Publ: Espec. but. Esp. Oceanogr. 23. 1997: 23-33 P UBUCACIONES ESPECIALES L"lSTlTUTO ESP.I\NOL DE O CEANOG RAFIAISSN; 021-1-7378 . ISBN: 81~191-O299-5

Ib Ministerio de Agriculrura, Pesca yAlimentacion , L997

Bathyal zones of the Mediter ranean continental slope:An attempt

c. C. Emig

Centre d'Ocean ologie de Marse ille (UMR-CNRS 6540) , Station Mari ne d 'Endoum e, Rue de la Batterie-des-Lions.13007 Marse ille, France.

Received Febru ary 1996. A ccepted August 1 99 6.

ABSTRACT

On the con tine n tal slop e, th e bathyal can be divided into two zones, the upper ba thya l andthe middle bath yal, at the shelf break, wh ich represen ts th e boun dary betwe en the coastal shelfenvironment an d the deep realm , located at about 100-110 m dep th. T he upper bathyal, p revi­ously considered a transi tional zone, is characteri sed by distin ct physical , geological an d biol ogi­cal features. Its bath ymen-ic ex tensio n is directly related to slope physiogra phy, and its lowerboun dary ge ne ra lly corresponds to the mud lin e. T his bel t is governe d by specific abio tic factorswith stee p physical grad ien ts (e.g., hyd ro dynam ics, salin ity, oxyge n , temperat ure , sedirnen ts}.Major change in tbe benthi c fauna is associated with major ch ange in these abi otic factors. Thethree main biocoeno ses are dominated by suspen sion-feed ing species, which are exclusive to th eMediter ran ean upper bathyal. Dep ending on water parameters, the limit between th e phytal andaphyta l systems ge ne ra lly occurs with in th e upper bathyal. The seco nd zone, the middle bath yal,is poorl y docum en ted; however, it can be divided in to several subzones whos e limi ts appear to bere lated to water-mass characteristics: the upper-midd le slope , the lower-m iddle slope , an d th elower slope . T he lower ba thyal zone deve lops into th e bathyal ri se and plain .

Key words: Medi terranean Sea, con tin en tal slope, bathyal zonation, bio logical oceanograp hy,physical oceanography, geolog ical oce anogra phy.

RESUMEN

L os zonas btuiales del talud con tinental medi ternineo: un en say o

En el talsui continental, el -piso bauai se puede dividi·r en banal superiory batial medio. Sn timue supe­ri ar (100-110 m) corresponds al reborde del talud, suuada entre dos domisuos muy distintos: el de la plala­[orma y el prof undo. Aruerumnerue considerada coma «zona de tronsicum» entreetpiso circaluoral y et ba­tial, el Ltuiol superior se caraaerua 1'01' sus particularidadesfisicas, geol6gicas y biol6gi.cas. 511, extensionbotimetrica dcpende de la Jisiografia deltalu d, qu e induce los efectos de los facnnes abuiticos y asi prooocauna disnibucum. del beruos en d auurones con dominancia de Los suspensivoros }' la casi ausencia de sedi­menuuion. El batial supenor comp rende generalmerue el limiieen tre el sistema fital y el ajtal segttn la trams­parencia de las aguas. El batial medi a empiexa a pm·t.i., de la. lin ea de fa ngo (mud line) qu.e marca un (',(J.t1/.+

bio abrupto de la energia. Aunque faltan estudios daa llados, a partir de Los datos disponibles se pu edejJrojJoner un esqlJ,mna de la. umacuni del batiai media: el talud: medic superior, el talud media inferun; etta­lud. inf erior. Por tanto, el boliol inf erior se exuende en la llanura batial.

Palabras cla ve: Mar M editemineo, talud continental, zonacum. batial, oceanografia biologica,oceanografiafisica; oceanografia geol6gica.

