The pelagie eross-shelf and alongshore boundaries of the NorthT7est Afriean upwelling region and their annual variability in terms

of zooplankton hiomass

L. POSTELAkademie der Wissenschaften der DDR. Institut für Meereskunde.

2530 Rostock-Warnemünde. Seestrasse 15, GDR.

SIMPOSIO INTERNACIONAL SOBRE LAS ÄREAS DE AFLORAMIENTOMÄS IMPORTANTES DEL OESTE AFRICANO

(CABO BLANCO Y BENGUE,LA)

INSTITUTO DE INVESTIGACIONES PESQUERAS

BARCELONA. 1985

Simp. lnt. Afl . O Afr., l t1st. lnv. Pesq., Barcelona 1985, v. I , pp.479-487ln t . Symp. Upw. W Af r . , Ins t . lnv . Pesq. , Barce lona 1985, v . I , pp .479-487

The pelagic cross-shelf and alongshore boundaries of the North

West African upwelling region antl their annual variabilitv in terms

of zooplankton biomass

L. POSTEL

Akaclcmie der Wissenschziften dcr DDR. Institut für Mcereskundc.

2,530 Rostock-Warncmünde. Seestrasse 15, GDR.

Key words: Upwcl l ing, boundar ies, physics, zooplankton biomass, NW AI l ica.Mots c lös ' . Upwel l ing, l in-r i tcs, physique, b iomasse zooplanctonique, NO d'A-1ricluc.

Palubras c la le: Af lo lan-r icnto, l imi tcs, l is ica, b iom:rsa zooplanclönica, NO clcÄf r ica.

SUMMARY: The cross-shelf and arlongshorc extent of thcr biological clfectsof coastal uprvelling is estimatccl on thc basis of zooplankton biomass dat:tfor thc pelagic zonc ol f North West Afr ica. Thc dc{ in i t ion ol 'a character is t icbiomass for thc adiaccnt pclagic zonc of the Tropical Ccntra l Al lant ic is usedas an aid. In cross-shcl f c l i rcc l ion, lhe "b io logical ' c f fects of colc l water up-rvcl l ing can be disccrnecl for sever:r l hundrcd k i lonretrcs, and of f CapBlanc (20" 5.5' N) it thereforc exccccl thc range of 1he "phvsicallv" definecluprvel l ing zone by a factor of 2.5-3.7. In a longshore di rcct ion the "physical 'and "bio logical" ef fects are both appalent over a d istance excecding 1000 km.Alongshorc scasonal variations arc \/ery probablv of thc same ordcr as themeridional migration of cold rvatcr uprvelling phenomena. In cross-shclf clircc-t ion var iat ions of f Nouakchott (17' 50' N) and Cap Vcrt (14' 1-5 ' N) appcarto be chic l ly o1'seasonal characlcr .

R E S U M E : L I , s t . r t t r L r - s p l . r . \ c r e r . L S , r R . \ N S \ E R S \ L r s r . r r . \ T l r t r J l \ \ L E s , D r . r . \ R r ' c r u \o'up'lvltLt.tuc t)u NoRD-ouEST o'AFnreult lr SES v;\RtATroNS ANlutlt-ris l\ tERr,lts lllBIor\IAssE z0opr.ANcToNtelrn. - On donne unc cvaluation de l'extension transvcr-salc ct latitudinale des changements biologiques produils par l'upwclling cotierpour la zonc pdlagique de l'Afrique Nord-Ouest, par movcn des valeurs de labiomase dc zooplancton. La difinition d'unc biomassc- caractdristique pourla zone pdlagique adiacentc de l 'At lant ique Centrale Tropicale est une donniendcdssaire pour l'analyse

Sur la d i rect ion l ransversalc, l 'uprvel l ing e\erce une in l luence "bio logique"jusqu'ä p lusicurs centaines de k i lomötres et d6passe de cet te l 'aqon dc 2.5ä 3.7 fois la zonc "physiqucment" ddtermindc dans la rdgion de Cap Blanc.Sur la d i rcct ion lat i tudinalc, on peut constater une extcnsion dc plus de1000 km du point dc vuc "physique" et "biologique". Les variations saison-niörcs latitr-rdinalcs correspondent ä cellcs du ddplacement meridional de l'up-rvel l ing c les caux f ro ides. Les var izr t ions t ransvcrsales c lans les rdgions c leNouakchol t c1 Cap Vcrt ont en pr incipe des caractdr is t iqucs saisonniörcs.

