Deep-Sea Corals Reveal Dramatic Shift in Pacific Ocean Ecosystem

It turns out that a dramatic, long-term shift in the Pacific Ocean ecosystem may have occurred. Scientists have discovered that long-lived deep-sea corals hold a record of changes in the marine food web that could reveal a bit more about the history of our marine environment and could tell them a bit more about the ocean's gyres.

Deep-sea corals are colonial organisms that can live for thousands of years. They feed on organic matter that rains down from the upper levels of the ocean. Because their skeletons are made out of a hard protein material that incorporates the chemical signatures of their food sources, scientists can use them to examine what exactly the environment was like in the past.

"They're like living sediment traps, recording long-term changes in the open ocean that we can't see any other way," said Matthew McCarthy, one of the researchers, in a news release.

While there are sediments on the ocean floor, these cores aren't very useful for the most recent millennia. In the open ocean of the North Pacific, sediment accumulates so slowly that the entire Holocene epoch, which is the past 12,000 years or so, is represented by less than four inches of sediment. That's why the researchers turned to coral skeletons.

In order to analyze these skeletons, the researchers combined carbon dating with a method for analyzing nitrogen isotopes in proteins. This allowed them to reconstruct records over the past 1,000 years. In the end, they found that a shift occurred around 1850 in the source of nitrogen feeding the surface waters of the open ocean. As a result of decreasing nitrogen inputs from subsurface water, the phytoplankton community became increasingly dominated by nitrogen-fixing cyanobacteria.

This shift in the ecosystem could actually be the result of the North Pacific Subtropical Gyre expanding and becoming warmer. There would be more stable layering of warmer surface water over cooler subsurface water. This increased "stratification" would limit the amount of nutrients delivered to the surface in nutrient-rich subsurface water. This means that large-scale shifts in ocean conditions switched on at the end of the Little Ice Age and are still continuing today.

The findings are changing scientists' notions about the stability of the open ocean gyres such as the North Pacific Subtropical Gyre. Although these open ocean gyres were once considered to be relatively static, nutrient-deprived "deserts," it seems that this may not be the case. The findings could be important for better understanding the oceanic ecosystem which could, in turn, impact environmental conditions across the globe.

The findings are published in the journal Nature.