Bathyal Zones of the Mediterranean Continental Slope: an Attempt
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Publ: Espec. but. Esp. Oceanogr. 23. 1997: 23-33 P UBUCACIONES ESPECIALES L"lSTlTUTO ESP.I\NOL DE O CEANOGRAFIA ISSN; 021-1-7378 . ISBN: 81 ~19 1 -O 299-5 Ib Ministerio de Agriculrura, Pesca yAlimentacion , L997 Bathyal zones of the Mediterranean continental slope: An attempt c. C. Emig Centre d'Ocean ologie de Marseille (UMR-CNRS 6540) , Station Mari ne d 'Endoum e, Rue de la Batterie-des-Lions. 13007 Marseille, France. Received Febru ary 1996. A ccepted August 1 99 6. ABSTRACT On the con tine ntal slop e, th e bathyal can be divided into two zones, the upper bathya l and the middle bath yal, at the shelf break, which represents th e boun dary betwe en the coastal shelf environment an d the deep realm , located at about 100-110 m dep th. T he upper bathyal, previ ously considered a transitional zone, is characterised by distin ct physical, geological an d biol ogi cal features. Its bath ymen-ic extension is directly related to slope physiography, and its lower boun dary ge ne rally corresponds to the mud line. T his belt is governe d by specific abiotic factors with stee p physical grad ients (e.g., hydro dynam ics, salin ity, oxygen , temperat ure , sedirnen ts}. Major change in tbe benthic fauna is associated with major ch ange in these abiotic factors. The three main biocoeno ses are dominated by suspen sion-feed ing species, which are exclusive to th e Mediterran ean upper bathyal. Dep ending on water parameters, the limit between th e phytal and aphyta l systems gene ra lly occurs with in th e upper bathyal. The second zone, the middle bath yal, is poorl y docum en ted; however, it can be divided into several subzones whos e limi ts appear to be related to water-mass characteristics: the upper-middle slope , the lower-m iddle slope , an d th e lower slope .T he lower bathyal zone deve lops into th e bathyal rise and plain . Key words: Medi terranean Sea, con tin en tal slope, bathyal zonation, biological oceanograp hy, physical oceanography, geological oce anography. RESUMEN L os zonas btuiales del talud continental medi ternineo: un en say o En el talsui continental, el -piso bauai se puede dividi·r en banal superiory batial medio. Sn timue supe ri ar (100-110 m) corresponds al reborde del talud, suuada entre dos domisuos muy distintos: el de la plala [orma y el profundo. Aruerumnerue considerada coma «zona de tronsicum» entre etpiso circaluoral y et ba tial, el Ltuiol superior se caraaerua 1'01' sus particularidadesfisicas, geol6gicas y biol6gi.cas. 511, extension botimetrica dcpende de la Jisiografia deltalu d, qu e induce los efectos de los facnnes abuiticos y asi proooca una disnibucum. del beruos en d auurones con dominancia de Los suspensivoros }' la casi ausenciade sedi menuuion. El batial supenor comp rende generalmerue el limiieentre el sistema fital y el ajtal segttn la trams parencia 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 puede jJrojJoner un esqlJ,mna de la. umacuni del batiai media: el talud: medic superior, el talud media inferu n; etta lud. inf erior. Por tanto, el boliol inferior 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 Meduermnean baih)'at zones INTRODU CTION edge sector is a poorly known e nviron m ent because it lies between two distinct zones of inter The upper part of the Mediterran ean continen es t, the coastal-shelf environment and th e dee p tal slo pe was previously regarded as a «transitio nal».. realms (figu re 2). T hus, it is n ot surprising th at th e zone between th e circalittora l zone and the bathyal shelf-to-slo pe transition remains a biological and zo ne (Peres and Picard , 1964; Carpine, 1970; Reyss, physical «no man's land» (Vanney an d Stanley, 1974; Falcone tti, 1980; Emig, 1985; Peres, 1982; 1983). Bella n-Santini, 1983; Ab e1l6, Vallad ares and The Mediterranean shelf-to-slope morphology Castellon , 1988). All these au thors have noted th e on soft substrates presents two major types of pro importan ce of th e batbym etric depth range files (figure I ), on which the shelf break occurs at betwee n ] 0()..1 50 to 200-300 m, which generally about 100 to 120 m. H owever, in Type Il, th e co nti includes the limi t be tween th e phytal and aphytal nental sh elf