macronutrients in the ocean - solas int · solas summer school, cargese august 05, catherine...

Solas Summer School, Cargese August 05, Catherine Jeandel 1

Macronutrients in the ocean

Catherine JeandelLEGOS (CNRS/CNES/IRD/UPS)

Toulouse, France


Contents

I- Basic featuresI-1 Photosynthesis, Organic matter compositionI-2 Redfield ratioI-3 Primary, new and regenerated productions

II- N, P, Si: oceanic cycles and distributionII-1 Vertical distribution, Limitation, Oxygen and AOUII-2 Horizontal distribution, thermohaline circulation, preformed nutrients and classical application

III- Deviation to the general behaviors: key features for SOLASII-1 Nitrification, denitrification and nitrogen fixationIII-2 Colimitation and the role of micronutrientsIII-3 Combinations of nutrients as tracersIII-3 An environment affected by human pressure: case study of the Med Sea


PhotosynthesisSunlight + nutrients → Organic matter → food web

Ecosystems run on energy: behind each cycle of massis transduction of bioenergy and production of heat

With the courtesy of D. Karl

P, N, C+ Si and Ca


Organic matter composition

Traditional Stoechiometric formula:106 CO2 +16 HNO3+H3PO4 +122H2O=(CH2O)106(NH3)16(H3PO4) +138 02

Stoechiometric ratios: Redfield ratiosP:N:C = 1:16:106 (Redfield et al, 1963) + 138 02

Oxygen and hydrogen too high?

Anderson (1995)106 CO2 + 16 HNO3+ H3PO4 + 78 H2O

= C106H175042N16P + 150 02

Composition of this organic matter (per weight)54.4 % proteins25 % carbohydrates16 % lipids4 % nucleic acids


New vs regenerated productions

Rivers and atmosphere(small and local)

a) Until the early 90’s:- steady state N cycle between inputsand export (Eppley and Peterson,79)

- NO3 → PON = New production(Dugdale and Goering, 1967)-In situ recycling of OM by bacteria→ NH3 → Regenerated production

Primary Production =New + Regenerated productions

NPRP

PP= NP+RP

b) During the 90’s- Major role of DOM: DON, DOPand DOC could be a net source ofnutrients for OM synthesis- role of bacteria in the surfaceecosystem becomes essential

c) Since the mid 90’sSurface ocean biological cyclingtakes into account the supply of Nto organisms by N2 fixation, asby NO3 and NH3 transportedthrough the atmosphere

Sarmiento and Gruber, 2003

N2 fixation


Contents

I- Basic featuresI-1 Photosynthesis, Organic matter compositionI-2 Redfield ratioI-3 Primary, regenerated new and exported productions

II- N, Si, P: oceanic cycles and distributionII-1 Vertical distribution, Limitation, Oxygen and AOUII-2 Horizontal distribution, thermohaline circulation, preformed nutrients and classical application

III- Deviation to the general behaviors: key features for SOLASII-1 Nitrification, denitrification and nitrogen fixationIII-2 Colimitation and the role of micronutrientsIII-3 Combinations of nutrients as tracersIII-3 An environment affected by human pressure: case study of the Med Sea


Oceanic nitrogen cycle

NitrificationStep 1 Ammonia oxidation:NH3 + 1½ O2 → NO2

- + H+ + H2OStep 2 Nitrite oxidation:NO2

- + 1½ O2 → NO3-

DenitrificationNO3

- + 6H+ + 5e- → ½N2 + 3H2O

Nitrogen fixationN2 + 8H+ + 8e- + 16ATP --> 2NH3

+ H2 + 16ADP + 16Pi (biological)

AssimilationNO3

-, NH3

Production/reduction of N20Depends on circumstances ([O2], [NO2], etc)

Heine, 2003Solas Summer School, Cargese August 05, Catherine Jeandel 8

Nitrogen cycle: different stepsI- Nitrogen fixationThe microbial conversion of molecular nitrogen (N2) to ammonia (NH3 NH4+ in solution).Fixation converts nitrogen gas to a salt that organisms can use. In other words, higher organisms are completely dependent on micro-organisms for the nitrogen atoms in their proteins, nucleic acids, etc.

II- Nitrogen assimilationAmmonia can be incorporated into organic molecules such as nucleic acids.Algae are able to do assimilative nitrate reduction, i.e. they use nitrate as a nitrogen source by reducing it and incorporating the nitrogen atoms into organic molecules.