23

c. C. Emig

INTRODU CTION

The upper part of the Mediterran ean continen­tal slo pe was previously regarded as a «transitio nal»..zone between th e circali tto ra l zone and the bathyalzo ne (Peres and Picard , 1964; Carpine, 1970; Reyss,1974; Falcone tti , 1980; Emig, 1985; Peres, 1982;Bella n-Santin i, 1983; Ab e1l6 , Vallad ares andCas tellon , 1988) . All these au thors have no ted th eim portan ce of th e batbym etric depth rangebetwee n ] 0()..1 50 to 200-300 m, which generallyincludes the limi t be twee n th e phytal and aphytalsystems (Emig, 1989b ). Bu t th ey failed to take in toaccount th e importance of abio tic fac to rs, i.e .,slo pe physiography and physical characteris tics, inorder to exp lain th e bio logi cal specificiry of th iszone (Emig, 1985, 1989a, 1989b) whose limits havenever been clear ly defined. H owever, Ercegovic(1957) an d Peres and Picard (1964) defined th eextension of the «transitional» zone , from thelower boundar y of th e circalitto ral , i.e., th e lowerlimit of multicellular algae, down to th e lower limitof unicellular algae. Conse quently, the «true»ba thyal zone was considere d to begin beyond th etransitional zon e, be tween 200 and 500 m. O th erauthors use arbitrary (15 0 m ) or economical (200m) zone defin itio ns of th e upper lim it of thebathyal, based on artificial considera tions, neverscien tific on es.

A review of the northwestern Mediterraneanbathyal zone, with new information fro m sub­mersible cam paigns along th e con tin ental slope,makes it possib le to reco nsider the position andimpor tan ce of th is zone on th e co n tinen tal slope.On the other hand, few faunistic surveys were pe r­fo rm ed in the Mediterran ean bathyal, and com­parisons of data an d results are d ifficu lt, becau seth ey were obtained in different ways. Co nsequently,n either local nor biogeographic comparisons canbe made. Neverth eless, except th e upper bathyalzo ne , the o ther ba thyal zones are un likely to bed efined and characterised at present, al though age nera l tendency can be discerned .

Upper bathyal zone

T he upper lim it of th e bathyal zone lies a t th econ tin en tal shel f break, defined by physical , ge o­logical and bio logical charac teristic s (figures 1 and2). Although of primar y im p ortan ce, because ofmaj or gradient change s, the Medi ter ran ean sh elf-

24

Meduermnean baih)'at zones

edge sector is a poorl y known e nviron m entbecause it lies between two distinct zones of inter­es t, the coastal-shelf environment and th e dee prealms (figu re 2). T hus, it is n o t surprising th at th eshelf-to-slo pe transition remains a bio logical andphysical «no man 's land» (Vanney an d Stanley,

1983) .

The Mediterranean shelf-to-slope morphologyon soft substrates presents two major types of pro­files (figure I ) , on which the shel f break occurs atabout 100 to 120 m. H owever, in Type Il , th e co nti­nental sh elf break is ge ne rally co ns idered to be at adepth of abou t 150 m, which corresponds to th eedge of a large an d fla t bathyal offshore te rrace ;but th e tr ue co ntinental shelf break , ofte n missed ,lies shorewar d and co rresponds to th e edge of ashort scarp of about 5--20 ID (figure 1) (Ernig,1989a, 1989b; Savoye and Piper, 1993) .

T he lower limit of th e u pper bathyal zone isin dicated by the mud line, wh ich is kn own to in d i­cate an abrupt change in the en vironmental condi­tio ns (Blake an d Doyle, 1983) and serves as anenergy-level marker (Stanley, Addy an d Behrens,1983) . Located a depth of between 160 and 300 m,this limi t is rel ated d irectly to slo pe mo rphology(figure I ), which gove rns th e influence of the pre­vailing abiotic factors: th e weake r the inclin e theshal lower in depth is this limit (Em ig, 1989a,1989b). Thus, like th e upper limit of th e upperbathyal, th e lower limit does n ot co incide with abathymetric expression .

