University of South Florida
Scholar Commons
Marine Science Faculty Publications
College of Marine Science
3-15-2000
Plankton Community Composition, Production,
and Respiration in Relation to Dissolved Inorganic
Carbon on the West Florida Shelf, April 1996
Gary L. Hitchcock
University of Miami
Gabriel A. Vargo
University of South Florida St. Petersburg, [email protected]
Mary-Lynn L. Dickson
University of Rhode Island
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Scholar Commons Citation
Hitchcock, Gary L.; Vargo, Gabriel A.; and Dickson, Mary-Lynn L., "Plankton Community Composition, Production, and Respiration
in Relation to Dissolved Inorganic Carbon on the West Florida Shelf, April 1996" (2000). Marine Science Faculty Publications. 117.
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105,NO. C3, PAGES 6579-6589,MARCH 15, 2000
Planktoncommunitycomposition,
production,and respiration
in relation to dissolvedinorganiccarbon on the West Florida
Shell April 1996
Gary L. Hitchcock
Rosenstiel
Schoolof MarineandAtmospheric
Science,
University
of Miami,Miami,Florida
GabrielA. Vargo
Department
of MarineScience,
University
of SouthFlorida,St.Petersburg
Mary-Lynn Dickson
Graduate
School
of Oceanography,
University
of RhodeIsland,Narragansett
Abstract.In April 1996theFloridaShelfLagrangian
Experiment
examined
dissolved
inorganiccarbon(DIC) dynamicsonthe West
Florida
Shelf. DIC concentrations
increased
over
-1
-1
2 weeksatanaverage
rateof 1 gmolkg d in a patchof theintentionally
released
tracers
sulfur
hexafluoride
(SF6)
andhelium
3 (3He).Approximately
20%oftheincrease
wasdueto
air-seaexchange
withtheremaining
80%attributed
to plankton
respiration
[Wanninkhofet
al., 1997].Herewepresent
particulate
matterconcentrations,
phytoplankton
production,
and
community
respiration
ratesfromthetracerpatchthatsuggest
thatheterotrophs
dominated
thecommunity
afterthetermination
of a spring
bloom.Duringtheexperiment,
chlorophyll
a
andphaeopigment
concentrations
declined
from> 1.5to< 0.5ggL-•,with75-85%oftotal
chlorophyll
a in the< 5gmsizefraction.
Particulate
mattercomposition,
withmeanratiosof
particulate
organic
carbon:chlorophyll
a > 200andparticulate
organic
nitrogen:chlorophyll
a
> 100,suggests
thatphytoplankton
werea minorcomvonent
of theplankton
biomass.
Rates
ofdailygross
primary
production
estimated
bytheH2180
method
averaged
69+ 5 mmol
C
d (n = 3) whfiedarkrespiration
rates,estimated
fromdarkbottleincubations,
werea•p-
proximately40+ 3 mmol
Cm'2d-1.Netcommunity
production
rates
(6ñ 6 mmol
Cm-2d-1)
weremuchlowerthanrespiration
rates.Thusrespiration
ratesnearlybalanced
phytoplankton
production.
Lightrespiration
rateswereestimated
fromgross
production
minusnetcommu-
nityproduction
(-51•-8mrnol
Cm-2d-1)andexceeded
dark
respiration.
Plankton
community
respiration
rates,corrected
for autotrophic
carbonfixation,weremorethansufficient
to accountfortheobserved
increase
of DIC withinthetracerpatch.
1. Introduction
'highnutrient-low
biomass'waters,whereCO2fugacityde-
creases
afteriron enrichment[Cooperet al., 1996;de Baar et
Plankton
communities
alterthedissolved
inorganic
carbon al., 1995].
(DIC)content
of oceanic
surface
waters
through
theprocesses Whileseveralstudieshaveexamined
planktonproductivity
of photosynthesis
andrespiration.
In surface
waters,DIC con- andDIC in theopenocean,fewerhaveexamined
theseproccentrations
decrease
if primaryproduction
ratesexceedthe
esseson continental
shelves[Frankignoulle
et al., 1996].In
combined
influxof carbon
fromtheatmosphere
andrespira- comparisonto the open sea, continentalshelf environments
tion [Robertson
and Watson,1995]. In the NorthAtlantic
are characterized
by high ratesof carbondeposition
dueto
Ocean,for example,
DIC concentrations
declinewhenprienhanced
nutrientinputs,highproductivity
rates,and an inmaryproductivity
increases
duringthe springbloom[Chipflux of organicmatterfrom terrestrialsources[Wollast,
manet al., 1993]. Measurements
of air-seaCO2exchange
in
contribute
to coastalwatersasa sigtheNorthAtlanticshowthatspatial
distributions
of primary 1993]. Theseprocesses
environment
in theglobalocean[Smith
producers
correlatesignificantly
with variationsin surface nificantheterotrophic
and
Mackenzie,
1987].
Nevertheless,
ratesof photopCO2at lengthscales
< 100km [Watson
et al., 1991]. Pro- autotrophiccarbon fixation can reducehigh
DIC levels in shelf
ductivityalsoinfluences
the spatialdistribution
of DIC in
surface
waters.Codispoti
et al. (1986)observed
thatphotosyntheticcarbonuptakereducedpCO2from > 300 to < 125
ppmduringa springbloomin theBeringSea.In April 1990,
DIC concentrations
on GeorgesBankinitiallydecreased,
and
Copyright2000 by theAmericanGeophysical
Union.
thenincreased,
atrates
ofseveral
I.tmolC kg-•d-t asthespring
bloomdeveloped
andsubsequently
declined[Sambrotto
and
Langdon,1994]. In contrastto temperate
and boreallati-
Papernumber 1998JC000293.
0148-0227/00/1998JC00029359.00
6579
6580
HITCHCOCK ET AL.: PLANKTON PRODUCTION ON THE WEST FLORIDA SHELF
tudes,few studieshavebeenmadein subtropical
or tropical from 24ø N, 84ø W reveal that net particle displacementin
of theShelfis of theorderof a fewkmd'• with
shelfenvironments
wherephytoplankton
biomassand pro- thisregion
ductivityratesarerelativelylow.
corresponding
along-shoretrajectoriesof hundredsof kiloIn April 1996 the Florida Shelf LagrangianExperiment metersper month[Weisberget al., 1996]. Theselengthscales
(FSLE)characterized
the dissolved
inorganiccarbonsystem agree with the observeddisplacementof the tracer patch
on the West Florida Shelf [Wanninkhofet al., 1997]. The [Wanninkhofet al., 1997].
main objectiveof the studywas to quantifyair-seagasexProductivitystudiesindicatethat the West Florida Shelf is
changeratesin a subtropicalshelfecosystem.Sincecarbon oligotrophic.Annualproductivityestimatesrangefrom 30 to
dynamics
on continental
shelvesarecomplicated
by advection 180g C m-2yr4 [Vargo
et al., 1987;El-Sayed,
1972;Wroand mixing of oceanic,coastal,and riverinewaters [De- blewskiand 0 'Brien, 1973] with maximumratesin bloomsof
Grandpreet al., 1998],the FSLEwasdesigned
as a "patch" the dinoflagellateGymnodiniumbreve Davis. However,most
study throughthe releaseof the intentionaltracerssulfur of the production measurementshave been made during
hexafluoride
(SF6)andhelium
3 (3He).TheLagrangian
de-
nonbloomperiodswhen G. breve is absentfrom surfacewa-
signof the experiment
providedinvestigators
with the op- tersorpresent
atbackground
concentrations
of< 103cellsL-•
portunityto follow DIC dynamicsin a taggedwatermassfor [Geeseyand Tester, 1993]. Shorewardof the 40-m isobath
2 weeks. Duringthe study,DIC concentrations
in the mixed thedailyproduction
ratesaretypically
< 500mgC m-2d4
layerincreased
at anaverage
rateof- 1 lamolkg'• d-•,with withsurface
chlorophyll
a concentrations
< 1 lagL4 [Vargo
et
20% of the increasedue to the influx of atmospheric
CO2 al., 1987, Table 1). Chlorophylla concentrations
on the mid[Wanninkhofet
al., 1997].Theremaining80% of theincrease dletoouter
Shelfcanexceed
1 lagL'• forseveral
weeks
inthe
wasattributed
to plankton
respiration.
