Largest continuous ecosystem on earth has undergone major shifts

Study finds that changes in the composition of phytoplankton community of North Pacific Subtropical Gyre relate to large-scale regional climate phenomena

[Deutsche Version folgt in Kürze]
It is unparalleled: the subtropical North Pacific Ocean has recently gone through a change of plankton regime that enhances nitrogen-fixing cyanobacterial production. Scientists of the Universities of California, Colorado and Kiel have discovered multiple regime shifts in plankton community composition corresponding to large-scale climate changes during the past 1000 years. These findings have major implications for understanding past and future changes in open ocean productivity and how carbon is removed and exported from the atmosphere to the ocean floor. The study has been published yesterday (Thursday, November 26) in the journal “Science Express”.
Dr. Kelton McMahon of the Ocean Science Department at the University of California explains: “We applied a cutting-edge compound-specific stable isotope fingerprinting approach to the skeleton of organic-based deep-sea corals. The corals that we used are particularly long-lived and the skeleton gives us the opportunity to look back in time”. Briefly, deep-sea corals act as living sediment traps, filtering out sinking organic matter, including the remains of phytoplankton, from the ocean surface. The carbon isotope signatures of the essential amino acids in the sinking organic matter are recorded in the organic skeleton of the deep-sea corals. These isotope fingerprints provide a unique, diagnostic tracer of the phylogenetic identity of the plankton groups that made that sinking organic matter. “In this way we were able to reconstruct a long term, high resolution record of the relative contribution of different phytoplankton groups back through time in the North Pacific Subtropical Gyre”, McMahon adds.
Dr. Thomas Larsen, research scientist of the Cluster of Excellence “The Future Ocean” at Kiel University is one of the co-authors of the study. He developed the analytical approach that made it possible to track the source and fate of essential amino acids from producers to consumers. Larsen: “It is an approach taking advantage of naturally occurring stable isotope patterns and it can be applied in widely different settings. For example, it can identify contribution of essential amino acids from gut bacteria to their animal host or it can document bacterial activity in ocean sediments.”
The study concludes that the nitrogen-fixing cyanobacteria increased over the last 150 years owing to the large-scale rising sea surface temperatures and a likely decrease in the trade winds. This resulted in less mixing of bottom and surface waters in turn decreasing nutrient availability. The lower nutrient availability resulted in a regime shift towards cyanobacteria that convert atmospheric nitrogen to bioavailable nutrients. Nevertheless, the authors find it unlikely that the observed shift to increased cyanobacterial production and associated removal of atmospheric carbon dioxide will be enough to counteract the harmful effects of rising carbon dioxide. In the future, conditions may become less favorable for nitrogen-fixing cyanobacteria because of ocean acidification and phosphate limitations.
Original publication:
“Millennial-scale plankton regime shifts in the subtropical North Pacific Ocean”.
Kelton W. McMahon, Matthew D. McCarthy, Owen A. Sherwood, Thomas Larsen, Thomas P. Guilderson. aaa9942

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