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Fanny Monteiro

University of Bristol, Geographical Sciences, NERC Research Fellow/Lecturer

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My general interest is to understand what drives marine ecosystem, biogeochemical cycles and climate to interact. I look in particular into the cycling of nitrogen, phosphorus, iron, oxygen and carbon using mathematical and numerical methods in close comparison with observations. I am currently involved with projects related to:- Coccolithophore and foraminifera ecology- Oceanic Anoxic Events of the Mesozoic using an Earth system model (GENIE)- Marine nitrogen cycle (nitrogen fixation, nitrification and denitrification)- Marine ecosystem and plankton diversity using an adaptive ecosystem model (MITgcm)
Supervisors: Mick Follows (Ph.D. supervisor)
Address: School of Geographical Sciences
University Road
Bristol, BS8 1SS
UK

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University of Bristol

Rachel A. Foster

Abdellah Laazizi

Margaret Mulholland

Ministry of Earth Sciences

National Academies of Sciences, Engineering and Medicine

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Papers by Fanny Monteiro

Distribution of diverse nitrogen fixers in the global ocean

Global Biogeochemical Cycles, 2010

We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, al...

Global SEM coccolithophore abundance compilation

Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the o... more Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the organic carbon pump through the primary production and the ballast of organic matter, and to the carbonate pump through the production of calcium carbonate. Here we compiled all available scanning electron microscopy (SEM) coccolithophore abundance observations. Taxa were standardized following NannoTax3 to a species level where possible. Subspecies (e.a. C. leptoporus subsp. leptoporus and C. leptoporus subsp. quadriperforatus) were grouped as single species. The database contains 2556 abundance observations from 35 different publications. The data span the period of 1993-2017, with observations from all ocean basins and all seasons, and at depths ranging from the surface to 5000 m. We limited our compilation to SEM observations (or observations which further identified samples with SEM) because SEM provides greater detail of coccolithophore diversity than more commonly used polarized li...

Haplo-diplontic life cycle expands coccolithophore niche

Biogeosciences, 2021

Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-dipl... more Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-diplontic life cycle. The haplo-diplontic life cycle allows coccolithophores to divide in both life cycle phases and potentially expands coccolithophore niche volume. Research has, however, to date largely overlooked the life cycle of coccolithophores and has instead focused on the diploid life cycle phase of coccolithophores. Through the synthesis and analysis of global scanning electron microscopy (SEM) coccolithophore abundance data (n = 2534), we find that calcified haploid coccolithophores generally constitute a minor component of the total coccolithophore abundance (≈ 2 %-15 % depending on season). However, using case studies in the Atlantic Ocean and Mediterranean Sea, we show that, depending on environmental conditions, calcifying haploid coccolithophores can be significant contributors to the coccolithophore standing stock (up to ≈ 30 %). Furthermore, using hypervolumes to quantify the niche of coccolithophores, we illustrate that the haploid and diploid life cycle phases inhabit contrasting niches and that on average this allows coccolithophores to expand their niche by ≈ 18.8 %, with a range of 3 %-76 % for individual species. Our results highlight that future coccolithophore research should consider both life cycle stages, as omission of the haploid life cycle phase in current research limits our understanding of coccolithophore ecology. Our results furthermore suggest a different response to nutrient limitation and stratification, which may be of relevance for further climate scenarios. Our compilation highlights the spatial and temporal sparsity of SEM measurements and the need for new molecular techniques to identify uncalcified haploid coccolithophores. Our work also emphasizes the need for further work on the carbonate chemistry niche of the coccolithophore life cycle.

The haplo-diplontic life cycle expands niche space of coccolithophores

Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-dipl... more Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-diplontic life cycle. The haplo-diplontic life cycle allows coccolithophores to divide in both life cycle phases, and has been proposed to allow coccolithophores to expand their niche space. To-date research has however largely overlooked the life cycle of coccolithophores, and has instead focused on the diploid life cycle phase. Through a synthesis of global scanning electron microscopy (SEM) coccolithophore abundance data (n = 2534), we show that the haploid life cycle phase contributes significantly to coccolithophore abundance, constituting ≈18 % of species abundance for which haploid-diploid pairs are defined. Using hypervolumes to quantify the niche space of coccolithophores, we furthermore show that the haploid and diploid life cycle phases inhabit contrasting niches, and that this allows coccolithophores to expand their niche space by ≈17 %. Our results highlight that future coccolithophore research should consider both life cycle stages, as omission of the haploid life cycle phase in current research limits our understanding of coccolithophore ecology. 1 Introduction Coccolithophores are marine phytoplankton that produce calcium carbonate platelets, called 'coccoliths', which can be seen from space when coccolithophores bloom. Coccoliths eventually rain down into the ocean interior or serve as ballasts as they are incorporated into faecal pellets and aggregates, which drives the carbonate pump and increases organic carbon export rates to the deep sea (Klaas and Archer, 2002). Through the production of coccoliths, coccolithophores produce ≈1.5 Pg of inorganic carbon per year (

Mechanistic models of oceanic nitrogen fixation

Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more

Investigating the benefits and costs of spines and diet on planktonic foraminifera distribution with a trait-based ecosystem model