Herewe reportpigment spring(Gilbeseta!., 1996). This springbloomon the West
concentrations,
particulatemattercomposition,
and commu- Florida Shelf may be initiatedby an influx of allochthonous
nity production
andrespiration
ratesfromthe tracerpatch. nutrientsfrom riverine sourceson the northernGulf coast,inThe observations
supportthe conclusions
of Wanninkhof
et cludingthe MississippiRiver and Mobile Bay, or from onal. [1997],sincerespiration
ratesby the predominantly
het- shoretransportof nutrient-rich,deep water upwelled at the
erotrophicplanktoncommunitywere morethan sufficientto Shelf break. In April 1996 we observedthat surfacechloroaccount for observed increases in DIC.
phyll a and phaeopigmentconcentrations
at mid-Shelf de-
The WestFloridaShelfis at the easternboundaryof the clined
frominitiallevelsof 1.5to < 0.5lagL4. OurobservaoligotrophicGulf of Mexico. Nutrient concentrationsare tions thereforelikely coincidedwith the terminationof the
relativelylow in surfacewaterson the Shelf,althoughnutri- 1996 springbloom.
entlevelsareoftenenhanced
followingwindevents[Fanning
et al., 1982]. Circulationon the inner Shelf is primarily
drivenby windsand tides [Marmorino,1983;Mitchurnand 2. Methods
Clarke, 1986]. The experimentwas locatedat 29øN, 84ø W,
with the tracerreleasedalongthe 25-m isobath(Figure1). 2.1. Particulate Analyses
Long-termacousticDopplercurrentprofiler(ADCP) records
Particulatematterwas analyzedfor chlorophylla, phaeopigrnent,
andparticulate
organiccarbon,nitrogen,andphosphorus.Chlorophylla concentrations
were measuredon samples collected at 20 stations from 30-L Niskin bottles
86 ø
84ø
mounted on a General Oceanics rosette with a SeaBird 11
82 ø
conductivity-temperature-depth
(CTD). Duplicatealiquotsof
200 mL eachwere filteredonto Whatmanglassfiber filters
(GF/F) at a vacuumof 50 mm of Hg. The filters were ex-
30 ø
tracted in 10 mL of methanol for 4 hours in the dark at room
x
28 ø -
".•v",..., "}_
i
x. ,". b•";',.
'/-'.,•
',.
)
'•
;
temperature.
Chlorophylla and phaeopigrnent
concentrations
weredetermined
from fluorescence
readingsbeforeandafter
acidificationon a TurnerDesignsModel 10 fluorometerfollowingthe methodof Holm-Hansenand Riemann[1978].
'E'T
I•
The fluorometer
was calibratedwith purifiedchlorophylla
purchased
from SigmaChemicalCorporation.
Particulateorganiccarbon(POC) and particulateorganic
nitrogen(PON) concentrations
were measuredat 12 stations
(Table 1) from duplicate200 mL aliquotsfilteredontoprecombustedWhatmanGF/F filters (2 hoursat 450øC). The
26 ø
filterswere storedovera dessicant
at-20øC and assayedin
thelaboratory
ona CarloErba1600carbonnitrogenhydrogen
(CHN) analyzer[Sharp, 1974]. Blanksof precombusted
filtersandstandards
wereanalyzedat the beginningandendof
eachday. Particulatephosphate
was measured
by the combustionmethodof SolorzanoandSharp[ 1980]on duplicate
samplesfiltered onto precombusted
Whatman GF/F filters
acid-rinsed,
glassscintilFigure 1. Bathymetryof the West Florida Shelf. The tracers andstoredfrozenin precombusted,
lation vials.
werereleased
onthe25-misobath
at 29øN, 84øW (dot).
', ',,,,,., -,., ,,
-
*-"'"
I
HITCHCOCK
ET AL.: PLANKTON
PRODUCTION
ON THE WEST FLORIDA SHELF
6581
Table 1. Sampling
Activityat CTD Stations
for Particulate
MatterandPigment
Concentrations,
PrimaryProductionRates,andNutrientMeasurements
Day
GMT
CTD
Latitude,øN Longitude,øWDepth,m
1
2
3
1331
1907
1246
8
11
15
29.049
28.038
29.066
3
2146
17
28.934
4
4
5
6
7
8
8
9
10
11
14
0827
1135
1220
0012
0320
0810
2226
1007
1012
0819
0809
18
19
25
28
34
40
43
45
54
58
72
29.017
28.967
29.066
28.999
29.099
29.063
29.144
29.041
28.493
28.947
29.063
Activity
SF6
15
5
10
part.
part.
part.
NS
NS
130
84.084
5
nut.
242
84.013
84.083
84.001
84.092
84.067
83.986
83.966
84.078
84.064
84.102
83.986
5
10
11
10
10
5
10
5
10
5
5
prod.1
part.
part.
part.
part.
prod.2
part.
part.
part.
prod.3
prod.4
161
95
106
60
83
25
23
33
34
21
20
84.111
84.022
84.031
14
1020
73
29.063
84.000
5
nut.
19
14
16
2214
1633
75
93
29.091
29.073
84.010
84.101
20
5
part.
pan.
5
11
Day 1 corresponds
to April 1, with time in UT and latitudeand longitudein decimaldegrees.
Depthsof discrete
samples
arein meters.SF6concentrations
arein partspertrillionby volume.The
tracerinjection
wasonApril 1;conductivity-temperature-depth
(CTD) 8 wasontheinjection
line.NS,
no samples;
part.,particulate
matterandpigmentconcentrations;
prod.,primaryproduction
rates;nut.,
nutrient measurements.
concentrations
in initial,light,anddarkbottlesby the Winkler
methodutilizinga potentiometric
endpointtitrator.Seawater
samples
weredrawnfromthe Go-Flobottlesinto quartz125mL flasks, flushedwith severalhundredmilliliters of seawater,andthen closedwith groundglassstoppers.Four initial O2 samplesanda total of 16 light and darkbottleswere
2.2. Grossand Net Production and Respiration Rates
filled with seawaterfrom each of the four samplingdepths.
Samples
for production
andrespiration
rateswerecollected
The O2flasksweresuspended
with the grossproductionsamfrom 2, 5, 10, and 20 m at four CTD stations(Table 1).
plesat theircollectiondepthin acryliccarousels.Automatic
Grossandnet production
ratesare onlyavailablefromthree
temperature
recorders(OnsetComputerCorporation,Bourne,
of the stationssincethe in situarraywas loston day 11 (proMassachusetts)
were placedin eachcarouselto monitortemductivity3). Grossprimaryproduction
ratesweremeasured
perature.
by the methodof Benderand Grande[1987] from samples
After the in situarraywas retrievedat dusk,quadruplicate
incubated
for 24 hourswithH2•80[alsoseeBender
et al.,
light anddarkbottleswere fixed with reagentsfor determin1987]. All seawatersamplesfor production
and respiration
ing photoperiod
(daylight)ratesof NCP and DCR. The remeasurements
were collectedat 0400 local time (0800 UT) to
mainingfour light bottlesand four dark bottlesfrom each
ensurethatphytoplankton
werenotexposed
to directsunlight
depthwerethenplacedin the on-deckincubator
to completea
beforeincubation. Two samplesfrom each depthwere en24-hour incubation. The O2 sampleswere titrated after
riched
with200laLof 97.2atom%
H2•80(Cambridge
Isotope
reachingthermalequilibriumwith an automated
titrationsysLaboratories,Andover, Massachusetts)
for 24-hour incubatem. The titrationswererun to a potentiometric
endpointwith
tions.TheH2•SO-enriched
samples
werethensuspended
from a RadiometerAutomaticBurette(ABU) and a TitrationMana surfacefloat at their depthof collection.The float was deager(TIM) 90 controller.The analyticalprecisionin laboraployedbeforedawnandtrackedby a VHF beaconuntil it was
torysamples,
calculated
asstandard
deviations,
is + 0.04% for
retrievedat sundown. At duskthe grossproductionsamples
iodatestandards
and+ 0.07% for replicateO2 samples.At sea
were placed in a darkenedon-deckincubatorcooled with
the analyticalprecisionis + 0.15% for replicateoxygensamflowingsurfaceseawaterandincubateduntil dawn.
plesdrawnfrom the sameGo-Flobottle.Precisionfor NCP
Two samplesfrom the Niskin bottleswere collectedfor
Verticalprofilesof scalarirradiance
weremeasured
with a
LiCor LI-193S 4• sensorloweredfrom the stem of the ship.
A total of sevenprofileswere takenwithin 2 hoursof local
noonwhilethe stemwaspositionedtowardthe Sun.
andDCRratesisestimated
tobe+ 0.2lamol
O2L'•.
initial•SOconcentrations.
Approximately
50 mLof seawater
were dispensedinto preevacuated150 mL glassflaskspoi-
soned
with200txLof saturated
HgC12.Thedissolved
•802
was extractedfollowing the procedureof Emerson et al.
