Coupling and decoupling of biogeochemical cycles in marine ecosystems [Elektronische Ressource] / vorgelegt von Sönke Hohn

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• Biologie

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• Publié par: universitat_bremen
• Publié le: 01 janvier 2009
• Nombre de lectures: 42
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Couplinganddecouplingof
biogeochemicalcyclesinmarine
ecosystems
Dissertation
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
- Dr. rer. nat. -
am Fachbereich 2 (Biologie/Chemie)
der Universitat¨ Bremen
vorgelegt von Son¨ keHohn
Erstgutachter: Prof. Dr. Dieter Wolf-Gladrow
Zweitgutachter: Prof. Dr. Andreas Oschlies
Bremen, January 22, 20092Contents
Preface iii
1 GeneralIntroduction 1
1.1 Ecologicalstoichiometry................................ 2
1.2 Ecological modelling ...... 4
1.3 Studyregions...................................... 7
2 Modelling primary productivity in a shallow coastal tidal basin (S. Hohn, C. Volk¨ er,
J.E.E.van Beusekom, M.Schartau) 11
2.1 Introduction ....................................... 11
2.2 Modeldescription... 12
2.2.1 Physicalsetup.................................. 12
2.2.2 Salinitymodel ..... 15
2.2.3 Ecosystemmodel................................ 16
2.2.4 Modelruns....... 2
2.3 Results..................................... 23
2.3.1 Salinitymodel ..... 23
2.3.2 Referencerun.................................. 25
2.3.3 Impactoffilterfeders ....... 30
2.4 Discusion........................................ 30
2.4.1 Hydrodynamics . . ... 30
2.4.2 Riverine runoff . . . .............................. 31
2.4.3 Transport budget . ... 3
2.4.4 Bentho-pelagiccoupling............................ 3
2.4.5 Impactoffilterfeders ....... 34
2.4.6 Limitations................................... 35
2.5 Apendix....... 35
2.5.1 Statevariables ................................. 35
3 A model of the carbon:nitrogen:silicon stoichiometry of diatoms based on metabolic
processes (S. Hohn, C.Volk¨ er, D.A.Wolf-Gladrow) 43
3.1 Introduction ....................................... 43
3.2 Modeldescription... 4
3.2.1 Parameterization of algal physiology . . . ................... 44
3.2.2 Integration of the physiological model; comparison with laboratory studies . 48
3.3 Results.......................................... 51
3.3.1 Experiment1...... 51
i3.3.2 Experiment2.................................. 51
3.3.3 Experiment3.. 52
3.3.4 alternativeSiuptakekinetics.......................... 5
3.3.5 Globalmodelrun.......... 57
3.4 Discusion.................................. 57
4 CouplinganddecouplingofsiliconandnitrogencyclesintheSouthernOcean (S.Hohn,
C.Volk¨ er, M.Losch, S. Loza, D.A.Wolf-Gladrow) 63
4.1 Introduction ....................................... 63
4.2 Modeldescription... 6
4.2.1 Thegeneralcirculationmodel......................... 6
4.2.2 REcoMSO............. 6
4.2.3 Processes ......................... 70
4.2.4 Forcing and initialization ...... 73
4.2.5 Modelparameters................................ 74
4.3 Results.............. 75
4.3.1 Parameterstudies................................ 75
4.3.2 Referencerun...... 79
4.3.3 Decoupling of silicon and nitrogen cycles ................... 86
4.4 Discussion....................... 89
5 Highresolution modelling (M.Losch,M.Schroder¨ , S. Hohn, C.Volk¨ er) 97
5.1 Introduction ....................................... 97
5.2 TheModel....... 98
5.2.1 Biogeochemicalmodel............................. 98
5.2.2 PhysicalmodelandCS510configuration.. 98
5.3 Scalability ........................................ 99
5.4 Results.........10
5.5 ConcludingRemarksandOutlok...........................101
6 GeneralDiscussion 105
6.1 Couplingofelementalcycles..............................105
6.2 Parametervaluesandparameterizations..108
6.3 Proposal for future research . ..............................111
Zusammenfassung 129
iiPreface
The global cycles of biologically important elements in the ocean are coupled to each other via the
production of biomass (Fig. 1). Once incorporated into organic molecules and compartimented in
cellular structures, the fate of biogeochemical elements is linked until bacterial breakdown of or-
ganic molecules, remineralization, releases the elements again as dissolved inorganic or, to a smaller
amount, organic nutrients. Remineralization processes thus again decouple the fluxes of different
biogeochemical elements from each other (Fig. 1).
