Organic carbon budget at the sediment–water interface on the Gulf of Lions continental margin

Abstract

Oxygen uptake rates were measured at the sediment–water interface along a shelf-slope transect in the western Gulf of Lions (northwestern Mediterranean Sea). Values ranged from 0.6 to 48.0 mmol O 2 m −2 d −1, and decreased from inshore to the continental slope. The overall mean was 36.8±11.2 mmol O 2 m −2 d −1 for the coastal zone, 11.2±1.4 mmol O 2 m −2 d −1 for the shelf and 2.4±1.8 mmol O 2 m −2 d −1 for the slope. Slope stations exhibited significantly different values according to their location in the canyon axis (average 3.7±2.1 mmol O 2 m −2 d −1) or open slope sites (average 1.5±0.9 mmol O 2 m −2 d −1). Independent on the location, sediment oxygen uptake reached maximum rates in late spring, when fluxes were up to 4 times higher than in other seasons. Rates were significantly related ( r=0.92, n=16, p<0.001) to meiofauna density and consistent with the spatial patterns of bacterial potential respiration (Tholosan, 1999). This suggests that the rates of sediment oxygen uptake recorded during our experiment at least partially resulted from spatial and seasonal variability of the biological components (micro- and meiobenthos). In this study we attempt to construct an organic carbon (OC) budget for the different investigated sites, with the “input” term represented by the arrival of particulate organic carbon (POC) in proximity to the seafloor and the “output” term represented by the sum of the OC accumulated in the sediment plus the POC remineralized at the sediment–water interface. Input and output terms are approximately balanced at the slope stations, where, according to our estimation, about 50% of the deposited POC is accumulated in the sediment, while the remaining 50% is remineralized. Conversely, on the shelf, a lower percentage of the POC input (about 31%) is able to accumulate in the surficial sediment, while a higher proportion (about 69%) is remineralized at the sediment–water interface.