[1991].The•gO2
samples
wereanalyzed
in thelaboratory
of
Daily oxygenproductionrateswere convertedto carbon
equivalents
by the followingequation:
Gc = {(No2/1.4)+ ((Go2- No2)/1.1)}
(1)
M. Benderusing a FinniganMAT 252 massspectrometer.
The analyticalprecisionof the grossproductionmeasurement where
gross
production
from•gOincubations
(Go2)andNCP
(No2)from the Winkler titrationsare expressedas mmol 02
onreplicate
samples
was+_0.07ø/oo.
Ratesof net communityproduction(NCP) and dark com- m'2t4 andGcisgross
carbon
production
expressed
asmmolC
munityrespiration(DCR) were estimatedfrom changesin O2 m'2t'•. A photosynthetic
quotient
(PQ)of 1.4wasselected
to
6582
HITCHCOCK
ETAL.:PLANKTON
PRODUCTION
ONTHEWESTFLORIDASHELF
29.1ø
L
29.00_]
28.9ø
29'1ø-
29'0
o
28.9 ø
29.1ø
_
%•
5
.
29'00
0 ,,•,zi•
28.9ø-
"
-84.3ø
-84.2ø
-84.1o
.84.0o
_83.9o
_83.8o
O
.83.7o
Longitude
Figure
2. Surface
distributions
inthetracer
patch
11days
afterrelease
for(a)temperature,
(b)salinity,
and(c)
relative
chlorophyll
a fluorescence.
Thedotted
lineinthetemperature
map(a)shows
theship
track
during
thesurvey.Thefluorescence
mapincludes
thestation
positions
forproductivity
andparticulate
samples
(solid
diamonds),
andadditional
stations
forchlorophyll
a (open
diamonds).
Thespatial
extent
oftheSF6patch
isindicated
bythe5
partspertrillionbyvolume(pptv)contour
(dashed
line).
tionsincreased
from100nmolkg'• in themixedlayerto a tionsincreased
from-• 0.25ggL'• inthesurface
layerto 0.9
maximum
of 150nmolkg'• at25m. In general,
nitrate
con- ggL'• below
thepycnocline
(Figure
40. Thedecline
inchlo-
centrations
at CTD 74 were50% higherthanthoseat CTD rophylla andphaeopigment
concentrations
duringthe first
17,varying
from250to350nmolkg'• inthewatercolumn. few daysof the studyand the low initial chlorophyll
Pigment
profilesat CTD 75 weretypicalof the vertical a:phaeopigment
ratiossuggest
thatthe phytoplankton
comdistributions
of chlorophyll
a andphaeopigment
during
the munitywasin declinesoonafterthetracerwasinjected.
lastweekofthestudy.
Chlorophyll
a exceeded
phaeopigrnent Chlorophylla concentrationswere measuredin three
atalldepths,
withmaximum
chlorophyll
a andphaeopigment
sizefractions
onApril8 (CTD 43) andApril 16 (CTD 93).
concentrations
in thebottomlayer.Chlorophyll
a concentra-Size-fractionated
samples
werecollected
fromfive depths
HITCHCOCK
ET AL.: PLANKTON
PRODUCTION
convertnet oxygenproductionratesto net carbonproduction,
with a PQ of 1.1 to convertoxygenrespiration
ratesto carbon
equivalents
[Laws,1991]. Respiration
ratesin the light(RL)
were estimatedby subtracting
the NCP ratesfrom the gross
ON THE WEST FLORIDA
SHELF
6583
surfacesampleslisted in Table 1 the mean POC:Chl a ratio
was226'1. Particulateorganicnitrogenconcentrations
varied
fourfold,
from1.65-9.72txmol
L'l, withnoapparent
relationship to the variabilityin chlorophylla (Figure 3). As with
•80production
rates
following
theprocedure
of Grande
etal. POC, the weight-basedratiosof PON:Chl a were high com[1989].
3. Results
3.1. Pigment and Particulate Distributions
As discussed
by 14/anninkhof
et al. [ 1997],the surfacedistributionof SF6at 3, 7, and 11 daysafter injectionrevealed
that the tracerfield expandedin response
to wind and tides
(14/anninkhofet
al., 1997,Figure1). Herewe presentthe corresponding
surfacedistributions
of temperature,
salinity,and
chlorophyll
fluorescence
fromthefinal survey(Figure2). The
surfacefieldsarenot trulysynoptic
sincethe survey(Figure
2a) required40 hoursto complete.Nevertheless,
a weakgradientis consistent
in the surfacetemperature,
salinity,and
fluorescence
distributions
at 84øW. Surfacetemperature
increasedfrom 16.5øCat the easternedgeof the patch,near
83.7øW,to a maximumof 17.4øCat 84.2øW(Figure2a). The
paredto thosein algal cultures,with a mean of 102:1. Particulateorganicphosphorus(POP) concentrations
exhibited
lessvariabilitythandid PON, rangingfrom 0.46-0.79 txmolL'
•, with a meanweight-based
ratioof POP:Chla of 3.1:1
(grams:grams).As discussedbelow, this POP:Chl a ratio is
comparable
to that in oligotrophicwaters.
The molarratiosof particulate
organiccarbon,nitrogen,
andphosphorus
in thetracerpatchdidnotcorrespond
to Redfieldratios.
Themeanmolarratioof POC:PON
(moles:moles)
was 3.36 + 1.6:1, about half the Redfield ratio of 6.6:1. Mo-
lar ratiosof POC:PONwere5'1-6'1 on days5-7, otherwise
theratios(3:1-5:1)werelowerthanthosein phytoplankton
cultures.Molar ratios of PON:POP declined from a maximum
of 100:1in thefirst2 daysof thestudy,to between
33:1and
76:1 duringthe lastweek. The meanmolarratioof PON:POP
was76 + 34:1, considerably
higherthanthe Redfieldratioof
16'1. Similarly,molarratiosof POC:POPwerehigherthan
thermal gradientwas centeredbetween 84.0ø and 84.1øW, theRedfieldratioof 106:1,varyingfroma minimumof 163:1
where surfacetemperaturesincreasedfrom 16.8ø to 17.2øC. to a maximum of 224:1. The mean POC:POP ratio was 180 +
24:1.
Salinityincreased
from 35.0 at the easternedgeof the patch
Verticalprofilesof properties
fromtheCTD castsshowthe
to 35.6 at thewestemmargin(Figure2b). A surfacegradient
layerdeepened
by - 5 rn duringthestudy.CTD stawas alsoevidentat 84.10øW,wheresalinityincreasedfrom surface
35.2 to 35.4.
tion17wascompleted
soonafterthetracerinjection
and- 10
hours
before
the
first
productivity
station
(Table
1).
Density
Chlorophyllfluorescencevaried by less than twofold
and
SF6
tracer
profiles
from
CTD
17
show
that
the
mixed
acrossthe width of the tracerpatch.The tracerpatch,as deextending
to 15rn
finedbythe5 partspertrillionbyvolume(pptv)SF6contour, layerwas10rndeep,withthepycnocline
thedensity
andtracerprofilesclearly
extended
from83.7øto 84.3øW(Figure2c). Stations
for par- (Figure4a).Although
ticulate,nutrient,andproductivity
samples
wereconcentratedshowstratification,dissolvedammoniaand nitrate concentraandbottom
layers.
Ammonia
in thecenterof thepatch,wheremaximum
chlorophyll
a fluo- tionsweresimilarin thesurface
from150to 300nmolkg'• between
thesurface
rescence
valuescoincidedwith the weak surfacegradientin (NH3)varied
(NO3)concentrations
ranged
from150
temperature
andsalinity(Figure2c). Lowerfluorescence
val- and25 m,whilenitrate
4b). Nitriteconcentrations,
in conueswere foundto the eastand west(Figure2c). Surface to225nmolkg'• (Figure
trast,
increased
with
depth
from-•
150
nmol
kg
'•
at
the
surface
samples
for totalchlorophyll
a (chlorophyll
+ phaeopigment)
layer
to
300
nmol
kg
'•
at
25
m.
indicatedthat total pigmentconcentrations
rangedfrom a
minimum
of 0.5txgL'• along
thewestern
edgeofthesurvey The vertical pigmentdistributionsshow that maximum
werealwaysin thebottomlayer. At CTD 17,
to a maximum
of 0.7 txgL4 nearthecenter.
Thisrange
of concentrations
in the surfacelayerwere0.90 txg
concentrations
is comparable
to the surfacepigmentconcen- chlorophylla concentrations
L
'l,
with
corresponding
phaeopigment
concentrations
of 1.10
trationsobserved
in the tracerpatchduringthe lastweekof
the study(Figure3a).
ggL'• (Figure
4c).In thebottom
layer,chlorophyll
a concen-
increased
to 3.2txgL'l, withphaeopigment
at6.0txg
Chlorophyll
a concentrations
decreased
in thesurface
layer trations
duringthefirstweekof theexperiment.
