Publ: Espec. but.
Esp.
Oceanogr. 23. 1997: 23-33
P UBUCACIONES ESPECIALES
L"lSTlTUTO ESP.I\NOL DE O CEANOG RAFIA
ISSN; 021-1-7378 . ISBN: 81~19 1-O299-5
Ib Ministerio d e Agri culrura, Pesca y Alimentacion , L997
Bathyal zones of the Mediter ranean continental slope:
An attempt
c.
C. Emig
Ce ntre d'Ocean olo gie d e Mar se ille (UMR-CNRS 6540) , Station Mari ne d 'Endo um e, Rue de la Batterie-des-Lions.
130 07 Marse ille, France.
Received Febru ary 1996. A ccepted August 1 99 6.
ABSTRACT
O n the con tine n tal slop e, th e bath yal can be divide d into two zones, the upper ba thya l and
the mid dl e bath yal, at the shelf break, wh ich represen ts th e bo un dary betwe en th e coastal shelf
envir on men t an d the dee p realm , located at about 100-110 m d ep th. T he up per bathyal, p revio usly co nsidered a transi tional zo ne, is ch ar acteri sed by d istin ct p hysical, geological an d biol ogical features. Its bath ym en-ic ex ten sio n is dir ectly related to slope p hysiogra p hy, and its lower
bo un dary ge ne ra lly co rresp onds to th e m ud lin e. T h is bel t is gover ne d by sp ecific ab io tic factors
with stee p physical grad ien ts (e.g., hyd ro dynam ics, salin ity, oxyge n , temperat ure , sedirnen ts}.
Major ch ange in tbe ben thi c fau n a is associated with m ajor ch ange in these abi otic factor s. Th e
three main biocoeno ses ar e domin ated by suspen sion-feed in g speci es, which ar e exclusive to th e
Mediter ran ean upp er bat hyal. Dep endi ng on water para m eters, th e limit between th e p hytal and
aphyta l systems ge ne ra lly occu rs with in th e upp er bathyal. The seco n d zo n e, th e middl e bath yal,
is poorl y do cum en ted ; however, it ca n b e divided in to several subzones whos e limi ts ap pe ar to be
re lat ed to water-ma ss characteristics: th e u pper-m idd le slope , the lower-m iddl e slope , an d th e
lower slope . T he lower ba thyal zo ne deve lops into th e bath yal ri se an d plain .
Key words: Medi terranean Sea, con tin en tal slope, bathyal zo nation, bio logical oceanograp hy,
p hysical ocean og rap hy, geolog ical oce an ogra phy.
RESUMEN
L os z ona s btuiales del talud con tinental m edi ter nineo: un en say o
En el talsui continental, el -piso bauai se puede dividi·r en banal superior y 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» entre etpiso circaluoral y et batial, el Ltuiol superior se caraaerua 1'01' sus particularidades fisicas, geol6gicas y biol6gi.cas. 511, extension
botimetrica dcpende de la Jisiografia deltalu d, qu e indu ce los efectos de los facnnes abuiticos y asi proooca
una disnibucum. del beruos en d auurones con dominan cia de Los suspensivoros }' la casi ausencia de sedimenuuion. El batial supenor comp rende generalmerue el limiie en 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 ede
jJrojJoner un esqlJ,mna de la. umacuni del batiai media: el talud: medic superior, el talud media inferun ; et talud. inf erior. Por tanto, el boliol inf erior se exuende en la llanu ra batial.
Palabras cla v e: M ar M editemin eo, talud continental, zonacum. batial, oceanografia biologica,
oceanografiafisica; oceanografia geol6gica.
23
c.
C. Emig
INTRODU CTION
T he up pe r part o f th e Mediterran ean contine ntal slo pe was previ ously regarded as a «transitio n al»..
zo n e between th e circ ali tto ra l zone and the bathyal
zo n e (Peres and Picard , 1964; Carpine, 197 0; Reyss,
1974; Falcon e tti, 1980; Emig, 1985; Peres, 1982;
Bella n -Santin i, 1983; Ab e1l6 , Vallad a res and
Cas tellon , 1988) . All these au th ors have no ted th e
im porta n ce of th e batb ym e tric d epth rang e
be twee n ] 0()..1 50 to 200-300 m , wh ich ge n erally
includ es th e limi t be twee n th e p hytal an d aphytal
systems (Emig, 1989b ). Bu t th ey failed to ta ke in to
acco unt th e impo rtance of ab io tic fac to rs, i.e .,
slo pe physiogra p h y an d physical characte ris tics, in
order to ex p lain th e bio logi cal specificiry of th is
zo ne (Emig, 1985, 1989a, 1989b ) wh ose limits h ave
n ever been clear ly define d. H owever, Ercegovic
(1957) an d Peres and Picard (1964) d efined th e
ex tensio n of the «transitio nal» zo ne , from the
lower boundar y of th e circalitto ral , i.e., th e lower
lim it of mul ticellular algae, d own to th e lowe r limit
o f u nic ellular algae. Conse quently, the «true»
ba thyal zo n e was co nside re d to beg in beyond th e
tr ansitional zon e, be tween 200 and 500 m . O th er
au thors use arb itrary (15 0 m ) or eco nom ical (200
m ) zo ne defin itio ns of th e u p pe r lim it of th e
ba th yal, based o n a rtificial co nside ra tions, n ever
scien tific on es.