III- DeaminationConversely, organic molecules containing nitrogen are deaminated during decomposition of organic materials, producing ammonia.

IV- NitrificationBacteria collaborate to oxidize ammonia. The first oxidation product is the nitrite anion (NO2

-) Nitrite is further oxidized by bacteria mainly of the genus Nitrobacter, producing the nitrate (NO3

-) anion.

V- Denitrification (or nitrate reduction) Dissimilative nitrate reduction involves the microbial reduction of nitrate, producing nitrogen gas.Note that only microorganisms carry out nitrogen fixation, nitrification and denitrification.


Nitrogen oceanic cycle: important features

I- Oxidation numbers of relevant nitrogenous compounds:

-3: NH3, R-NH20 : N2+3: NO2

-

+5: NO3-

II- Examples:NH3 -----> R-NH2

The incorporation of ammonia into organic molecules does not involve oxidation or reduction. Both forms of nitrogen have the same oxidation state.

NO3- + 8 e- -----> R-NH2

The incorporation of nitrate into organics requires an input of 8 electrons (reduction).


Silicon Oceanic cycle

Tréguer et al, 19951012 moles Si/y

Particulate bioSi

Major input

Major output


-Diatoms are amongthe largest phyto.

- Siliceous external skeleton(frustule: 2 valves)

- Dominant in the highlatitudes, coastal andupwelling areas

- Often the first speciesto appear after theintroduction of nutrients

- When Si becomes too lowdiatoms are replaced by non-siliceous phyto.


- A calcareous phytoplancton species: Coccolithophorids- Prymnesiophytes form an outer shell of CaCO3 plates: coccoliths- those are formed inside the shell and are replaced as frequentlyas one every 15 mn- Preserved (high cliffs,as along the Channel Sea). - Appeared 170 millions years ago

-E Huxleyi is the most ubiquist: present everywhere exceptedin the Polar Seas

-highest blooms: subarctic North Atlantic, North Pacific andlow latitude marginal seas etc…


Phosphorus Cycle

Very rapid processes


Phosphorus oceanic cyclePhosphorous cycle : simpler than Si and N

Phosphorus has only one form, phosphate: PO4

Heavy molecule: part of an organism, a rock, or dissolved in water

Weathering: PO4 goes into solution. Riverine is the main pathway for PO4 into theocean.

Autotrophs take this phosphorous up : constituent of cell membranes, DNA, RNA, and ATP ( adenosine triphosphate, respiration cycle).

Heterotrophs (animals) obtain their phosphorous from the plants they eat.

Animals may also use phosphorous as a component of bones, teeth and shells.

When animals or plants die (or when animals defecate), phosphate may be returnedto sea water by decomposers (bacteria). There, it can be taken up by another plant and used again.

This cycle will occur over and over until at last the phosphorous is lost at the bottomof the deepest parts of the ocean, where it becomes part of the sedimentary rocks forming there.

Ultimately, this phosphorous will be released if the rock is brought back to the earthsurface and weathered.

Humans often mine rock rich in phosphorous: fertilizer.

This human use greatly accelerates the phosphorous cycle and may cause local overabundance of phosphorous (see the Med sea budget at the end of this course)

Local abundance of phosphate can cause overgrowth of algae in the water: eutrophication.


N and 02: Vertical profiles(Pacific Ocean)

The S minimum corresponds to a water formed in theNorth

Note the mirror shapes of N and oxygen profiles

Surface waters are completely depleted in nitrate:

Nitrate is aLIMITING ELEMENT

for the biological activity

ST

02 NO3


Bio-limiting and bio-intermediate elements(Broecker and Peng, 1982)

Surface waters are nevercompletely depleted in carbon

Same for barium, involved in biological cycles

Both are BIO-INTERMEDIATE elements

On the opposite, Si (as N and P) can be completelysurface depleted.

Those are LIMITING elements

C Alk

Si Ba


N and P: what kind of limiting nutrients?Biologists: Nitrogen is regarded as the limiting nutrientfor phytoplankton growth and export production in mostregions

Indeed, the amount of available N is microbial-sensitiveGeochemists: regard phosphorus as the limiting nutrienton very long timescales

The amount of P in the ocean will mainly reflect thebalance between river input and loss to the sediments: biolimiting nutrient on very long timescales(glacial/interglacial state of the ocean)

Actually, things are not so simple Imbalances in which denitrification exceeds nitrogenfixation over periods of several thousands of years couldchange oceanic export production by significantamounts (Codispoti,1989)OM production controlled by the nutrients having thelowest concentration relative to the needs for growth:

« limiting » depends on the balance between nutrient supply and demand.