Physical characteristics

The upper bathyal zo ne is a high -energy secto rcharacte rised by grad ient variations of th e prevail­ing abiotic factors, i.e. , hydrodynamics, salin ity,tem pera tu re, oxygen , sed imen ts (figure 2) . In th eentire northwestern basin there is a main current,th e Liguro-Provencal-Catalan current, that fo llowsthe co n tinental shelf break and co inc ides with ape rmanen t sh elf/ slope density front; thi s flow isin tensified by th e p revailing win d s and separatesth e co n tinental fresh waters on the she lf fro m thesa ltie r an d usually warmer open-sea waters (Salatand Fo n t, 1987; Millot, 1987; Wang et al., 1988;Emig, 1989a; Monaco et al., 1990; Fo n t, 1990; Emigan d Garcia-Carrascosa, 199 1; H uthnan ce , 1992) .

The near-bo ttom curren t, weak or abse nt on thelower par t of th e contine ntal shelf, increases by sev­eral tens of meters a t th e level of the shelf edge; its

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Depth in m P c 5 %. 02 mill Water masses Lower BATHYAL

Figure 2. Diagram of diffe re nt,parameters ofth e surrounding water column in meBathya l Zone (drawn after data from var­ious authors}.

bathymetric ex tension, velo city and direction ared irectly related to slope physiography (figure 1) .T he velocity shows a ver tical gradient (figure 1) andvaries fro m about 0.5 up to 2 kno ts or more, an dsometimes causes large ripple marks o r undulationson th e bo ttoms (Reyss and Soyer, 1965; Emig, 1987) .Consequen tly, sedimentatio n is absent or weak inthe upper bathyal, which appears to be a resuspen­sion lone with minor accumulation of particles andin which the fauna is characterise d by high densitiesof suspension-feeders. At th e lower limit of th e sec­to r, th e bo ttom curren t velocity drops off by severaltens of meters, while the su bstra te becomes muddy(Reyss and Soye r, 1965; Emig, 1989a; Emig andGa rcia-Car rascosa, 1991) : the mud line is eas ilydetected in the case of a ch an ge fro m sandy tomuddy substrates, but remains pro blematical whenthis lin e occurs shoreward to the shelf edge.

In the surroun din g water co lu m n severalo th e r g ra d ien ts charac te r ise th e up per bathyal in

26

the northwestern Mediterran ean Sea (figure 2) :low annual tempe rature var iatio n (abo ut 2 DC)wh ich be comes co ns tant beyond abo ut 200 md ep th (th e Medi terranean h omo th ermy) , lowsalinity increase, and sharp decrease of oAJ'genwhi ch, howeve r, is kn own to be higher than bio­logical consump tion. T h e lower boundary corre­sponds to th e lowe r lim it of the fro n tal structure,across which the hydrol og ical pro per ties ch angesharp ly (Huang a nd Su , 1991), an d to th e begin­ning of the oxyge n m ini mum laye r (OML) whichis related [ 0 the Levan tine intermed iate waterm ass with a salinity m ax imum an d homo th er my(figure 2) .

T he CjN ra tio in th e sed irne n ts decre asesfrom 12 to 10 (Carpine, J970) . T he change ofsediment oc cur in some tens o f meters, related tothe hydrodynamics, at bo th lim its of the UpperBathyal (Emig, 1989a; Emig and Garcia-Carrascosa,199J ) .