RESUMEN: Los r . r v r r r . s p r r . i c r t os , rR . \ \ s \ t .RS \ t . t . s l LAT t ru t , lN \L r s , u r . r . l n r c r r i lDE AFr.oRANrrENTo DEL N0RoESTE DF: Äl.RrcA y sus Yr\Rrr\croNES ANITALES REFERTDAS ABToMASA zooplelrcrdutc,r. - Sc da una evaluacidn de la extensidn transvcrsal y la-titudinal de los efectos biolögicos provocados por el afloramiento costero enla zona pelägica del NO de Äfr ica, basändosc en los valores de la b iomasaz<.roplanctönica. La c ' lehnic iön de una biomasa caractcr is t ica para la zona adva-

480 L. pos'lF.r-

centc del At länt ico Ccntra l Tropicer l cs un c lato ncccsar io para electuzrr c lanäl is is .

En la c l i recciön t ransversal , los efcctos ubio lögicos, dcl af loran' r iento puedendist ingr, r i rse hasta var ios ccntenafes c le k i lömctros v, f rente a Cabo Blanco(20'-55'N), sobrepasa por consiguicntc cn 2,5 a 3,7 veccs la zona c le af lora-miento def in ic la of is icamente". En la d i reccic in lat i tudinal , ambos cfcctos, <f i -s icos, y "b io lögicoso, se mani f iestan sobrc una distancia c le mäs de 1000 km.Las var iaciones estacionales lat i tudinales responden probablcmente al des-plazamiento mer id ional dc los fendmenos de af loramiento. Las rar iacionestransversales f rente a Nouakchott v cabo Verclc parecen tencr un caräcterpr incipalmente estacional .

INTRODUCTION

coastal upwellings in the subtropics and tropics are, comparecl with sur-rounding waters, anomalies. This is shown by the field ctistribution of physical,chemical and biological parameters. Due to the hierarchic nature of theelements of an ecosystem (corresponding to different trophic levels), thesedifferences are not identical in time and/or in space. According to TurRror(1978), nutrient consumption in a waterbody while it is drift ing after up-well ing, and after seasonal termination of upwelling action, restores theoriginal statc typical for these latitudes.

Bo.re and rouczar (1978) showed that it is necessary to know horv largesuch areas are. The following paper tries to delineate the pelagic zones af-fected by upwelling on the basis of "biological" criteria. This has been cioneby defining a characteristic zooplankton biomass (ZpB) for the pelagic zoneof the Tropical Central Atlantic. Measurements taken at different seasonsare used to es l imate seasonal var iat ions in space.

MATERIAL AND METHODS

The data base consists of ZPB dry weights obtained on board the R/V"4. von Humboldt" f rom 1970 to 1976 (Tab. l ) . These formed par t of Jarge

TABLE 1

Working arrcas and clatcs of irrvcst igations

Working areo, Dct te of invest igat ion

Bah ia de Ga rne t ( 25 ' N ) t o Cap Ve r r ( 14 " 45 'N )

Bahia c le Garnet (2-5 ' N) to Cap Vcrga (10. N)

Bahia de G:rrnet (25" N) to Cap Blanc (20" 55'N)

27. 8. to 8. 9.197028.10 . to 8 .1 1 .197020. 6. to 8. 7.197224.12.1972 t<: 10. 1.197323. 2. ro 14. 3.197314. 5. to 30. 5.197412. 2 . to 17 . 2 .1976

BIOLOGICAL BOUNDARIES OF NW AI.'RICAN UPWELLING

1 6 0 W

481

20"

ZPB 10mg m-3_ - l n n l O 7 "

- - M o n ' 1 9 7 3- ' - Moy 19 74- - J u n . 1 9 7 2- - S e p . 1 9 7 0- . - N o v . 1 9 7 0

Frc. 1.-Var iat ion in space and in t ime of the area wi th ZPBsabove 10 mg m ' off the Northwest African coast.

^ . 1LOiletr0

I

c . e tLnc

' 1 0 m o m - 3 \- \

i t

. .,11. ..,,:

I,. POSTEI

scale studies performed in the course of a standard station programme t-rffthe North West African coast (Fig. 1). The mesozooplankton was obtainedwith a WP-2 net (TnlNren, 1968) by vertical hauls from depths down to 200 m.Dry weights were determined at 60'C using the method of Lovecnovs (1966).The 1970 values were obtained by KöHlnn (1972). Statistical analysis rvas basedon TaueeNrrnru (1969).