Most of the biogeochemical models have elected N as driving variable, adapted further for P or C fluxes usingRedfield ratios


The N/P ratio on a global scale




P as a limiting nutrient ?


P as a limiting nutrient (2):the global distribution of primary production fits

the P minima and maxima, except in the Southern Ocean



Horizontal discrimination of macronutrients

Libes,1992


Why this Pacific to Atlantic deep enrichment ?

From Broecker and Peng, 1982

« The trash effect »


The horizontal distribution of essential elements

oxygen

silicon

nitrate


Apparent Oxygen Utilisation: AOU

The respiration of Particulate Organic matter causes theremoval of oxygen from the thermocline and below.

AOU = (02)sat –02(meas) : in µmoles02/kg sw

(02)sat : assumes that oxygen is at saturation with theatmosphere at the ocean’s surfaceAOU expresses the consumption of oxygen by remineralization: it increases with increasing distance from the deep water formation locationAs (02)sat is temperature dependent, the expression of AOU eliminates the T effect, and therefore most of theadvection effect.However: oxygen is generally SUPERsaturated (biology), but can also be UNDER saturated (convection): therelative AOU value is more important than its absolutevalue.Redfield: 138 moles O2 consummed for 1 mole P remineralized

(Anderson: 150/1)


Horizontal distribution of AOU

AOU


Preformed nutrients

Some nutrients are present in newly formed deepwaters

Suggesting that phytoplankton growth did not consume all the available nutrients (other limiting factors: T, OML, Fe…)

Amounts of nutrients in a water mass: sum of the« preformed » + « produced by POM remineralization»

Example:(PO4) = (PO4)° +AOU/138

Can be used as a conservative tracerExample: Northern Component Water (PO4)° = 0.73Southern Component Waters (PO4)° = 1.2


Example(Broecker and Peng, 1982)

Conservative mixing between NCW and SCW

NCW

SCW


Preformed phosphate section (global ocean)


Sarmiento et al, 2004

Si* = [Si(OH)4] – [NO3-]

-10 to-15 µmole/kg in the SAMW formation regions

Si* will be nearly conserved when SAMW will spread within theocean = powerful water mass tracer

Si* is an indicator of diatom requirements (negative Si* induceslow diatoms production)

SAMW (σθ=26.8) formationarea

Definition and concept of Si*


Global maps of nutrient properties along σθ=26.8

Si*

Depth of σθ=26.8

Zonal mean of the« export ratio »ie the vertical Si(OH)4gradient vs that of NO3(between the first upper 100m and the next 100m)



Origin of low-Si* waters?Physical and biological processes



Contents

I- Basic featuresI-1 Photosynthesis, Organic matter compositionI-2 Redfield ratioI-3 Primary, regenerated new and exported productions

II- N, P, Si: oceanic cycles and distributionII-1 Vertical distribution, Limitation, Oxygen and AOUII-2 Horizontal distribution, thermohaline circulation, preformed nutrients and classical application

III- Deviation to the general behaviors: key features for SOLASII-1 Nitrification, denitrification and nitrogen fixationIII-2 Colimitation and the role of micronutrientsIII-3 An environment affected by human activities:

case study of the Med Sea


Anaerobic destruction of Organic MatterDenitrification = N sink

When the rate of 02 removal exceeds its rate of supply by thermohaline circulation: anaerobic microorganisms willmetabolize any remaining OM

Denitrification(CH2O)106(NH3)16(H3PO4) + 84.8 NO3

- →106 CO2 + 148.8 H2O +42.4 N2 + 16 NH3 + H3PO4

Sulfate reduction(CH2O)106(NH3)16(H3PO4) + 53 SO4

2- →106 CO2 + 106 H2O + 16 NH3 + 53 S2- + H3PO4

Note that the remineralized N and sulfur remain in reducedform (no oxygen)

Hydrolysis of CO2 causes the reduced forms to be converted to NH4+

and HS-


Oxygen-deficient intermediate or deepwaters

Libes, 1992


Denitrification affects Redfield ratio(Libes, 1992)

Redfield remineralization ratio

Loss of N


Nitrate vertical profile in deficient O2 areas(Broecker and Peng, 1982)


Microbiological N2 fixation

•Discovered in the late 19th century in soil bacteria

•H. B. Bigelow (1931): “The possibility that so-called N2 fixers may also fertilize seawater must be taken into account”

•R. Dugdale discovered N2 fixation in the Sargasso Sea in 1961

•Process was considered to be negligible in pre-JGOFS era, but significant during JGOFS

With the courtesy of D Karl


Nitrogen fixation, nitrification = N source

I- Atmospheric N2 fixation Trichodesmium (blue-green algae)

utilizes inexhaustible pool of dissolved N2 as nitrogen source.