Publ. Espec Inst. Esp. Oceanogr 23. 1997: 23-33

c. c. emig

Geological characteristics

T he Recent imprin t, since th e last rise in sealevel (the Wurrn glacia tio n , 17 000 years), is clearl yevident in the upper bathyal : th e continen tal shelfbreak lies a t the outer edge of th e deposits forminga narrow prograding wedge or pri sm located at th eou ter part of th e con tin en tal shelf (figure 1). T he .shelf edge is in equilib rium wi th th e present en vi­ronmental co n d itions ; its progra da tion d uring theLate H ol ocene has been negligible and its depthcoincides with a «depositio na l equilibr ium » level

(Mougeno t, Boillot an d .J. P. Rehault, 1983;Monaco et al., 1990; Co u r p and Monaco , 1990 ;Savoye and Piper, 1993). The relative absence ofRecent sed im ent supply in the upper bathyaJ (fig­ure 1) shows that sed im ents are mainly re worke drelict (Wurrnian) sed imen rs with th e OCCLU'fenCe ofQuate r nary thanatocoenoses visib le by submarineo bservation and generally loca ted a t a depth ofbetween 180 to 20 0 m (Gautie r and Picard , 1957 ;Blan c, 1968; Em ig, 1987 ) .

Biological characteristics

The circali ttoral ex tends d own to the lower limitof m ulti cellular algae (Peres and Picard, 1964) :from subm arin e obser vations of th e coastal detri ticbioco enosis (CD) , ca lca reous red algae do n otextend beyond th e shelf-break and, afte r a shor t

transitional zone of some tens of m ete rs, thebathyal de tritic sand biocoenosis (BDS) begins, or

at th e sides of submarin e ca nyo ns, tbe shelf-edgedetri tic biocoenosis (SD) (figure 1). The SD hasbeen traditional ly considered a circalit to ral bio­coenosis, som etimes co nfuse d with the BDS, but

belon gs with out an y doub t to th e upper bathyal(Laubier and Em ig, 1993). T he continental shelf

edge corresp onds to th e upper limi t of the di strib­u tion of a t least two excl usive u pp er bathyalspec ies, th e brachiopod Gryphus vitreus (Born,1778) in the BDS an d the crinoid Leptornetra pha­langiurn (Mulle r, 1841) in the SD, while the mudline corresponds to the lower limit o f their exten­SIOn .

The su bstratum of the BDS is well-sorted sand(gravel, co arse and fine sand ), clogged by a finefractio n which can reach 60 %, an d co ntains a largedetritic p ro portion of small h ard substrates ofen dogenous origin (fragments of mollu sc and bra­ch iopod sh ells, of sponges, bryozo ans, co rals, grav-

Publ. Espec Inst. &p. Oceanogr: 23. 1997: 23-33

M editerranean bathyal zones

el s and pebbles) , a character istic of the BDS bot­tom (Falconetti, 1980; Emig, 1989a, 1989b) . Thesu bstratum of th e SD is sand y mud (gravel , sand,mud) (P ica rd, 1965; Emig, unpubli sh ed data).

On m uddy substrata, the tr ansiti on from the cir­calitto raJ m uddy de tritic (MD) and terrigenousmud shelf (TMS) biocoenoses to an upper bathyalmud biocoenosis (UBM) remains hypothetical,due to lack of research. However, fr om the data ofG uille (1970) , Picard (1971) , Salcn-Picard (1982) ,Alber te lli, Della Croce an d Fraschetti (1991) andAlbertelli and Fraschetti (1992), one may arguethat there is a faunistic change a t the level of th eshelf edge, with the ap pearance of domin antspecies , particularly the oph iu r id Amphiura fili­

[ormis (Muller, 1776), reaching 8-34 % of th e fauna,accord ing to Sale n-Pica rd (1982) and Alber te lli,De lla Croce and Fraschetti (19 91) , and th e poly­chaete Maldane glebifex Grube, 1860, while th e char­

acteris tic species from both circalittoral MD andTMS biocoenoses are abse n t in th e u p pe r bathyal.In th is h ypothetical UBM, some species can reachhigh densiti es in some areas, i.e ., the echinode rrnBrissopsis lyrifera Fo rbes, 1841 , the p ennatulari aFu n icula quadrangularis (P allas, 1766) an d the

sponge Thenea muricata (Bowerbank, 1858).Within th e upper bathyal biocoenoses, the Iau­

