Archive for upper ocean

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

Powerful new tools for working with Argo data

Posted by mmaheigan 
· Thursday, June 9th, 2022 

No single program has been as transformative for ocean science over the past two decades as Argo: the fleet of robotic instruments that collect measurements of temperature and salinity in the upper 2 km of the ocean around the globe. The Argo program has been instrumental in revealing changes to ocean heat content, global sea level, and patterns of ice melt and precipitation. In addition, Biogeochemical Argo—the branch of the Argo program focused on floats with additional biological and chemical sensors—has recently shed light on topics such as regional patterns of carbon production and export, the magnitude of carbon dioxide air-sea flux in the Southern Ocean (thanks to the SOCCOM project), and the dynamics modulating ocean oxygen concentrations and oxygen minimum zones. While Argo data are publicly available in near-real-time via two Global Data Assembly Centers, there tends to be a steep learning curve for new users seeking to access and utilize the data.

To address this issue, a team led by scientists at NOAA’s Pacific Marine Environmental Laboratory developed a software toolbox available in two programming languages for accessing and visualizing Argo data— OneArgo-Mat for MATLAB and OneArgo-R for R. The toolbox includes functions to identify and download float data that adhere to user-defined time and space constraints, and other optional requirements like sensor type and data mode; plot float trajectories and their current positions; filter and manipulate float data based on quality flags and additional metadata; and create figures (profiles, time series, and sections) displaying physical, biological, and chemical properties measured by floats. Examples of figures created using the OneArgo-Mat toolbox are given below (Figure 1).

Figure 1. Example figures created using the OneArgo-Mat toolbox: (A) the trajectory of a float deployed in the North Atlantic from the R/V Johan Hjort in May of 2019, (B) a time series of dissolved oxygen at 80 dbars from that float, and (C) a vertical section plot of nitrate concentrations along the float track from the surface to 300 dbars. The black contour line in panel C denotes the mixed layer depth (MLD) based on a temperature criterion and the red line denotes the depth of the time series shown in panel B. The effects of seasonal phytoplankton blooms are evident in panel C, with mixed layer shoaling in the spring followed by drawdown of nitrate in the surface ocean. Panel B shows that, as the mixed layer deepens through the winter, the oxygen concentration at 80 dbars increases as a result of the oxygenated surface waters reaching that depth. The MATLAB code to download the required data and create all of these plots is shown (D).

The OneArgo-Mat and OneArgo-R toolboxes are intended for newcomers to Argo data, seasoned users, data managers, and everyone in between. For this reason, toolbox functions are equipped with options to streamline float selection, data processing, and figure creation with minimal user coding, if desired. Alternatively, the toolbox also provides rapid and straightforward access to the entire Argo database for experienced users who simply want to download up-to-date profile data for further processing and analysis. The authors hope these new tools will empower current Argo data users and entrain new users, especially as the US GO-BGC Project and US and international Argo partners move toward a global biogeochemical Argo fleet, which will create myriad new opportunities for novel studies of ocean biogeochemistry.

 

Authors
Jonathan Sharp – Cooperative Institute for Climate, Ocean, and Ecosystem Studies (CICOES) & NOAA Pacific Marine Environmental Laboratory (PMEL)
Hartmut Frenzel – CICOES & NOAA PMEL
Marin Cornec – University of Washington & NOAA PMEL
Yibin Huang – University of California Santa Cruz & NOAA PMEL
Andrea Fassbender – NOAA PMEL

A new proxy for ocean iron bioavailability

Posted by mmaheigan 
· Monday, July 26th, 2021 

In many oceanic regions, iron exerts strong control on phytoplankton growth, ecosystem structure and carbon cycling. Yet, iron bioavailability and uptake rates by phytoplankton in the ocean are poorly constrained.

Recently, Shaked et al. (2020) (see GEOTRACES highlight), established a new approach for quantifying the availability of dissolved Fe (dFe) in natural seawater based on its uptake kinetics by Fe-limited cultured phytoplankton. In a follow up study published in GBC, this approach was extended to in situ phytoplankton, establishing a standardized proxy for dFe bioavailability in low-Fe ocean regions.

As explained in the short video lecture above, Yeala Shaked, Ben Twining, and their colleagues have analyzed large datasets collected during 10 research cruises (including 3 GEOTRACES section and process cruises) in multiple ocean regions. Dissolved Fe bioavailability was estimated through single cell Fe uptake rates, calculated by combining measured Fe contents of individual phytoplankton cells collected with concurrently-measured dFe concentrations, as well as modeled growth rates (Figure). Then the authors applied this proxy for: a) comparing dFe bioavailability among organisms and regions; b) calculating dFe uptake rates and residence times in low-Fe oceanic regions; and c) constraining Fe uptake parameters of earth system models to better predict ocean productivity in response to climate-change.

