Archive for phosphorus

The ocean is shifting toward phosphorus limitation

Posted by mmaheigan 
· Friday, February 28th, 2025 

Biogeochemical models predict that ocean warming is weakening the vertical transport of nutrients to the upper ocean, with severe implications for marine productivity. However, nutrient concentrations across the ocean surface often fall below detection limits, making it difficult to observe long-term changes.

In a recent study in PNAS, we analyzed over 30,000 nitrate and phosphate depth profiles observed between 1972 and 2022 to quantify nutricline depths, where nutrient concentrations are reliably detected. These depths accurately represent nutrient supplies in a global model, allowing us to assess long-term trends. Over the past five decades, upper ocean phosphate has mostly declined worldwide, while nitrate has remained mostly stable. Model simulations support that this difference is likely due to nitrogen fixation replenishing upper ocean nitrate, whereas phosphate has no equivalent biological source.

Figure caption: Five decades of global and regional nutricline depth data reveal declining phosphate-to-nitrate trends. Nutricline depths were defined based on threshold concentrations of 3 μmol kg−1 nitrate (TNO3) and 3/16 μmol kg−1 phosphate (TPO4). Site-specific trends were quantified for each unique pair of geographic coordinates where sufficient data was available (TNO3, n = 1,859 sites; TPO4, n = 1,641 sites). Shown are 95% confidence intervals (CI95%) calculated for each median trend by generating 10,000 bootstrap samples. The curves over the histograms depict the kernel densities. The sets of error bars from top to bottom are the interquartile ranges of TNO3 and TPO4 from a monthly climatology, the total observations, and the total observations with added measurement error.

These findings suggest that the ocean is becoming more limited in phosphorus. This decline could make phytoplankton less nutritious for marine animals. Fish larvae growth rates correlate with phosphorus availability in the ecosystem, so intensifying phosphorus limitation may greatly impact fisheries worldwide.

 

Authors
Skylar Gerace (University of California, Irvine)
Jun Yu (University of California, Irvine)
Keith Moore (University of California, Irvine)
Adam Martiny (University of California, Irvine)

@UCI_OCEANS

A new Regional Earth System Model of the Mediterranean Sea biogeochemical dynamics

Posted by mmaheigan 
· Thursday, November 19th, 2020 

The Mediterranean Sea is a semi-enclosed mid-latitude oligotrophic basin with a lower net primary production than the global ocean. A west-east productivity trophic gradient results from the superposition of biogeochemical and physical processes, including the biological pump and associated carbon and nutrient (nitrogen, phosphorus) fluxes, the spatial asymmetric distribution of nutrient sources (rivers, atmospheric deposition, coastal upwelling, etc.), the estuarine inverse circulation associated with the inflow of Atlantic water through the Gibraltar Strait. The complex and highly variable interface between land and sea throughout this basin add a further layer of complexity in the Mediterranean oceanic and atmospheric circulation and on the associated biogeochemistry dynamics, emphasizing the need for high-resolution truly integrated Regional Earth System Models to track and understand fine-scale processes and ecosystem dynamics.

In a recent paper published in the Journal of Advances in Modeling Earth System, the authors introduced a new version of the Regional Earth System model RegCM-ES and evaluated its performance in the Mediterranean region. RegCM-ES fully integrates the regional climate model RegCM4, the land surface scheme CLM4.5 (Community Land Model), the river routing model HD (Hydrological Discharge Model), the ocean model MITgcm (MIT General Circulation model) and the Biogeochemical Flux Model BFM.

A comparison with available observations has shown that RegCM-ES was able to capture the mean climate of the region and to reproduce horizontal and vertical patterns of chlorophyll-a and PO4 (the limiting nutrient in the basin) (Figure 1). The same comparison revealed a systematic underestimation of simulated dissolved oxygen (which will be fixed by the use of a new parametrization of oxygen solubility), and an overestimation of NO3, possibly due to uncertainties in initial and boundary conditions (mostly traced to river and Dardanelles nutrient discharges) and an overly vigorous vertical mixing simulated by the ocean model in some parts of the Basin.

Figure.1 Distributions of chlorophyll-a mg/m3 (top) and PO4 mmol/m3 (bottom) in the Mediterranean Sea as simulated by RegCM-ES.

Overall, this analysis has demonstrated that RegCM-ES has the capabilities required to become a powerful tool for studying regional dynamics and impacts of climate change on the Mediterranean Sea and other ocean basins around the world.

