Quantitative distribution of deep-sea meiobenthos in the northwestern Mediterranean (Gulf of Lions)

Abstract

The distribution of metazoan meiofauna abundances, in relation to environmental factors and “food indicators”, was examined in 29 surface sediment samples taken between 672 and 2367 m in five canyons on the northwestern Mediterranean margin in summer and autumn 1986. Almost all the parameters measured decreased significantly, with increasing water depth. Meiofauna densities were dominated by nematodes (92.4%), and ranged from 36 to 1005 individuals 10 cm −2. They were positively correlated with sediment water content (30–70%), organic carbon (2.7–7.8 mg g −1) and nitrogen (0.3–1.1 mg g −1) concentrations, chloroplastic pigment levels (0.35–8.29 μg g −1) as well as viable bacterial counts [9 × 10 −3 −3 × 10 −5 Colony Forming Units (CFU) ml −1], and microbial total uptake (5.8–50.6% after 4 h) and respiration (2.3–19.0%) of 14C-glucose. They were only poorly or were not correlated with sediment granulometry (56.7–97.0% < 40 μm), total microbial uptake (7.1–51.0% after 4 h) or respiration (2.0–24.0%) of 14C-glutamic acid. Principal Component Analysis was used to describe and evaluate associations of parameters and samples. The quantitative distribution of meiofauna along the margin is well reflected by the distribution of “food indicators”; this can be explained by “distance from land masses” and “water depth”. In the Gulf of Lions, meiofaunal abundance decreases more rapidly with increasing depth than in other oceans, because the margin includes two different systems: (1) the upper slope, which is comparatively rich and active due to continental and coastal (high primary productivity) influence, related to rivers that induce downslope transport of organic-rich fine sediment; (2) the lower slope and basin mainly fuelled by the low offshore productivity, combined with relative high degradation rates in the warm (13°C from 100 m, to the seabed) waters. The comparison of summer and autumn samples revealed the existence of temporal variability (meiofauna abundances doubled from summer to autumn over the entire area), related to seasonal inputs of organic matter into the benthos.