University of Groningen
Factors controlling phytoplankton growth and species composition in the Antarctic
Ocean
Buma, Anita
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Publication date:
1992
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Buma, A. G. J. (1992). Factors controlling phytoplankton growth and species composition in the Antarctic
Ocean s.n.
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Chapter lX
fluenceof iron and nitrate
t1.
Lsof the picoplanktonic alga
Summary
) zone,australspring 1988,
This researchwas designedto shedmore light upon the factorscontrolling
dynarnrcsof ron in scawateÍ.
oteins, RNA, DNA and
Mar. Biol. l0 : 44-51.
. kptodinium ririle gen. et sp.
phytoplanktongrowth, standingstock,speciescompositionand successionin
Antarctic waters.
The spatialand seasonaldistributionof variousphytoplanktoncommunitieswas
investigatedduring two surveysin the Atlantic s€ctorof the SouthernOcean.During
orophyll a- and à- containing
both cruisesin situ chlorophyllc levels were low, remainingfar below the maximal
teocystis pouchetii blooms in
special
expectedlevel at nutrientdepletion.Applying specificpigmentmeasurements,
attentionwas paid to cells in the nanoplanklonsize range(2-20 pm), sincefew data
i h e l fS c i .2 3 : 1 7l - 1 8 2 .
mshy in providing a source of
were availóle on the quantitativecontribution of this phytoplankton fraction to total
biomassin Antarcticecosystems.
In the australautumn(APSARA) the compositionof
the phytoplanktonshowedlarge scalesimilarities.Only small differenceswere found
r a triggering tactor fbr red tide
betweenthe water massesunderinvestigation.Pigmentpatternsrevealedthat green
;land , South Orkneys, The
algaeand dinoflagellateswere of minor qrxmtitative importancein the area,whereas
predominatedover diatoms.During the australspring(EPOS)the
Prymnesiophyceae
-189.
ndancein westem Antarctic and
r80.
rtcnoitlsof the Chrysophyceae.
M.V. Nella Dan.Januaryt985
rire phytoplanktonanalysedby
compositionof the phytoplanktonshowedmuch more spatialand temporalvariation.
During the coarseof this surveya diatom dominatedphytoplanktonice edgebloom
was followed by a small celledbloom, with Cryptophyceaereachingmonospecific
bloom conditionsat the end of the cruise.This successionwas found to be relatedwith
lirill grazing,as found by other EPOSteams.Greenalgaeand Prymnesiophyceae
tbrmed a large fractionof the under-icephytoplanktonstock.Diatomswere found to
persistonly in the Weddell ScotiaConfluenceduring this season.The high relative
importanceof nanophytoplanktonthroughout implies that microbial pathwaysaÍe of
greatsignifrcancein Antarcticfood chain dynamics.
No "clean"studyhad everbeencarriedout beforeto testthe hypothesisofFe
limitation of phytoplankton growth in Antarctic waters.During the secondcruise
(EPOS)the effect of iron on both phytoplanktondevelopmentand community structure
was studied.Theimpact of iron on phytoplanktongrowth was investigatedby
monitoring nutrient utilization, phytoplankton pigment increaseand total organic
carbon formation after enrichmentof natural phytoplankton populations with Fe. The
effects of manganese,copper and zinc were studiedoccasionally, showing virtually no
139
effect.For most planktoncommunitiesFe was found to stimulatethe formationof
rnore
chlorophyllc , total particulatecarbonand the assimilationof niffateandphosphate.
Furthersupportwas providedby division ratecalculationsfor variousphytoplanklon
woul
andr
species,suggestingenhancedlevelsof division rate after Fe addition.
The effectsof metaladditionson the structureof the first trophic levelsin the
kinet
variouswater systemswere studiedby applying microscopyand specificpigment
basec
measurements.The
additionof Fe selectivelyfavouredthe growth of diatom species.
were
The simultaneousFe-mediatedstimulationof zooplankon (hereciliate) activity and
lnten
biomasscausedadditionalshiftsin community structurein the Fe-enrichedbottles
prese
towardsdiatom dominance,throughselectiveciliate grazingupon smallcelled
new "
phytoplanklon.Theseobservationsimplied first of all that Fe exertsa selective
implit
pressureon phytoplanktonspeciesin situ .In systemswhich carry elevatedambient
thepe
levelsof Fe, suchas neritic regionsand marginalice zones,the growth of diatomsand
There
consequentlythe relativeuptakeof nitrateversusammoniummay be favoured.It also
not ne
implied that Fe-fertilizationof the SouthemOceanwould dramaticallychangethe
behav
structureof the food web by generatingshiftsin phytoplanktoncommunitiestowards
capab
diatomdominance,and probablyat the sametime to a proportionalincreasein new
conflr
productionand subsequent
vertical particleflux.