Marine Micropaleontology, 2021

Planktonic foraminifera are important calcifiers in the modern ocean. Despite this importance, th... more Planktonic foraminifera are important calcifiers in the modern ocean. Despite this importance, the main functions of foraminifera's test and ornamentation such as spines are unclear. Spinose species dominate the plank-tonic foraminifera population in subtropical oligotrophic gyres, while non-spinose species dominate in deeper waters and at high latitudes suggesting that spines help foraminifera in food-limited areas. Here we take a novel approach to investigate the benefits of spines on foraminifera foraging using a 0-D trait-based ecosystem model. The model considers the traits of size, calcification, spines, passive feeding, and diet. We assess how the presence of spines impact foraminifera diet and fitness via a series of simulated environments representing oligo, meso-and eutrophic settings at different temperatures. We find that independent of diet, non-spinose taxa need to be more size-generalist predators than other zooplankton species to maintain their population. In contrast, spinose species benefit from a relatively higher surface-to-volume ratio compared to non-spinose species, which allows them to be as generalist as other zooplankton groups. In agreement with observations, we find that herbivory is the most successful diet in cold environments, while carnivory allows foraminifera to be more successful in warm environments.

EcoGEnIE 1.0: plankton ecology in the cGEnIE Earth system model

Geoscientific Model Development

We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE)... more We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE) that explicitly accounts for the growth and interaction of an arbitrary number of plankton species. The new package (ECO-GEM) replaces the implicit, flux-based parameterisation of the plankton community currently employed, with explicitly resolved plankton populations and ecological dynamics. In ECOGEM, any number of plankton species, with ecophysiological traits (e.g. growth and grazing rates) assigned according to organism size and functional group (e.g. phytoplankton and zooplankton) can be incorporated at runtime. We illustrate the capability of the marine ecology enabled Earth system model (EcoGEnIE) by comparing results from one configuration of ECOGEM (with eight generic phytoplankton and zooplankton size classes) to climatological and seasonal observations. We find that the new ecological components of the model show reasonable agreement with both global-scale climatological and local-scale seasonal data. We also compare EcoGEnIE results to the existing biogeochemical incarnation of cGEnIE. We find that the resulting globalscale distributions of phosphate, iron, dissolved inorganic carbon, alkalinity, and oxygen are similar for both iterations of the model. A slight deterioration in some fields in Eco-GEnIE (relative to the data) is observed, although we make no attempt to re-tune the overall marine cycling of carbon and nutrients here. The increased capabilities of EcoGEnIE in this regard will enable future exploration of the ecological community on much longer timescales than have previously been examined in global ocean ecosystem models and particularly for past climates and global biogeochemical cycles.

EcoGEnIE 1.0: plankton ecology in the cGEnIE Earth system model

Geoscientific Model Development

We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE)... more We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE) that explicitly accounts for the growth and interaction of an arbitrary number of plankton species. The new package (ECO-GEM) replaces the implicit, flux-based parameterisation of the plankton community currently employed, with explicitly resolved plankton populations and ecological dynamics. In ECOGEM, any number of plankton species, with ecophysiological traits (e.g. growth and grazing rates) assigned according to organism size and functional group (e.g. phytoplankton and zooplankton) can be incorporated at runtime. We illustrate the capability of the marine ecology enabled Earth system model (EcoGEnIE) by comparing results from one configuration of ECOGEM (with eight generic phytoplankton and zooplankton size classes) to climatological and seasonal observations. We find that the new ecological components of the model show reasonable agreement with both global-scale climatological and local-scale seasonal data. We also compare EcoGEnIE results to the existing biogeochemical incarnation of cGEnIE. We find that the resulting globalscale distributions of phosphate, iron, dissolved inorganic carbon, alkalinity, and oxygen are similar for both iterations of the model. A slight deterioration in some fields in Eco-GEnIE (relative to the data) is observed, although we make no attempt to re-tune the overall marine cycling of carbon and nutrients here. The increased capabilities of EcoGEnIE in this regard will enable future exploration of the ecological community on much longer timescales than have previously been examined in global ocean ecosystem models and particularly for past climates and global biogeochemical cycles.

Why marine phytoplankton calcify

by Fanny Monteiro and U. Riebesell

Science advances, Jul 1, 2016

Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic o... more Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic organisms in the ocean and are at risk from global change. To better understand how they will be affected, we need to know "why" coccolithophores calcify. We review coccolithophorid evolutionary history and cell biology as well as insights from recent experiments to provide a critical assessment of the costs and benefits of calcification. We conclude that calcification has high energy demands and that coccolithophores might have calcified initially to reduce grazing pressure but that additional benefits such as protection from photodamage and viral/bacterial attack further explain their high diversity and broad spectrum ecology. The cost-benefit aspect of these traits is illustrated by novel ecosystem modeling, although conclusive observations remain limited. In the future ocean, the trade-off between changing ecological and physiological costs of calcification and their benefit...

Antarctic ice sheet fertilises the Southern Ocean

Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability o... more Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.