A review of the no rthwestern Mediterranean
b at hyal zone, with n ew inform a tio n fro m submersible cam paigns along th e co n tin e ntal slope,
makes it p ossib le to reco nsider the p osition a nd
imp o r tan ce of th is zo n e o n th e co n tine n tal slope.
O n the other han d , few faunistic surve ys we re pe rfo rm ed in th e Med ite rran ean bathyal, and co mparisons of d ata an d results are d ifficu lt, becau se
th ey were ob tai n ed in different ways. Co nseq u e n tly,
n either local n or b iogeograp h ic co m parisons can
be mad e. Neverth eless, exce pt th e u p per bath yal
zo ne , th e o the r ba thyal zo nes a re un likely to be
d efined a nd characte rised at present, al though a
ge n e ra l tendency can be discerned .
Up pe r bath yal zone
T h e upper lim it of th e b a th yal zo n e lies a t th e
co n tin e n tal shel f b reak, d efined by physical , ge ological and bio logical ch arac teristic s (figu res 1 an d
2). Although of primar y im p ortan ce, beca use of
maj or gradient c han ge s, the Medi te r ran ean sh elf-
24
Meduerm nean baih)'at zones
edge sector is a p o orl y known e nv iron m e nt
because it lies between two distinct zo nes of interes t, the coasta l-shelf e nviron m e nt and th e d ee p
realms (figu re 2). T h us , it is n o t surprisin g th at th e
sh elf-to-slo pe transition remains a b io logical and
p h ysical «n o m an 's la nd» (Vanney an d Stanley,
1983) .
T h e Mediterranean shelf-to-slo pe m o r phology
on soft substrates presents two m ajor types of profiles (fig ure I ) , on which the sh el f break occurs a t
abo u t 100 to 120 m . H owever, in Typ e Il , th e co n tinental sh elf break is ge ne rally co ns id ered to be a t a
d epth of abo u t 150 m , wh ich corresp onds to th e
edge of a lar ge an d fla t bath yal offsho re te rrace ;
but th e tr u e co n tine ntal sh elf break , ofte n m issed ,
lies shorewar d and co rrespon ds to th e edge of a
short scarp of about 5--20 ID (figure 1) (Ernig,
1989a, 1989b; Savoye and Piper, 1993) .
T h e lowe r lim it of th e u pper bathyal zo ne is
in dica ted by th e m u d lin e, wh ich is kn own to in d icate an abrup t cha nge in the en vironme ntal co nd itio ns (Blake an d D oyle, 1983) and serves as an
e ne rgy-level mark er (Stanley, A ddy an d Be h rens,
1983) . Located a depth o f betwee n 160 an d 300 m ,
this limi t is rel ated d ir ectly to slo pe mo r p hology
(figure I ), wh ich gove rns th e influence o f th e p revailing a biotic fact ors: th e weake r th e in clin e th e
sh al lower in d e pth is this limit (Em ig , 1989a,
1989b). Thus, like th e up per limit of th e upper
bath yal, th e lower limit d o es n ot co incide with a
b athym etric ex p ressio n .
Physical ch ar acteristics
T h e upper bathya l zo ne is a hi gh -en e rgy secto r
characte rised by grad ie nt variations of th e prevailin g a b iotic factors, i.e. , hydrodynamics, salin ity,
tem pera tu re, oxygen , se d imen ts (fig ure 2) . In th e
entire northwestern basin there is a main current,
th e Liguro-Prove n cal-Catalan curre nt, tha t fo llows
the co n tin ental shelf break and co inc ides with a
pe rmanen t sh elf/ slope d e nsity front; thi s flow is
in ten sified by th e p revailing win d s an d se parates
th e co n tine ntal fresh wate rs on the she lf fro m th e
sa ltie r an d usu ally warm er open-sea waters (Salat
an d Fo n t, 1987; Millot, 1987; Wang et al., 1988;
Em ig, 1989a; Mon aco et al., 1990; Fo n t, 1990; Em ig
an d Garcia-Carrascosa, 199 1; H uthnan ce , 1992) .