The data suggest that dFe species are highly available in low-Fe settings, likely due to photochemical reactions in sunlit waters.

Figure 1: The new bioavailability proxy (an uptake rate constant-kin-app) was calculated for ~1000 single cells from multiple ocean regions. For each cell, the iron quota was measured with synchrotron x-ray fluorescence (left panel), a growth rate was estimated from the PISCES model for the corresponding phytoplankton group (right panel), and the dissolved Fe concentration was measured concurrently (middle panel).

Authors:
Y. Shaked (Hebrew University and Interuniversity Institute for Marine Sciences)
B.S. Twining (Bigelow Lab)
A. Tagliabue (University of Liverpool)
M.T. Maldonado (University of British Columbia)
K.N. Buck (University of South Florida)
T. Mellett (University of South Florida)

References:
Shaked, Y., Twining, B. S., Tagliabue, A., & Maldonado, M. T. (2021). Probing the bioavailability of dissolved iron to marine eukaryotic phytoplankton using in situ single cell iron quotas. Global Biogeochemical Cycles, e2021GB006979. https://doi.org/10.1029/2021GB006979

Shaked, Y., Buck, K. N., Mellett, T., & Maldonado, M. T. (2020). Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics. The ISME Journal, 1–12. https://doi.org/10.1038/s41396-020-0597-3

 

Joint highlight with GEOTRACES – read here.

When GEOTRACES‐based synthesis efforts improve global iron-cycling understanding

Posted by mmaheigan 
· Friday, December 18th, 2020 

Authors of a recent paper published in Global Biogeochemical Cycles conducted a detailed study of the residence times of total and dissolved iron (Fe) in the upper layers (0-250m) of the global ocean. Using historical (1980-2007) and recent GEOTRACES data, they compiled an impressive data set comprising dissolved, filtered and trap-collected particulate Fe spanning different biogeochemical oceanographic provinces. They also used indirect isotopic approaches to calculate Fe export from the surface layers (e.g., based on thorium-234-uranium-238 disequilibrium). The study revealed that upper ocean residence times of total Fe consistently fell between 10 and 100 days, despite a broad range of total Fe inventories and ocean biogeochemical settings. Conversely, dissolved Fe residences times were longer and more variable, cycling on sub annual to annual time scales. In addition to these detailed insights on upper ocean Fe cycling, these new data sets will help constrain the rate constant for total Fe export, an important term for exploring links between ocean Fe cycling and the global carbon cycle in ocean biogeochemical models.

Figure Caption: In-situ iron concentration and export (Ftot) estimates from numerous GEOTRACES efforts were combined with prior study results to constrain the residence time of iron in the upper ocean (diagonal lines, lower panel). Broad patterns in iron residence times emerged when contrasting coastal and open regions (pink vs. white), as well as with high and low latitude zones (black vs. white). Despite clear regional differences, however, the majority of residence times for total iron fell into a small range between 10 and 100 days.

 

Authors:
E. E. Black (former WHOI, current Dalhousie University, Lamont Doherty Earth Observatory)
S. S. Kienast (Dalhousie University)
N. Lemaitre (Institute of Geochemistry and Petrology, Zürich, Switzerland)
P. J. Lam (University of California, Santa Cruz)
R. F. Anderson (Lamont Doherty Earth Observatory)
H. Planquette (University Brest)
F. Planchon (University Brest)
K. O. Buesseler (WHOI)

This is a joint highlight with GEOTRACES

Hurricane-driven surge of labile carbon into the deep North Atlantic Ocean

Posted by mmaheigan 
· Thursday, February 27th, 2020 

Tropical cyclones (hurricanes and typhoons) are the most extreme episodic weather event affecting subtropical and temperate oceans. Hurricanes generate intense surface cooling and vertical mixing in the upper ocean, resulting in nutrient upwelling into the photic zone and episodic phytoplankton blooms. However, their influence on the deep ocean is unknown.

Figure 1. (a) Particulate organic carbon (POC) flux and percentage of the total mass flux (yellow) (top panel); fluxes (middle panel) and POC-normalized concentrations (bottom panel) of diagnostic lipid biomarkers for phytoplankton-derived and labile material, zooplankton, bacteria, and other (see legend); (b) Lipid concentrations (left panel) and POC-normalized concentrations (right panel) of diagnostic lipid biomarkers for the same sources as in (a) (see legend) measured two weeks after Nicole’s passage (25-29 Oct. 2016). Shown for reference are total lipid concentration profiles in April 2015 (dark gray, typical post spring bloom conditions) and Nov 2015 (light gray, typical minimum production period).

