Dynamics of suprabenthos off the Ebro Delta (Catalan Sea: western Mediterranean): Spatial 13 and temporal patterns and relationships with environmental factors

Dynamics of suprabenthos (hyperbenthos) composition and biomass have been simultaneously analyzed at two sites (S1, S2) off the Ebro River Delta (western Mediterranean). The stations, separated by ca. 5 km, differed in terms of depth (S1, 47 m; S2, 61 m), distance to the river mouth (S2 south of S1 and farther from the mouth) and fishing activity (S1 is a fishing ground; S2 is in an area closed to fishing). Peracarids (gammaridean amphipods, mysids, and cumaceans) were dominant among suprabenthic taxa. Seasonality was the main explanation for changes in taxonomic composition, with two seasonal groups indicated by MDS analyses (late summer–autumn, August–September, and November 2003; early summer, June and July samples). Peracarids at both S1 and S2 showed a peak of abundance in early July, with the highest densities reaching 5400 individuals (100 m)⁻² at S2. There was a sharp decrease of density in late July (S1) and August (S2), then an increase in August (S1) and in September (S2), respectively. A secondary peak of abundance occurred in November (S1) and December (S2). There was, therefore, a similar picture in the dynamics of suprabenthic peracarids at both sites, though with a delay of 1 month at the deeper S2. This pattern coincided with changes in river discharge (specifically, a decrease of suprabenthos when influx was below 200 m³ s⁻¹ in the period June–September 2003), and with the formation of a thermal gradient (also in June–September 2003) between S1 and S2 associated with the 15 °C isotherm. In addition, the decrease of suprabenthos in late July–August also coincided with the massive occurrence of mucilaginous aggregates close to the bottom (at between 0 and 4 m above the bottom), in the usual habitat of suprabenthos. C/N ratios in sediments (an indicator of the degree of degradation of organic matter (OM)) increased during this peak abundance of mucilaginous aggregates. The impoverishment of sediments in total organic carbon (TOC) was parallel to the decline of suprabenthos in late July–August, which is consistent with negative or non-significant correlations (evidenced both by Spearman's r and multi-regression models) between suprabenthos densities and TOC. Zooplankton taxa (e.g., copepods, chaetognaths, and fish/crustacean larvae living in the entire water column) showed significant correlations with a number of environmental factors (basically temperature), in contrast to suprabenthos. In conclusion, suprabenthos abundance was influenced by a number of natural factors, both intra-annual (e.g., OM quality, river discharges) and inter-annual (e.g., accumulation of mucilaginous remains). Probably due to their swimming capability, suprabenthos were not influenced by trawling activity. Considering the high P/B exhibited by suprabenthic crustaceans in comparison to infauna, this compartment likely has an important role in regulating food webs in those communities submitted to high fishing impact.