The near-bo ttom cu rren t, weak or abse nt o n th e
lower par t of th e co n tine n tal sh elf, in creases by seve ral tens of meters a t th e level of th e shelf edge; its
PubL Espec. Inst. Esp. Oceanogr: 23. J997: 23-33
n, with
,,§'
~
",,"
~
~
~
""o-
~
ec
:--;1
....
;g
~
l;:
r
~
~
r-
J'
~
:?
<#'
C;
Profile Type I
Figure 1. Block-d iagram s of th e bathyal bio coe noses d istr ibu tion alo ng th e uppe r con tine n tal slope sec tor accordin g to th e slope gradie nt on th e two main p rofile types, 1 an d
the sides sh owing th e sed ime n tatio n prism sinc e th e last Wiirm glaciatio n (hatch ing ) . Fo r profil e location s (P rove nce and Corsica) an d exp lan ation see Emig (1989a,
1989 b, 1989c) , Em ig and Oarcta-Carrascosa (199 1) . Th e number s in d ica te the depth in me tres. Between do tte d lin es, th e strongest velocity zone of th e near-bo ttom CU lTC n LS
an d th eir main di rectio ns (arrows) . Biocoenos es: (UBM): u pp er barh yal mud ; (SD) : shelf detritic; (BDS): bath yal d eu-itic sa nd ; (MBM): midd le bathyal mud .
Profile Type II
.5'
,,'b'
~
~
~
[
.
't
~
il
I
X
"
~
-G
,
[)
n
;p
1':
c.
C. Emig
Mediterranean bathyal w nes
1000
Depth in m
"' '''~ '''.J''- ~ '''' '''
~ ~
P c
•
38 .4-38.5
4 .5-4.7
5 %.
02 mill
~
"' .... ..... "' ..... "' ,...'"
M editerranean
Deep
Water masses
Figure 2. Diagram o f diffe re nt, parame ters o fth e surro und ing water column in
ious au thors}.
bathymetric ex tension, velo city and direction are
d irec tly relat ed to slope physiog ra p hy (figure 1) .
T h e velocity shows a ver tical gradi ent (figure 1) a nd
varies fro m abo ut 0.5 up to 2 kn o ts or m ore, an d
some times causes large ripple marks o r undulations
on th e bo ttoms (Reyss and Soyer, 1965; Em ig, 1987) .
Conseq ue n tly, sedi mentatio n is absent o r weak in
th e uppe r bathyal, whic h appears to be a resuspensio n lone with minor accum ulation of particles and
in wh ich th e fau na is ch aracterise d by high d e nsities
of suspension-fee ders. At th e lowe r limit o f th e secto r, th e bo ttom curren t velocity drops off by seve ral
te ns of meters, while th e su bstra te becomes m ud dy
(Reyss a n d Soye r, 1965; Emig, 1989a; Em ig a nd
Ga rcia-Car rascosa, 1991) : the m ud line is eas ily
de tected in th e case of a ch an ge fro m san dy to
muddy su bstrates, b ut re main s p ro ble mati cal when
th is lin e occurs sh oreward to th e sh elf edge.
In th e surro un din g wa ter co lu m n several
o th e r g ra d ie n ts cha rac te r ise th e u p per bath yal in
26
Lower BATHYAL
me Bathya l Zo ne (drawn after data from var-
th e northwestern Mediterran ean Sea (fig ure 2) :
low an n ual tempe rature var iatio n (abo u t 2 DC)
wh ich be co m es co ns tant beyon d a bo ut 200 m
d e p th (th e Medi terranean h omo th e rm y) , low
salinity increase, and sharp decrease of oAJ'gen
whi ch, h oweve r, is kn own to be hi g h e r tha n b iological consump tion . T h e lowe r b oun dar y co rrespon ds to th e lo we r lim it of the fro n tal structure,
across whic h the hydrol og ical p ro pe r ties ch a ng e
sh a r p ly ( H u a n g a n d Su , 199 1), a n d to th e beginning of th e oxyge n m ini mu m laye r (O ML) wh ich
is related [ 0 the Levan tin e intermed iate wa ter
m ass with a salinity m ax imu m an d h o m o th er my
(fig u re 2) .
