Studying)the)influence)of)ver2cal)
mixing)on)phytoplankton)growth))
with)a)quasi<2D)NP)model)
L."Hahn>Woernle1 "
"H.A."Dijkstra1
"
"H.J."van"der"Woerd2"
1"IMAU,"University"of"Utrecht,"The"Netherlands."""2"IVM,"Vrije"Universiteit"Amsterdam,"The"Netherlands."
l.hahn>[email protected]"
2."Measurement"Data"
Can"we"make"use"of"satellite"ocean"
color"data"to"esHmate"verHcal"mixing?"
1."MoHvaHon"
In" summer" 2009" and" spring" 2011,"
physical," opHcal," chemical," and"
biological" profiles" were" measured"
along"the"transect"shown"in"Fig."2"as"
part"of"the"STRATIPHYT"project."The"
summer" ver2cal) mixing) coefficient)
KT)(Fig."3a)"shows"high"values"at"the"
surface" for" the" whole" transect"
separaHng" a" strongly" mixed) layer"
(ML)" from" the" moderately" mixed"
water"column"below"[2]."
"
In" Fig." 3b" the" summer" Chl>a"
concentraHon" along" the" transect" is"
given." The" data" shows" a" transiHon"
from" a" deep) chlorophyll) maximum)
(DCM)" in" the" south" to" an" upper"
chlorophyll) maximum) (UCM)" in" the"
north."The"difference"in"the"depth"of"
the" growth" regime" is" strongly"
coupled" to" the" annual" evoluHon" of"
t h e" m i x i n g" p r o fi l e ." S p r i n g"
measurements"of"northern"laHtudes"
(not"shown)"show"deep"mixing"while"
southern" staHons" are" straHfied"
having" significant" consequences" for"
the" nutrient" availability" in" the" ML"
and"therefore"on"the"phytoplankton"
growth"over"the"year.""
"
Fig."3a:"VerHcal"Mixing"Coefficient"(Summer"‘09)"
Fig."1:"MODIS"Aqua"Satellite"Data"(oceancolor.gsfc.nasa.gov/)"
Fig."1"shows"the"chlorophyll<a)(Chl<a))surface"concentraHon"(indicator"for"
phytoplankton"abundance)"measured"by"MODIS"Aqua."The"data"is"ploRed"
over"3"years"along"a"track"in"the"North"AtlanHc"(Fig."2)."
"
The"Chl>a"concentraHon"follows"the"annual"cycle"of"daylight."AddiHonally"
strong" localized" maxima," which" sustain" only" for" short" periods,"
characterize" the" growth" behavior" of" phytoplankton." The" origin" of" these"
maxima" is" assumed" to" be" strongly" coupled" to" the" verHcal" mixing" of" the"
water"column"since"it"provides"nutrient>rich"water"from"below"[1]."
"
The" aim" of" this" project" is" to" develop" a" phytoplankton>turbulence"
model"in"which"surface"phytoplankton"satellite"data"is"assimilated"to"
determine"properHes"of"the"verHcal"mixing."
"
Here" we" apply" observed" verHcal" mixing" to" an" advecHon>diffusion>
reacHon"phytoplankton)growth)model)(PGM)"to"study"the"effect"of"
the"different"mixing"profiles"on"the"phytoplankton"distribuHon."We"
find" that" the" relaHve" nutrient" flux" is" an" effecHve" measure" which"
correlates" the" strength" of" the" verHcal" mixing" to" surface" Chl>a"
concentraHon."
3."Model"&"Method"
Fig."4:"PGM"scheme"
x"
The" PGM" simulates" the" growth" of" one"
phytoplankton" species" dependent" on" the"
availability" of" light" and" nutrients" in" a" mixed"
water"column"(based"on"[3],"schemaHc"set>up"in"
Fig."4).""
"
The" local" phytoplankton) concentra2on) P%
depends" on" 4" processes" (Eq." 1)" on" which" the"
verHcal" mixing" has" different" influences." It"
acHvely"moves"the"cells"in"the"water"column"and"
weakens"the"effect"of"the"sinking."Since"verHcal"
mixing"also"alters"the"nutrient)density)N"due"to"
resupply" of" deep," nutrient>rich" water" to" the"
euphoHc"layer"(Eq."2),"its"strength"influences"the"
rate"of"growth"in"nutrient"limited"systems."
"
x"
(1)"
"
In"order"to"understand"the"influence"of"
different" verHcal" mixing" profiles" we"
proceeded"in"two"steps:"
"
1.  PGM"parameters"were"calibrated"to"
reproduce" in>situ" data" for" 3"
reference" staHons:" the" Southern"
staHon" at" 41N" (DCM)" and" the"
Northern" staHon" at" 61N" (UCM),"
both" of" the" summer" cruise" (red"
boxes" Fig.3)," and" of" the" spring"
cruise"the"Transi2on"staHon"at"46N"
(UCM"to"DCM,"not"shown).""
