine species in the Yellow Sea during the last Ice Age. The study was published in the journal Geophysical Research Letters.
Looking into the deep past
The East Asian winter monsoon causes cooler temperatures and stronger northern winds, often bringing snowstorms which cause damage to agriculture and infrastructure on land. Little had been known about how this meteorological system affects the ocean.
To get a long view of the impact of the East Asian winter monsoon on the marine environment and tease out any patterns, the team looked to the fossil record. They examined a long core of sediment drilled from the seabed, looking for small creatures called ostracods.
Ostracods are millimetre-scale crustaceans – tiny shrimp-like animals. Ostracods live across the world, but only certain species are adapted to Arctic conditions. They are possessed of calcium carbonate shells that can be well preserved as fossils available even in small sediment cores. That affords the opportunity to look far into the past to the distribution of earlier Arctic species. By seeing which species lived in the Yellow Sea at different points in history, they could tell when the region was colder.
Matching this with climate information that records the strength of the East Asian winter monsoon allowed the researchers to see its affect on the ostracod populations.
They identified two periods of migration of Arctic ostracods into the Yellow Sea, which corresponded with when the East Asian winter monsoon was stronger – when it produced colder temperatures and stronger winds. These periods were from 120,000–100,000 years ago and from 30,000–15,000 years ago.
In the modern Yellow Sea, there is a region at the bottom of the sea that is colder and saltier, called the Yellow Sea Bottom Cold Water zone, where two species of cold-adapted ostracods live. The appearance of more cold-adapted ostracod species during the two periods of stronger East Asian winter monsoon identified in the study suggests that the system plays a key role in making the zone colder, or reduces sea-surface temperatures, both of which would allow more Arctic species to move in.
Looking to the future
What does this mean for the East Asian winter monsoon and the Yellow Sea today? The sediment core records hundreds of thousand years of history, allowing long-term trends to be determined. “Fossil records, in this case ostracods, in sediment cores are a very useful indicator, telling us about marine communities and ecosystems in deep time, far beyond the range of biological observations normally cover just a few decades,” according to Yasuhara.
Wang notes that the Yellow Sea is getting warmer. Since many fewer Arctic species of ostracod live in the region today than in the periods of strengthened East Asian winter monsoon , the effect of warming could wipe out these species in the Yellow Sea. This would restrict their range, pushing them further north and making them more vulnerable to extinction altogether with continued warming. The Arctic itself is known to have warmed nearly four times faster than the rest of the world during the past few decades.
“In the Yellow Sea we know colder-adapted species are declining, and our study shows that they can be strongly affected by changes to the coldness of the water,” Wang said. “We need to protect these species, and only by understanding what affects them can we make plans for their conservation.”