The potential role of the Antarctic Ice Sheet in global biogeochemical cycles

Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir... more Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir of organic carbon and metabolically active microbial cells. At the ice-bed interface, subglacial lake and sedimentary environments support low diversity microbial populations, adapted to perennial cold, anoxia and lack of light. The dynamic exchange of water between these shallow environments conveys meltwaters and associated sediments into the coastal ocean. This, together with the release of iceberg-rafted debris to more distal coastal environments, could be important for sustaining primary productivity in the iron-limited Southern Ocean, via the release of associated nutrients and bioavailable iron. We estimate the magnitude and review the wider impacts of the potential export of nutrients (N, P, C, Si and bioavailable Fe) dissolved and associated with suspended sediments in Antarctic runoff and entombed in iceberg rafted debris. Located beneath subglacial lakes and the subglacial till complex are deep sedimentary basins up to 14 km thick, located largely around the Antarctic periphery. These sedimentary basins are largely hydrologically decoupled from shallower lake and till environments by the presence of highly consolidated sediments which limit the penetration of glacial meltwaters to depth. They provide extensive habitats for sustained microbial activity over Ma timescales, and are likely to be a focal point for the anaerobic cycling of organic carbon and other elements in the deep sub-surface. Organic carbon buried in these basins during ice sheet formation is thought to be microbially cycled to methane gas, and the methane largely stored as hydrate within sediments, stabilised by the high pressure/low temperature conditions. We conclude that the export of nutrients and biogenic gases from deep and shallow subglacial Antarctic environments designates Antarctica as a potentially important component of the Earth's carbon cycle, and highlight the importance of evaluating these potential impacts further via global and regional-scale biogeochemical modelling.

Nutrients as the dominant control on the spread of anoxia and euxinia across the Cenomanian-Turonian oceanic anoxic event (OAE2): Model-data comparison

The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in th... more The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in the oxygen and sulfur cycles of the ocean, potentially resulting from changes in oxygen supply (via oxygen solubility and ocean circulation) and in marine productivity. We assess the relative impact of these mechanisms, comparing model experiments with a new compilation of observations for seafloor dysoxia/anoxia and photic-zone euxinia. The model employed is an intermediate-complexity Earth system model which accounts for the main ocean dynamics and biogeochemistry of the Cretaceous climate. The impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations. The model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with OAE2. Observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content (responsible for 85% of the change). Higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments, which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation. The model also shows that the presence of seafloor anoxia, as suggested by black-shale deposition in the proto-North Atlantic Ocean before the event, might be the result of the silled shape and lack of deep-water formation of this basin at the Late Cretaceous. Overall our model-data comparison shows that OAE2 anoxia was quasi-global spreading from 5% of the ocean volume before the event to at least 50% during OAE2.

On nitrogen fixation and preferential remineralization of phosphorus

Regional and global nitrogen fixation rates are often estimated from geochemical tracers related ... more Regional and global nitrogen fixation rates are often estimated from geochemical tracers related to N* (= NO3 − 16PO4). However the patterns of this tracer reflect the influence of numerous processes including nitrogen fixation, denitrification, remineralization of organic matter, variable stoichiometry, atmospheric deposition and physical transport. Here we have used idealized models to illustrate how preferential remineralization of organic phosphorous may explain observed features of N* distribution in the North Atlantic Ocean, including a subsurface maximum and an increased temporal variability in the mid-thermocline. If preferential remineralization of phosphorus is key in shaping the oceanic distribution of N*, published estimates of nitrogen fixation may be underestimating the marine nitrogen fixation rate by as much as a factor of three.

Interconnection of nitrogen ﬁxers and iron in the Paciﬁc Ocean: Theory and numerical simulations

We examine the interplay between iron supply, iron concentrations and phytoplankton communities i... more We examine the interplay between iron supply, iron concentrations and phytoplankton communities in the Pacific Ocean. We present a theoretical framework which considers the competition for iron and nitrogen resources between phytoplankton to explain where nitrogen fixing autotrophs (diazotrophs, which require higher iron quotas, and have slower maximum growth) can co-exist with other phytoplankton. The framework also indicates that iron and fixed nitrogen concentrations can be strongly controlled by the local phytoplankton community. Together with results from a three-dimensional numerical model, we characterize three distinct biogeochemical provinces: 1) where iron supply is very low diazotrophs are excluded, and iron-limited nondiazotrophic phytoplankton control the iron concentrations; 2) a transition region where nondiazotrophic phytoplankton are nitrogen limited and control the nitrogen concentrations, but the iron supply is still too low relative to nitrate to support diazotrophy; 3) where iron supplies increase further relative to the nitrogen source, diazotrophs and other phytoplankton coexist; nitrogen concentrations are controlled by nondiazotrophs and iron concentrations are controlled by diazotrophs. The boundaries of these three provinces are defined by the rate of supply of iron relative to the supply of fixed nitrogen. The numerical model and theory provide a useful tool to understand the state of, links between, and response to changes in iron supply and phytoplankton community structure that have been suggested by observations.