Fig."2:"StraHphyt"staHons"(projects.nioz.nl/straHphyt)"
5."Conclusions"
"  Applying"in"situ"verHcal"mixing"to"the"PGM"at"3"
calibrated"reference"staHons"shows"strong"sensiHvity"
of"the"modeled"phytoplankton"growth"to"the"verHcal"
mixing."
"  The"relaHve"nutrient"flux"ρN"correlates"to"the"surface"P"
and"hereby"connects"verHcal"mixing"characterisHcs"to"
surface"Chl>a"data."
"  The"results"of"the"sensiHvity"analysis"encourage"us"that"
we"will"achieve"our"main"aim:"determining"properHes"
of"the"verHcal"mixing"based"on"ocean"color"data.")
Reference"staHon"
Fig."3b:"Chlorophyll>a"(Summer"’09)"
References)
[1]" Mahadevan," A.," E." D’Asaro," C." Lee," and" M.J." Perry," 2012," Eddy>driven" StraHficaHon"
IniHates"North"AtlanHc"Spring"Phytoplankton"Blooms."Science,"337(6090):54>56."
[2]"Jurado,"E.","H.J."van"der"Woerd,"and"H.A."Dijkstra,"2012,"Microstructure"measurements"
along" a" quasi>meridional" transect" in" the" northeastern" atlanHc" ocean." J." Geophys." Res.>
Oceans,"117:C04014."
[3]" Ryabov," A.," L." Rudolf," and" B." Blasius," 2010," VerHcal" distribuHon" and" composiHon" of"
phytoplankton"under"the"influence"of"an"upper"mixed"layer."J."Theor."Bio.,"263(1):120>133."
4."Model"results"
Southern"StaHon"
Northern"StaHon"
(2)"
To" measure" the" effect" of" single"
verHcal" mixing" profiles" on" the"
phytoplankton" growth," the" rela2ve)
nutrient)flux)ρN"was"defined"as:"
i
ρ Ni =
∂N (z)
∂z
i = 1,…, 30
∂N ref (z)
ref
KT (z)
(3)"
∂z
KTi (z)
where" ref" indicates" the" quanHHes"
of" the" correspondent" reference"
2.  In>situ" verHcal" mixing" profiles" of" staHon." The" notaHon" ā" stands" for"
both" seasons" (in" total" 30)" were" the"verHcal"average"of"a."
applied" to" each" reference" staHon"
and" were" run" for" 500" days" keeping"
model"parameters"unchanged."
Post<it)comments)
Reference"staHon"
Fig."5:"PGM"results"for"P"based"on"the"Southern"(leq)"and"Northern"(right)"staHon"with"summer"verHcal"mixing"(Fig.3a)"
The" PGM" results" in" Fig." 5" show" that" the" phytoplankton"
concentraHon" does" not" only" depend" on" the" different" model"
calibraHon" (compare" Southern" to" Northern" staHon)" but" also"
strongly" on" the" characterisHcs" of" the" verHcal" mixing." While"
summer"verHcal"mixing"leads"to"different"states"of"the"DCM"for"the"
Southern" staHon," the" Northern" staHon" remains" mainly" in" a" UCM"
with"varying"intensity"for"the"same"verHcal"mixing."
"
In"Fig."6"the"depth"integrated"P"is"ploRed"as"total"biomass"against"
the" depth" of" the" nutricline." Systems" which" where" not" nutrient"
limited"can"be"found"on"the"y>axis."Nutrient"limited"systems"show"
a" trend" towards" more" biomass" for" a" shallowing" nutricline." For"
these" systems" the" relaHve" nutrient" flux" (Eq." 3)" is" ploRed" against"
the" normalized" surface" P" (mean" over" first" 20m" divided" by"
reference" surface" P)" in" Fig." 7." The" weak" nutrient" flux" at" the"
Southern"staHon"leads"to"a"wide"range"of"ρN"and"a"separate"x>axis"
is"chosen.""
)
)
A"clear"correlaHon"for"which"
)higher)surface)P%occurs)with)increasing)ρN%
"is"found"for"all"3"reference"staHons."
"
My)homepage)
QuesHon?"
"Comment!"
"Contact.""
!"
THANK"YOU"FOR"YOUR"INTEREST."
Fig."6:"Total"biomass"as"funcHon"of"the"depth"of"the"
nutricline"for"the"3"reference"staHons"
Fig."7:"Normalized"surface"P"as"funcHon"of""ρN"for"
the"3"reference"staHons"