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How do ocean microbes share the job of denitrification?

Posted by mmaheigan

· Monday, March 31st, 2025

Denitrification is a crucial multi-step process for ecosystem productivity and sustainability because some of its steps can result in the loss of the essential nutrient nitrogen or the production of greenhouse gas nitrous oxide. We do not understand why microbial functional groups conducting different steps of denitrification can coexist in the ocean and why certain groups are more abundant than others.

In a recent study published in PNAS, we uncover ecological mechanisms that govern the coexistence of these microbes. For the microbial groups utilizing different nitrogen substrates, the “stronger” groups rely on the “weaker” groups to feed them nitrogen (with respect to the organic substrates that they compete for), enabling them to coexist. For the groups competing for the same nitrogen substrates, microbes that invest more to build longer denitrification steps win the competition when nitrogen is limiting, but lose the game when nitrogen is repleted and organic carbon is limiting. The spatial and temporal variability of nutrients in the ocean allows these microbes to be observed in the same water mass.

Figure caption: Temporal and spatial heterogeneity in nutrients promotes the coexistence of functionally diverse denitrifiers in the ocean.

These hypothesized coexistence patterns help us predict where and when nitrogen loss and nitrous oxide production may occur. As human activities continue to alter marine nutrient balances, these predictions help us better anticipate ocean responses and design better strategies for mitigating negative anthropogenic impacts on the ocean.

Authors
Xin Sun (Carnegie Institution for Science) @xinsun-putiger.bsky.social
Emily Zakem (Carnegie Institution for Science) @carnegiescience.bsky.social

Nutrient management improves hypoxia in the Chesapeake Bay despite record-breaking precipitation and warming

Posted by mmaheigan

· Friday, August 26th, 2022

Hypoxia is currently one of the greatest threats to coastal and estuarine ecosystems around the world, and this threat is projected to get worse as waters warm and human populations continue to increase. Over the past 35-years, a massive effort has been underway to decrease hypoxia in the Chesapeake Bay by reducing nutrient input from land. Despite this effort, record-breaking precipitation in 2018-2019 fueled particularly large hypoxic volumes in the Bay, calling into question the efficacy of management actions.

Figure 1. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

In a recent paper published in Science of the Total Environment, researchers used empirical and numerical modeling to quantify the impact of nutrient management efforts on hypoxia in the Chesapeake Bay. Results suggest that if the nutrient reduction efforts beginning in 1985 had not taken place, hypoxia would have been ~50–120% greater during the average discharge years of 2016–2017 and ~20–50% greater during the wet years of 2018–2019. The management impact was most pronounced in regions of the Bay where the hypoxia season would have been 60-90 days longer if nutrient reductions did not occur (Figure 1).

Although these results suggest that management has reduced hypoxic conditions in the Bay, additional analysis revealed that warming temperatures have already offset 6-34% of this improvement. This highlights the importance of factoring in climate change when setting future management goals.

Figure 2. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

Authors:
Luke T. Frankel (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Aaron J. Bever (Anchor QEA)
Romuald N. Lipcius (Virginia Institute of Marine Science, William & Mary)
Gopal Bhatt (Pennsylvania State University; Chesapeake Bay Program)
Gary W. Shenk (USGS; Chesapeake Bay Program)

Contrasting N2O fluxes of source vs. sink in western Arctic Ocean during summer 2017

Posted by mmaheigan

· Wednesday, October 20th, 2021

During the western Arctic summer season both physical and biogeochemical features differ with latitude between the Bering Strait and Chukchi Borderland. The southern region (Bering Strait to the Chukchi Shelf) is relatively warm, saline, and eutrophic, due to the intrusion of Pacific waters that bring heat and nutrients in to the western Arctic Ocean (WAO). Because of the Pacific influence, the WAO is one of the most productive stretches of ocean in the world. In contrast, the northern region (Chukchi Borderland to the Canada Basin) is primarily influenced by freshwater originating from sea ice melt and rivers, and is relatively cold, fresh, and oligotrophic. A frontal zone exists between the southern region and northern region (~73°N) due to the distinct physicochemical contrast between mixing Pacific waters and freshwater. These regions support distinct bacterial communities also, making the environmental variations drivers extremely relevant to nitrous oxide (N₂O) dynamics.

