Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Observed declines in upper ocean phosphate-to-nitrate availability
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.  more » « less
Award ID(s):
2023545
PAR ID:
10573765
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
PNAS
Date Published:
Journal Name:
Proceedings of the National Academy of Sciences
Volume:
122
Issue:
6
ISSN:
0027-8424
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; (, ASLO meeting 2021)
    null (Ed.)
    Dissolved organic phosphorus (DOP) has a dual role in the surface ocean as both a product of primary production and as an organic nutrient fueling primary production and nitrogen Fixation, especially in oligotrophic gyres. Though poorly constrained, understanding the geographic distribution and environmental controls of surface ocean DOP concentration is critical to estimating distributions and rates of primary production and nitrogen Fixation in the global ocean. Here we pair DOP concentration measurements with a metric of phosphate (PO43-) stress (P*), satellite-based chlorophyll a concentrations, and iron stress estimates to explore their relationship with upper 50 m DOP stocks. Our results show that PO43- and iron stress work together to control surface DOP concentrations at basin scales. SpeciFcally, upper 50 m DOP stocks decrease with increasing phosphate stress, while alleviated iron stress leads to either surface DOP accumulation or loss depending on PO43- availability. Our work suggests an interdependence between DOP concentration, inorganic nutrient ratios, and iron availability, and establishes a predictive framework for DOP distributions in the global surface ocean. 
    more » « less
  2. (, AGU Advances)
    Abstract Nutrient availability is a major driver of ocean biodiversity and productivity, yet long‐term global changes remain poorly constrained. Using over 14 million nitrate and phosphate measurements collected between 1925 and 2025, we quantify long‐term trends in nutrient concentrations across ocean biomes and depths. We find that surface waters in oligotrophic and mesotrophic regions show significant declines in phosphate, whereas nitrate is also slightly decreasing in oligotrophic but increasing in mesotrophic regions. Many coastal areas exhibit nutrient enrichment. Subsurface waters reveal widespread nitrate accumulation, suggesting an imbalance driven by biological nitrogen fixation and reduced vertical mixing. Comparison with Earth system models indicates that current simulations underestimate the pace of nutrient shifts. Our results highlight a large‐scale biogeochemical reorganization of ocean nutrient distributions that may intensify under future climate warming. 
    more » « less
  3. ; ; (, Global Biogeochemical Cycles)
    Surface ocean marine dissolved organic matter (DOM) serves as an important reservoir of carbon (C), nitrogen (N), and phosphorus (P) in the global ocean, and is produced and consumed by both autotrophic and heterotrophic communities. While prior work has described distributions of dissolved organic carbon (DOC) and nitrogen (DON) concentrations, our understanding of DOC:DON:DOP stoichiometry in the global surface ocean has been limited by the availability of DOP concentration measurements. Here, we estimate mean surface ocean bulk and semi‐labile DOC:DON:DOP stoichiometry in biogeochemically and geographically defined regions using newly available marine DOM concentration databases. Global mean surface ocean bulk (C:N:P = 387:26:1) and semi‐labile (C:N:P = 179:20:1) DOM stoichiometries are higher than Redfield stoichiometry, with semi‐labile DOM stoichiometry similar to that of global mean surface ocean particulate organic matter (C:N:P = 160:21:1) reported in a recent compilation. DOM stoichiometry varies across ocean basins, ranging from 251:17:1 to 638:43:1 for bulk and 83:15:1 to 414:49:1 for semi‐labile DOM C:N:P, respectively. Surface ocean DOP concentration exhibits larger relative changes than DOC and DON, driving surface ocean gradients in DOC:DON:DOP stoichiometry. Inferred autotrophic consumption of DOP helps explain intra‐ and inter‐basin patterns of marine DOM C:N:P stoichiometry, with regional patterns of water column denitrification and iron supply influencing the biogeochemical conditions favoring DOP use as an organic nutrient. Specifically, surface ocean marine DOM exhibits increasingly P‐depleted stoichiometries from east to west in the Pacific and from south to north in the Atlantic, consistent with patterns of increasing P stress and alleviated iron stress. 
    more » « less
  4. ; ; ; ; ; (, Deep Sea Research Part II: Topical Studies in Oceanography)
    Oligotrophic ocean regions are characterized by strong nutrient limitation, low standing phytoplankton biomass, and highly efficient nutrient recycling. During the BLOOFINZ-IO expedition (February 2022), we quantified nutrient inventories, primary productivity and N2 fixation in the Argo Basin of the eastern Indian Ocean, the sole known spawning ground for Southern Bluefin Tuna. Low concentrations of surface nitrate (<0.02 μmol L−1) and persistent residual phosphate indicated N as the limiting macronutrient, with photophysiological indices consistent with iron colimitation. Depth-integrated net primary production (NPP), from 14C-based in-situ incubations during 4 Lagrangian experiments, averaged ~460 mg C m−2d−1, generally agreeing with mean satellite-based NPP estimates (459 mg C m−2d−1) but with spatial discrepancies. Nitrogen fixation provided a consistent new nitrogen source, contributing ~16 % to local NPP in the upper euphotic zone. Gross primary production (GPP), from fast-repetition-rate-fluorometry-based estimates of electron transport, revealed significant autotrophic respiration losses, with mean GPP:NPP ratios of ~1.8 consistent with metabolic costs under nutrient limitation. Net community production (NCP) from O2/Ar ratios averaged ~20 % of NPP in the upper 30 m. This result, in combination with N2 fixation measurement, indicates that N2 fixation supports most of the export production in the region. Together, the multimethod approach revealed a recycling-dominated ecosystem affected by episodic mixing events, where primary productivity is maintained primarily through efficient nitrogen recycling and physiological photoacclimation. These results provide a comprehensive baseline of bottom-up support of ecosystem productivity for the Argo Basin for assessing future climate-driven changes in stratification, nutrient cycling, and food-web dynamics. 
    more » « less
  5. ; ; ; ; (, Global Biogeochemical Cycles)
    Abstract Dissolved iron (dFe) plays an important role in regulating marine productivity. In high nutrient, low chlorophyll regions (>33% of the global ocean), iron is the primary growth limiting nutrient, and elsewhere iron can regulate nitrogen fixation by diazotrophs. The link between iron availability and carbon export is strongly dependent on the phytoplankton iron quotas or cellular Fe:C ratios. This ratio varies by more than an order of magnitude in the open ocean and is positively correlated with ambient dFe concentrations in field observations. Representing Fe:C ratios within models is necessary to investigate how ocean carbon cycling will interact with perturbations to iron cycling in a changing climate. The Community Earth System Model ocean component was modified to simulate dynamic, group‐specific, phytoplankton Fe:C that varies as a function of ambient iron concentration. The simulated Fe:C ratios improve the representation of the spatial trends in the observed Fe:C ratios. The acclimation of phytoplankton Fe:C ratios dampens the biogeochemical response to varying atmospheric deposition of soluble iron, compared to a fixed Fe:C ratio. However, varying atmospheric soluble iron supply has first order impacts on global carbon and nitrogen fluxes and on nutrient limitation spatial patterns. Our results suggest that pyrogenic Fe is a significant dFe source that rivals mineral dust inputs in some regions. Changes in dust flux and iron combustion sources (anthropogenic and wildfires) will modify atmospheric Fe inputs in the future. Accounting for dynamic phytoplankton iron quotas is critical for understanding ocean biogeochemistry and projecting its response to variations in atmospheric deposition. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Text Encoded Conversion
  • XML
  • Save / Share this Record
  • Send to Email