T he C jN ra tio in th e sed irne n ts dec re ases
fro m 12 to 10 (Carp ine, J970) . T h e ch a nge o f
sediment oc cur in some tens o f meters, related to
the hyd rodyn amics, at bo th lim its of the U p pe r
Bathyal (Em ig, 1989a; E mig a nd Garcia-Carrascosa,
199 J ) .
Publ. Espec Inst. Esp. Oceanogr 23. 199 7: 23-33
c. c. emig
Geological characteristics
T he Recent imprin t, since th e last rise in se a
level (the Wurrn glacia tio n , 17 000 yea rs), is cle arl y
evide nt in the u pper bathyal : th e continen tal shelf
bre ak lies a t the outer edge of th e deposits forming
a n arrow progradin g we dge or pri sm lo cated at th e
ou ter p art of th e con tin en tal shelf (figure 1). T he .
shelf edge is in equilib rium wi th th e present en vironm ental co n d itio ns ; its progra da tion d uring the
Late H ol ocene h as been n egligible and its d epth
coincides wi th a «d e p ositio n a l e quilibr iu m » level
(Mougeno t, Boillot a n d .J. P. Rehault, 19 83;
Mon a co et al., 1990; Co u r p and Monaco , 1990 ;
Savoye a n d Pipe r, 1993). T he relative abse nce of
Recen t sed im e nt sup ply in the upper bathyaJ (figure 1) sh ows that se d im e nts are mainly re wor ke d
relict (Wu rrnian) sed imen rs with th e OCCLU'fen Ce of
Q uate r nary thanatocoenoses visib le b y submarine
o bservation and ge n erally lo ca ted a t a d epth of
betwee n 180 to 20 0 m (Gau tie r and Pi card , 1957 ;
Blan c, 1968; Em ig, 1987 ) .
Biological characteristics
The circali ttoral ex te n ds d own to the lower limit
of m ulti cellular algae (Pere s and P icard, 1964) :
from su b m arin e obse r vations of th e co astal d etritic
bioco enosis (CD) , ca lca reou s red a lgae do n ot
exte n d beyond th e shelf-break and, afte r a sho r t
tr ansitional zo ne of some tens of m e te rs, the
bath yal d e tritic san d biocoenosis (BDS) be gins, or
at th e sides of su b marin e ca nyo ns, tbe shelf-edge
d etritic bioc oenosis (SD) (figure 1). The SD h as
be en traditional ly conside re d a circalit to ral biocoen os is, som etimes co n fuse d with the BDS, but
bel on gs with out an y do ub t to th e upper bathyal
(La ubier an d Em ig, 1993). T he co ntine ntal shelf
e dge corresp onds to th e upper limi t of th e di stribu tion of a t least two excl usive u p p er bathyal
sp ec ie s, th e brachiopod Gryphus vitreus (Born,
1778) in the BDS an d the crinoid Leptornetra phalangiurn (Mu lle r, 1841) in the SD, wh ile the mud
line corre sponds to the lower limit o f their exten SIOn .
The su bstratum of the BD S is well-sorted san d
(gravel, co arse and fine san d ), clogged by a fine
fractio n wh ich can reach 60 %, an d co ntai ns a large
d etritic p ro portion of small h ard su bstrates of
en dogeno us origin (fragments of mollu sc a n d brach io pod sh ells, of sp onges, b ryozo ans, co rals, grav-
Publ. Espec Inst. &p. Oceanogr: 23. 1 99 7: 23-33
M editerran ean bathyal zones
el s an d p eb bles), a cha racter istic of the BD S bottom (Falconetti, 1980; Emig, 1989a, 1989b ) . The
su bstratum of th e SD is sand y mud (gravel , sand,
mud ) (P ica rd, 1965; Emig, unp ubli sh ed data).
On m uddy substr ata, th e tr a nsiti on from the circalitto raJ m uddy d e tritic (MD) and terrigenous
mud shelf (TMS) biocoeno ses to an upper bathyal
mud b iocoenosis (UBM) remains hypothetical,
du e to lack of research. However, fr om the data of
G uille (1970) , Picard (197 1) , Salc n-Picard (1982) ,
Al ber te lli, Dell a Croce an d Fraschetti (199 1) and
Albertelli and Fraschetti (1992), one may argue
th at there is a faunistic change a t the level of th e
shelf e dge, with the ap p earance of dom in ant
species , p articularly the op h iu r id Amphiura fili[ormis (Muller, 1776), reaching 8-34 % of th e fauna,
acco rd ing to Sale n-Pica rd (1982) and Alb e r te lli,
De lla Croce an d Frasche tti (19 91) , and th e p olychae te Maldane glebifex Grube, 1860, wh ile th e characteris tic sp e cies from both circalittoral MD and
TMS bio coenoses are abse n t in th e u p pe r bathyal.
In th is h yp othetical UB M, so me species can reach
high densiti es in so me areas, i.e ., the echinode rrn
Brissopsis lyrifera Fo rbes, 1841 , the p e nnatulari a
Fu n icula quadrangularis (P allas, 1766) an d the
sponge T hen ea muricata (Bowerban k, 1858).