A recent study published in Scientific Reports examined the role of the WAO as a source and a sink of atmospheric N₂O. There are obvious differences in N₂O fluxes between southern Chukchi Sea (SC) and northern Chukchi Sea (NC). In the SC (Pacific water characteristics dominate) N₂O emissions act as a net source to the atmosphere (Figure 1a). In the NC (freshwater dominant) absorption of atmospheric N₂O into the water column suggests that this region acts as a net sink (Figure 1a). The positive fluxes of SC occurred with relatively high sea surface temperature (SST), sea surface salinity (SSS), and biogeochemically-derived N₂O production, whereas the negative fluxes of NC were associated with relatively low SST, SSS, and little N₂O production. These linear relationships between N₂O fluxes and environmental variables suggest that summer WAO N₂O fluxes are remarkably sensitive to environmental changes.

Figure 1. (a) Map of the sampling stations using the Ice Breaking R/V Araon during August 2017. The sampling locations were coloured with N2O fluxes (blue to red gradient, see color bar; sink, air → sea (−), and source, sea → air (+). The southern Chukchi Sea (SC) extends from Bering Strait to Chukchi Shelf and the northern Chukchi Sea (NC) extends from Chukchi Borderland and Canada Basin. The frontal zone arises between SC and NC (black dotted line). (b) Illustration showing future changes in the distribution of the WAO N2O flux constrained by the positive feedback scenario of increasing inflow of Pacific waters and rapidly declining sea-ice extent under accelerating Arctic warming.

This study suggests a potential scenario for future WAO changes in terms of accelerating Arctic change. Increasing inflow of the Pacific waters and rapidly declining sea-ice extent are critical. The increasing inflow of warm nutrient-enriched Pacific waters will likely extend the SC N₂O source region northward, increasing productivity, and thereby intensifying nitrification. All of which would lead to a strengthening of the WAO’s role as an N₂O source. A rapid loss of the sea ice extent could ultimately lead to a sea-ice-free NC, and again, a northward shift, which would result in a diminished role of the NC as an N₂O sink (Figure 1b). While improving our understanding of WAO N₂O dynamics, this study suggests both a direction for future work and a clear need for a longer-term study to answer questions about both seasonal variations in these dynamics and possible interannual to climatological trends.

Authors:
Jang-Mu Heo (Department of Marine Science, Incheon National University)
Sang-Min Eom (Department of Marine Science, Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)
Hyo-Ryeon Kim (Department of Marine Science, Incheon National University)
Joo-Eun Yoon (Department of Marine Science, Incheon National University)
Il-Nam Kim (Department of Marine Science, Incheon National University)

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)

A role for tropical nitrogen fixers in glacial CO2 drawdown

Posted by mmaheigan

· Wednesday, December 4th, 2019

Iron fertilization of marine phytoplankton by Aeolian dust is a well-established mechanism for atmospheric carbon dioxide (CO₂) drawdown by the ocean. When atmospheric CO₂ decreased by 90-100 ppm during previous ice ages, fertilization of iron-limited phytoplankton in the high latitudes was thought to have contributed up to ¹/₃ (30 ppm) of the total CO₂ drawdown. Unfortunately, recent modeling studies suggest that substantially less CO₂ (only 2-10 ppm) is sequestered by the ocean in response to high latitude fertilization.

The limited capacity for high latitude CO₂sequestration in response to iron enrichment motivated the authors of a new study published in Nature Communications to address how lower latitude phytoplankton could contribute to CO₂ drawdown. The authors used an ocean model to show that in response to Aeolian iron fertilization, dinitrogen (N₂) fixers, specialized phytoplankton that introduce bioavailable nitrogen to tropical surface waters, drive the sequestration of an additional 7-16 ppm of CO₂ by the ocean.

Figure 1: Scenarios of Fe supply to the tropical Pacific. In the low iron scenario, analogous to the modern climate, N₂ fixation (yellow zone and dots) is concentrated in the Northwest and Southwest subtropical Pacific where aeolian dust deposition is greatest. Non-limiting PO₄ concentrations (green zone and dots) exist within the tropics and spread laterally from the area of upwelling near the Americas and at the equator (blue zone). In the high Fe scenario, analogous to the glacial climate, N₂ fixation couples to the upwelling zones in the east Pacific, enabling strong utilisation of PO₄, the vertical expansion of suboxic zones (grey bubbles) and a deeper injection of carbon-enriched organic matter (downward squiggly arrows).