Biogeographical controls on the marine nitrogen fixers

We interpret the environmental controls on the global ocean diazotroph biogeography in the contex... more We interpret the environmental controls on the global ocean diazotroph biogeography in the context of a three-dimensional global model with a self-organizing phytoplankton community. As is observed, the model’s total diazotroph population is distributed over most of the oligotrophic warm sub-and-tropical waters, with the exception of the South eastern Paciﬁc Ocean. This biogeography broadly follows temperature and light constraints which are often used in both ﬁeld-based and model studies to explain the distribution of diazotrophs. However the model suggests that diazotroph habitat is not directly controlled by temperature and light, but is restricted to the ocean regions with low ﬁxed nitrogen and suﬃcient dissolved iron and phosphate concentrations. We interpret this regulation by iron and phosphate using resource competition theory which provides an excellent qualitative and quantitative framework.

Iron conservation by reduction of metalloenzyme inventories in the marine diazotroph Crocosphaera watsonii

The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open oc... more The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ∼40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans. A global numerical model of ocean circulation, biogeochemistry and ecosystems suggests that Crocosphaera’s ability to reduce its iron-metalloenzyme inventory provides two advantages: It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation than if they did not have this reduced iron requirement.

Distribution of diverse nitrogen fixers in the global ocean

We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, although Trichodesmium analogs dominate the diazotroph population in much of the North and tropical Atlantic Ocean and the Arabian Sea, while unicellular‐diazotroph analogs dominate in the South Atlantic, Pacific and Indian oceans. This pattern reflects the relative degree of nutrient limitation by iron or phosphorus. The model suggests in addition that unicellular diazotrophs could add as much new nitrogen to the global ocean as Trichodesmium.

On the interannual variability of nitrogen ﬁxation in the subtropical gyres

Time-series observations of geochemical tracers and diazotroph abundances in the northern subtrop... more Time-series observations of geochemical tracers and diazotroph abundances in the northern subtropical gyres suggest variability in nitrogen ﬁxation on interannual and longer timescales. Using a highly idealized model of the biogeochemistry and ecology of a subtropical gyre, we explore the previously proposed hypothesis that such variability is regulated by an internal “biogeochemical oscillator.” We ﬁnd, in certain parameter regimes, self-sustained oscillations in nitrogen ﬁxation, community structure and biogeochemical cycles even with perfectly steady physical forcing. During the oscillations of nitrogen ﬁxation, “blooms” of diazotrophs occur at intervals between a year and several decades, consistent with the observed variability. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters. The oscillatory solutions occur in a relatively small region of parameter space, but one in which the relative ﬁtness of diazotrophs and non-diazotrophs are closely matched and the time-averaged biomass of each class of phytoplankton is maximized.

Mechanistic models of oceanic nitrogen fixation

Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and iron cycles have important implications for the control of primary production and carbon storage in the ocean. The biological process of nitrogen fixation is thought to be particularly important where the ocean is nitrogen limited and oligotrophic. This thesis examines some of the mechanisms responsible for the distribu- tion, rates and temporal variability of nitrogen fixation and its geochemical signature in the modern ocean. I employ simple analytical theories and numerical models of ecosystems and biogeochemical cycles, and closely refer to direct observations of the phytoplanktonic community and geochemical tracers of the marine nitrogen cycle. Time-series observations of geochemical tracers and abundances of nitrogen fixers (or diazotrophs) in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. I use a highly idealized, two-layer model of the nitrogen and phosphorus biogeochemistry and ecology of a subtropical gyre to explore the previously proposed hypothesis that such variability is regulated by an internal biogeochemical oscillator. I find, in certain parameter regimes, self- sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters, suggesting a period of several years to several decades for the North Pacific subtropical gyre regime, but would likely be shorter (only a year or so) for the North Atlantic Ocean. Geochemical tracers such as DINxs (=NO3−16PO3) measure the oceanic departure from the Redfield ratio. DINxs is often used to estimate the rate of nitrogen fixation in the ocean, by quantifying the tracer accumulation along isopycnals. However this tracer reflects an interwoven set of processes including nitrogen fixation, but also denitrification, atmospheric and riverine sources, differential remineralization and complex transport pathways. I examine analytical solutions of the prognostic equation of DINxs and an idealized three-dimensional model of the basin-scale circulation, biogeochemical cycles and ecology of the North Atlantic Ocean. The two approaches demonstrate that the observations of a subsurface maximum in the North Atlantic Ocean and the temporal variability at the station BATS of DINxs can be explained simply by preferential remineralization of organic phosphorus relative to nitrogen. A further analysis reveals that the current geochemical estimates based on inorganic forms of phosphorus and nitrogen underestimate integrated nitrogen fixation rates by a factor of two to six, by neglecting the preferential remineralization effect.
Most current understanding of oceanic nitrogen fixation is based on the Trichodesmium, though unicellular cyanobacteria, diatom-diazotroph associations (DDA) and heterotrophic bacteria might be as important in adding nitrogen into the ocean. I employ a self-assembling global ocean ecosystem model to simulate diverse phytoplanktonic diazotrophs in the global ocean and examine how temperature, oligotrophy, iron and phosphate limitations influence the global marine diazotroph distribution. Analogs of Trichodesmium, unicellular diazotrophs and DDA are successful in the model, showing very similar distributions with observations. The total diazotrophic population is distributed over most of the oligotrophic warm (sub)tropical waters in the model. The model demonstrates that temperature is not the primary control, but suggests instead that diazotroph biogeography is restricted to the low fixed nitrogen oceanic regions which have sufficient dissolved iron and phosphate. The theory of resource competition is used to map out regions of iron and phosphate regulation of diazotroph distribution. The theory suggests that diazotrophs are largely regulated by iron availability, in particular in the Pacific and Indian Oceans. The iron cycle is currently not well enough constrained to confidently predict the diazotroph distribution in global ocean models.