Within th e upper bathyal bio coeno ses, the Iaun istic d istribution deve lo ps in to belts re la te d to th e
prevaili n g fa ctors (hydrodynamics an d th e related
sedimentary cond itions). Consequen tly, the se bel ts
are main ly suspe n sio n-fe e d e r o nes. The direction
an d velo ci ty of th e bOtt0111 currents crea te liv e distribu tional zo nes in the BDS (tile h ighest velocity
zon e is re ported in figure 1) , i.e ., th e di stributional zo ne s of Gryphus uitreus, whi ch can reach up to
700 individuals.z m ? (Em ig, 1987 , 1989a , 1989b;
Emig a nd Arna ud, 1988; Emig an d GarciaCarrascosa, 199 1) or the d istributio n belt (between
100 to 140-150 m ) of the ec h in odcrrn Neolampas rostellata Aga ssiz, 1869 re p o rted previously by
Fa lco ne tti (1980) as belonging to th e circa littoral.
In th e SD , the crinoid Leptomeira phalangiu m can
reach up to 30-50 in d ivid u a ls/ m ' (Reyss, 19 71 ) an d
in th e U BM Amphiura filifo,.,nis re aches lip to 28
in d ivid uals ymz (Albenelli and F ra sch e tti, 199 2) .
Su cb a b el t distribution explains the relative h eterogeneity of the fau na be yond tile sh elf-e dge, as
reporte d by previo us a utho rs, as well as th eir a rgumen ts for co nsi dering the upp er bathyal a transitio nal zo ne.
The upper bathyal biocoenoses seem to be charac te rise d by lo we r 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 su bstrate,
the lower the richn ess. H owever, the analysis of
recently p u bli shed d a ta by Albertelli, De lla Croce
and Frasch etti (1991) , Al bertelli and Fraschetti
(1992) an d Tse lep ides and Eleftherio u (1992)
from m u ddy sh elf-to-u p p er slo pe tr ansects sh ow
that the mac rofauna increases in spec ies and individ uals, mainly of su sp ension-fe ed ers a nd d etritusfe ede rs, in the up pe.r bathyal zone (figu re 3) .
Within the SD an d m iddle bathyal mud (MBM)
bio coe nose s o n the northern Mediterranean coast,
there is a geograph ic decrease in r ich ness fro m the
west to the east (Ca rp in e, 197 0) , wh ich n ee d s confirm ation because o pposite to the cyclonic circulation.
Th e extension of circalittoral speci es into the
upp er bathyal occurs in general d own to a depth of
130-140 m , where th eir p re se n ce is prob ably limited by abio tic factors, i.e. , ligh t, p ress u re or/ and
hydrodynamics, particularly for polychaetes an d
echinoderrns, wh ile the ex tens ion of 1vfBM species
in to th e u pper bathya l is limited by the sub str ate, at
leas t in th e SD and BDS. N ever theless, species
which occur at the same time in the circalittoral
and b a th yal are generally co nsidered as circ alittoral
sp ec ies which ex te n d into the bathyal, rare ly th e
contrary. For example, several brachi op od species,
i.e., Megerlia truncata (Linnaeus, 1767) a nd Megathiris
detru n cata (Gmelin , 1790) , co nsidered resp ectively as
a sh allow-water species and an eurybathic species
(Logan, 1979) , h ave their maximum density in
the upp er bath yal, be twee n 100 an d 150 m (Em ig,
19S8).
Phytal-aphytal boundary
T his boundary can be estimated by the percentage of the Gryphus vi/reus shells infested with th e
green alga Ostreobiu m sp . Bornet and Flahau t.
Occurring generally within th e upp e r bathyal zo ne,
i ts dep th varies in relation to the water transparency, i.e ., betwee n LSO an d 210 m in normal water
co nditi ons in Corsica, but it can be restricted to
125-135 m u nder turbid wa ter con d itions (Emig ,
19S9c ). Along the P rove n cal coast, be can se o n ly a
small number of shells is infested with Ostreobium
sp. , this boundary is estima te d at abou t 150 m
de pth . Such resu lts in d icat e that the phytal -ap hytal
boundary is inde pendent of benthic zo natio n , and
that its depth can flu ctu a te eve n with in a restricte d
28
Mediterranean balh)'al zones
geographic area, contradicting Bella n-San tini ' s
(19S3) statemen t that this boundary lies at th e
lower limi t of the circalittoral zone, as well as the
definition of the lower limi t of the transitional zone
proposed by Ercegovic (1957) and Peres and
Picard (1964) .