 

Authors:
Marco Reale (Abdus Salam International Centre for theoretical physics-ICTP, National Institute of Oceanography and Experimental Geophysics-OGS)
Filippo Giorgi (Abdus Salam International Centre for theoretical physics-ICTP)
Cosimo Solidoro (National Institute of Oceanography and Experimental Geophysics-OGS)
Valeria Di Biagio (National Institute of Oceanography and Experimental Geophysics-OGS)
Fabio Di Sante (Abdus Salam International Centre for theoretical physics-ICTP)
Laura Mariotti (National Institute of Oceanography and Experimental Geophysics-OGS)
Riccardo Farneti (Abdus Salam International Centre for theoretical physics-ICTP)
Gianmaria Sannino (Italian National Agency for New Technologies, Energy and Sustainable Economic Development-ENEA)

Biogeochemical controls of surface ocean phosphate

Posted by mmaheigan 
· Tuesday, November 12th, 2019 

Phosphorus availability is important for phytoplankton growth and more broadly ocean biogeochemical cycles. However, phosphate concentration is often below the analytical detection limit of the standard auto-analyzer technique. Thus, we know little about geographic phosphate variation across most low latitude regions. To address this issue, a global collaboration of scientists conducted a study published in Science Advances on combined phosphate measurements using high-sensitivity methods that yielded a detailed map of surface phosphate (Figure 1).

Figure 1: Fine-scale global variation of surface phosphate. Surface phosphate measured using high-sensitivity techniques revealed previously unrecognized low latitude differences in phosphate drawdown.

The study’s new globally expansive phosphate data set revealed previously unrecognized low-phosphate areas, including large regions of the Pacific Ocean—really low phosphate in the western North Pacific and to a lesser extent in the South Pacific. Although atmospheric iron input and nitrogen fixation are commonly described as regulators of surface phosphate, this study shows that shifts in the elemental stoichiometry (N:P:Fe) of the vertical nutrient supply play an additional role. Previous studies and climate models have suggested that the availability of phosphate is a first-order driver of ocean biogeochemical changes. Interestingly, this study suggests that marine ecosystems are more resilient to phosphate stress than previously thought. These findings underscore the importance of accurately quantifying nutrients at low concentrations for understanding the regulation of ocean ecosystem processes and biogeochemistry now and under future climate conditions.

And the data are of course available in BCO-DMO!

 

Authors:
Adam C. Martiny (University of California, Irvine)
Michael W. Lomas (Bigelow Laboratory for Ocean Sciences)
Weiwei Fu (University of California, Irvine)
Philip W. Boyd (University of Tasmania)
Yuh-ling L. Chen (National Sun Yat-sen University)
Gregory A. Cutter (Old Dominion University)
Michael J. Ellwood (Australian National University)
Ken Furuya (The University of Tokyo)
Fuminori Hashihama (Tokyo University of Marine Science and Technology)
Jota Kanda (Tokyo University of Marine Science and Technology)
David M. Karl (University of Hawaii)
Taketoshi Kodama (Japan Fisheries Research and Education Agency)
Qian P. Li (Chinese Academy of Sciences)
Jian Ma (Xiamen University)
Thierry Moutin (Université de Toulon)
E. Malcolm S. Woodward (Plymouth Marine Laboratory)
J. Keith Moore (University of California, Irvine)

Role for iron in controlling microbial phosphorus acquisition in the ocean

Posted by mmaheigan 
· Thursday, October 12th, 2017 

In the subtropical North Atlantic, dissolved inorganic phosphorus (DIP) concentrations are depleted and might co-limit N2 fixation and microbial productivity. There are relatively large pools of dissolved organic phosphorus (DOP), but microbes need an enzyme to access this P source. One such alkaline phosphatase (APase) enzyme requires zinc (Zn) as its activating cofactor. This has been known for almost 30 years. However, recent crystallography studies revealed that two other widespread APase enzymes contain Fe. Via this requirement, Fe availability could regulate microbial access to the DOP pool.

As detailed in a recent publication in Nature Communications (Browning et al. 2017), this hypothesis was tested on a cruise across the tropical North Atlantic by adding Fe and Zn to incubated seawater and monitoring changes in bulk APase using a simple fluorescence assay. Adding Fe significantly increased APase activity in seawater samples collected in areas that were far-removed from coastal and aerosol Fe sources. Despite seawater Zn concentrations being much lower than Fe, it appeared not to be limiting.

 

Iron (Fe) and zinc (Zn) enrichment experiments conducted in the DIP-depleted tropical North Atlantic suggested that Fe, not Zn, could limit alkaline phosphatase activity (APA). DIP*=DIP–DIN/16, and represents excess DIP availability assuming a 16-fold higher microbial N requirement. Results in the bar chart represent a subset of treatments from one experiment (out of eight conducted).

DIP is depleted in surface waters of the tropical North Atlantic because inputs of North African aerosol Fe stimulates N2 fixation and leads to microbial drawdown of DIP. If the modern ocean is a good analog for the past, the lack of APase stimulation following experimental Zn addition could reflect limited evolutionary selection for Zn-containing APase. In general, DIP is only substantially depleted where there is enhanced Fe input fueling N2 fixation; it therefore follows that any significant requirement for APases might be restricted to these relatively high-Fe, low-Zn waters.

On a shorter timescale, growing anthropogenic nitrogen input to the ocean relative to phosphorus could result in more prevalent oceanic phosphorus deficiency. Corresponding iron inputs might then serve as an important control on phosphorus availability for microbes in these regions.

