1
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The role of nutricline depth in regulating ocean’s
2
carbon cycle.
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4
Pedro Cermeño1, Stephanie Dutkiewicz2, Roger P. Harris3, Mick Follows2, Oscar
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Schofield1 & Paul G. Falkowski1,4
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7
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Coastal Sciences, Rutgers University, 71 Dudley Rd, New Brunswick, 08901, New
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Jersey, USA.
Environmental Biophysics & Molecular Ecology Program, Institute of Marine &
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Massachusetts Institute of Technology, Cambridge, MA 02139-4307 USA
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Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, UK
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Department of Earth and Planetary Science, Rutgers University, 610 Taylor
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Road, Piscataway, 08854, New Jersey, USA.
Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Ave.,
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Carbon uptake by marine phytoplankton, and its export below the
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thermocline, lowers the partial pressure of carbon dioxide (pCO2) in the
18
upper ocean, and stimulates the diffusion of CO2 gas from the atmosphere1.
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Conversely, precipitation of calcium carbonate by marine planktonic
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calcifiers such as coccolithophorids increases pCO2, and promotes its
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outgassing2,3. Over the past ~50 million years, these carbon exchange fluxes
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between the atmosphere and the ocean have been largely controlled by the
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1
balance between diatom and coccolithophorid abundance, contributing
2
significantly to the regulation of atmospheric pCO2 and Earth’s climate4,5.
3
Yet, the mechanisms that control this critical balance remain poorly
4
understood. The ecological distribution of these two phytoplankton
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functional groups has been linked to their distinct abilities to compete for
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nutrients6. Here we analyse phytoplankton community composition across
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latitudinal gradients in the Atlantic Ocean to show that the balance between
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diatoms and coccolithophorids is dependent upon the nutricline depth, a
9
proxy of nutrient supply to the upper mixed layer of the ocean. Coupled
10
atmosphere-ocean general circulation models (GCMs) predict a dramatic
11
reduction in the nutrient supply to the euphotic layer as a result of increased
12
ocean stratification7-9. Our findings indicate that, by shifting phytoplankton
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community composition, this causal relationship may lead to a decreased role
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of the biological pump in sequestering atmospheric CO2, implying a positive
15
feedback in the climate system. These results are fundamental to understand
16
the connection between upper ocean dynamics, the calcium carbonate-to-
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organic C production ratio and atmospheric pCO2 variations on time scales
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ranging from seasonal cycles to geological transitions.
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Diatoms and coccolithophorids are derived from a common Rhodophyte
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ancestor, yet the evolutionary trajectories have channelled these two
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phytoplankton functional groups into distinct ecological strategies that have
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significant biogeochemical implications2,3,4,6,10,11. From an evolutionary
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perspective, they constitute an example of the classical ecological divergence of r-
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and K-strategists6. Diatoms, as opportunists, efficiently exploit resources in
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unstable, rarefied environments6,11,12. Coccolithophorids, possessing high affinity
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for nutrients and low resource requirements, may grow under quiescent conditions
3
1
characteristic of stratified, open ocean waters11,12,13. The ecological distribution of
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these two phytoplankton functional groups has been hypothesised to be linked to
3
the mechanisms that supply nutrients into the upper mixed layer of the ocean6,14.
4
Here, we use data of phytoplankton biomass and species composition along with
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concurrent measurements of hydrographic variables from four Atlantic
6
Meridional Transects (AMTs) (Fig. 1a, see also Supplementary Table 1) in order
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to elucidate the mechanisms that control the balance between diatoms and
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coccolithophorids in the ocean and their role in the carbon cycle.