to25
However,therewas a steadyincreaseof chlorophyll a in the control bottles(no
additon)aswell, and the completeutilization of a major nutrient (nitrate,phosphateor
retard
silicate)within a few weeksafter the startof the experiments.This provedthat ambient
uansp
chara
Fe levelsin the Weddell-ScotiaSeasare high enoughto principally supportrapid
or whi
build-up of phytoplanktonpigmentand utilizationof nutrients.It also implied that the
condit
experimentalconditionsin the controlswere more favourablefor phytoplankton
growth thanthe conditionsin the field, obviouslyeliminatingone (or more)growth
light c
impac
suppressingfactor which only existsin the field. Thereforeintensegrazingpressureby
140
largeherbivoressuchas Euphausiasuperba was suggestedto play a key role in crop
Conc
control,as theseorganismsare underrepresented
after initial samplingfor this type of
specie
experiment.Although to a lesserextentthanin theFe-enrichedbottles,the plankton
factor
compositionin the control bottlesshiftedtowardsdiatom-and microzooplankton
poor li
dominanceas well. This finding of the simultaneousdevelopmentof large diatomsand
factors
ciliatesto the observedhigh numbersfurthermoresuggestedthe importanceof
under
top-downconffol ofphytoplankton by zooplanktonin the field. It wasconcludedthat
contro
althoughFè playsa role as a rate limiting factor,grazingseemsan importantfeaturein
the oth
keepingnutrientconcentrationshigh and phytoplanktonstandingstockslow, at leastin
ultima
the areainvestieateddurine EPOS.Futureresearchshouldfocuson the role of Fe in
food u
rtetheformation of
moreremoteareasof the Antarctic.Here the presumedlow(er) ambientFe levels
itrateandphosphate.
would havea more pronouncedimpacton phytoplanktongrowth, speciescomposition
rariousphytoplankton
and new production.
dition.
One study focussedon featuresof growth performanceand photoadaptation
rophiclevelsin the
This study,
kineticsunderchanginglight conditionsin threeAntarctic nanoflagellates.
d specificpigment
basedon semi-continuousculture experiments,demonstratedfirst of all that thesealgae
rth of diatomspecies.
were capableof photoadaptation
to 1owlight fluxes.Division ratesat saturatinglight
: ciliate)activity and
intensitiesat the experimentaltemperaturefell within rangesgiven for polar diatoms,as
Fe-enriched
bottles
presentedin the literature. The reportedlong times required Íbr the establishmentof
on smallcelled
new "balanced"division ratesand constantfluorescencelevelsafter a light transition,
rertsa selective
implied poor photoadaptational
behaviourin the testedorganisms.However,during
arry elevatedambient
the period of unbalancedgrowth intemal buffers seemedto supporthigh division rates.
,growthof diatomsand
Therefore overall growth performanceof thesealgae during transient stateperiods was
Laybe favoured.It also
not negatively affected by long periods of unbalancedgrowth and poor kinetic
fically changethe
behaviour with respectto changesin cell characteristics.Polar algae do not seem
communitiestowards
capableof adaptationto largedown-shiftstepsin the light intensity,which was
lnal increasein new
confrmed in this study.Stronglight sfess was observedafter a shift down from 400
n thecontrolbottles(no
to 25 pE.m'z.s-1:for at leastone month following the shift growth was sffongly
retardedandcellularchlorophyllcontentandcell sizekept their "sun" adapted
t (nitrate,phosphate
or
characteristics.This implied that surfacephytoplanklon communities which are
his provedthatambient
transportedfrom the surfaceto deeperwatersby a wind-mixed layer deepeningevent,
ally supportrapid
or which are advectedbeneathsea-ice,may haveproblems to adapt to low in siz light
It alsoimplied that the
conditions.Featuresof susceptibilityto stronggradientsin irradiancemay havean
x phytoplankton
ne (or more)growth
impacton speciescomposition,competitionand surcessionin the dynamicAntarctic
light climate.
:nsegrazingpressureby
rlay a key role in crop
Conclusions. It is concludedfrom this thesisthat phytoplanktonproduction and
mplingfor this typeof
speciescomposition in Antarctic watersis governedby a complex of inter-dependent
bottles,theplankon
factors. Phytoplankton growth may be restrictedby temperature,iron availability or
nicrozooplankton
poor light conditions,wherebya balancebetweenphytoplanktongrowth and loss
:nt of largediatomsand
factors (like grazing) is established.Thereforeelimination of one rate limiting factor
: importanceof
under certain circumstanceswould allow phytoplankton growth to escapegrazing
, It wasconcludedthat
control,leadingto occasionalincreasedlevelsofphytoplankÍon stocksorblooms. On
an importantfeaturein
the other hand,eventhough grazingis the proximatecontrol,the supplyof iron might
rg stockslow, at leastin
ultimately regulateproductivity by principally influencing speciescompositionand
food web structwe. Furthermorethe relative importanceof rate and stock limiting
rson therole of Fe in
141
factorswill differ for the variousecosystems
encounteredin the SouthernOcean.In
openremotepartsof the SouthemOceanrate limiting factorslike iron levelsand light
conditionswill havea strongimpacton phytoplanktongrowth and species
composition.This is not likely in neritic regionsand marginalice zones.Here strong
gnzing pressurelikely preventsthe phytoplankton from exploiting the available
nutrientsin surfacewaters.It is ofimportance for future researchin high-nutrient,low
chlorophyll regionsof the world's oceans,to considerthe interactionof rate- and stock
limiting factors,ratherthanto focuson the role of one singlelimiting factor.
142