Initialchlorophyll
a C-1.
A final productivitystationwas occupiedat CTD 73 2
concentrations
varied
from1.0to 1.5txgL-• atthetimeofthe
tracer release,with corresponding
phaeopigmentconcentra- weeksafter the tracerinjection(Table 1). Vertical profiles
tionsat 1.5-2.0txgL'• (Figure
3a). Chlorophyll
a: phaeopig-compiledfromdataat CTD stations73-75 wererepresentative
observedduringthe lastweekof
mentratioswere initially < 1 andthenincreasedto > 1 as sur- of the propertydistributions
the
experiment.
By
day
14
the
mixed layerhad deepenedto
face pigmentconcentrations
declined. Five daysafter the in15
rn
with
maximum
SF6
concentrations
of 25 pptv in the
jection,chlorophylla concentrations
in the centerof the patch
of SF6in the mixedlayer
were0.5-1.0txgL'l, withphaeopigment
concentrations
at0.5- centerof the patch. Concentrations
1.5txgL'•. Oneweekaftertheinjection,
thechlorophyll
a and of CTD 73-75 were -• 20 pptv, indicatingthat the stations
phaeopigment
concentrations
in the surfacelayerof the tracer were near the tracer patch center. In contrastto the initial
profiles(e.g.,CTD 17), SF6waspresentin the bottomlayerat
patch
ranged
from0.25to0.45txgL'•.
5 pptv(Figure4d). As seenin the first week of the study,disRatiosof particulateorganiccarbonto chlorophyll(Chl) a
solved inorganic nitrogen concentrationswere variable
(POC:Chla) andparticulateorganicnitrogento chlorophylla
(PON:Chl a) suggestthat phytoplankton
were a minor com- throughoutthe water column. At C•D 74 the NH3 concenranged
from100to 250nmolkg'• withmaximum
ponentof the particulatematterin surfacewaters. Particulate trations
(Figure4e). Concentrations
of NO2
organiccarbonconcentrations
variedabouttwo-fold, from 8.3 levelsnearthe pycnocline
to 17txrnol
L'• (Figure
Bb),withcorresponding
weight-based
in the surfacelayerhad decreased,
althoughmaximumconratios of POC:Chl a (grams:grams)at 97:1-310:1. For the
centrations
were still in the bottomlayer. Nitrite concentra-
6584
a
HITCHCOCK
3
ET AL.' PLANKTON
PRODUCTION
ON THE WEST FLORIDA SHELF
,
20
m
(Figure
5).
Although
in
situ
light
intensi
at
20
m
were < 15% of surfacevalues,grossproductionratesfrom 20
mwere
- 70%ofthose
atthesurface.
Gross
production
rates
•
i•
-
gmol02 ggChla-:h-:atthesurface,
decreasing
to0.2gmol
02 gg Chl a-• h'• at 20 m. Theseassimilation
indices
are
_
Chla
&t
I
II
normalizedto chlorophylla yield assimilationindicesof 1
I
'I
comparableto thosereportedfor planktoncomunitiesin the
oligotrophiccentralNorth Pacific Ocean [Williamsand Purdie, 1991]. The meannet communityproductionrates(NCP)
from 12-hourin situ incubationsduringthe photoperiodde-
1
b 20_
creased
from2 gmol02 kg'• nearthesurface
to • 1 •tmol02
kg4 at thebaseof theeuphotic
zone.In general,
theNCP
& lO
from the photoperiodincubationswere twice thosefrom 24hour(day-night)incubations
(NCP•2versusNCP24,Figure5).
.•,.z•,pOP(xlOO)
' At 20 m, grossproductionwas balancedby lossesto respiration, resultingin a mean 24-hour NCP rate of • 0 gmol C
-
POC
..
0
I I
10
I
I
I
I
15
'
'
'
kg-1.
'
20
Days
Figure 3. (a) Concentrations
of chlorophyll(Chl) a and phaeopigment(Ph) in surfacelayerof the tracerpatch.(b) Molar concentrationsof particulateorganiccarbon(POC), particulateorganicnitrogen(PON), andparticulateorganicphosphorus
(POP)
in surfacewatersof the tracerpatch.Particulateorganicmatter
A comparison
of darkcommunityrespiration(DCR) rates
from the 12- and 24-hourincubationsshowsthat respiration
rates during the photoperiodwere approximatelytwofold
higher than at night. The photoperiodrespirationrates
(DCR•2)
wereabout-1.4+ 0.5gmolO2kg'• 12h'•,withlittle
variationwith depth(Figure5). At nightthe meanDCR was
-0.8 + 0.4gmolO2kg-• 12h-1. Therateof lightrespiration
(POM:POC,PON,POP)concentrations
arein gg L-•, except (RL),estimatedfromthe differencebetweengrossproduction
that the particulateorganicphosphorus
concentrations
havebeen
increased 100-fold.
andNCP:2,exceeded
the maximumratesof darkrespiration.
However, there was little variation in RL throughoutthe
euphoticzone. Oxygenconsumptionrates were approxi-
mately
-2.5 gmolO2kg'• 12h'• throughout
theupper
20 m
betweenthe surfaceand 25 m. At both stationsthe > 12-gm
sizefractionaccountedfor < 10% of total chlorophylla in the
mixed layer,while the 5- to 12-gm size fractioncontainedat
most 15% of total chlorophylla. At CTD 43, the > 12-•tm
size fractionalso represented< 10% of total chlorophylla,
and the 5- to 12-gm size fraction contained 13-16%. The
highestproportionof the largersize fractionswas in the pycnocline. The < 5-gm size fraction, in contrast,represented
75-85% of the total chlorophylla biomassthroughoutthe
water column at both stations.
Thus the size fraction data in-
dicatethat the phytoplanktoncommunitywas dominatedby
picoplankton,or small nanoplankton,throughoutthe water
column, with larger forms accountingfor a slightly higher
proportionof pigment biomassin the pycnocline.Microscopicobservationsof whole water samplesconfirmedthat
small, 1-gm diameter,unicellularphytoplankton
were dominant in the mixed layer. Examinationof 4'-6-diamidino-2phenylindole(DAP)I-stainedsamplesfrom the mixed layer
with an epifluorescent
microscope
revealedthatthe cellshad
thetypicalorangeandred fluorescence
signature
of Synechococcus spp.
3.2. Community Productionand RespirationRates
Sevenlightcastswerecompletedbetweendays4 and 14 to
estimate
vertical
extinction
coefficients
Ka(m-:)in thetracer
patch.Valuesfor Karangedfrom0.20to 0.15m': between
days4 and8 andthendecreased
to - 0.10m':fortheremainder of the study. The corresponding
euphoticzone depths
rangedfrom 20 m to the bottom.Sincethe depthof the
euphotic
zonewas> 20 m, theeuphoticzonealwaysextended
belowthe mixedlayer.
Mean grossproductionratesdecreasedfrom a maximumof
4.5gmol02kg':d-:atthesurface
to 3.4gmol02 kg-• d'• at
(Figure5).
Carbon-basedrates of productionand respirationinte-
gratedthroughthemixedlayerindicatethata largefractionof
grossproductionwas consumedby respiration. Daily gross
production
based
ontheH2•80method
variedfrom65to 75
mmolC m'2,witha meanof 69mmolC m-2(G24
inFigure
6).
Rates of NCP estimatedfrom the Winklet method, in contrast,
were14.7-20.6
mmolC m'2 in thephotoperiod
incubations
(N•2),decreasing
to 12.1to-0.9 mmolC m-2in the24 hour
incubations(N24). Absoluteratesof dark communityrespiration were higherthan net communityproduction,at-25 + 7
mmolC m'2 duringthe 12-hour
photoperiod
and-40 + 4
mmolC m'2over24 hours.
Thedifference
in darkrespiration
rates at 12 and 24 hours reflects the reduced rate of dark res-
piration
atnight,equivalent
to-15 + 4 mmolC m-2. Theenhanceddarkrespirationobservedduringthe photoperiod
was
exceededby light respiration(compareRm2andRLin Figure
6). Thus the planktoncommunityin the tracerpatch was
characterized
by ratesof grossproductionthat were almost
balancedby respiratorylosses,resultingin low ratesof daily
net production.
Ratiosof darkrespiration:net
productionare usefulin assessingthe ability of phytoplantkon
to meet the respiration
demandsof the plankton community.The ratios of dark
communityrespiration:netcommunity production(DCR12:
NCP12)during the photoperiodvaried from a maximumof
2.25:1 on day 4 to a minimumof 0.9:1 on day 14. The mean
ratioof DCR•2:NCP•2for the threeexperiments
was 1.5'l, indicatingthatthe respirationdemandsof the planktoncommunity exceeded
the net productionby phytoplankton.