Distribution of diverse nitrogen fixers in the global ocean

Global Biogeochemical Cycles, 2010

We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, al...

Global SEM coccolithophore abundance compilation

Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the o... more Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the organic carbon pump through the primary production and the ballast of organic matter, and to the carbonate pump through the production of calcium carbonate. Here we compiled all available scanning electron microscopy (SEM) coccolithophore abundance observations. Taxa were standardized following NannoTax3 to a species level where possible. Subspecies (e.a. C. leptoporus subsp. leptoporus and C. leptoporus subsp. quadriperforatus) were grouped as single species. The database contains 2556 abundance observations from 35 different publications. The data span the period of 1993-2017, with observations from all ocean basins and all seasons, and at depths ranging from the surface to 5000 m. We limited our compilation to SEM observations (or observations which further identified samples with SEM) because SEM provides greater detail of coccolithophore diversity than more commonly used polarized li...

Haplo-diplontic life cycle expands coccolithophore niche

Biogeosciences, 2021

Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-dipl... more Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-diplontic life cycle. The haplo-diplontic life cycle allows coccolithophores to divide in both life cycle phases and potentially expands coccolithophore niche volume. Research has, however, to date largely overlooked the life cycle of coccolithophores and has instead focused on the diploid life cycle phase of coccolithophores. Through the synthesis and analysis of global scanning electron microscopy (SEM) coccolithophore abundance data (n = 2534), we find that calcified haploid coccolithophores generally constitute a minor component of the total coccolithophore abundance (≈ 2 %-15 % depending on season). However, using case studies in the Atlantic Ocean and Mediterranean Sea, we show that, depending on environmental conditions, calcifying haploid coccolithophores can be significant contributors to the coccolithophore standing stock (up to ≈ 30 %). Furthermore, using hypervolumes to quantify the niche of coccolithophores, we illustrate that the haploid and diploid life cycle phases inhabit contrasting niches and that on average this allows coccolithophores to expand their niche by ≈ 18.8 %, with a range of 3 %-76 % for individual species. Our results highlight that future coccolithophore research should consider both life cycle stages, as omission of the haploid life cycle phase in current research limits our understanding of coccolithophore ecology. Our results furthermore suggest a different response to nutrient limitation and stratification, which may be of relevance for further climate scenarios. Our compilation highlights the spatial and temporal sparsity of SEM measurements and the need for new molecular techniques to identify uncalcified haploid coccolithophores. Our work also emphasizes the need for further work on the carbonate chemistry niche of the coccolithophore life cycle.

The haplo-diplontic life cycle expands niche space of coccolithophores

Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-dipl... more Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-diplontic life cycle. The haplo-diplontic life cycle allows coccolithophores to divide in both life cycle phases, and has been proposed to allow coccolithophores to expand their niche space. To-date research has however largely overlooked the life cycle of coccolithophores, and has instead focused on the diploid life cycle phase. Through a synthesis of global scanning electron microscopy (SEM) coccolithophore abundance data (n = 2534), we show that the haploid life cycle phase contributes significantly to coccolithophore abundance, constituting ≈18 % of species abundance for which haploid-diploid pairs are defined. Using hypervolumes to quantify the niche space of coccolithophores, we furthermore show that the haploid and diploid life cycle phases inhabit contrasting niches, and that this allows coccolithophores to expand their niche space by ≈17 %. Our results highlight that future coccolithophore research should consider both life cycle stages, as omission of the haploid life cycle phase in current research limits our understanding of coccolithophore ecology. 1 Introduction Coccolithophores are marine phytoplankton that produce calcium carbonate platelets, called 'coccoliths', which can be seen from space when coccolithophores bloom. Coccoliths eventually rain down into the ocean interior or serve as ballasts as they are incorporated into faecal pellets and aggregates, which drives the carbonate pump and increases organic carbon export rates to the deep sea (Klaas and Archer, 2002). Through the production of coccoliths, coccolithophores produce ≈1.5 Pg of inorganic carbon per year (

Mechanistic models of oceanic nitrogen fixation

Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more

Investigating the benefits and costs of spines and diet on planktonic foraminifera distribution with a trait-based ecosystem model

Marine Micropaleontology, 2021

Planktonic foraminifera are important calcifiers in the modern ocean. Despite this importance, th... more Planktonic foraminifera are important calcifiers in the modern ocean. Despite this importance, the main functions of foraminifera's test and ornamentation such as spines are unclear. Spinose species dominate the plank-tonic foraminifera population in subtropical oligotrophic gyres, while non-spinose species dominate in deeper waters and at high latitudes suggesting that spines help foraminifera in food-limited areas. Here we take a novel approach to investigate the benefits of spines on foraminifera foraging using a 0-D trait-based ecosystem model. The model considers the traits of size, calcification, spines, passive feeding, and diet. We assess how the presence of spines impact foraminifera diet and fitness via a series of simulated environments representing oligo, meso-and eutrophic settings at different temperatures. We find that independent of diet, non-spinose taxa need to be more size-generalist predators than other zooplankton species to maintain their population. In contrast, spinose species benefit from a relatively higher surface-to-volume ratio compared to non-spinose species, which allows them to be as generalist as other zooplankton groups. In agreement with observations, we find that herbivory is the most successful diet in cold environments, while carnivory allows foraminifera to be more successful in warm environments.