RESULTS AND DISCUSSION

The mean ZPB in the Central Atlantic at 30 W betrveen 1 N and 15 Nover the whole year was less than 10 mg'm-" dry u'eight (KAISER and Posrpl.1979).In contrast, no such low ZPB which remainecl constant regardless ofseason was found in regions rvhere substantial vertical nutrient l lux occurred,for example on the equator at the same longitud or off Cap Blanc in theNorth West African uprvell ing region. A ZPB of l0 mg.m " was thereforeconsidered the boundary between tl-rc pelagic zone "biologically" marked bt'coastal upwelling and its tropical and subtropical surroundings.

In order to locate this boundary in the cross-shell direction, the positionof the 10 mg.m-" iso l ine obta ined f rom s ix large scale studies (Posrut . , 1979)rvas p lot ted in F igure l . Thc th ick l ine shows thc cxt reme posi t ions, thosefarthest off-sl-rore being measured only in the northern part of the areastudied. Nevertheless, the 27 times that this isoline was reachcd in the courseof the programmc showed that there \vas some relationship betureen thedistancc from the coast and the mean ZPB obtained along the rvhole oceano-graphic t ransect . For a l inear corre lat ion ( r : 0 .8981), th is dependence is s ig-nificant at the 99.9 jto level. Thc two parameters were best l i tted by a 3rddegree polynomial (F ig.2) . This re lat ionship permi t ted the calculat ion of thernissing values for the maximum extent of the area with exceeding l0 mg'm ".It was also used in case of crossing thc oceanographic section more than onetime by the chosen isopleth. These points are denoted by "*" in Table 2.Apart from the cxtreme values for the dilferent regions, Table 2 shou,s thcdates on which the measurements were made and, as the differences betrveenextremes, the amplitudes of the cross-shelf variations.

Cusstrvc (1971) found by comparing surface temperature and ZPB tharthe zone with elevated plankton contents extended 2.5 times farther per-pendicular to the coast than the upwelling zone deiined by physical data. Thecorresponding factor for the region off Cap Blanc is 2.5-3.7 . Here we regardthe distance of the shelf edge from the coasl (72 km) as the "physical" bound-ary because according to CostLot'tr and B,rneEn (1980) the coldest water offNorth West Africa is restricted to this region. The "biological" bounclary canbe expressed as the mean d is tance of the 10 mg.m- ' isopleth f r t - rm the coast ,i.e. 244 km with a confi.dence interval of + 44 km which is significanl at the95 ozo level.

BIOLOGICAL BOUNDARIES OF N\ \ ' AFRICAN UPWELLING

y = - 3 1 . 2 1 1 7 1 6 0

+ 1 2.0758000x0.' l 940554x'

+ 0 .0012827x"

483

L ! 0

Fr<; . 2. - Distance (km) of thecoast (10' -25' N) re lated

T I r - I - i - i t30 r .3 50 6t t l lmg r 3 l g i .

10 mg m-r' isopleth from the Northwest Africanto the cross-shel f averaged ZPB (mgm ") .

TABLE 2

Distance f rom the coast (km) of the 10 mg'm- 'ZPB isopleth and range of var iat ion(max.-min.) in the diffcrent marine arcas off Northwest Africa at the different dates

of invcstigatiorrs.

Working area Maximum Date Mininrum Date McLx.-min

Bahia de Garnet (25' N)Bahia de Gorrei (23' N)Cap Blanc (20" 55'N)Nouakchot t (17 '50 'N)Cap Ver t (14" 45 'N)

Cap Roxo (12 'N)Cap Verga (10" N)

197209',2792642t2

296+435

13.-14.3.19736.1 1 .1970

2.-4.9.19705.-7.3.19732.-3.3.1973

24.-25.5.197427.-28.2.197323.-25.2.1973

t2.-1,3.2.197622.-24.6.19722s.-26.6.197230.8-1.9.1970

27,8.1,970

6.7.19727 .-8.7.1972

I57

19029:

1

t29t76

1,9615289

235211

1,67259

484 L. posrEt-

Delineation of the "biologically" defined upwelling region by means of thel0 mg'm-'t isoline is more complicated in the alongshore direction than inthe cross-shelf direction due to local singularit ies. In the southern part of theregion investigated, off Cap Roxo and Cap Verga (Fig. 1), the river input leadsalso to high plankton standing stocks even when there is no upwelling(ScHrnrarurR et al.,1975). Our chosen criterion therefore cannot be used unti lfurther north where the 10 mg.m " isopleth follows more zonal direction(Fig. 1) between Cap Roxo and Cap Vert in June (1972) and off Nouakchottin September (1970). The directions and positions of these ZPB isolines coin-cide with those of the isopleths for physical parameters such as temperatureand salinity which are also perpendicuiar to the coast (ScnEuAtNDA e/ aI.,1975).The northward migration from June to September suggests some dependenceon the seasonal meridional migration of uprvell ing phenomena (Fig. 3). Thisis shown visually by comparison with Figure 4 where the "annual" variationsamounting to the mean ZPB's for the different sea areas are compared withthe "annual" mean. The period with above-average ZPB is shown separatelyin the top part of the Iigure. This shows, as indicated by physical oceano-graphic results (Fig.3), that the period lasting almost the whole year can beobserved only in the northern area, including the region off Cap Blanc. Thesteepest gradient in the duration of above-average ZPB is found between CapBlanc and Nouakchott. The turning point in the meridional migration of"physically" defined upwellings is also situated in this area (Fig.3). We cantherefore assume that the '"physical" southern boundary of the upwellingregion and its meridional breadth of variation are also "biologically" valid.An analogous correlation may hold for the northern boundary as well. Figure 4

o o o s Nq

I -kr i"r/+th, I

I t "rr* It "r t"*

- lShoJ fe r ( 1972 )

t -t't". II rr.r"r

_lS c h e m o i n d o ,N e h r i n g ,S c h u l z { ' 1 9 7 5 )

t -hl"/d.h, Il.rr."/ort"*^l

Woos te r ,Boku n ,Mc Lo in ( 1976 )

-'^tQr!*;l

lL nr ttr".Iortu r-n

S p e t h .D e t l e f s e n ,S i e r t s ( 1 9 7 8 )

Frc. 3. - Mean meridional extent and seasonal variationof the cold water upwelling region off Northwest Africa.

BIOLOGICAL BOUNDARIES OF NW AFRICAN UPWELLING

B d G o r n e l

B d . G o r r e '

C B l o n c

N o u o k c h o t t

C V e r t

L X O X O

C. Vergo

485

J F ' I ' 4 A M ] . J ' A ' S ' O ' N ' D

r.0

0

50

0

50

0r.0

0

50

01 0 0

l m q m - 3 1

50

0

Ftc. 4. - Mean ZPB (mg m-") in the different areas off the Northwest African coastat the different dates of investisations.

G o r r e i 2 3 o 0 0 ' N

, \ _N o u o k c h o t t 1 7 o 5 0 ' N

C V e r t 1 1 . o 4 5 ' N

C R o x o 1 2 0 0 0 ' N

C Ve rgo 10 "00 'N

J ' F ' f . , 1 ' A ' M ' I t

f - A r a ] ö t N r D r

486 L. posrEI-

and Table 2, for example show that in February (1976) when the northernboundary of the cold water upwelling region reaches its southernmost point(F ig.3) , theZPB of f Bahia de Garnet a lso remains below the l0 mg.m-: ' level .

The region with elevated zooplankton levels therefore extends more than1000 km in the alongshore, and up to several hundred kilometres in the cross-shelf direction. Although the "physical" and "biological" boundaries coincidein space "- parallel to the coast, perpendicular to it we found the "biological"boundary more than 2-3 times farther offshore than the "physical" boundary.

Alongshore variabil ity in the migration of the upwelling region dependson annual variations.

In more southerly sea areas this behaviour is modified by local singularit ies.Upwelling occurs throughout almost the whole year in the northern part ofthe area. Cross-shelf variabil ity is therefore of a seasonal nature exceedingoff Nouakchott and Cap Vert (in terms of the relationship between ZPB andthe extent of the region with above-average plankton levels).

*- Along the t ime scale, the bio logical reacr ion t ime (c. f ' . Vtxocnloorr e/ 41. , 1973) causesphase lag rvhich must be taken into account.

B I O L 0 ( ; t C A l , U { ) L N I ) A R I U S O } r \ \ N A I j R I C A N l r P W E l . l . l N ( - ;

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