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Across latitudinal gradients in the ocean, the availability of inorganic
10
nutrients exerts a major control on phytoplankton biomass, primary productivity
11
and community structure15. Typically, high latitude, temperate and upwelling
12
systems receive substantial amounts of nutrients from deep waters by wind-driven
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vertical mixing and fluid transport along isopycnal layers. In contrast,
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phytoplankton communities inhabiting low latitude environments, such as
15
stratified, subtropical gyres, largely rely on local recycling and diapycnal nutrient
16
fluxes to sustain their standing stocks. In general, high nutrient conditions
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enhance diatom productivity. Coccolithophorids, instead, are thought to be more
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widely distributed in oligotrophic ocean waters. Contrary to conventional
19
wisdom, our data analysis shows that the biomass (and biovolume) of both,
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diatoms and coccolithophorids, increases consistently from subtropical regions to
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temperate and upwelling systems in response to increased nutrient supply (Fig.
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1b). Though both phytoplankton groups exhibited similar latitudinal trends in
23
biomass, our results reveal striking differences in their ranges of variation.
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Diatom biomass increased over four orders of magnitude from low latitudes to
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temperate and upwelling systems, whereas coccolithophorid standing stocks
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varied by two orders of magnitude (Fig. 1b). These latitudinal variations in diatom
4
1
and coccolithophorid biomass are consistent with their patterns of diversity (Fig.
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1c). Again, their ranges of variation were remarkably different, with the number
3
of diatom species varying two-fold more than coccolithophorids (Fig. 1c).
4
We found a striking correspondence between the coccolithophorid-to-
5
diatom (C/D) ratio and the nutricline depth (Fig. 2). Specifically, our analysis
6
indicates that, across the entire latitudinal gradient considered, coccolithophorids
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rapidly rise to dominance relative to diatoms as the water column stratifies and
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the nutricline deepens in the ocean.
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When physical mixing increases, the upper mixed layer penetrates the
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nutricline, thereby providing nutrients to the euphotic zone, and either the
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nutricline shoals or nutrients become elevated throughout the water column (i.e.,
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zero-depth nutricline in this study). Conversely, when stratification increases, the
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upper mixed layer is deprived of nutrients, which leads to a progressive deepening
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of the nutricline. The depth of the nutricline may thus be used as a proxy of
15
nutrient supply into the upper mixed layer of the ocean. In fact, the primary
16
productivity of phytoplankton is negatively correlated with the depth of the
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nutricline (Supplementary Fig. 1). This inverse relationship suggests a strong
18
coupling between the position of the nutricline in the water column, the rate of
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nutrient supply into the euphotic layer, and the photosynthetic performance of
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phytoplankton.
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Different factors, such as the availability of silicic acid16, the mineral used
22
by diatoms to construct their cell walls, or the saturation state of seawater with
23
respect to calcium carbonate, a key controller of calcification17, have been put
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forward to explain the distribution of diatoms and coccolithophorids in different
5
1
marine environments. For instance, the low silicate-to-nitrate input ratios
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characteristic of the equatorial Pacific Ocean are thought to limit diatom
3
productivity and C drawdown in this high nutrient-low biomass ocean region16.
4
Though these factors play a role, our study, covering temperate, upwelling,
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tropical, subtropical, and equatorial ecosystems, lends weight to the view6 that
6
upper ocean turbulence and nutrient supply dynamics exert a dominant control
7
upon the ecological selection and evolutionary success of these two
8
phytoplankton functional groups in the ocean. In line with this, a suite of
9
competition experiments in chemostats have shown that the population dynamics
10
of diatoms and coccolithophorids can be controlled by simply varying the rate of
11
nitrate supply with the C/D-ratio going up and down in response to steady-state
12
and nutrient pulsing conditions, respectively (unpublished results). Remarkably,
13
the strong linkage between the nutricline depth and phytoplankton community
14
composition reported here arises despite that, in nature, ecological systems are
15
influenced by manifold and ever changing environmental templates and complex
16
biotic interactions.
17
Our field observations illustrate very well how the balance between diatoms
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and coccolithophorids is primarily driven by variations in diatom biomass and
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diversity, with coccolithophorids exhibiting comparatively minor response to
20
environmental variability (Fig. 1b,c). But why do these phytoplankton groups
21
exhibit such different ranges of variability?