n istic d istribution develo ps in to belts re la ted to th eprevailing factors (hydrodynamics an d th e relatedsedimentary cond itions). Consequen tly, these bel tsare main ly suspensio n-feeder o nes. The direction

an d veloci ty of th e bOtt0111 currents create live dis­tribu tional zones in the BDS (tile h ighest velocityzone is re ported in figure 1) , i.e ., th e di stribution­al zo ne s of Gryphus uitreus, whi ch can reach up to700 individuals .z m? (Em ig, 1987, 1989a, 1989b;Emig a nd Arnaud, 1988; Emig an d Garcia­Carrascosa, 1991) or the d istributio n belt (between100 to 140-150 m ) of the ech in odcrrn Neolampas ros­tellata Aga ssiz, 1869 rep o rted previously byFa lco netti (1980) as belonging to th e circalittoral.In th e SD, the crinoid Leptomeira phalangiu m canreach up to 30-50 ind ivid uals/ m ' (Reyss, 19 71) an din th e UBM Amphiura filifo,.,nis reaches lip to 28in d ivid uals ymz (Albenelli and Fra sch e tti, 199 2) .

Su cb a bel t distribution explains the relative het­erogeneity of the fauna beyond tile shelf-edge, asreported by previo us authors, as well as th eir a rgu­men ts for co nsidering the upp er bathyal a transi­tio nal zone.

The upper bathyal biocoenoses seem to be char­ac te rised by lower spe cies and individual rich n ess

27

c. C. Emig

when compared to the ove rlying circalittoral bi o­coenoses; in general, the muddier the substrate,the lower the richn ess. However, the analysis ofrecently published da ta by Albertelli, Della Croceand Frasch etti (1991) , Albertelli and Fraschetti(1992) an d Tse lep ides and Eleftheriou (1992)from m u ddy sh elf-to-u pper slope tr ansects showthat the mac rofauna increases in spec ies and indi­viduals, mainly of suspension-feeders and detri tus­feeders, in the uppe.r bathyal zone (figu re 3) .Within the SD an d m iddle bathyal mud (MBM)bio coenoses on the northern Mediterranean coast,there is a geograph ic decrease in r ichness fro m thewest to the east (Carp in e, 197 0) , which n eeds con­firmation because o pposite to the cyclonic circula­tion.

Th e extension of circalittoral species into theupp er bathyal occurs in general down to a depth of130-140 m , where their p resence is probably limit­ed by abio tic factors, i.e. , ligh t, p ressu re or/ andhydrodynamics, particularly for polychaetes an dechinoderrns, wh ile the ex tens ion of 1vfBM speciesin to th e upper bathyal is limited by the substrate, atleas t in th e SD and BDS. Never theless, specieswhich occur at the same time in the circalittoraland ba th yal are generally co nsidered as circ alittoralspec ies which ex ten d into the bathyal, rare ly th econtrary. For example, several brachi op od species,i.e. , Megerlia truncata (Linnaeus, 1767) and Megathirisdetruncata (Gmelin , 1790) , co nsidered respectively asa shallow-water species and an eurybathic species(Logan, 1979) , h ave their maximum density inthe upper bathyal, be tween 100 an d 150 m (Em ig,19S8).

Phytal-aphytal boundary

This boundary can be estimated by the percent­age of the Gryphus vi/reus shells infested with th egreen alga Ostreobiu m sp . Bornet and Flahau t.Occurring generally within th e upper bathyal zone,i ts dep th varies in relation to the water transparen­cy, i.e ., between LSO and 210 m in normal water

co nditions in Corsica, but it can be restricted to125-135 m under turbid water con d itions (Emig ,19S9c). Along the Proven cal coast, becan se o n ly asmall number of shells is infested with Ostreobium

sp. , this boundary is estimated at about 150 m

depth . Such results in d icat e that the phytal -aphytalboundary is independent of benthic zonation , andthat i ts depth can flu ctua te even with in a restricted

28

Mediterranean balh)'al zones

geographic area, contradicting Bellan-San tini ' s(19S3) statemen t that this boundary lies at th elower limi t of the circalittoral zone, as well as thedefinition of the lower limi t of the transitional zoneproposed by Ercegovic (1957) and Peres andPicard (1964) .