Whenthe
corresponding
respirationand net productionratesare compared from the 24-hour incubations, the mean ratio
(DCR24:NCP24)declinedto < 0.2:1.
The sum of the mean
HITCHCOCK
ET AL.' PLANKTON
PRODUCTION
ON THE WEST FLORIDA
SHELF
6585
100
SF•2•tv)
300 400
0
-5
,
.
-
-I0
_........'"
I_
,
,
-
-20
-25
25.0
25.5
26,0
0
100 200
øt
300
400
500
0
I
i
!
1
2
4
6
8
10
0.g
1.0
D• (nmol
kg'• )
Chl a (pg !-I)
SF6 (pptv)
10
0
{
0
15
_
20
i
-5
E -10'
o
-15
-2O
-25
i'"
i
25.5
25.0
26.0
0
•'
'l
100
200
I
I
300
400
DIN(rimkg'a)
500
0.0
0.2
0.4
0.6
Chla(lagl'l)
Figure4. Verticalpropertyprofilesfromconductivity-temperature-depth
(CTD) station17 for (a) ot (solidline)
andSF6(dot),(b) ammonia
(NH3),nitrite(NO2)andnitrate(NO3)concentrations,
and(c) chlorophyll
a (dots)and
phaeopigment
(triangles).
(d-f) Corresponding
properties
fromCTD 74 and75. Nitrogendataare courtesy
of K.
FanningandR. Masserini,Universityof SouthFlorida.DIN, dissolved
inorganicnitrogen.
I
RL
•
II
R24 R12 N24
/.-•
DCR24and NCP24from the Winkler potentiometric
titrations
I
G24
N12
-5-
is43mmolC m-224h'•. Thisestimate
of gross
production
is
62%of thatderived
fromtheH2•80incubations;
thedifferenceis dueto light respiration.
-10 -
4. Discussion
-15 -
-20 '
I
-4
I
-2
I
I
0
I
2
I
I
I
4
Brnol
O:time
Our observationsindicatethat the FSLE studytook place
duringthe transitionfrom a springbloom to a microheterotroph-dominated
community.During this transition,high rates
of planktonrespirationcontributedCO2 to the mixed layer.
This interpretation
is basedon the temporalpatternin chlorophyll a and phaeopigmentconcentrations,the particulate
mattercomposition,and planktonproductionand respiration
rates.
Betweensummerandwinterthe surfacechlorophylla conFigure5. Production
andrespiration
ratesasa functionof depth centrationson the oligotrophicWest Florida Shelf are typi,
from threeproductivitystations.The right half containsgross cally< 0.5 !.tgL-• [Vargoet al., 1987;Tomas,
1995]. In
production
ratesfrom•SOincubations
(G24)andthenetcommu-
spring,high pigmentconcentrations
extendsouthfrom Cape
San
Blas
with
chlorophyll
a
concentrations
on the middleto
hour(N•:) and24-hour(N:4) incubations.The leR half contains
et al., 1996]. Oceancolor
darkcommunity
respiration
ratesfrom 12-hour(R•:) and24-hour outerShelf> 1.0!.tgL-• [Gilbes
(R24)incubations
and estimatesof light respirationrates(Rz). imageryfrom 1979 to 1986 showsthatthe featuresdevelopin
Symbolscorrespond
to mean_+2 standard
deviations.
Februaryto May. As the featurespropagatesouth,total chlo-
nityproduction
ratesfromWinklerpotentiometric
analyses
of 12-
6586
HITCHCOCK
80
ET AL.: PLANKTON
PRODUCTION
ON THE WEST FLORIDA
SHELF
1995], the low initial ratiossuggestthat grazingpressuremay
have contributedto the decreasein phytoplanktonbiomass
observedduringthe first few daysof the study. In summer,
chlorophylla concentrations
exceed phaeopigmenton the
WestFloridaShelf [Tomas,1994], sothe transitionfrom high
chlorophylllevelsduringthe springbloomto "typical"summer conditions can occur within a week.
The elementalcompositionof particulatematterindicates
that a largefractionof organicmatterwas associated
with the
microbialfood web (in the senseof Rassoulzadegan
[1993]),
rather than phytoplankton. In phytoplankton cultures,
POC:Chl a ratiosvary from minimumvaluesof- 20, under
optimalgrowthconditions,to typicalvaluesof 100-150under
conditionsof high light, low temperatures,or nutrientstress
[e.g., Laws et al., 1985; Sakshaugand Holm-Hansen,1977;
Kana and Glibert, 1987]. AlthoughPOC:Chl a ratioscanexceed300 underhigh light levels,the averageirradiancein the
Day4
Day8
Day11 Day14
watercolumnduringFSLE was 35% of incidentsurfaceirra-80
diance. The POC:Chl a ratio in naturalphytoplanktonpopulationshasbeenestimatedfrom flow cytometricmethodsand
Figure6. (top)Meangrossproductivity
rates,ascarbon,from epifluorescence
microscopy[Bucket al., 1996]. Thesemeth24-hour•gOincubations
(G24)andnet community
productivity
ods
have
demonstrated
that the phytoplankton
POC:Chla rafrom 12-hour(N•2)and24-hour(N24)O2incubations
for days4,
8, and14. No productivity
ratesareavailablefor productivity
3 tio in the North Atlantic varies as a function of latitude, with
(day11) sincethein situbottleswerelost. (bottom)Meandark ratios _>150 in oligotrophicwaters of the subtropicalgyre.
community
respiration
ratesfrom 12-hour(Rt•2) and 24-hour Althoughthe inherentvariabilityin POC:Chl a ratioslimits
(RD24)
02 incubations,
withlightrespiration
rates(RL).RLis esti- their applicationas estimatorsof autotrophiccarbonbiomass
matedfrom the differencebetweengrossproductivity(G24)and [Banse,1977; Geideret al., 1997], averageratiosof POC:Chl
12-hournetcommunity
productivity
(N•2)rates.
a on the West Florida Shelf (> 200) were higherthan ratios
typicallyfound in phytoplankton
and cyanobacteria
cultures
under nutrient-limited
conditions.
The POC:Chl
a ratios in
the tracerpatchare alsohigherthanthosereportedby Bucket
rophylla (chlorophylla plus phaeopigment)
concentrationsal. [1996] for the oligotrophicNorth Atlantic gyre. Thus a
canreach3-8I.tgL-•. These
relatively
highpigment
concen- largefractionof the nondetritalPOC in the tracerpatchwas
microheterotrophs.
trationscan persistfor a month,as verifiedby in situ sam- probablyassociated
Ratiosof PON:Chl a supportthis conclusion.Batchculpling in 1992 and 1993 [Gilbeset al., 1996; Tomas,1995].
The surfacepigmentconcentrations
observedin 1992 and tures of Skeletonemacostatum(Grev.) Cleve and Pavlova
1993 were similar to the total chlorophylla levels initially lutheri (Droop) Green exhibit PON:Chl a ratios of 8-20 in
presentin the tracerpatch. Our observations
in 1996 suggest logarithmicto stationaryphaseculturesunderN-, P-, or FeandHolm-Hansen,1977]. Thus
that the postbloomtransitionof surfacepigmentconcentra- limitedconditions[Sakshaug
tionsis rapid,with chlorophylla andphaeopigment
decreas- averagePON:Chl a ratios _>100'1 in the tracer patchwere
nearlyfive-fold higherthan thosein phytoplankton
cultures.
ingfrom> 1.5to< 0.5ggL'• inlessthanaweek.
Althoughthe factorsthat triggerthe onsetof the spring Molar ratiosof POC:PONalsosuggestthatthe majorfraction
pigmentmaximaon the West FloridaShelf havenot been of particulateorganic carbon and nitrogen was associated
The averagePOC:PONratioof 3.1'1
conclusively
identified,phytoplankton
growthis likely en- with microheterotrophs.
hancedby the input of allochthonous
nutrients. There are was lessthan half the Redfieldratio (C:N = 6.6:1) andwithin
severalpotentialnutrientsources,including(1) the riversof the rangeobservedin culturedmarinebacteria[Goldmanet
the northwestFloridacoast,(2) the upwellingof dense,nutri- al., 1979], as well as heterotrophicorganismsin Georgia
ent-rich subsurfacewater along the Shelf break, (3) Loop coastalwaters[Hopkinsonet al., 1989]. Detritalmaterial,in
Currentintrusions,or (4) the advectionof nutrient-richMis- contrast,is relatively depletedof organicnitrogen [Banse,
sissippiRiver water ontothe Shelf [Gilbeset al., 1996]. Al- 1974], with POC:PONratios> 30:1 [Sambrottoand Langthoughthe datesof initiationof the 1996 springbloomarenot don, 1994].
