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Biogeochemical and physical drivers of deep European margin ecosystems

Final Activity Report Summary - BIODEM (Biogeochemical and physical drivers of deep European margin ecosystems)

Ocean margins are areas of special interest in the field of biogeochemical processes governing exchanges between the continent and the open ocean, and specially those of the carbon cycle. Also, owing to their high biodiversity, ocean margin biota represents an enormous genetic resource which we are only just beginning to appreciate. Major questions about the functioning of marine ecosystems on the continental margin concern their interrelationship/dependence on the natural drivers (physical and biogeochemical processes), as they are strongly physically mediated and potentially affected by extreme perturbations.

The project BIODEM aimed to understand the biogeochemical and physical drivers of European deep margin ecosystems. Based on a detailed characterisation of hydrological (speed and direction of bottom currents, turbulence and mixing), geological (vertical and lateral particle fluxes, mass accumulation rates) and geochemical (organic matter quality, grain size) forcing conditions, the project aimed at investigate i) what are the physical factors controlling marine ecosystems, ii) what are the biogeochemical factors controlling marine ecosystems, and iii) how do these natural drivers influence biological communities.

Results obtained by BIODEM add to the growing evidence that physical exchange processes such as dense shelf water cascading (i.e. cooling and sinking of shelf waters that cascade through the submarine canyons following a flushing pattern) are responsible for a large transfer of particulate matter from the continental shelf of the Gulf of Lion to the deep Mediterranean basin. Indeed, strong temperature and current speed anomalies recorded in winter 2005-06 in the Western Gulf of Lions were the signal of the occurrence of a major 4-month length cascading event down to the rise, with current speed maxima recorded at the onset (early January, velocities up to 70 cm s-1) and at the end (March) of the event. Both cascading pulses of dense shelf water coincided with maxima total mass fluxes and sand content recorded by the sediment traps, highlight the ability of cascading waters to erode and transport large amounts of coarse sediments downslope.

However, each cascading pulse resuspended and transported organic matter of different quality (as a measure of source and degradation state). The first pulse of organic matter showed low delta13COC and N/C and high delta15N values, indicative of the arrival of organic matter of terrestrial origin. In contrast, the second pulse showed high delta13COC and N/C and extremely low delta15N values, indicative of the transfer of organic matter with a higher marine contribution. Overall, results suggest that dense shelf water cascading resuspend and transport to the deep Mediterranean basin organic matter of different origin and introduced to the system by external (river discharge) or internal (primary production) forcings, which in turn might alter the quality of the sedimentary organic matter deposited on the sea floor within a couple of months of difference. The abrupt change in the quantity and the quality of particulate organic matter along with the physical and chemical conditions may have immediate impact on the benthic fauna and bacterial assemblages, which may respond distinctly depending on their ecology and feeding habits.