Subtropical Atlantic and Pacific: enhances the PP beyond thelimits given by the pool of nitrate.

Metabolic switch: alter P/N ratios, could provide an efficient mechanism of « pulsed »export

In such a case: P, Fe…eventuallylimit the productivity


With the Courtesy of D KarlSolas Summer School, Cargese August 05, Catherine Jeandel 40

Is the oceanic N budget in balance or not?

Large uncertainties affect the marine N budget

Some studies indicate that total denitrification (sink) isexceeding total nitrogen fixation (source)

Is the ocean loosing N? (Codispoti, 1995)

Hypothesis of McElroy (1983): loss during warm periods, gain during glacial times.

Could explain the CO2 drop during glacial periods

Steady State (Liu, 1979) or dynamic (Codispoti, 1995)?Problem is the extrapolation of sparse data

Gruber and Sarmiento (1997) proposes an exhaustive studyquasi-conservative tracer N*

Based on the large-scale distribution of N and P: eliminatesmost of the problems associated with the extrapolation.


The present day marine N budget is close to equilibrium(Gruber and Sarmiento, 1997)


The Atlantic case: phosphorus deficiency

BATS: DIP concentration in the Sargasso Sea is 2 orderof magnitude lower than in the Pacific (HOT)

Phosphorus is yetunderstood as a limitingnutrient in the Atlantic

N2 fixation uses nitrogenase, which requires high amountof iron

Iron input is much more important in the Atlantic thanin the remote Pacific

N vs P limitation in thecontemporary ocean

is closely linked to eolianFe supply

Wu et al, 2000

N/P is higherat BATSthan at HOT(1 y of data)


What about the Med Sea?(Marty et al: Dyfamed TSS)


Seasonal N/P variation at Dyfamed(8 y of data: Marty et al, 2002)

N/P =16

Successive limiting conditions in the Med Sea

P-limited(picoplankton)DOC export

N-limited(larger plankton)

POC export


Nutrient dynamics in many oceanic areas

•Past Dogma: N limits biomass accumulation and production rates

•Contrariant Viewpoint: P or some trace nutrient limits biomass accumulation and production rates

•New Hypothesis: There is a systematic, temporal alternation between N and P/Fe control of plankton processes, resulting from complex interactions between the ocean and the atmosphere, that may have important consequences for biogeochemical cycling rates and processes in the sea


Interannual tendency of the Med Sea(Bethoux et al, 1998, 2002)

P inputs increase by 3% per year

human activities, via riversand atmosphereP concentrations in deepwaters increased by 0.5% per year since 40 years

Despite this increaseN:P constant with time but larger than in the otheroceans. Cannot result fromN:P of inputs. Likely reflectsN2 fixation (large amount of Fe, Saharan) Si:P = 32 in the Ionian Sea, 21 in the AP basin, largerthan anywhere else.However, Med Sea is not atsteady state and theincrease of P and N couldshortly induce a change in the algual species of thetrophic chain (towards non-siliceous dominating) withrisk of eutrophisation


Conclusions

Macronutrients (together with micronutrients as Fe, or vitamins…) are essential to sustain the oceanicecosystem dynamics (transduction of bioenergy andproduction of heat)

Productivity in the ocean is maintained by rapid nutrientrecycling (and not high nutrient loading)

This constrains nutrient vertical profiles as well as theirlarge scale horizontal distribution

Preformed nutrient can be used as conservative tracers, and added as useful variables in ocean circulation models.


Conclusions (2)

Our understanding of the nutrient dynamics and vital roles of micro-organisms has considerably progressed during the last two decades, mostly the 90’s.

It is ~ established that the nitrogen budget of the ocean isbalanced (ocean is not loosing nitrate).

N2 fixing organisms and aperiodic blooming of diatoms + alternative N-controlled and P-controlled conditions are keyprocesses that could yield pulses of export, and change ecological assemblages, with a potential impact on the C cycle and eutrophication.