These results provide evidence of a tropical ocean CO₂ sequestration pathway, the mere existence of which is hotly debated. Importantly, the study describes an additional mechanism of CO₂ drawdown that is complementary to the high latitude mechanism. When combined, their contributions elevate iron-driven CO₂ drawdown towards the expected 30 ppm, making iron fertilization a driver of a stronger biological pump on a global scale.

Authors:
Pearse Buchanan (University of Liverpool, University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate System Science)
Zanna Chase (University of Tasmania)
Richard Matear (CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes)
Steven Phipps (University of Tasmania)
Nathaniel Bindoff (University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes, Antarctic Climate and Ecosystems Cooperative Research Centre)

Ocean microbes drive fluctuating nutrient loss

Posted by mmaheigan

· Tuesday, May 28th, 2019

The removal of bioavailable nitrogen (N) by anaerobic microbes in the ocean’s oxygen deficient zones (ODZs) is thought to vary over time primarily as a result of climate impacts on ocean circulation and primary production. However, a recent study in PNAS using a data-constrained model of the microbial ecosystem in the world’s largest ODZ revealed that internal species oscillations cause local- to basin-scale fluctuations in the rate of N loss, even in a completely stable physical environment. Such ecosystem oscillations have been hypothesized for nearly a century in idealized models, but are rarely shown to persist in a three-dimensional ocean circulation model.

Figure caption. Ecological variability in the basin-scale rate of nitrogen loss over time (left) and in the local-scale contribution of autotrophic anammox to total N loss (right) in a model with unchanging ocean circulation. In the left panel, colors represent model simulations with different biological parameters. In the right panel, colors represent distinct locations within the ODZ in the standard model simulation.

These emergent ecosystem dynamics arise at the oxic-anoxic interface from O₂-dependent resource competition between aerobic and anaerobic microbes, and leave a unique geochemical fingerprint: infrequent spikes in ammonium that are observable in nutrient measurements from the ODZ. Non-equilibrium ecosystem behavior driven by competition among aerobic nitrifiers, anaerobic denitrifiers, and anammox bacteria also generates fluctuations in the balance of autotrophic versus heterotrophic N loss pathways that help reconcile conflicting field observations.

These internally driven fluctuations in microbial community structure partially obscure a direct correspondence between the chemical environment and microbial rates, a universal assumption in biogeochemical models. Because of the fundamental nature of the underlying mechanism, similar dynamics are hypothesized to occur across wide-ranging microbial communities in diverse habitats.

Authors:
Justin L. Penn (University of Washington)
Thomas Weber (University of Rochester),
Bonnie X. Chang (University of Washington, NOAA)
Curtis Deutsch (University of Washington)

See also the OCB2019 plenary session: Anthropogenic changes in ocean oxygen: Coastal and open ocean perspectives (Monday, June 24, 2019)

Dramatic Increase in Chlorophyll-a Concentrations in Response to Spring Asian Dust Events in the Western North Pacific

Posted by mmaheigan

· Tuesday, October 23rd, 2018

According to Martin’s iron hypothesis, input of aeolian dust into the ocean environment temporarily relieves iron limitation that suppresses primary productivity. Asian dust events that originate in the Taklimakan and Gobi Deserts occur primarily in the spring and represent the second largest global source of dust to the oceans. The western North Pacific, where productivity is co-limited by nitrogen and iron, is located directly downwind of these source regions and is therefore an ideal location for determining the response of open water primary productivity to these dust input events.

Figure 1. Daily aerosol index values (black squares) and chlorophyll-a concentrations (mg m-3, circles) during the spring (a) 2010 (weak dust event), (b) 1998 (strong dust event) in the western North Pacific. Color scale represents difference between mixed layer depth (MLD) and isolume depth (Z0.054) that indicates conditions for typical spring blooms; water column structures of MLD and isolume were identical in the spring of 1998 and 2010. Dramatic increases in chlorophyll-a (pink shading, maximum of 5.3 mg m-3) occurred in spring 1998 with a lag time of ~10 days after the strong dust event (aerosol index >2.5) on approximately April 20 compared to constant chlorophyll-a values (<2 mg m-3) in the spring of 2010.

