2025-02-04 カリフォルニア大学校アーバイン校(UCI)
＜関連情報＞
- https://news.uci.edu/2025/02/04/climate-change-is-overhauling-marine-nutrient-cycles-uc-irvine-scientists-say/
- https://www.pnas.org/doi/10.1073/pnas.2411835122
上層海域のリン酸塩から硝酸塩への変換可能性の減少が観測される Observed declines in upper ocean phosphate-to-nitrate availability
Skylar D. Gerace, Jun Yu, J. Keith Moore, and Adam C. Martiny
Proceedings of the National Academy of Sciences Published:February 4, 2025
DOI:https://doi.org/10.1073/pnas.2411835122
Significance
Earth System Models (ESMs) predict that ocean nutrients are decreasing from ocean warming with potentially severe impacts on marine life globally. However, direct observations have yet to support predictions due to the low detectability of nutrients throughout the ocean surface. Here, we quantified the depths where nitrate and phosphate reached well-detected concentrations through time over five decades. The temporal trends of these depths revealed that upper ocean phosphate is mostly declining, while nitrate is mostly stable. Using ESMs, we demonstrate that this difference is likely due to decreasing iron stress for nitrogen fixation, which replenishes nitrate with increasing stratification. These findings suggest that phosphorus limitation is expanding throughout the ocean, consequently bringing many important implications for marine ecosystems.
Abstract
Climate warming is increasing ocean stratification, which in turn should decrease the nutrient flux to the upper ocean. This may slow marine primary productivity, causing cascading effects throughout food webs. However, observing changes in upper ocean nutrients is challenging because surface concentrations are often below detection limits. We show that the nutricline depth, where nutrient concentrations reach well-detected levels, is tied to productivity and upper ocean nutrient availability. Next, we quantify nutricline depths from a global database of observed vertical nitrate and phosphate profiles to assess contemporary trends in global nutrient availability (1972–2022). We find strong evidence that the P-nutricline (phosphacline) is mostly deepening, especially throughout the southern hemisphere, but the N-nutricline (nitracline) remains mostly stable. Earth System Model (ESM) simulations support the hypothesis that reduced iron stress and increased nitrogen fixation buffer the nitracline, but not phosphacline, against increasing stratification. These contemporary trends are expected to continue in the coming decades, leading to increasing phosphorus but not nitrogen stress for marine phytoplankton, with important ramifications for ocean biogeochemistry and food web dynamics.