Middle bathyal zone

There is a rapid transitio n over several tens ofmete rs from the upper bathyal zone to the middle

bathyal zone, i.e., fro m the u pper bathyal b io­coenoses to the MBM . The substrate becomesm uddy, with th e appearance of burrowin gs havingvarious diameters (Em ig and Arnaud , 19S8; Emigan d Carcia-Carrascosa, 1991) . O ver the co n tinen­

tal slope in the middle bathyal , with in creasin gdep th, a ge neral decrease in the size of the m ac­robenthos is recorded (Carpine, 1970; Peres , 1985;Sarda an d Cartes, 1993; Stefanescu and Cartes,1992) , as well as a quanti ta tive decrease in th e mac­robenthos (fig ure 3) and an in crease in d ivers ityrelated to increased en vironmental stability, associ­ated wi th in creasin g dep th (Sanders, 1965; Sandersand Hessler, 1969; H aed rich , Rowe and Po l1on i,

19S0; Abell6, Vall adares and Ca ste1l6n , 19S8) .Nevertheless, fo ur of th e five grenadier fish speciesoccurring in the m idd le bathyal fo llow th e bigger­deeper ph enom enon (Massu ti , Morales-Nin andStefanescu, 1995).

There are several faunistic stock changes withinthe middle bathyal zone, which consequen tly canbe divided in to severa l d istributional zones (figure4) , based m ai nly on recent data from crustaceanand fish distribution . However, these zones can berel ated to several previous proposals, such as th a tof Per es (19S5) , who d ivided the up per slope intotwo zones below the transitional one: from 200 to500 m and from 50 0 to 1 000 m. Car p ine (19 70)proposed the followi ng limits: transi tional zone,15 0-250 m ; upper bathyal , 250-550 m ; m idd lebathyal , 550-2000 m; an d lower bathyal, I 500­3000 m .

The cr ustacean assemblages occurring o n theupper an d on th e lower-middle slo pe correspondto hi ghly d ifferen tia ted faunis tic communities,whose d ifferences can be a ttr ibu te d to hyd rologica ldifferences. The upper community is associatedwith the warmer intermediate waters, and thelower zone with the deep-water mass, which has afairly co nstan t temperature throughout the year

Pubt. Espec. Inst. Esp. Oceanogr 23. 1997: 23-33

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c. C. Emig

CIRCALlTTORALlOO Shelf break 120160 UPPER BATHYAL

250

Upper-middle slope

480510

650

Lower-middle slope 800

....I<>J:I-<m

1200W....I0 1400

9:E

Lowerslope

1000

I<Rise / plain"

2200

LOWER BATHYAL

Figure 4. Diagram of bath yal zonation along theMediterranean continental slope, with the main faunistic

changes and boundaries.

(Abe Il6, Valladares and Caste1l6n, 1988) . T hegreatest abundance of mysids seems to occur onthe midd le slo pe (Cartes an d Sorbe, 1995) . Thepopulations of Aristeus antennatus (Risso, 1816) canbe divided into seaso nal groups on the up per-mid­dle slope, but such gro u ps are less defined in thelowe r-m idd le slo pe , and th e se aso na l effectbecom es very sligh t on the lower slope (Cartes,1994).

