Analyses of the total organic carbon (TOC), total nitrogen (TN), C/N ratio and isotopic (δ13C and δ15N) composition of organic matter (OM) in surficial sediments, integrated with hydrographic measurements, have been used to evaluate the sources and distribution of sedimentary organic matter on the Catalonian shelf and slope (northwest Mediterranean Sea). We examined the effects of the variations on meiofauna. Settled OM in February, April and October displayed δ13C relatively closer to marine algae (mean −22.4 ‰ ± 0.1) than that in July (−23.3, −23.7 and −22.7 ‰ at 60, 600 and 800 m, respectively). Lowest δ13C (−23.7 ‰) and high C/N ( 10), indicative of mixed continental and marine organic contributions, were detected principally at the head of the Buscarró Canyon (at 600 m) in July, ca. 2–3 months after the maximum river discharge. We observed significant changes in total meiofaunal density among seasons in the canyon, with the highest values in July and October. On the adjacent slope at 800 m there was no significant increase of total meiofauna in those months. Although export of marine organic matter from surface layers was the main food source in the study area (on average 71.2 % estimating marine and terrestrial OC by binary models; 68–71 % at the most by SIAR models), advection of organic matter from the continental edge and/or resuspension of organic matter along submarine canyons may seasonally alter the composition of deposits on the slope, particularly in July with % OC of terrestrial origin reaching 48.7 % in Buscarró canyon, shifting the basal food-web of deep ecosystems. © 2015, The Oceanographic Society of Japan and Springer Japan.

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