break is ge ne rally co nsidered to be at a systems (Emig, 1989b ). Bu t th ey failed to take in to depth of abou t 150 m, which corresponds to th e account th e importance of abiotic facto rs, i.e., edge of a large an d flat bathyal offshore te rrace; slo pe physiography and physical characteristics, in but th e tr ue co ntinental shelf break , ofte n missed , order to exp lain th e biologi cal specificiry of this lies shoreward and co rresponds to th e edge of a zone (Emig, 1985, 1989a, 1989b) whose limits have short scarp of about 5--20 ID (figure 1) (Ernig, never been clear ly defined. H owever, Ercegovic 1989a, 1989b; Savoye and Piper, 1993) . (1957) an d Peres and Picard (1964) defined th e T he lower limit of th e u pper bathyal zone is extension of the «transitional» zone , from the in dicated by the mud line, wh ich is kn own to in di lower boundar y of th e circalitto ral , i.e., th e lower cate an abrupt change in the en vironmental condi limit of multicellular algae, down to th e lower limit tio ns (Blake an d Doyle, 1983) and serves as an of unicellular algae. Conse quently, the «true» energy-level marker (Stanley, Addy an d Behrens, bathyal zone was considere d to begin beyond th e 1983). Located a depth of between 160 and 300 m, transitional zone, be tween 200 and 500 m. O th er this limi t is rel ated directly to slo pe mo rphology authors use arbitrary (15 0 m ) or economical (200 (figure I ), which gove rns th e influence of the pre m) zone defin itio ns of th e upper lim it of the vailing abiotic factors: th e weake r the inclin e the bathyal, based on artificial considerations, never shallower in depth is this limit (Em ig, 1989a, scientific on es. 1989b). Thus, like th e upper limit of th e upper A review of the northwestern Mediterranean bathyal, th e lower limit does n ot co incide with a bathyal zone, with new information fro m sub bathymetric expression. mersible cam paigns along th e con tin ental slope, makes it possib le to reco nsider the position and impor tan ce of th is zone on th e co n tinen tal slope. Physical characteristics On the other hand, few faunistic surveys were pe r fo rm ed in the Mediterranean bathyal, and com The upper bathyal zo ne is a high -energy sector parisons of data an d results are difficu lt, becau se characte rised by grad ient variations of th e prevail th ey were obtained in different ways. Co nsequently, ing abiotic factors, i.e., hydrodynamics, salinity, n either local nor biogeographic comparisons can tem pera ture, oxygen, sedimen ts (figure 2) .In th e be made. Neverth eless, except th e upper bathyal entire northwestern basin there is a main current, zo ne, the o ther ba thyal zones are unlikely to be th e Liguro-Provencal-Catalan current, that follows d efined and characterised at present, although a the co n tinental shelf break and co inc ides with a ge neral tendency can be discerned . pe rmanent sh elf/ slope density front; thi s flow is in tensified by th e p revailing win d s and separates th e co n tinental fresh waters on the she lf fro m the Upper bathyal zone saltier an d usually warmer open-sea waters (Salat and Fo n t, 1987;Millot, 1987; Wang et al., 1988; T he upper lim it of th e bathyal zone lies at th e Emig, 1989a; Monaco et al., 1990; Fo n t, 1990; Emig con tin en tal shelf break, defined by physical, ge o an d Garcia-Carrascosa, 199 1;H uthnan ce, 1992). logical and biological characteristics (figures 1 and The near-bo ttom curren t, weak or abse nt on the 2). Although of primar y im p ortan ce, because of lower par t of th e contine ntal shelf, increases by sev maj or gradient change s, the Mediter ran ean sh elf- eral tens of meters at th e level of the shelf edge; its 24 PubL Espec. Inst. Esp. Oceanogr: 23. J997: 23-33 ;p [) 1': n ~ -G, X "~ r- ~ ~ ~ ~ ,,'b'",," J' .5' ~ ~ :? ~r ,,§' C;<#' l;: .... ;g :--;1 ec ~ Profile Type I Profile Type II I ~ il Figure 1. Block-diagrams of the bathyal biocoe noses d istribu tion along the uppe r con tine n tal slope sector accordin g to the slope gradient on th e two main profile types, 1 and ~ n, with the sides showing th e sedimen tation prism since the last Wiirm glaciatio n (hatch ing) .For profile location s (Provence and Corsica) and explanation see Emig (1989a, [ 1989b, 1989c), Em ig and Oarcta-Carrascosa (199 1).