Acco rding to data from Alberielli and Catta ne o(1985) and Albertelli , Della Croce and Fraschetti

30

Mediterranean. bathj'al zones

(1991) , on muddy substrates in th e Gulf of Ge n ovadeposit-feeders represen t respectively 63 % in theupper bathyal an d 37 % in th e upper-m iddle slopeof th e middle bathyal, wh ile suspens ion-feedersrepresent respectively 14 and 7%, and predators 20an d 57 %. Along a Cretan tran sect (Tse lep ides andElef th eriou, 1992), polychaete deposit-feeders aredominant between 200-400 m (more th an 50 %)and carnivores (more than 49 %) be low 500 m . Inthe Catalan Sea, th e upper-middle slope co mm uni­ty (245-485 m ) shows a predominanc e ofmesopelagic species (Cartes, So rbe and Sarda,1994); th e fish asse mblage on the lower-middleslope, between 1000 and 1425 m , is domin ated byLepidion. lepidion (Risso , 1810) with five subdomi­nant species (Stefanescu, Lloris and Rucabado,1993) .

So me au thors (Abcllo , Val lad ares and Cas te llon,1988; Mura an d Ca u, 1994) indicate a change inth e decapod faun a between 650 and 800 m.Acco rding to Sarda an d Ca rtes (1993) , the d iet ofthese decapods consists lar gely of in£auna or epi­fauna, an d there are differences in th e co mposi­tio n of th e diet above an d beyond this limit.

On th e lower slope , the re is evidence of fau nis­tic changes, as well as drastic changes in theamount of available energy and a sharp re d uct ionof th e available trophic resources related to the dis­tribu tion of mesopelagic organ isms and hydrologi­cal characteristics (Cartes, 1993; Stefanescu , Llori sand Ru cabado, 1993; Sarda an d Cartes , 1993) _Thefauna is dominated by demersal fish species withlow ene rgy cos t, and small epibe ruhic and ben­thopelagic inve rtebrates (Stefanescu, Lloris andRucabado, 1993) . T he absence of mesopelagicorganisms below 1 000-1 200 m is reflected by anim poverish ment of th e benth ic ecosystem. Cartes(1993) indica tes th at one of th e most importantpreys, th e decapod Calocasis macandreae Bell, 1846,d isappea rs below "[ 000-1200 m (figure 3). In de m­ersal fish assemblages there is an abrupt decreasein values and a subseque nt hom ogene ity at greaterdepths (figure 3); the asse mblage is d ontinated byBathsptetois mediterraneus Baucho t, 1962, with twosubdom inan t species (Ste fa nescu, Lloris an dRucabado, 1993) _

Canyon valleys

The canyon valleys are subj ect to spec ific abi ot­ic conditions, and their macrofauna co mposition is

Publ: Espec. Ins t. Esp. Oceanogr. 23. 1997: 23-33

c. C. Emig

too poorly documented to allow compariso n (seeMonaco et al., 1990; Bourcier, Stora and Cerino,1993; Gerino et aI., 1995) . T hese valleys are geo­mo rphologically important in concen trating andtransp or ting sedimen t to deeper areas (Mo naco etal., 1990), with h igh dynamic water circulation(Mille t, 1990). Their biologica l im portance is due

to the input of b igh prod uctivity with abundant .resources, and the occur rence of recru itm ent areasco ncen tr ating juvenil es of certain species, particu­larly in th e middle ba th yal zo ne. Fa un a b iom assand abun dance are higher than those recorded o nth e sur round ing slope areas.

DISCUSSION

T he zone herein defined as th e upper bathyal",..as previo usly considered a transitional zone. notonly in th e Mediterranean but in the ""orld O cean(see Zenkevitch, 1963; Peres, 1982; Blake andDoyle , 1983; Zezin a, 1994 ) . However, thi s zo nebegin s at the con tinen tal she lf-edge (geomorpho­logica l she lf-break) which is ch aracte r ised by bi o­

logical, physical and ge ological expression, andbeyo nd which the upper bathyal develops as a bel twhose bathyrnetric extension is di rectly rela ted toslo p e physiography (figure 1) . Con se q uen tly,future stu d ies of bathyal fau na shou ld always indi­ca te the slo pe profile from whi ch the da ta havebeen re por ted; rhi s is also valid for th e middlebathya]. In the bathyal , th e major change in faunais directly associa ted with major environ m entalchanges, wh ich pl ay a key role in d etermina tio n ofthe fauna across the shelf-break. T h e biologicalex tension alo ng the slope is given by th e bio­coenoses occurring in th e upper bathyal , dominat­ed by hi gh densities of exclusive suspen sion-feederswhose distributional belts should provide informa­tion regard ing flo w str uctures in th e shelf-to-slo peco n text.