 

Authors:

Tom Browning (GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany)
Eric Achterberg (GEOMAR) 
Jaw Chuen Yong (GEOMAR)
Insa Rapp (GEOMAR)
Caroline Utermann (GEOMAR) 
Anja Engel (GEOMAR)
Mark Moore (Ocean and Earth Science, University of Southampton, Southampton, UK)

 

Sinking particles as biogeochemical hubs for trace metal cycling and release

Posted by mmaheigan 
· Thursday, September 14th, 2017 

The extent to which the return of major and minor elements to the dissolved phase in the deep ocean (termed remineralization) is decoupled plays a major role in setting patterns of nutrient limitation in the global ocean. It is well established that major elements such as phosphorus, silicon, and carbon are released at different rates from sinking particles, with major implications for nutrient recycling. Is this also the case for trace metals?

A recent publication by Boyd et al. in Nature Geoscience provides new insights into the biotic and abiotic processes that drive remineralization of metals in the ocean.  Particle composition changes rapidly with depth with both physical (disaggregation) and biogeochemical (grazing; desorption) processes leading to a marked decrease in the total surface area of the particle population. The proportion of lithogenic metals in sinking particles also appears to increase with depth, as the biogenic metals may be more labile and hence more readily removed.

Findings from GEOTRACES process studies revealed that release rates for trace elements such as iron, nickel, and zinc vary from each other. Microbes play a key role in determining the turnover rates for nutrients and trace elements. Decoupling of trace metal recycling in the surface ocean and below may result from their preferential removal by microbes to satisfy their nutritional requirements. In addition, the chemistry operating on particle surfaces plays a pivotal role in determining the specific fates of each trace metal. Teasing apart these factors will take time, as there is a complex interplay between chemical and biological processes. Improving our understanding is crucial, as these processes are not currently well represented by state-of-the-art ocean biogeochemical models.

Figure caption: Rapid changes in the characteristics of sinking particles over the upper 200 m as evidenced by: a) differential release of trace metals from sinking diatoms; b) changes in proportion of lithogenic versus biogenic materials; and c) ten-fold decrease in total particle surface area.

 

Authors:
Philip Boyd (IMAS, Australia)
Michael Ellwood (ANU, Australia)
Alessandro Tagliabue (Liverpool, UK)
Ben Twining (Bigelow, USA)

 

Relevant links:
GEOTRACES Digest: Iron Superstar

Joint workshop with GEOTRACES in August 2016: Biogeochemical Cycling of Trace Elements within the Ocean

Do rivers supply nutrients to the open ocean?

Posted by mmaheigan 
· Wednesday, May 24th, 2017 

Rivers carry large amounts of nutrients (e.g., nitrogen and phosphorus) to the sea, but we do not know how much of that riverine nutrient supply escapes biological and chemical processing in shallow coastal waters to reach the open ocean. Most global ocean biogeochemical models, which are typically unable to resolve coastal processes, assume that either all or none of the riverine nutrients entering coastal waters actually contribute to open ocean processes.

While we know a good deal about the dynamics of individual rivers entering the coastal ocean, studies to date have been limited to a few major river systems, mainly in in developed countries. Globally, there are over 6,000 rivers entering the coastal ocean. In a recent study, Sharples et al (2017) devised a simple approach to obtain a global-scale estimate of riverine nutrient inputs based on the knowledge that low-salinity waters entering the coastal ocean tend to form buoyant plumes that turn under the influence of Earth’s daily rotation to flow along the coastline. Using published data on such flows and incorporating the effect of Earth’s rotation, they obtained estimates of typical cross-shore plume width and compared them to the local width of the continental shelf. This was used to calculate the residence time of riverine nutrients on the shelf, which is the key to estimating how much of a given nutrient is consumed in shelf waters vs. how much is exported to the open ocean.

Global distribution of the amount of riverine dissolved inorganic nitrogen that escapes the continental shelf to reach the open ocean.

The results indicate that, on a global scale, 75% (80%) of the nitrogen (phosphorus) supplied by rivers reaches the open ocean, whereas 25% (20%) of the nitrogen (phosphorus) is consumed on the shelf (e.g., fueling coastal productivity). Limited knowledge of nutrient cycling and consumption in shelf waters represents the primary source of uncertainty in this study. However, well-defined global patterns related to human land use (e.g., agricultural fertilizer use in developed nations) emerged from this analysis, underscoring the need to understand how land-use changes and other human activities will alter nutrient delivery to the coastal ocean in the future.

 

Authors:
Jonathan Sharples (School of Environmental Sciences, University of Liverpool, UK)
Jack Middelburg (Department of Earth Sciences, Utrecht University, Netherlands)
Katja Fennel (Department of Oceanography, Dalhousie University, Canada)
Tim Jickells (School of Environmental Sciences, University of East Anglia, UK)

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