EcoGEnIE 1.0: plankton ecology in the cGEnIE Earth system model

Geoscientific Model Development

We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE)... more We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE) that explicitly accounts for the growth and interaction of an arbitrary number of plankton species. The new package (ECO-GEM) replaces the implicit, flux-based parameterisation of the plankton community currently employed, with explicitly resolved plankton populations and ecological dynamics. In ECOGEM, any number of plankton species, with ecophysiological traits (e.g. growth and grazing rates) assigned according to organism size and functional group (e.g. phytoplankton and zooplankton) can be incorporated at runtime. We illustrate the capability of the marine ecology enabled Earth system model (EcoGEnIE) by comparing results from one configuration of ECOGEM (with eight generic phytoplankton and zooplankton size classes) to climatological and seasonal observations. We find that the new ecological components of the model show reasonable agreement with both global-scale climatological and local-scale seasonal data. We also compare EcoGEnIE results to the existing biogeochemical incarnation of cGEnIE. We find that the resulting globalscale distributions of phosphate, iron, dissolved inorganic carbon, alkalinity, and oxygen are similar for both iterations of the model. A slight deterioration in some fields in Eco-GEnIE (relative to the data) is observed, although we make no attempt to re-tune the overall marine cycling of carbon and nutrients here. The increased capabilities of EcoGEnIE in this regard will enable future exploration of the ecological community on much longer timescales than have previously been examined in global ocean ecosystem models and particularly for past climates and global biogeochemical cycles.

EcoGEnIE 1.0: plankton ecology in the cGEnIE Earth system model

Geoscientific Model Development

We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE)... more We present an extension to the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE) that explicitly accounts for the growth and interaction of an arbitrary number of plankton species. The new package (ECO-GEM) replaces the implicit, flux-based parameterisation of the plankton community currently employed, with explicitly resolved plankton populations and ecological dynamics. In ECOGEM, any number of plankton species, with ecophysiological traits (e.g. growth and grazing rates) assigned according to organism size and functional group (e.g. phytoplankton and zooplankton) can be incorporated at runtime. We illustrate the capability of the marine ecology enabled Earth system model (EcoGEnIE) by comparing results from one configuration of ECOGEM (with eight generic phytoplankton and zooplankton size classes) to climatological and seasonal observations. We find that the new ecological components of the model show reasonable agreement with both global-scale climatological and local-scale seasonal data. We also compare EcoGEnIE results to the existing biogeochemical incarnation of cGEnIE. We find that the resulting globalscale distributions of phosphate, iron, dissolved inorganic carbon, alkalinity, and oxygen are similar for both iterations of the model. A slight deterioration in some fields in Eco-GEnIE (relative to the data) is observed, although we make no attempt to re-tune the overall marine cycling of carbon and nutrients here. The increased capabilities of EcoGEnIE in this regard will enable future exploration of the ecological community on much longer timescales than have previously been examined in global ocean ecosystem models and particularly for past climates and global biogeochemical cycles.

Why marine phytoplankton calcify

by Fanny Monteiro and U. Riebesell

Science advances, Jul 1, 2016

Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic o... more Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic organisms in the ocean and are at risk from global change. To better understand how they will be affected, we need to know "why" coccolithophores calcify. We review coccolithophorid evolutionary history and cell biology as well as insights from recent experiments to provide a critical assessment of the costs and benefits of calcification. We conclude that calcification has high energy demands and that coccolithophores might have calcified initially to reduce grazing pressure but that additional benefits such as protection from photodamage and viral/bacterial attack further explain their high diversity and broad spectrum ecology. The cost-benefit aspect of these traits is illustrated by novel ecosystem modeling, although conclusive observations remain limited. In the future ocean, the trade-off between changing ecological and physiological costs of calcification and their benefit...

Antarctic ice sheet fertilises the Southern Ocean

Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability o... more Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.