Our traditional picture of the Atlantic Ocean as a regionpredominantly controlled by N (or Si) availability, isreconsidered, together with the role of Fe, likely coupled to N fixation and feedbacks to P demands.

Nutrient combinations (Si*) can be used to trace water massand nutrient status of a given area

Systems as coastal seas, or the Med Sea, submitted to a strong anthropic pressure, could see their phyto-ecologychange from siliceous to non siliceous dominated species.


Perspectives (related to SOLAS objectives)

Because P is a staff of lifeNeed to understand P delivery and export completely, as well as controls on local recycling (comprehensivechemical characterization of DOP)

We must remain vigilant: the sea is source of our life, and weare already in a period of significant human impact on global nutrient cycles

It is almost certain that we have already changed pre-industrial processes

A variety of essential nutrients can influence (and alter) the C sequestration in the deep ocean

The study of Fe-N-P coupling/decoupling must be a main priority for the coming year.This implies understanding their full cycles (input, microbial and biological transformation, outputs).

It appears that the sea surface metabolically diverse microbial soup is able to react quickly to external forcing andclimate change

Future strategies should implement complementary« high frequency time series studies » , fieldexperimentation, satellite observations andcomprehensive ecosystem model-data comparisons.


Some relevant papers on nutrient fate in the ocean (Catherine Jeandel, 2005)

Ammerman J.W. et al, Phosphorus deficiency in the Atlantic: an emerging paradigm in oceanography, EOS, vol 84, N°18, 6 may 2003Bethoux J-P et al, The Mediterranean Sea: a miniature ocean for climatic and environmental studies and a case for the climatic functioning of the North Atlantic, Progress in Oceanography, 44, 131-146, 1999.Bethoux et al, Nutrients in the Med Sea, mass balance and statistical analysis of concentration respect to environmental changes, Marine Chemistry, 63, 155-169, 1998.Codispoti L.A. and J.P. Christensen: Nitrification, denitrification and nitrous oxide cycling in the eastern Tropical South Pacific Ocean. Marine Chemistry, 16, 277-300, 1985.Codispoti L.A Is the ocean losing nitrate? Nature, p 724, 1995Dugdale et al. The role of silicate pump in driving new production, Deep Sea Res., 42, 687-719, 1995.Gruber N. and Sarmiento J., Global patterns of marine nitrogen fixation and denitrification . Global Biogeochem. Cycles (end page 260), 1997.Karl D.M. et al., The role of nitrogen fixation in the biogeochemical cycling in the subtropical North Pacific ocean, Nature, 388, 533-538, 1997Karl D.M et al, Ecological nitrogen-to-phosphorus stoichiometry at station ALOHA Deep Sea Res., 48, 1529-1566, 2001.Karl D.M. Nutrients dynamic in the deep blue sea Trends in Microbiology, 10, p 410- 418, 2002. A VERY USEFUL PAPER!Libes Susan, An introduction to the oceanic biogeochemistry, 1992Marty JC et al: author of a paper + editor of the DSR (part II) special issue on the DYFAMED time serie station, 2002Peterson T.D., Whitney F., Harrison P.J, Macronutrient dynamics in an anticyclonic mesoscaleeddy in the Gulf of Alaska Deep Sea Res. II, 52 (2005) 909-932.Ragueneau et al (14 authors!) A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleo-productivity proxy Global Planet change 317-365 (2000)Sachs J.P. and Repetta D. J, Oligotrophy and nitrogen fixation during Eastern Mediterranean Sapropel event, Science, 286, 2485-2488, 1999.Sarmiento J and Gruber N. Ocean biogeochemical dynamic, A WONDERFUL LECTURE! www.atmos.ucla.edu/~gruber/teaching/teaching.fr-textbook-htmlSarmiento et al, High-latitude controls of thermocline nutrients and low latitude biological productivity, Nature, 427, 56-60Tréguer P et al, The silica balance in the world ocean : a reestimate. Science, 268, p 375-379, 1995.Thingstad T.F et al, P limitation of the heterophic bacteria and phytoplankton in the NW Mediterranean, Limnology and Oceanogr., 43, 88-94, 1998.Wu J. et al, Phosphate depletion in the western North Atlantic Ocean. Science, 289, 759-762, 2000.

macronutrients in the ocean - solas int · solas summer school, cargese august 05, catherine...

Documents