Composition and variability of downward particulate matter fluxes in the Palamós submarine canyon (NW Mediterranean)

Abstract

To study the temporal and spatial variability of downward particle fluxes in the Palamós Submarine Canyon, seven sediment traps were moored inside and in the vicinity of the canyon from March to November 2001. Total mass fluxes, major constituent (organic carbon, opal, calcium carbonate and lithogenics) contents and fluxes, and 210 Pb activity of particulate matter were obtained from two consecutive deployments at intervals of 10 and 12 days respectively. Downward particle fluxes measured at the Palamós Canyon head were 2 to 9 times higher than those measured in other northwestern Mediterranean canyons, and this relation increases drastically at 1200 m depth, where observed particle fluxes are 1 to 2 orders of magnitude higher than those reported in other surrounding canyons at similar depths. The highest near-bottom downward particle fluxes were not recorded in the canyon head but in the mid-canyon axis during late spring/summer, as a result of sediment gravity flows triggered by trawling activities at the canyon rims. In comparison to the adjacent open slope, Palamós Canyon is a prime site for the focusing and across-margin transference of total and organic matter. Off-shelf sediment transport was enhanced during a severe storm in November 2001, when a sharp increase in downward particulate fluxes was observed in the whole canyon both near the bottom and at intermediate waters. Despite the dominance of lithogenic particles all year round, a siliceous bloom affected the whole study area in March-April. An asymmetrical pattern was observed between the north and south canyon walls, with higher mean downward fluxes in the latter case, a fact related to the flow regime inside the canyon. The spatial-temporal distribution of total mass fluxes and major constituents defined two domains in the Palamós Canyon: an "inner" domain (up to 1200 m depth) constricted by the canyon topography and mainly influenced by a lateral transport of particles resuspended from the adjoining shelf and upper slope, and an "outer" domain, where slope dynamics and seasonal trends are more important in determining the composition and amount of downward particulate fluxes.

Chronic and intensive bottom trawling impairs deep-sea biodiversity and ecosystem functioning

“…Data from near-bottom current meter deployments within La Fonera Canyon showed a closed circulation inside the canyon confinement with low velocities (<20 cm/s) oriented along the local bathymetry and showing frequent current reversals at inertial frequencies (40,42). However, those measurements and further monitoring studies revealed that the near-bottom water turbidity and the sediment dynamics in this submarine canyon is heavily dominated, both in its magnitude and temporal patterns, by trawling-induced sediment resuspension at the fishing ground (17)(18)(19)(40)(41)(42). The local trawling fleet in this submarine canyon targets the deep-sea shrimp Aristeus antennatus (Risso, 1816) and operates on a daily basis (with interruption only during the weekend) and year round, down to ca.…”

“…The general circulation is governed by a baroclinic current along the continental slope toward the southwest, in geostrophic balance with the density front (40). The direction of the prevailing currents in the area means that a substantial part of the along-margin sediment transport bypasses the canyon head and enters the canyon directly from the northern canyon flank (41). Data from near-bottom current meter deployments within La Fonera Canyon showed a closed circulation inside the canyon confinement with low velocities (<20 cm/s) oriented along the local bathymetry and showing frequent current reversals at inertial frequencies (40,42).…”

“…The two otterboards are heavy (from hundreds of kilograms to tons) and can imprint furrows in the sediment with the resulting sediment resuspension. The sweeplines and the footrope also cause sediment resuspension (41). In La Fonera submarine canyon, there are two main areas that are routinely visited: the Sant Sebastià fishing ground along the , and maturity index (C).…”

Flux and composition of settling particles across the continental margin of the Gulf of Lion: the role of dense shelf water cascading

“…That suggests a continuous and variable export of material from the euphotic zone. However, in continental slopes, where TMF values are 3 orders of magnitude higher, the pelagic flux would represent a minimum part Martín et al, 2006;Zúñiga et al, 2009). Heussner et al (2006) reviewed this issue in the GoL area and differentiated between the lateral (resuspended plus rebound) and the pelagic component of the POC flux.…”

“…In those environments, settling material is subjected to biological, physical and chemical processes among which the remobilization basinwards supposes the principal way of shelf erosion. Actually, experiments carried out in the Western Mediterranean Sea show differences of three orders of magnitude when comparing settling particles in the slope with the open basin, underlining the importance of continental shelves as a source of material for the adjoining slope environment Martín et al, 2006;Zúñiga et al, 2009). Suspended particles on the continental shelf are mainly advected to the slope through intermediate or bottom nepheloid layers (Durrieu de Madron and Panouse, 1996), resulting in larger particulate fluxes closer to the slope bottom Heussner et al, 1999Heussner et al, , 2006.…”