The valid ity of th e bathyal biocoenoses needs tobe esta blished with new data, using fau nistic den sityand biom ass an d th e effects of almost abi otic char­acters, to define cir ca litto ral and baihyal speciesand the exclusive species of a biocoenosis, whi chcan only be stenotop ic species (Arn aud and Emig,1987). T he dis tr ibutional zo nes of fauna deter­mined by abi otic factors need a careful bathyrnetricand spatial choice of sampling along th e slope.

In th e Atlan tic , in faunal density increases in theu pperm ost slo pe zone (Blake and Doyle, 1983).

Publ. Espec: Inst. Esp. Oceonogr: 23. I 997: 23-33

Mediterranean bathyal zones

T his also seem s to be the ca se in th e Med i­terranean upper bathyal, as suggested by th e dat aof Alb er te lli an d Cattaneo (198 5) , Albertelli, DcllaCroce an d Frasche tti (.1 991) , Albertelli andFrasche tti ( .1 992) and Tsele pides and Eleftherio u(1992) on mudd y bottoms (figu re 3) . H owever, th especies and individ ua l ri chness glo ba lly decreases

from circalit toral to bathyal in. the Mediterranean,as a lso established by Sh errnan et aL (.1988) in thenortheast Atlantic for the density and biornass offour m ajor taxonomic groups.

Th e distributional zones in th e Medite rraneanmid dle bathyal (figure 4) can be com pared withth e depth boundary on th e Atlan tic co n tinentalslo pe at 400, 700 an d 1200 m , which are considerednearly un iversal by Hecker (1990) . Nevertheless ,this author indica tes th e fo llo wing zo nes : upperslo pe 180-600 m , upper-middle slope 600-1 300 m ,

lower-middle slope 1400-1600 m , lower slo pe .1 600­2000 m , an d a con tin en tal rise fr om abou t 2000 m .

Acco r ding to Van ney and Stan ley (.1 983 ) , thesh elf-ed ge appears as a boun dary of importan cecomparable to the coastline , separa ting the coastal­shelf environmen t from the deep realms; it is thepoint wh ere th e first maj o r change occurs in thep hysical gradie n t, and co nsequen tly, in the bio logi­cal gra d ien t, wh en entering the deep sea . T h isu pper lim it of th e deep-sea zone, the bathyal,depending bathymetrically o n th e dep th of th esh elf-edge, is expressed by biological , physical andgeomorphological criter ia wh ich have to coin cidewith each other and with th e lower boundary of thecircalittoral zone.

T h e transi tion from the upper bathyal to themiddle bathyal, whose bathymetric lim it dependso n local environmental factors governed by phys­iography, al so follows su ch multidisciplinary crite­ri a . It co r responds in genera l to the mud lin e,located between abo u t 165 and 300 111, dependingo n th e slo pe physiography, in the n orthwestMed ite r ra nean, where this line al so in dicates ad rastic change in th e energy leve l, as reported inother ocean areas by Stan ley, Addy and Behrens(1983) .

As a result of stee p gra d ie n ts in physical con di­tions, the upper bathyal zone rep resen ts an under­estimated barrier fro m shelf to deep sea, occu r ringin the World O cean. In the Mediterranean, su ch ap hysical barrier m ay partly explain th e sharp faunadecrea se in th e middle bathyal and the bi ologicalpover ty of the Med ite r ranean deep sea. On theo ther hand, a lo ng-botto m transport p assing

31

c. C. E1I1ig

through the canyon axes a ppear as p a th ways acrossthe sh e lf break to the Mediterranean deep sea(Courp and Monaco, 1990) .

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