22
Microbial plankton are ubiquitous in the world oceans. Their global
23
biogeography is determined by local environmental factors that select for a
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particular species based on its optimal growth potential. According to this
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‘universal distribution, local selection rule,’ the patterns depicted here for diatoms
6
1
and coccolithophorids reflect their divergent life histories and survival
2
strategies6,11,12,13. Diatoms, afforded by their high maximum growth rates, luxury
3
nutrient uptake, and ability to store inorganic nutrients in vacuoles, exploit
4
unstable environments where resources are supplied in excess relative to
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biological demand6,12,14, and diatoms bloom. But adaptation to unstable
6
environments imposes severe nutritional constraints under stable conditions.
7
Prolonged stability, such as increased ocean stratification, drives plankton
8
ecosystems to operate near their carrying capacity (i.e., resource demand-to-
9
supply ratio close to unity) and, under these conditions, ecological selection
10
depends primarily on the ability of each individual species to compete for limited
11
resources18. With the exception of species that migrate vertically and take up
12
inorganic nutrients in the vicinity of the nutricline, or in symbiotic association
13
with nitrogen fixers, diatoms are at a disadvantage under strong resource
14
limitation due to their high nutritional requirements and high half saturation
15
constants12. Consequently, across contrasting marine environments, the
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distribution of diatoms is inextricably tied to ample variations in biomass and
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diversity (Fig. 1b,c). In contrast, coccolithophorids have lower maximum nutrient
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uptake and growth rates12,19. Instead, they possess low half saturation constants
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for nutrient uptake and small intracellular nutrient quotas (i.e., low nutritional
20
requirements)12. These ecophysiological features allow coccolithophorids to
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inhabit a broad array of open oceanic environments without succumbing to
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dramatic variations in standing stocks (Fig. 1b,c).
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The impact of nutrient limitation on the competitive success of diatoms and
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coccolithophorids has profound implications. Changes in the C/D-ratio may alter
25
the efficiency of the biological pump by controlling the balance between two key
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processes, i) export of carbon into the ocean interior, and ii) release of CO2 due to
7
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coccolithophorid calcification (i.e., ‘alkalinity pump’, each mole CaCO3
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precipitated results in ~0.7 mole CO2 released, assuming present CO2
3
concentrations, temperature T = 12 C and salinity S = 35) (ref. 3). Consequently,
4
for a given rate of organic productivity, an increase in the C/D-ratio will lead to a
5
decreased role of the biological pump in sequestering atmospheric CO2 (refs. 3,
6
20). Our analysis shows that major changes in the C/D-ratio are linked to the
7
stabilisation-destabilisation of the water column (Fig. 2). Low latitude, stratified
8
ecosystems are dominated by coccolithophorids (C/D-ratio >> 1), and therefore
9
increased stratification bears little effect on the CaCO3-to-organic C production
10
ratio. However, in unstable environments, characterised by a significant
11
contribution of diatoms, the onset of water column stratification (and subsequent
12
nutrient depletion) causes a rapid floristic shift, increases the CaCO3-to-organic C
13
mass ratio, and reduces to a far greater extent the efficiency of the biological
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pump (Fig. 2).
15
Current evidence and modelling work suggest that climate warming will
16
increase ocean stratification, and hence reduce nutrient exchange between the
17
ocean interior and the euphotic layer7-9. Recent reports highlight that, indeed,
18
these climate driven trends in upper ocean stratification and primary productivity
19
across the central, oligotrophic gyres are already detectable and may be occurring
20
faster than model expectations21. Yet, predicting the impact that these changes
21
may exert on the ecological functioning of marine plankton ecosystems remains
22
elusive22. In order to investigate possible shifts in the distribution of diatoms and
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coccolithophorids caused by climate warming, we projected our empirical model
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between the coccolithophorid-to-diatom (C/D) ratio and the nutricline depth onto
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a coupled atmosphere-ocean general circulation model (GCM) (see
26
Supplementary information for details). This three dimensional GCM was forced
8
1
by the Intergovernmental Panel on Climate Change (IPCC) IS92a ‘continually
2
increasing’ CO2 scenario (880 p.p.m.v. in the year 2100). By the end of this
3
century, in this model configuration and CO2 scenario, the increased upper ocean
4
stratification reduces the supply of macronutrients to the euphotic layer, deepens
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the nutricline and leads to a reduction by 14 % in global oceanic primary
6
productivity (Supplementary Figs. X). The projection of our empirical model
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shows dramatic increase in the C/D-ratio across vast areas in the North Atlantic,
8
but particularly pronounced in temperate ecosystems (Fig. 3). By the year 2100,
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the C/D-ratio in the North Atlantic temperate region increases by >80 % relative
10
to year 2000. In both hemispheres, the expansion of subtropical systems into
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temperate, equatorial and upwelling regions causes striking shifts in ecosystems
12
characterized in the present day by a significant contribution of diatoms (Fig. 3).
