Bathymetric changes in the distributions of particulate organic matter and associated fauna along a deep-sea transect down the catalan sea slope (Northwestern Mediterranean)

In March 1998 (BBD1) and September 1998 (BBD2) two oceanographic cruises were undertaken in the Catalan Sea (North-western Mediterranean) during which a transect was sampled down a bathymetric gradient encompassing depths of ~189 m to ~1645 m. There were five stations along the transect nominally at depths of ~200 m (Station 1), 400 m (Station 2), 600 m (Station 3), 1200 m (Station 4) and 1600 m (Station 5). At each station the main physical parameters of the water column were measured and the sediments were sampled to assess their biochemical characteristics and the abundances and demographic structure of the macro- and meiobenthos. During March, the whole water column was homogeneous for both temperature and salinity, whereas in September it was thermally stratified, as is typical of summer conditions. Fluorescence was measured throughout the water column while profiling with a CTD, and the primary production (PPeu) was deduced from satellite imagery. Both fluorescence and primary production were higher in March (average PPeu: 181 gC m−2) than in September 1998 (average PPeu: 102.8 gC m−2). Also during March, the organic carbon and enzymatically hydrolizable amino acids (EHAA) contents of the sediments were found to increase with depth, and a peak in lipid concentrations was observed at one of the deeper stations (Station 4: 0.254 μg mgDW−1). During September, organic carbon, lipid and amino acid contents and the C/N ratio, all decreased with depth, except at St. 5. Overall concentrations of most sedimentary organic compounds were higher in March than in September, indicating a greater availability of food to the bathyal communities in spring. Total abundances of meiobenthos and of copepod nauplii were positively correlated with lipid concentrations, but not with any of the other organic parameters measured. The abundances of near-bottom zooplankton and suprabenthos (vagile macrobenthos) showed no significant correlations with any of the properties of the sedimentary organic content. However, at the level of population structure, during March abundances of Furcilia and Calyptopis larval stages of euphausiids were higher, and recruitment peaked in several of the deposit-feeding species (e.g. the cumaceans Cyclaspis longicaudata and Platysympus typicus, and the amphipod Scopelocheirus hopei). In contrast, no recruitment peaks were detected for species potentially feeding on meiofauna (i.e. the isopod Munnopsurus atlanticus and the amphipods Bathymedon spp. and Rhachotropis caeca). Hence the only responses detected in the population structure of the benthic/benthopelagic fauna over the short time scales of this study were in recruitment patterns, and these appeared to be dependent on autoecological features such as the trophic guild of the individual species.