A recent study in Geophysical Research Letters included an analysis of the spatial dynamics of spring Asian dust events, from the source regions to the western North Pacific, and their impacts on ocean primary productivity from 1998 to 2014 (except for 2002–2004) using long-term satellite observations (daily aerosol index data and chlorophyll-a). Geographical aerosol index distributions revealed three different transport pathways supported by the westerly wind system: 1) Dust moving predominantly over the Siberian continent (>50°N); 2) Dust passing across the northern East/Japan Sea (40°N‒50°N); and 3) Dust moving over the entire East/Japan Sea (35°N‒55°N). The authors observed that strong dust events could increase ocean primary productivity by more than 70% (>2-fold increase in chlorophyll-a concentrations, Figure 1) compared to weak/non-dust conditions. This result suggests that spring Asian dust events, though episodic, may play a significant role in driving the biological pump, thus sequestering atmospheric CO₂ in the western North Pacific.

Another recent study reported that anthropogenic nitrogen deposition in the western North Pacific has significantly increased over the last three decades (i.e. relieving nitrogen limitation), whereas this study indicated a recent decreasing trend in the frequency of spring Asian dust events (i.e. enhancing iron limitation). Further investigation is required to fully understand the effects of contrasting behavior of iron (i.e., decreasing trend) and nitrogen (i.e., increasing trend) inputs on the ocean primary productivity in the western North Pacific, paying attention on how the marine ecosystem and biogeochemistry will respond to the changes.

Authors:
Joo-Eun Yoon (Incheon National University)
Il-Nam Kim (Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)

Widespread nutrient co-limitation discovered in the South Atlantic

Posted by mmaheigan

· Thursday, March 15th, 2018

Unicellular photosynthetic microbes—phytoplankton—are responsible for virtually all oceanic primary production, which fuels marine food webs and plays a fundamental role in the global carbon cycle. Experiments to date have suggested that the growth of phytoplankton across much of the ocean is limited by either nitrogen or iron. But simultaneously low concentrations of these and other nutrients have been measured over large areas of the open ocean, raising the question: Are phytoplankton communities only limited by a single nutrient?

Authors of a study recently published in Nature tested this by conducting nutrient addition experiments on a GEOTRACES cruise in the nutrient-deficient South Atlantic gyre. Seawater samples were amended with nitrogen, iron, and cobalt both individually and in various combinations. Concurrent nitrogen and iron addition stimulated increased phytoplankton growth, yielding a ~40-fold increase in chlorophyll a. Supplementary addition of cobalt or cobalt-containing vitamin B₁₂ further enhanced phytoplankton growth in several experiments.

Experiments conducted throughout the southeast Atlantic GEOTRACES GA08 cruise transect (left panel) demonstrated that nitrogen and iron had to be added to significantly stimulate phytoplankton growth (right panel). Supplementary addition of cobalt (or cobalt-containing vitamin B12) stimulated significant additional growth.

In addition to co-limited sites, the study identified ‘singly’ and ‘serially’ limited sites. These limitation regimes could be predicted by the measured ambient seawater nutrient concentrations, demonstrating the potential for using nutrient datasets to make confident predictions about limitation at larger spatial scales, an approach that is being more widely used in programmes like GEOTRACES,.

Finally, a complex, state-of-the-art biogeochemical ocean model suggested a much smaller extent of nutrient co-limitation than the experiments indicated. Authors attributed this to relatively restricted microbial and nutrient diversity in the model. These findings have implications for how such models are constructed if they are to represent nutrient co-limitation in the ocean and accurately project changes in ocean productivity in the future.

Authors:
Thomas J. Browning (GEOMAR)
Eric P. Achterberg (GEOMAR)
Insa Rapp (GEOMAR)
Anja Engel (GEOMAR)
Erin M. Bertrand (Dalhousie University)
Alessandro Tagliabue (University of Liverpool)
Mark Moore (University of Southampton)

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)

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)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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