13
Our prognostic analysis indicates that in a climate warming scenario, the
14
progressive stratification of oceanic systems will increase the C/D-ratio, reducing
15
the potential of marine plankton ecosystems to drawdown atmospheric CO2 (ref.
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20). The model finds that oceanic regions subjected to the influence of the
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expansive subtropical gyres, such as the North Atlantic drift province or tropical
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upwelling systems, could be particularly susceptible to this biogeochemical
19
control (Fig. 3). Aside from shifting the C/D-ratio, increased ocean stratification
20
and nutrient limitation may decrease primary productivity and export fluxes7,
21
especially those associated with diatoms23. The efficiency of the biological pump
22
could further change if, as expected, the invasion of anthropogenic CO2 into the
23
ocean alters the rates of coccolithophorid calcification and/or phytoplankton C-
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consumption (hence deviating the C/N stoichiometry from a static Redfield
25
ratio)24,25. A sensitivity analysis indicates that the biogeochemical control
26
described in this study (positive feedback on climate) could be of magnitude
9
1
comparable, but inverse, to experimentally-observed reductions in
2
coccolithophorid calcification (negative feedback) (Supplementary Fig. X). If, on
3
the contrary, coccolithophorid calcification increases, as recent reports suggest26,
4
the storage capacity of the surface ocean for CO2 would decrease, which, united
5
to the smaller ability of marine plankton ecosystems to drawdown atmospheric
6
CO2, would cause a larger reduction of the biological pump’s efficiency.
7
Margalef’s thesis that external energy input determines phytoplankton
8
community structure6 is empirically demonstrated here. Over the past ~50 million
9
years, diatoms and coccolithophorids have responded incessantly to upper ocean
10
turbulence and nutrient exchange dynamics, contributing significantly to the
11
regulation of atmospheric pCO2 and Earth’s climate4,5,14,27. Consistent with the
12
geological record and the classical ‘Mandala’ of Margalef, our results indicate
13
that the relative distribution of diatoms and coccolithophorids is strongly
14
dependent upon the mechanisms that supply nutrients into the upper mixed layer
15
of the ocean. We suggest that this mechanistic connection is majorly responsible
16
for the ecological succession and long-term regime shifts of these two
17
phytoplankton functional groups in the ocean. If so, these taxonomic shifts, and
18
their associated impacts on net air-sea CO2 exchange, would be linked
19
mechanistically to the seasonal dynamics of the upper mixed layer, interannual
20
variations in climatic forcing22,28, contemporaneous trends in anthropogenic
21
climate warming20,22,27, and the historical climate change14,27,29.
22
METHODS SUMMARY
23
Physical, chemical and biological variables were obtained from four Atlantic
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Meridional Transect cruises (50N-50S). Micromolar concentrations of inorganic
25
nutrients were determined on fresh samples using standard techniques. For each
10
1
station, the depth of the nutricline was taken to be the first depth where nitrate
2
was detected (> 0.05 μM). Samples for identification and counting of
3
phytoplankton were analysed using an inverted microscope (Utermhol’s
4
technique). Cell numbers were expressed in terms of carbon using biovolume
5
estimates and volume to carbon conversion factors.