In October 2016, Category 3 Hurricane Nicole passed over the Bermuda time-series site (Oceanic Flux Program (OFP) and Bermuda Atlantic Time-Series site (BATS)) in the oligotrophic NW Atlantic Ocean. In a recent study published in Geophysical Research Letters, authors synthesized multidisciplinary data from hydrographic and phytoplankton measurements and lipid composition of sinking and suspended particles collected from OFP and BATS, respectively, after Hurricane Nicole in 2016. After the hurricane passed, particulate fluxes of lipids diagnostic of fresh phytodetritus, zooplankton, and microbial biomass increased by 30-300% at 1500 m depth and 30-800% at 3200 m depth (Figure 1a). In addition, mesopelagic suspended particles were enriched in phytodetrital material, as well as zooplankton- and bacteria-sourced lipids (Figure 1b), indicating particle disaggregation and a deep-water ecosystem response.

These results suggest that carbon export and biogeochemical cycles may be impacted by climate-induced changes in hurricane frequency, intensity, and tracks, and, underscore the sensitivity of deep ocean ecosystems to climate perturbations.

Authors:
Rut Pedrosa-Pamies (Marine Biological Laboratory)
Maureen H. Conte (Bermuda Institute of Ocean Science and Marine Biological Laboratory)
JC Weber (Marine Biological Laboratory)
Rodney Johnson (Bermuda Institute of Ocean Science)

Upwelled hydrothermal Fe stimulates massive phytoplankton blooms in the Southern Ocean

Posted by mmaheigan 
· Tuesday, July 9th, 2019 

Joint feature with GEOTRACES

Figure 1a: Southern Ocean phytoplankton blooms showing distribution, biomass (circle size) and type (color key).

In a recent study, Ardyna et al combined observations of profiling floats with historical trace element data and satellite altimetry and ocean color data from the Southern Ocean to reveal that dissolved iron of hydrothermal origin can be upwelled to the surface. Furthermore, the activity of deep hydrothermal sources can influence upper ocean biogeochemical cycles of the Southern Ocean, and in particular stimulate the biological carbon pump.

Authors:
Mathieu Ardyna
Léo Lacour
Sara Sergi
Francesco d’Ovidio
Jean-Baptiste Sallée
Mathieu Rembauville
Stéphane Blain
Alessandro Tagliabue
Reiner Schlitzer
Catherine Jeandel
Kevin Robert Arrigo
Hervé Claustre

Alternative particle formation pathways identified in the Equatorial Pacific’s biological pump

Posted by mmaheigan 
· Tuesday, November 27th, 2018 

The ocean is one of the largest sinks of atmospheric carbon dioxide (CO2) on our planet, driven in part by CO2 uptake by phytoplankton in the upper ocean during photosynthesis. Eventually, a portion of the resulting organic carbon is transported to depth, where it is sequestered from the atmosphere for centuries or even millennia. Our current understanding of the biological pump is based on the export of organic material in the form of large, fast-sinking (hundreds of meters per day) particles. However, using lipids as biomarkers, a recent study from the Equatorial Pacific Ocean published in JGR Biogeosciences showed that fast-sinking particles are refractory and distinctly different from plankton in the mixed layer, whereas slow-sinking particles were more labile and had a more similar composition to mixed layer particles (Fig. 1).

Figure 1. Particle lipid compositions for different particle fractions: ML = homogenous mixed layer particles, SU = suspended, SS = slow-sinking, and FS = fast-sinking of a) labile compounds known as unsaturated fatty acids synthesized by phytoplankton that provide a lot of energy for heterotrophs and b) sterols, including cholesterol (dark blue), which can be a biomarker for heterotrophy. Mixed layer particles are the most labile, showing the least degree of heterotrophic reworking, as expected. However, fast-sinking particles are most dissimilar from those in the mixed layer, with only a small proportion of labile compounds and a high degree of heterotrophic reworking.

The authors proposed a slower, less efficient export pathway, by which phytoplankton initially aggregate to smaller, slower-sinking detrital particles and then gradually form highly degraded, larger particles that sink to depth. Since smaller particles are respired more rapidly than larger particles, the proportion of phytoplankton-captured atmospheric CO2 being stored in the deep ocean is likely reduced, particularly in regions dominated by smaller phytoplankton such as the Equatorial Pacific. This study clearly demonstrates the need for improved representation of a wider range of particle dynamics in models of the ocean’s biological pump.

 

Authors:
E. L. Cavan (University of Tasmania, previously University of Southampton)
S. Giering (National Oceanography Centre)
G. Wolff (University of Liverpool)
M. Trimmer (Queen Mary University London)
R. Sanders (National Oceanography Centre)

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