The potential role of the Antarctic Ice Sheet in global biogeochemical cycles

Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir... more Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir of organic carbon and metabolically active microbial cells. At the ice-bed interface, subglacial lake and sedimentary environments support low diversity microbial populations, adapted to perennial cold, anoxia and lack of light. The dynamic exchange of water between these shallow environments conveys meltwaters and associated sediments into the coastal ocean. This, together with the release of iceberg-rafted debris to more distal coastal environments, could be important for sustaining primary productivity in the iron-limited Southern Ocean, via the release of associated nutrients and bioavailable iron. We estimate the magnitude and review the wider impacts of the potential export of nutrients (N, P, C, Si and bioavailable Fe) dissolved and associated with suspended sediments in Antarctic runoff and entombed in iceberg rafted debris. Located beneath subglacial lakes and the subglacial till complex are deep sedimentary basins up to 14 km thick, located largely around the Antarctic periphery. These sedimentary basins are largely hydrologically decoupled from shallower lake and till environments by the presence of highly consolidated sediments which limit the penetration of glacial meltwaters to depth. They provide extensive habitats for sustained microbial activity over Ma timescales, and are likely to be a focal point for the anaerobic cycling of organic carbon and other elements in the deep sub-surface. Organic carbon buried in these basins during ice sheet formation is thought to be microbially cycled to methane gas, and the methane largely stored as hydrate within sediments, stabilised by the high pressure/low temperature conditions. We conclude that the export of nutrients and biogenic gases from deep and shallow subglacial Antarctic environments designates Antarctica as a potentially important component of the Earth's carbon cycle, and highlight the importance of evaluating these potential impacts further via global and regional-scale biogeochemical modelling.

Nutrients as the dominant control on the spread of anoxia and euxinia across the Cenomanian-Turonian oceanic anoxic event (OAE2): Model-data comparison

The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in th... more The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in the oxygen and sulfur cycles of the ocean, potentially resulting from changes in oxygen supply (via oxygen solubility and ocean circulation) and in marine productivity. We assess the relative impact of these mechanisms, comparing model experiments with a new compilation of observations for seafloor dysoxia/anoxia and photic-zone euxinia. The model employed is an intermediate-complexity Earth system model which accounts for the main ocean dynamics and biogeochemistry of the Cretaceous climate. The impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations. The model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with OAE2. Observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content (responsible for 85% of the change). Higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments, which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation. The model also shows that the presence of seafloor anoxia, as suggested by black-shale deposition in the proto-North Atlantic Ocean before the event, might be the result of the silled shape and lack of deep-water formation of this basin at the Late Cretaceous. Overall our model-data comparison shows that OAE2 anoxia was quasi-global spreading from 5% of the ocean volume before the event to at least 50% during OAE2.

On nitrogen fixation and preferential remineralization of phosphorus

Regional and global nitrogen fixation rates are often estimated from geochemical tracers related ... more Regional and global nitrogen fixation rates are often estimated from geochemical tracers related to N* (= NO3 − 16PO4). However the patterns of this tracer reflect the influence of numerous processes including nitrogen fixation, denitrification, remineralization of organic matter, variable stoichiometry, atmospheric deposition and physical transport. Here we have used idealized models to illustrate how preferential remineralization of organic phosphorous may explain observed features of N* distribution in the North Atlantic Ocean, including a subsurface maximum and an increased temporal variability in the mid-thermocline. If preferential remineralization of phosphorus is key in shaping the oceanic distribution of N*, published estimates of nitrogen fixation may be underestimating the marine nitrogen fixation rate by as much as a factor of three.

Interconnection of nitrogen ﬁxers and iron in the Paciﬁc Ocean: Theory and numerical simulations

We examine the interplay between iron supply, iron concentrations and phytoplankton communities i... more We examine the interplay between iron supply, iron concentrations and phytoplankton communities in the Pacific Ocean. We present a theoretical framework which considers the competition for iron and nitrogen resources between phytoplankton to explain where nitrogen fixing autotrophs (diazotrophs, which require higher iron quotas, and have slower maximum growth) can co-exist with other phytoplankton. The framework also indicates that iron and fixed nitrogen concentrations can be strongly controlled by the local phytoplankton community. Together with results from a three-dimensional numerical model, we characterize three distinct biogeochemical provinces: 1) where iron supply is very low diazotrophs are excluded, and iron-limited nondiazotrophic phytoplankton control the iron concentrations; 2) a transition region where nondiazotrophic phytoplankton are nitrogen limited and control the nitrogen concentrations, but the iron supply is still too low relative to nitrate to support diazotrophy; 3) where iron supplies increase further relative to the nitrogen source, diazotrophs and other phytoplankton coexist; nitrogen concentrations are controlled by nondiazotrophs and iron concentrations are controlled by diazotrophs. The boundaries of these three provinces are defined by the rate of supply of iron relative to the supply of fixed nitrogen. The numerical model and theory provide a useful tool to understand the state of, links between, and response to changes in iron supply and phytoplankton community structure that have been suggested by observations.

Biogeographical controls on the marine nitrogen fixers

We interpret the environmental controls on the global ocean diazotroph biogeography in the contex... more We interpret the environmental controls on the global ocean diazotroph biogeography in the context of a three-dimensional global model with a self-organizing phytoplankton community. As is observed, the model’s total diazotroph population is distributed over most of the oligotrophic warm sub-and-tropical waters, with the exception of the South eastern Paciﬁc Ocean. This biogeography broadly follows temperature and light constraints which are often used in both ﬁeld-based and model studies to explain the distribution of diazotrophs. However the model suggests that diazotroph habitat is not directly controlled by temperature and light, but is restricted to the ocean regions with low ﬁxed nitrogen and suﬃcient dissolved iron and phosphate concentrations. We interpret this regulation by iron and phosphate using resource competition theory which provides an excellent qualitative and quantitative framework.