The elemental composition (stoichiometry) of biomass appears to be relatively uniform in ma-
rine ecosystems (Redfield et al., 1963). The average ratios of C:N:P in marine biomass is found
to be 106:16:1, respectively (Redfield et al., 1963). This ratio can be explained by a combination
of different organic molecules that have characteristic C:N:P ratios (Geider and La Roche, 2002).
Neutral lipids and carbohydrates do not contain nitrogen or phosphorus but only carbon, oxygen and
hydrogen. Phospholipids additionally a phosphate group associated to the glycerol. Pro-
teins are rich in nitrogen and also contain carbon, oxygen, hydrogen and small amounts of sulfur.
Enzymes, belonging to the functional group of proteins, can also contain small amounts of metal
ions in their reaction centres. DNA is a combination of saccharides, nitrogen rich organic bases and
phosphates. And ribosomes that are needed for DNA transcription are especially rich in phosphorus.
Many other different structural and functional molecules are combined in biomass, making organ-
isms a combination of different chemical elements that perform complex chemical reactions called
life. The common cellular structure underlying all organisms causes the similarity of the biochemical
composition of the biomass of different organisms.
However, different species have evolved different physiological requirements for chemical ele-
ments due to different realized metabolic pathways (Fig. 1). Cyanobacteria for example are capable
of splitting the triple bond between two nitrogen atoms and thus transform gaseous nitrogen into
reactive nitrogen. Cyanobacteria have also developed high requirements for iron because the en-
zyme nitrogenase, that performs the splitting of gaseous nitrogen, contains large amounts of iron.
Other phytoplankton organisms require less iron but depend on the availability of reactive nitrogen.
Diatoms take up dissolved silicon in the form of silicic acid and build cell walls of amorphous hy-
drated silica. Diatoms are thus also dependent on the availability of silicon to perform cell division.
Coccolithophores produce small plates of calcite that are attached to the cell surface and thus couple
calcium to the fluxes of carbon. The biominerals silica and calcite, which are the most important
biominerals in phytoplankton, can have high geological importance due to sedimentation and accu-
mulation at the sea floor.
From the perspective of biogeochemistry, species can be combined to so called phytoplankton
functional types (PFT) of calcifiers, nitrogen fixers, silicifiers, DMS producers, and various others,
depending on similarities in their physiological properties. The combination of different species or
functional groups in the plankton community of different ocean regions leads to differences in the
average stoichiometric composition of the produced biomass. The chemical composition of biomass
iiiFigure 1: The elemental cycles of carbon, nitrogen, phosphorous, silicon and iron, and many other biolog-
ically important elements, are coupled to each other via the formation of biomass. Different organisms may
havedifferentrequirementsfornutrition. Whenorganismsdie,theyaddtothepoolofdeadorganicmaterial,
detritus. Decompositionofdetritusreleasestheelementsfrombeingincorporatedintoorganicmoleculesand
particlesandagaindecouplesthefateofelementsfromeachother.
is thus only uniform in a statistical sense and the coupling of biogeochemical elements varies on the
level of species compositions.
Environmental conditions also determine the chemical composition of phytoplankton organisms
as nutrient uptake is strongly dependent on nutrient availability. Even though the fluxes of different
elements are coupled via formation of organic molecules, the combination of molecules in the cell
can vary due to physiological responses to the environment. For example, when algae grow under
nitrogen-limiting conditions, nitrogen uptake is reduced and cellular nitrogen, protein, and chloro-
phyll contents decline, raising the intracellular C:N ratio. The cellular content of protein strongly
affects metabolic activity as most metabolic reactions are catalized by enzymes. At low irradiance
levels, photosynthetic carbon fixation is inhibited and the intracellular C:N ratio decreases. As most
metabolic reactions also require energy in form of ATP or NADH, which are provided via photosyn-
thesis or respiration of starch or lipids, carbon deficiency due to light-limitation also reduces cellular
activity. The coupling of biogeochemical elements may thus also vary on the species or individual
level.
Iron is only needed in small amounts in phytoplankton cells but iron deficiency has various effects
on cellular stoichiometry. Iron is involved in the assimilation of nitrogen because it occurs in the
reaction centre of the enzyme nitrate reductase. Iron is also involved in photosynthesis as it is part of
the electron transport chain and the photosystems. Different roles of iron in the cellular metabolism
of different elements induce different regulations of elemental composition under iron-limitation. In
diatoms, increased Si:N and Si:C ratios are observed when