Compositionratios of POC:POPand PON:POPfurther
known,changesin chlorophylla:phaeopigment
ratiossuggest
that a heterotrophic
planktoncomzooplankton
grazingmayhavecontributed
to the reductionin supportan interpretation
pigmentconcentrations.(There is little interferencefrom munitycontributedto a large proportionof the particulate
chlorophyllb in the estimationof phaeopigment
in this re- matterin the surfacelayer of the tracerpatch.Phosphorusdominatethe planktonbiomassin
gion. Threeyearsof observations
indicatethatchlorophyllb limited bacterioplankton
concentrations
are typically < 5% of chlorophylla on the the oligotrophicsurfacewatersof the SargassoSea, where
WestFloridaShelf (G. Vargo, unpublisheddata, 1999).) On mean POC:POP ratios are- 260:1 [Comer et al., 1997]. In
April 2, chlorophylla:phaeopigment
ratiosin the tracerpatch thesewaters,PON:POPratiosvary from 24:1 to 75:1, similar
rangedfrom0.6:1to 0.8:1;the ratioincreased
to > 1 by April to the mean PON:POP ratio of 69'1 on the West Florida
biomassin the surfacewatersof the
8. Sincephaeopigments
are productsof microzooplankton Shelf. Bacterioplankton
Gulf of Mexico is phosphorus-limited
[Pomeroy
and macro-zooplankton
grazers [e.g., Buck and Newton, oligotrophic
HITCHCOCK
ET AL.: PLANKTON
PRODUCTION
ON THE WEST FLORIDA
SHELF
6587
et al., 1995],although,to our knowledge,similarstudieshave
ceeddarkrespirationat saturatinglight intensities[Grandeet
not been conducted on the West Florida Shelf. While we lack
al., 1989]. However, while the Mehler reaction consumes
oxygenin the light reactionsof photosynthesis,
carbonsubstratesare not oxidized. Thus light respirationmay have reducedgrossphotosynthesis
ratesthroughthe consumptionof
oxygenbut may have releasedrelativelylittle, if any, CO2 to
bacterialdensitiesin the tracerpatch,or phosphorus
dynamics
for the planktoncommunity,we hypothesizethat a phosphorus-limitedbacterioplankton
populationdominatedplankton
biomassfollowingthe end of the springbloom [cf. Ducklow
et al., 1986].
Ratiosof POP:Chl a in the tracerpatchalso suggestthat P
availabilitylimited the biomassof the planktoncommunity.
The mean ratio of POP:Chl a in the tracer patch (3.1:1) is
similar to that in phytoplanktonculturesunder P-limiting
conditions[e.g., Sakshaugand Holm-Hansen,1977, Table
III]. In naturalplanktoncommunities,POP:Chla ratiostypicallyrangefrom a minimumof 1:1 in eutrophicwaters,where
chlorophylla concentrations
are high,to maximumvaluesof
5:1 in oligotrophicwaters,where phytoplankton
biomassis
relativelylow [Harris, 1986]. Thusthe elementalcomposition of particulatemattersuggests
that at the terminationof
the springbloomthe particulateorganicmatterwas predominantly associatedwith microheterotrophs;
furthermore,the
planktoncommunitybiomassmay havebeenlimitedby phosphorusavailability. The relative abundanceof POC, PON,
and POP in the tracerpatchis similarto that observedin oceanic particulatematter as it sinks from the euphotic zone
[Knauer et al., 1979; Bishop et al., 1980]. In the open sea,
particulatematteris initially depletedof P, relativeto C and
N, asorganicmatteris recycledbelowthe euphoticzone.
Productionand respirationratesin the tracerpatchsupport
the conclusion
thatthe planktoncommunitybiomasswas predominantlyheterotrophic
with a high proportionof grossproduction consumedby respiration.Although phytoplankton
biomassdeclinedduring the study,the phytoplanktoncommunitywas activelyphotosynthesizing
with grossproduction
ratesand assimilationratiossimilarto thosereportedfor the
oligotrophicNorth PacificGyre [Williamsand Purdie, 1991].
Ratios of DCR•2:NCPa2in the tracer patchwere high, at an
averagevalueof 1.5:1. In phytoplankton
culturesthe ratio of
respiration:production
is typically < 0.4:1 [Langdon,1993;
Grandeet al., 1989], and in temperatecoastalplanktoncommunitiesthe ratio of DCR:NCP is low when primaryproduction ratesare high [Benderet al., 1987; Grandeet al., 1991].
On the West Florida Shelf, in contrast,dark respirationreducedgrosscarbonproductionby 90% (6 versus69 mmol C
the surface waters.
Irrespectiveof any potential effects of light respiration,
darkcommunityrespirationrateswere morethan sufficientto
accountfor the observedincreasein DIC in the tracer patch.
Wanninkhofet al. [1997] attributed80% of the observed1
lamol
kg-•d-• increase
inDICtorespiration.
Themeanrateof
DCR24
was-40mmolC m-2d'•, equivalent
toanincrease
of 2
mmolC m-3d-• (- 2 lamolkg'ad-a)in the20 m mixedlayer.
The CO2 contributionfrom dark respirationwould be reduced
bythenetproduction
of 6 mmolC m-2d'• (0.3lamol
C m-3da),yielding
a netincrease
of- 1.7mmolC m'3d-a(1.7lamol
DICkg-ad'a).Ourestimate
of carbon
contributed
tothewater
columnby the planktoncommunitywas thereforenearly70%
higherthanthe observedincreasein DIC. Otherfactorscould
accountfor the discrepancy,includingthe uncertaintybased
on a small number of productionand respirationmeasurements. We also have not quantifiedany contributionto the
DIC pool from the metabolismof dissolvedorganiccarbon
releasedduring phytoplanktonexcretion[see Biddandaand
Bennet, 1997a]. Although we cannot quantify these unknowns, we concludethat plankton community respiration
was the primary source of the observedincreasein DIC
within the tracerpatch.
Bacterioplankton
arethe primaryorganisms
responsible
for
respirationin the sea [Williams, 1981; Cole et al., 1988], and
as discussedabove,they may have dominatedplanktonbiomassin the tracer patch. Severalcarbonsourcespotentially
sustainedbacterioplanktonrespirationon the West Florida
Shelf. Rivefine dissolvedorganicmatter, principallyfrom
the MississippiRiver, likely providesa largeflux of organic
matter to the northern Gulf of Mexico.
Since riverine nutrient
sources
maytriggerthe onsetof the springpigmentmaximum
on the West Florida Shelf [Gilbeset al., 1996], riverinedissolvedorganicmatter(DOM) may alsoserveas a sourceof
organicmatterto fuel bacterioplankton
respiration. Phytoplanktonalsoprovidedan unknownquantityof organicmatter as theywere grazedby microheterotrophs.
A third potensuchas Synechococcus
spp.,
m-2d'l). Sincegross
production
wasan orderof magnitudetial sourceare photoautotrophs
higher than net production,and photoautotrophs
typically which release• 10% of their carbonproductionas dissolved
have DCR < NCP, the respirationdata suggestas well that organicmatter[Biddandaand Bennet,1997a,b]. Thus sevmicroheterotrophs
dominatedthe planktoncommunityme- eral sourcesof DOM on the West Florida Shelf can potentabolism.
tially supportbacterioplankton
respiration.
In temperatecoastalecosystems,
microbialrespiration
rates
In photoautotrophic
organisms,light respirationreduces
grossproductionthroughseveralmetabolicpathways.These reach a maximum 1-2 weeks after the termination of the
include (1) photorespirationand associatedglycollate me- springbloom [Blightet al., 1995]. The lag betweenpeak
tabolism;(2) the Mehler reaction,in which oxygenis pro- primaryproductivityandthe seasonalmaximumin microhetducedin photosystem
II and consumedin photosystem
I; and erotrophrespirationhasbeenattributedto the rate at which
[Azam et al.,
(3) chlororespiration,
in which NAD(P)H is oxidizedin chlo- DOM becomesavailableto bacterioplankton
roplasts[Beardall and Raven, 1990]. In phytoplanktoncul- 1994; Blight et al., 1995]. Sherr and Sherr [1996] propose
tures,light respiration(RL) typicallyequalsor exceeds,dark that the delay in primary productionand microheterotroph
may be regulated
respiration[Falkowski et al., 1985; Grande et al., 1989; respirationin temperatemarineecosystems
seasonal
changesin bacterialgrowthefficienKana, 1990], and in the tracerpatch,RLwas higherthan dark by temperature,
changesin the availabilityof DOM to bacterespiration(Figure6). The dominantphotoautotrophs
in the cies,or seasonal
tracerpatchwere Synechococcus
spp.,in which light respira- rioplankton. The rapid responseof the microheterotroph
tion is primarilyassociatedwith the Mehler reaction[Kana, communityto a decline in phytoplanktonbiomasson the
1992]. In Synechococcus
cultures,light respirationcan ex- West Florida Shelf could reflect, in part, the relativelywarm
6588
HITCHCOCK ET AL.: PLANKTON PRODUCTION ON THE WEST FLORIDA SHELF
springtemperatures
in this subtropicalenvironment.As de-
scribedabove,therearealsoseveralsources
of DOM in this
Cole,J. J., S. Findlay,andM. L. Pace,Bacterialproduction
in fresh
andsaltwaterecosystems:
A cross-system
overview,Mar. Ecol.
