Decades-old specimens solve a long-standing mystery
Van Houtan and others had suspected upwelling played a role in sardine population trends, but scientists only started measuring the process in Monterey Bay in 1946. Historic seaweed specimens, Van Houtan realized, might fill in the blanks for earlier years—similar to the way ice cores can help reconstruct carbon dioxide levels from times before researchers started collecting real-time measurements.
For the new study, the scientists relied on the fact that deeper water near Monterey typically hosts more of a particular nitrogen isotope, a rarer version of nitrogen with an extra neutron that makes each atom heavier. Looking at modern upwelling data and recently collected seaweed, they found that higher levels of this nitrogen in the plants’ cells corresponded with periods of more upwelling. Next they measured the isotope levels in 70 historic specimens of the red seaweed Gelidium, gathered from Monterey as far back as 1878. The results suggested a gradual increase in upwelling and then a dramatic decrease, which lined up with the sardine population’s growth and decline.
“This paper is an excellent example of the creative detective work of historical ecology,” says Loren McClenachan, a marine ecologist at Colby College, who was not involved in the research. “There are thousands and thousands of similar specimens in collections around the world, and applying similar methods could teach us a great deal about long-term ocean change.”
This article was originally published with the title “Seaweed Sleuths” in Scientific American 323, 4, 22 (October 2020)