SUBMARINE CANYONS AND OFFSHORE ADVECTION OF NERITIC PLANKTON

Increased offshore advection of migratory neritic plankton may occur b y the pathway of nearshore submarine canyons. The nocturnal distributi on of migratory bentho-hyponeustonic crustacea was investigated in the axis and on both sides of three submarine canyons, over the continent al shelf and slope of the northwestern Mediterranean. The three canyon s arc frequently exposed to strong northerly and northwesterly winds b lowing seaward. A horizontal surface haul and a vertical haul from bot tom to surface were successively achieved with an OMORI plankton net o ver each depth, from the isobath 50 m to more than 1000 m, in the midd le of the night. The Mysidacea Anchialina agilis, Haplostylus lobatus, Siriella clausii, Siriella norvegica and Leptomysis gracilis, and the Isopoda Eurydice truncata dispersed offshore along the axis of the ca nyons, far from the limits of their benthic habitat. In the three cany ons, an accumulation of animals occurred over the top of the talweg (b ottom line of a canyon), above 300 and 500 m. Animals were scarce on t he shelf edge (200 m), above the lateral sides of the canyons. This ac cumulation in the canyons probably results from passive transport that depends on both the specific current regime of the nearshore canyons and the local hydrodynamism. The usual current regime in canyons consi sts of up and down flow near the floor with a prevalence of the down-c anyon current. In the provencal canyons, the normal down-canyon curren t is strengthened by a refluxing current of the littoral vein of the E W Liguro-Provencal and, by strong northerly winds, by a discharge cur- rent. The prevalence of the down-canyon current tends to prevent anima ls transferred offshore at night from being carried back over the cont inental shelf during the day. In a northerly and northwesterly wind re gime, the landward subsuperficial compensation current can induce an i nshore transfer. Nevertheless, this transfer occurs solely during the morning and evening vertical migrations of the animals; it cannot comp ensate for the nocturnal surficial and diurnal near-bottom offshore tr ansfers. The determination of the energetic balance of these shelf-mar gin transfers depends on accurate knowledge of: a) the local hydrodyna mic conditions; b) the physiology, behaviour and survival ability of t he benthic neritic species subject to pelagic conditions or strong bat hyal hydrostatic pressures; c) the precise offshore diurnal vertical d istribution of the species within the water column.