Prog.Set., 43, 1-10, 1988.
Cooper,D. J.,A. J. Watson,andP. D. Nightingale,
Largedecrease
in
ocean-surface
CO2 fugacityin response
to in situ iron fertiliza-
region.
If the couplingbetweenautotrophicproductionand mition, Nature, 383, 511-513, 1996.
croheterotroph
respiration
is on the orderof a few daysat low
latitudes,then subtropicaland tropical environmentsmight Comer,J. B., J. W. Ammerman,E. R. Peele,and E. Bentzen,Phosphorus-limitedbacterioplankton
growth in the SargassoSea,
rapidlychangefrom CO2sinksto sources.The timescalefor
Aquat.Microb. Ecol., 13, 141-149, 1997.
a transitionfrom sink to sourcemay be much shorterat low deBaar,H. J. W., J. T. M. deJong,D.C. E. Bakker,B. M. Lfscher,
latitudesthan in temperateand borealwaters[seeRobinson
C. Veth, U. Bathmannn,
andV. Smetacek,
Importance
of iron for
planktonbloomsand carbondioxidedrawdownin the Southern
and Williams,1999]. We suggestthat futurestudieson subOcean,Nature, 373, 412-415, 1995.
tropicaland tropicalcontinentalshelvesexaminethe role of
DeGrandpre,M.D., T. R. Hammar,and C. D. Wirick, Short-term
temperatureand DOM availabilityas factorsregulatingthe
pCO2 and 02 dynamicsin Californiacoastalwaters,Deep Sea
degreeof couplingbetweencommunityproductionandrespiRes.,Part II, 45, 1557-1575, 1998.
ration in these environments.
Ducklow, H. W., D. A. Purdie, P. J. LeB. Williams, and J. M. Da-
vies, Bacterioplankton:
a sink for carbonin a coastalmarine
planktoncommunity,
Science,232, 865-867, 1986.
Acknowledgments. The authorsgratefully acknowledgeR.
Wanninkhoffor the invitationto participatein the FloridaShelfLa- E1-Sayed,S. Z., Primaryproductivityand standingcrop of phytoplankton,in Chemistry,PrimaryProductivity,and BenthicAlgae
grangianExperimentand for his comments
on the manuscript.The
of the Gulf of Mexico,editedby E1-Sayed,
S. Z., et al., pp. 8-13,
captainandcrewof theNOAA ShipBaldrigemadetheresearchposSet. Atlas Mar. Environ.,Folio 22, Am. Geogr.Soc.,New York,
sibleby their cooperation
in all phasesof the cruise.We especially
1972.
thankMichaelBenderfor providingthe laboratoryfacilitiesfor the
analysisof the grossO2productionsamplesandJosephOrchardofor Emerson,S. P., P. Quay,C. Stump,D. Wilbur, andM. Knox, 02, Ar,
N2and222Rn
in surface
waters
ofthesubarctic
ocean:
netbiologihis generous
assistance
in the analyses.The nitrogendatawereprocal 02 production,
GlobalBiogeochem.
Cycles,5, 49-69, 1991.
videdby K. Fanningand R. Masseriniof the Universityof South
Light-enhanced
Florida. Three anonymousreviewers improved the manuscript Falkowski,P. G., Z. Dubinsky,andG. Santistefano,
darkrespirationin phytoplankton,
Verh.Int. Ver. Theor.Angew.
throughtheir comments.This researchwas supported,
in part, by a
Lirnnol.,22, 2830-2833, 1985.
grantfromthe Officeof Naval Research,
N000149710131.
Fanning,K. A., K. L. Carder,and P. R. Betzer,Sedimentresuspensionby coastalwaters:A potentialmechanismfor nutrientrecyReferences
clingontheocean'smargins,DeepSeaRes.,29, 953-965, 1982.
Frankignoulle,
M., I. Bourge,C. Canon,and P. Dauby,Distribution
Azam,F., D.C. Smith,G. F. Stewart,
and/k.HagstrOm,
Bacteria- of surfaceseawaterpartial CO2pressurein the EnglishChannel
organicmattercouplingand its significancefor oceaniccarbon
andin the SouthernBight of the North Sea,Cont.ShelfRes., 16,
cycling,Microb. Ecol., 28,167-179, 1994.
381-395, 1996.
Banse,K., On theinterpretation
of datafor the carbonto nitrogenraGeesey,M., and P. Tester,Gymnodinium
breve:Ubiquitousin Gulf
tio of phytoplankton,
Lirnnol.Oceanogr.,19, 695-699, 1974.
of Mexico waters?,in ToxicPhytoplankton
Bloomsin the Sea,
Banse,K., Determiningthe carbon-to-chlorophyll
ratio of natural
editedby T. J. Smaydaand Y. Shimizu,pp. 251-255, Elsevier
Sci., New York, 1993.
phytoplankton,
Mar. Biol. Berlin,41,199-212, 1977.
Beardall,J., and J. A. Raven,Pathwaysandmechanisms
of respira- Geider,R. J., H. L. MacIntyre,andT. M. Kana, Dynamicmodelof
tion in microalgae.
Mar. Microb.Food Webs,4, 7-30, 1990.
phytoplankton
growthandacclimation:
Responses
of thebalanced
Bender, M. L., and K. D. Grande, Production,respiration,and
growthrate andthe chlorophylla: Carbonratio to light, nutrientchemistryof 02 in the upperwatercolumn.GlobalBiogeochern.
limitationandtemperature,
Mar. Ecol. Prog. Set., 148, 187-200,
1997.
Cycles,1, 49-59, 1987.
Bender,M., et al., A comparison
of four methodsfor planktonic Gilbes,F., C. Tomas,J. J. Walsh,andF. Mfiller-Karger,An episodic
community
production,
Limnol.Oceanogr.,32, 1085-1098,1987.
chlorophyll
plumeontheWestFloridaShelf,Cont.ShelfRes.,16,
1201-1224, 1996.
Biddanda,B., and R. Benner,Carbon,nitrogen,and carbohydrate
fluxesduringthe productionof particulateanddissolvedorganic Goldman,J. C., J. J. McCarthy,andD. G. Peavey,Growthrateinflumatterby phytoplankton,
Lirnnol.Oceanogr.,42, 506-518, 1997a.
enceon the chemicalcompositionof phytoplankton
in oceanic
waters,Nature, 2 79, 210-215, 1979.
Biddanda,B., and R. Benner,Major contributionfrom mesopelagic
planktonto heterotrophic
metabolismin the upperocean.Deep Grande,K. D., J. Man'a,C. Langdon,K. Heinemann,andM. Bender,
Sea Res.,Part I, 44, 2069-2085, 1997b.
Ratesof respiration
in thelightmeasured
in marinephytoplankton
18
Bishop,J. K. B., R. W. Collier, D. R. Kettens,and J. M. Edmond,
usingan O isotope-labeling
technique,
,I. Exp. Mar. Biol. Ecol.,
129, 95-120, 1989.
The chemistry,biology, and vertical flux of particulatematter
fromthe upper1500mof the PanamaBasin,Deep Sea Res.,27, Grande,K., M. L. Bender,B. Irwin, and T. Platt, A comparison
of
615-640, 1980.
netandgrossratesof oxygenproduction
as a functionof light inBlight, S. P., T. L. Bentley,D. Lefevre,C. Robinson,R. Rodrigues,
tensityin somenaturalplanktonpopulations
and in a SynechoJ. Rowlands,andP. J. LeB. Williams,Phasingof autotrophicand
coccusculture,,I. PlanktonRes., 13, 1-16, 1991.
heterotrophic
planktonmetabolism
in a temperatecoastalecosys- Harris, G. P., PhytoplanktonEcology: Structure, Function and
tem, Mar. Ecol. Prog. Ser., 128, 61-75, 1995.
Fluctuations,
384 pp., ChapmanandHall, New York, 1986.
Buck,K. R., andJ. Newton,Fecalpelletflux in DabobBay duringa Holm-Hansen,
O., and B. Riemann,Chlorophylla determination:
diatom bloom: contribution of microzooplankton,Limnol.