Middle bathyal zone
There is a rapid transitio n over several tens of
mete rs from the upper bathyal zone to the middle
bathyal zo ne, i.e., fro m the u p p er bathyal b iocoenoses to the MBM . T he su bstrate becomes
m ud dy, with th e a ppearance of burrowin gs havin g
various diameters (Em ig and Arnau d , 19S8; Emig
an d Carcia-Carrascosa, 1991) . O ver the co n tinen tal slope in the middle bathyal , with in creasin g
d e p th, a ge neral decrease in the size of the m acrobenthos is recorded (Carpine, 1970; Peres , 1985;
Sar d a an d Cartes, 1993; Stefan escu and Cartes,
1992) , as well as a quanti ta tive decrease in th e ma crobenthos (fig ure 3) and an in crease in d ivers ity
related to increased en vironmental stability, associated wi th in cre asin g d e p th (Sanders, 1965; Sa n ders
and Hessler, 1969; H aed rich , Rowe and Po l1on i,
19S0; Ab ell6, Vall ad are s and Ca ste1l6n , 19S8) .
Nevertheless, fo ur of th e five grenadier fish specie s
occurring in the m id d le bathyal fo llow th e biggerdeeper p h e n om e n on (Massu ti, Morales-Nin and
Stefanescu, 1995).
There are several faunistic stock changes within
th e mi d dle b athyal zone, which consequen tly can
be divided in to severa l d istributional zones (figure
4) , based m ai nly on recent data from crustacean
and fish distribution . However, these zo nes can be
rel ated to several previous proposals, su ch as th a t
of Per es (19S5) , who d ivided th e up p er slope into
two zones below the transitio nal one: fro m 200 to
500 m a nd 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 le
bathyal , 550-2000 m ; an d lower bathyal, I 5003000 m .
The cr us ta cean assemblages occurring o n the
up p er an d on th e lower- middle slo pe correspond
to hi ghly d iffere n tia te d faun is tic communities,
whose d ifferences can be a ttr ibu te d to hyd r ol ogi ca l
difference s. The upper community is associated
with the warmer intermediate waters, and the
lower zone with the deep -water mass, which has a
fairly co nstan t te m p e rature throughout the year
Pubt. Espec. Inst. Esp. Oceanogr 23. 1997: 23-33
""
'"
'"'"
~
:--;'l
:g
~
to
,.,
~
g
~
il
Cl
"
1;1
~
~
~
."
t;;
?t:
;p
200
I
e;j
200+10
I
0 /I
Depth in m
'.
I
I
0---
400
\
\,
\,
0
600
.,-800
~~~ - - - ~ - ~ - ~ - ~ ~~
<:>-- -OOi OlllllS$
o---a M ean abundance
i
0.2
0.4
CS)
1200
1400
1600
------------- ---------
Biornass dryWt glm2
1000
e---e Numb er IIf s p~cic slm~ ~ wi th biornass
0---0 Mean num ber of species/O. ! 10:
' K_~
400 +20
600 + 30
800 +40
50
ji 8 ~ species
1800
2 000
Figure 3. (A): Abu n da nce , species rich n ess a n d biom ass of the macro fauu a on the muddy bo ttom s in the shelf-to-slope sector of th e G ulf o f Ge noa (da m fro m AJbc n elli, Dclla
Croce an d Frasch eu.i, I99 J; AJbene lli an d Frasch eu.i, 1992) . (8 ) ; On a slo pe tran sect in th e Cre ta n Sea (Tsele pid cs an d Eleftheriou , J992) . (C): Biomass o f d em er sal fish co mmuni ties o n. the slope of Catalo nia (da ta fro m Stefa nescu , Llo ris an d Ruca bad o , 1993 ) .
<
.c.
~
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1 200 ru u~
~§ 1000
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..-
I ,...
~
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C. Emig
Mediterranean. bathj'al zones
CIRCALl TTORAL
lOO
Shelf brea k
160
120
UPPER BATHYAL
250
Upper-middle slope
480
510
650
800
Lower-middle slope
....I
<
>
J:
I-
<
m
1200
W
....I
0
140 0
9
:E
Lower slope
1000
I<Rise / plain"
2200
LOWER BATHYAL
Figure 4. Diag ram o f bath yal zona tion along the
Medi terranean co ntinental slope, with the main faunistic
changes and boundaries.
(Abe Il6, Valladares and Cas te1l6n, 1988 ) . T he
greatest abundance o f mysids seems to occu r o n
th e mid d le slo pe (Cartes an d Sorbe, 199 5) . The
populati o ns of Aristeus antennatus (Risso, 1816) can
be d ivid ed into seaso nal gr ou ps on th e up per-m idd le slope, but such gro u ps are less d efined in th e
lowe r-m idd le slo pe , and th e se aso na l effect
becom es very sligh t o n the lower slo pe (Cartes,
1994).