Iron conservation by reduction of metalloenzyme inventories in the marine diazotroph Crocosphaera watsonii

The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open oc... more The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ∼40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans. A global numerical model of ocean circulation, biogeochemistry and ecosystems suggests that Crocosphaera’s ability to reduce its iron-metalloenzyme inventory provides two advantages: It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation than if they did not have this reduced iron requirement.

Distribution of diverse nitrogen fixers in the global ocean

We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, although Trichodesmium analogs dominate the diazotroph population in much of the North and tropical Atlantic Ocean and the Arabian Sea, while unicellular‐diazotroph analogs dominate in the South Atlantic, Pacific and Indian oceans. This pattern reflects the relative degree of nutrient limitation by iron or phosphorus. The model suggests in addition that unicellular diazotrophs could add as much new nitrogen to the global ocean as Trichodesmium.

On the interannual variability of nitrogen ﬁxation in the subtropical gyres

Time-series observations of geochemical tracers and diazotroph abundances in the northern subtrop... more Time-series observations of geochemical tracers and diazotroph abundances in the northern subtropical gyres suggest variability in nitrogen ﬁxation on interannual and longer timescales. Using a highly idealized model of the biogeochemistry and ecology of a subtropical gyre, we explore the previously proposed hypothesis that such variability is regulated by an internal “biogeochemical oscillator.” We ﬁnd, in certain parameter regimes, self-sustained oscillations in nitrogen ﬁxation, community structure and biogeochemical cycles even with perfectly steady physical forcing. During the oscillations of nitrogen ﬁxation, “blooms” of diazotrophs occur at intervals between a year and several decades, consistent with the observed variability. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters. The oscillatory solutions occur in a relatively small region of parameter space, but one in which the relative ﬁtness of diazotrophs and non-diazotrophs are closely matched and the time-averaged biomass of each class of phytoplankton is maximized.

Mechanistic models of oceanic nitrogen fixation

Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and iron cycles have important implications for the control of primary production and carbon storage in the ocean. The biological process of nitrogen fixation is thought to be particularly important where the ocean is nitrogen limited and oligotrophic. This thesis examines some of the mechanisms responsible for the distribu- tion, rates and temporal variability of nitrogen fixation and its geochemical signature in the modern ocean. I employ simple analytical theories and numerical models of ecosystems and biogeochemical cycles, and closely refer to direct observations of the phytoplanktonic community and geochemical tracers of the marine nitrogen cycle. Time-series observations of geochemical tracers and abundances of nitrogen fixers (or diazotrophs) in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. I use a highly idealized, two-layer model of the nitrogen and phosphorus biogeochemistry and ecology of a subtropical gyre to explore the previously proposed hypothesis that such variability is regulated by an internal biogeochemical oscillator. I find, in certain parameter regimes, self- sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters, suggesting a period of several years to several decades for the North Pacific subtropical gyre regime, but would likely be shorter (only a year or so) for the North Atlantic Ocean. Geochemical tracers such as DINxs (=NO3−16PO3) measure the oceanic departure from the Redfield ratio. DINxs is often used to estimate the rate of nitrogen fixation in the ocean, by quantifying the tracer accumulation along isopycnals. However this tracer reflects an interwoven set of processes including nitrogen fixation, but also denitrification, atmospheric and riverine sources, differential remineralization and complex transport pathways. I examine analytical solutions of the prognostic equation of DINxs and an idealized three-dimensional model of the basin-scale circulation, biogeochemical cycles and ecology of the North Atlantic Ocean. The two approaches demonstrate that the observations of a subsurface maximum in the North Atlantic Ocean and the temporal variability at the station BATS of DINxs can be explained simply by preferential remineralization of organic phosphorus relative to nitrogen. A further analysis reveals that the current geochemical estimates based on inorganic forms of phosphorus and nitrogen underestimate integrated nitrogen fixation rates by a factor of two to six, by neglecting the preferential remineralization effect.
Most current understanding of oceanic nitrogen fixation is based on the Trichodesmium, though unicellular cyanobacteria, diatom-diazotroph associations (DDA) and heterotrophic bacteria might be as important in adding nitrogen into the ocean. I employ a self-assembling global ocean ecosystem model to simulate diverse phytoplanktonic diazotrophs in the global ocean and examine how temperature, oligotrophy, iron and phosphate limitations influence the global marine diazotroph distribution. Analogs of Trichodesmium, unicellular diazotrophs and DDA are successful in the model, showing very similar distributions with observations. The total diazotrophic population is distributed over most of the oligotrophic warm (sub)tropical waters in the model. The model demonstrates that temperature is not the primary control, but suggests instead that diazotroph biogeography is restricted to the low fixed nitrogen oceanic regions which have sufficient dissolved iron and phosphate. The theory of resource competition is used to map out regions of iron and phosphate regulation of diazotroph distribution. The theory suggests that diazotrophs are largely regulated by iron availability, in particular in the Pacific and Indian Oceans. The iron cycle is currently not well enough constrained to confidently predict the diazotroph distribution in global ocean models.