Improvements
in methodology,
Oikos,30, 438-447, 1978.
Oceanogr.,40, 306-315, 1995.
Hopkinson,C. S., Jr., B. Sherr,and W. J. Wiebe, Size fractionated
Buck, K. R., F. P. Chavez,and L. Campbell,Basin-widedistribumetabolism
of coastalmicrobialplankton,Mar. Ecol.Prog. Set.,
51, 155-166, 1989.
tionsof living carboncomponents
andthe invertedtrophicpyramid in the central gyre of the North Atlantic Ocean, summer, Kana,T. M., Light-dependent
oxygencyclingmeasuredby an oxy1993,Aquat.Microb. Ecol., 10, 283-298, 1996.
gen-18isotopedilutiontechnique,
Mar. Ecol.Prog.Set.,64, 293300, 1990.
Chipman,D. W., J. Marra, and T. Takahashi,Primaryproductionat
47ø N and 20ø W in the North Atlantic Ocean:a comparison
be- Kana, T. M., Relationshipbetweenphotosynthetic
oxygencycling
tweenthe •4Cincubation
method
andthemixedlayercarbon and carbonassimilationin Synechococcus
WH7803 (Cyanobudget,DeepSeaRes.,Part II., 40, 151-169, 1993.
phyta),,I. Phycol.,28, 304-308, 1992.
Codispoti,L. A., G. E. Friederich,andD. W. Hood,Variabilityin the Kana,T. N., andP.M. Glibert,Effect of irradiances
up to 2000 •tE
inorganiccarbonsystemover the southeastern
Bering Sea shelf
m'2s'• onmarine
Synechococcus
WH7803,
I, Growth,
pigmentaduringspring1980 andspring-summer
1981, Cont.ShelfRes.,5,
tion,andcell composition.
DeepSeaRes.,34, 479-495, 1987.
133-160, 1986.
Knauer,G. A., J. H. Martin,andK. W. Bruland,Fluxesof particulate
HITCHCOCK
ET AL.: PLANKTON PRODUCTION
ON THE WEST FLORIDA SHELF
6589
carbon,nitrogen,and phosphorusin the upper water columnof Tomas,C., Influenceof MississippiRiverwateron the WestFlorida
Shelf,in CoastalOceanographicEffectsof the 1993 Mississippi
thenortheastPacific,Deep SeaRes.,26, 97-108, 1979.
RiverFlooding,editedby M. J. Dowgiallo,specialNOAA report,
Langdon,C. The significanceof respirationin productionmeasurepp. 60-64, Natl. OceanicandAtmos.Admin.,CoastalOceanOff.,
mentsbasedon oxygen,ICES Mar. Sci. Sytnp.,197, 69-78, 1993.
SilverSpring,Md., 1994.
Laws, E. A., Photosyntheticquotients,new production,and net
communityproductionin the openocean,Deep SeaRes.,12, 143- Tomas,C., Coastalproductionand sportfishplanktondynamicson
167, 1991.
theFloridaShelf,Fl. Mar. Res.Inst. Proj. F-65, Fl. Dep. of EnviLaws,E. A., D. R. Jones,K. L. Terry, andJ. A. Hirata, Modification
ron.Prot.,36 pp., St. Petersburg,
F1, 1995.
in recentmodelsof phytoplanktongrowth:theoreticaldevelop- Vargo,G. A., K. L. Carder,W. Gregg,E. Shanley,C. Heil, K. A.
ments and experimentalexaminationof predictions,J. Theor.
Steidinger,andK. D. Haddad,The potentialcontribution
of priBiol., 114, 323-341, 1985.
maryproduction
by redtidesto theWestFloridaShelfecosystem,
Litnnol.Oceanogr.,32, 762-767, 1987.
Marmorino,G. O., Variability of current,temperature,and bottom
andbiologicalpropressureacrossthe West Florida continentalshelf, winter 1981- Wanninkhof,R., et al., Gasexchange,dispersion,
ductivityon the West Florida Shelfi Resultsfrom a Lagrangian
1982,J. Geophys.Res.,88, 4439-4457, 1983.
Mitchum, G. T., and A. J. Clarke, Evaluation of frictional, windtracerexperiment,Geophys.Res.Lett.,24, 1767-1770,1997.
forced long-wavetheory on the West Florida shelf, d. Phys. Watson,A. J., C. Robinson,J. E. Robinson,P. J. LeB. Williams, and
Oceanogr.,16, 1029-1037, 1986.
M. J. R. Fasham,Spatialvariabilityin the sink for atmospheric
carbondioxide in the North Atlantic, Nature. 350, 50-53, 1991.
Pomeroy,L. R., J. E. Sheldon,W. M. Sheldon,and F. Peters,Limits
Weisberg,R. H., B. D. Black,andH. Yang, Seasonal
modulation
of
to growth and respirationof bacterioplanktonin the Gulf of
thewestFloridacontinentalshelfcirculation,Geophys.Res.Lett.,
Mexico,Mar. Ecol. Prog. Ser., 117, 259-268, 1995.
23, 2247-2250, 1996.
Rassoulzadegan,
F., Protozoanpatternsin the Azam-Ammerman's
bacteria-phytoplankton
mutualism,in Trendsin Microbial Ecol- Williams, P. J. LeB., Microbial contribution to overall marine
plankton metabolism: Direct measurementsof respiration,
ogy,editedby R. GuerreroandC. Perdos-Alio,pp. 435-439, Span.
Oceanol. Acta, 4, 359-364, 1981.
Soc.for Microbiol., Barcelona,1993.
Robertson,J. E., and A. J. Watson. A summer-timesink for atmos- Williams, P. J. LeB., and D. A. Purdie, In vitro and in situ derived
ratesof grossproduction,net communityproductionandrespiraphericcarbondioxidein the SouthernOceanbetween88øW and
tion of oxygenin the oligotrophicsubtropicalgyre of the North
80ø E, Deep Sea Res.,Part I, 42, 1081-1091, 1995.
PacificOcean,Deep SeaRes.,38, 891-910, 1991.
Robinson,C., andP. J. LeB. Williams,Planktonnet communityproductionanddarkrespirationin the ArabianSeaduringSeptember Wollast,R. Interactionsof carbonandnitrogencyclesin the coastal
zone,in Interactionsof C, N, P and S Biogeochetnical
Cyclesand
1994, Deep Sea Res.,Part II, 46, 745-765, 1999.
Global Change,editedby R. Wollast, F. T. MacKenzie,and L.
Sakshaug,E., and O. Holm-Hansen, Chemical compositionof
Chou,pp. 195-210,Springer-Verlag,
New York, 1993.
Skeletonetna
costatutn(Grev.) Cleve and Pavlova (Monochrysis)
lutheri (Droop) Green as a functionof nitrate-,phosphate-,and Wroblewski, J. S., and J. J. O'Brien, A simulationof the mesoscale
distributionof the lower marinetrophiclevelsoff west Florida.
iron-limitedgrowth,d. Exp. Mar. Biol. Ecol., 29, 1 -34, 1977.
Invest.Pesq.,37, 193-244, 1973.
Sambrotto,R. N., and C. Langdon,Water columndynamicsof dissolvedinorganiccarbon(DIC), nitrogenand02 on GeorgesBank
duringApril, 1990, Cont.ShelfRes.,14, 765-789, 1994.
Sharp,J. H., Improvedanalysisfor "particulate"carbonandnitrogen,
Litnnol.Oceanogr.,19, 984-989, 1974.
M.-L. Dickson,GraduateSchoolof Oceanography,
Universityof
Sherr,E. B., and B. F. Sherr,Temporaloffset in oceanicproduction
andrespirationprocesses
impliedby seasonalchangesin atmos- RhodeIsland,SouthFerryRd., Narragansett,RI 02882-1197.
G. L. Hitchcock,RosenstielSchoolof Marine and Atmospheric
pheric oxygen:The role of heterotrophicmicrobes,Aquat. Microb. Ecol., 11, 91-100, 1996.
Science,Universityof Miami, 4600 RickenbackerCauseway,Miami,
Smith,S. V., and F. T. Mackenzie,The oceanas a net heterotrophic FL 33149-1098.(ghitchcock•rsmas.miami.edu)
system: Implications from the carbon biogeochemicalcycle,
G. A. Vargo,Departmentof Marine Science,Universityof South
GlobalBiogeochetn.Cycles,1, 187-198, 1987.
Florida, 140 7 AvenueS, St. Petersburg,
FL 33701.
Solorzano,L., and J. H. Sharp, Determinationof total dissolved
phosphorus
andparticulatephosphorus
in naturalwaters,Litnnol. (ReceivedDecember3, 1998; revisedSeptember22, 1999; accepted
Oceanogr.,25, 754-758, 1980.
December23, 1999.)