Acco rding to d ata from Alberielli a nd Ca tta ne o
(1985) and Albertelli , Della Cro ce and Fraschetti
30
(199 1) , on mud dy substrates in th e Gulf of Ge n ova
d eposit-feed e rs re prese n t resp ectively 63 % in th e
u p per bathyal an d 37 % in th e u p per-m iddle slo pe
of th e middl e bath yal, wh ile suspens ion-feed ers
represent respectively 14 and 7%, and predators 20
an d 5 7 %. Along a Cretan tran sect (Tse lep ides and
Elef th erio u, 1992), po lychaete d eposit-feed ers are
dom inant between 200-400 m (more th an 50 % )
and carnivores (more th an 49 %) be low 500 m . In
the Ca talan Sea, th e u pper-m iddle slo pe co m m unity (245-485 m ) shows a predom ina nc e of
m esopelagic species (Cartes, So rbe and Sarda,
1994); th e fish asse mblage o n the lower-m iddl e
slo pe, between 1000 and 1 425 m , is d o min ated by
Lepidion. lepidion (Risso , 1810) with five subdom inant species (Stefanescu, Llo ris and Ruca bado,
1993) .
So me au thors (Abcllo , Val lad ares and Cas te llo n,
1988; Mura an d Ca u, 1994) indicate a change in
th e d ecapod faun a betw een 65 0 a nd 800 m.
Acco rd ing to Sarda an d Ca rtes (1993) , the d iet of
these d ecapods co nsists lar gely of in£a una or epifa u na, an d there are d ifferences in th e co m positio n of th e di et above an d beyo nd this limit.
On th e lower slo pe , the re is evidence of fau ni stic changes, as well as drastic changes in the
amount of available energy and a sharp re d uction
of th e available trophic reso urces related to the d istribu tio n of m esopelagic o rgan isms and hyd rological charac teristics (Cartes, 1993; Stefanescu , Llori s
and Ru cabado, 1993; Sarda an d Cartes , 1993) _The
fauna is d o minated by d emersal fish species with
low e ne rgy cos t, and sm all e pibe ruhic and benthopelagic inve rteb rates (Stefanescu, Lloris and
Rucabad o, 1993) . T he a bse nce o f mesopelagic
o rga nisms below 1 000-1 200 m is reflected by an
im poverish m ent of th e benth ic ecosystem. Cartes
(1993) indi ca tes th at one o f th e most impo rtant
preys, th e deca po d Calocasis macandreae Bell, 1846,
d isap pea rs below "[ 000- 1200 m (figure 3). In de mersal fish asse mblages there is an abrup t d ecrease
in values and a su bse q ue nt hom ogene ity at greater
d e pths (figure 3); th e asse mblage is d ontinated by
Bathsp tetois mediterraneus Baucho t, 196 2, with two
su bdom inan t species (Ste fa nesc u, Ll oris an d
Ruca bado, 1993) _
Canyon valleys
The can yo n valleys ar e subj ect to spec ific abi otic co nditio ns, and their macrofauna co m position is
Publ: Espec. Ins t. Esp. Oceanogr. 23. 1997: 23-33
c. C. Emig
too poorly d ocumented to all ow com par iso n (see
Monaco et al., 1990; Bo urcier, Stora an d Cerino,
1993; Gerino et aI., 1995) . T hese valleys are geomo rph ol ogically important in concen tratin g and
transp or ting se dime n t to d eepe r areas (Mo naco et
al., 1990), with h igh dynamic water circulation
(Mille t, 1990). T heir biol ogica l im p ortance is du e
to the input of b igh prod uctivity with abundant .
re sources, and the occur rence of recru itm ent areas
co n ce n tr atin g juve nil es of certain species, p articularly in th e middle ba th yal zo n e. Fa un a b iom ass
a n d ab un dance are higher than those re corded o n
th e sur rou n d ing slope areas.
DISCU SSION
T he zone h erein defined as th e upper bathyal
",..a s previo usly considered a transitional zon e. no t
only in th e Mediterranean but in the ""orld O cean
(se e Zenkevitch, 1963; Peres, 1982; Blake and
Doyle , 1983; Zez in a, 1994 ) . However, thi s zo ne
be gin s at the con tinen tal sh e lf-edge (geomorphologi ca l sh e lf-break) which is ch aracte r ised b y bi ologi cal, physical a n d ge ological expression, and
beyo nd which th e upper bathyal d evelop s as a b el t
whose b athyrnetric exte n sion is di re ctl y rela te d to
slo p e physiograph y (figure 1) . Con se q ue n tly,
future stu d ies of bathyal fa u na shou ld always indica te th e slo pe profile from whi ch the d a ta h ave
b e en re por te d; rhi s is also valid for th e middle
bathya]. In the bathyal , th e major chan ge in fa una
is directly associa te d with major e nviron m e ntal
chan ges, wh ich pl ay a key role in d eter min a tio n of
the fauna ac ross the shelf-break. T h e biological
ex te nsion alo ng the slope is gi ven by th e bioco e noses occu rring in th e u pper b a th yal , dominated by hi gh d ensities of ex clusive su spen sio n -fee de rs
wh os e distributional belts sho uld provide information regar d ing flo w str uctures in th e shelf-to-slo pe
co n text.