6
The potential of marine phytoplankton to drawdown atmospheric CO2 was
7
calculated from the C/D(biomass)-ratio considering the opposing effects of
8
photosynthesis and calcification3. We assumed a fixed CaCO3-to-organic C
9
production ratio in coccolithophorids of 1. The moles of CO2 released by mole of
10
CaCO3 precipitated, also known as ‘Revelle factor’ were calculated from
11
analytical equations3.
12
The numerical simulation of change in ocean nutrient distribution was performed
13
using an earth system model of intermediate complexity. The model incorporates
14
an explicit C cycle model with a parameterization of the biological export
15
production limited by the availability of light and nutrients. This biogeochemical
16
component follows the fate of a macro-nutrient, dissolved organic matter,
17
alkalinity and dissolved inorganic carbon. Simulated changes in the nutricline
18
depth enabled to prognosticate the relative distribution of diatoms and
19
coccolithophorids over this century using our empirical relationships.
20
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Supplementary Information is linked to the online version of the paper at
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www.nature.com/nature.
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Acknowledgements We thank all those who participated in the collection of data, in particular D.
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Harbour for microscopy analyses, and S. Costas, A. Kahl, E. Marañón and Y. Rosenthal for
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discussion and comments on the manuscript. E. M. provided access to his primary productivity
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data. Atlantic Meridional Transect (AMT) data collection was supported by the UK Natural
5
Environmental Research Council through the AMT consortium (NER/O/S/2001/00680). P.C. was
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supported by Marie Curie Outgoing International Fellowship from the European Union.
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Author Contributions P.C., O.S. & P.G.F. designed the research. R.P.H. managed the
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phytoplankton database. P.C. conducted the analysis of phytoplankton data. S.D. & M.F.
9
performed the modelling section. P.C. wrote the paper.
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Author Information Reprints and permissions information is available at
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www.nature.com/reprints. The authors declare no competing financial interests. Correspondence
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and requests for materials should be addressed to P.C. (pedro@marine.rutgers.edu) or P.G.F
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(falko@marine.rutgers.edu).
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Figure Legends
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Figure 1. Phytoplankton distribution in the Atlantic Ocean. a, AMT cruises
3
track overlain on an annual climatology of chlorophyll showing high values
4
where surface nutrients are elevated. b, Latitudinal distribution of biomass
5
of diatoms (blue) and coccolithophorids (red). c, as b but for number of
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species. Solid lines in panels b and c are biomass and number of species
7
averaged each 5 of latitude. Dashed lines are the range of data values.
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Figure 2. Nutricline depth controls the coccolithophorid-to-diatom (C/D)
9
ratio. Data points are 5 zonal averages of C/D-ratio and nutricline depth
10
from four Atlantic latitudinal transects (solid circles) and two coastal
11
stations (open circles, see supplementary information). The left Y-axis is
12
represented on a log-scale in order to highlight the rapid shift from diatom
13
to coccolithophorid dominance in response to the onset of water column
14
stratification. Colour lines are the least square linear regression models:
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[C/D (biomass) = 0.14 + 0.14 * ND, r2 = 0.78, p < 0.0001], and [C/D
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(diversity) = 0.55 + 0.03 * ND, r2 = 0.83, p < 0.0001]. Changes (%) in the
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biological potential for atmospheric CO2 sequestration were estimated
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from the model equation considering the opposing effects of
19
photosynthesis and calcification (thick solid line, see Supplementary
20
information for details). Confidence intervals were obtained from the range
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of data values (as shown in Fig. 1b, thin solid lines). Note that major
22
changes are associated with the stabilisation/destabilisation of water
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column. Reduction in pump’s efficiency considering changes in the
24
buffering capacity of the ocean’s carbonate system in a future, high CO2
25
scenario was also displayed (dashed line, see Supplementary
26
information).
16
1
Figure 3. Projected changes in the distribution of diatoms and
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coccolithophorids over this century. a, Coccolithophorid to diatom (C/D)
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biomass ratio in year 2000. b, as a but for 2100 (assuming IPCC IS29 CO2
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‘continually increasing’ scenario).
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Figure 1.
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Figure 2.
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Figure 3.