The vali d ity of th e bathyal biocoenoses needs to
be esta blish e d with n ew d at a, usin g fa u nistic den sity
and bio m ass an d th e effe cts of almos t abi otic characters, to d efine cir ca litto ral and b aihyal sp ec ie s
and the exclusive sp e cies of a biocoenosis, whi ch
can only be stenotop ic sp e cies (Ar n a ud and Emig,
1987). T he dis tr ibutional zo n es of fauna de termined b y abi otic fa ctors n ee d a careful bathyrnetric
and sp atial choice of sampling alon g th e slope.
In th e Atlan tic , in fa u n al density incre ases in the
u ppe r m ost slo pe zo ne (Blake and Doyle, 1983).
Publ. Espec: Inst. Esp. Oceonogr: 23. I 997: 23-33
Mediterra nean bathyal zones
T h is also seem s to be the ca se in th e Me d iter ra nean uppe r bathyal, as su gge sted by th e d at a
of Alb er te lli an d Cattaneo (198 5) , Albertelli, Dclla
Croce an d Frasch e tti (.1 991) , Alberte lli and
Frasch e tti (.1 992) and Tse le pid es and Eleftherio u
(1992) on mudd y bottoms (fig u re 3) . H owever, th e
sp ecies and individ ua l ri chness glo ba lly decreases
from circalittoral to bathyal in. the Mediterranean,
as a lso established by Sh errnan et aL (.1988) in the
n ortheast Atlantic for the d ensity and biornass of
four m ajor ta xonomic groups.
Th e distributional zones in th e Medite rranean
mi d dle b athyal (figure 4) can be com pared with
th e d epth boundar y on th e Atlan tic co n tin ental
slo pe at 400, 700 an d 1200 m , wh ich a re con sid ered
n early un ive rsal by Hecker (1990) . Nevertheless ,
this author indica tes th e fo llo wing zo n es : upper
slo pe 180-600 m , upp e r-middle slo pe 60 0-1 300 m ,
lower-mi ddle slop e 1400-1 60 0 m , lower slo pe .1 6002000 m , an d a con tin e n tal rise fr om a bou t 2000 m .
Acco r ding to Van n ey an d Stan ley (.1 983 ), the
sh elf-ed ge ap p ears a s a boun d ary of importan ce
co mparable to the coastline , se par a ting the coastalshelf environmen t from the deep realms; it is the
point wh e re th e first maj o r change occurs in the
p hysical gradie n t, and co nsequ en tly, in th e bio logical g ra d ien t, wh en entering the deep se a . T h is
u p p er lim it of th e d eep-se a zone, the bathyal,
dependin g bathym etrically o n th e de p th of th e
sh elf-edge, is expresse d by biological , physical and
geomorph ol ogi cal criter ia wh ich have to coin cid e
with eac h other and with th e lower boundar y of the
circalittoral zo ne.
T h e tr ansi tion from the upper bathyal to the
middle bathyal, whose bathym etric lim it depends
o n loc al environmental factors govern e d by physiography, al so follows su ch multidisciplinary criteri a. It co r respon ds in genera l to the mud lin e ,
lo cated between ab o u t 165 and 300 111, depending
o n th e slo p e physiograph y, in the n orthwest
Med ite r r a n ean, where this line al so in d icates a
d rastic ch a nge in th e en ergy leve l, as reported in
other ocean areas by Stan ley, A ddy and Behrens
(1983) .
As a result of stee p gra d ie n ts in physical con di tions, the upper bathyal zo ne re p rese n ts a n unde re stimated barrier fr o m shelf to d e ep sea, occu r rin g
in the World O ce an. In the Me diterran e an, su ch a
p hysical barrier m ay partly explain th e sh arp fauna
d e crea se in th e middle bathyal and the bi ol ogical
pove r ty of the Med ite r ranea n deep sea. On the
o the r hand, a lo n g-botto m tran sport p assing
31
c.
C. E1I1ig
through the canyon axes a p pear as p a th ways across
th e sh e lf break to th e Mediterranean deep sea
(Courp and Monaco, 1990) .
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