Small-spatial scale changes in productivity of suprabenthic and infaunal crustaceans at the continental shelf of Ebro Delta (western Mediterranean)

Abstract

Production ( P) and production/biomass ratio ( P/B) were estimated from 21 species of peracarid and eucarid crustaceans belonging to two different compartments (infauna: 10 species and suprabenthos: 11 species) using the Hynes size–frequency method. Calculations were based on 7 cruises performed between December 2002 and November 2003 in the continental shelf of the Ebro river delta (ED, Catalan Sea: northwestern Mediterranean). Within ED, P and P/B were compared between two neighbouring sites, one located on 47 m (S1), the other at 61 m (S2). The possible effects of natural variability on production were considered. Sediments in ED were muddy (%clay between 40 and 65%). No significant trend in production comparing S1 and S2 was detected, with 50% of species showing higher P value in S1, and 50% in S2. A significant trend emerged from P/B estimates, with 18 of 21 species showing higher P/B at S1. Similarly significant trends for infauna and suprabenthos were found after analyzing both compartments separately. It is suggested that the more significant P/B found on S1 both among infauna and suprabenthos could be caused by the higher temperature and organic matter content (%TOC) at S1. In multi-linear regression (MLR) models, differences in P between S1 and S2 were greater with increasing differences in the density of species ( RD) between sites. Differences were greater in infauna than in suprabenthos, and greater among rare species (those with low dominance, %D). P/B differences between S1 and S2 were mainly explained by %D and RD, and to a lesser extent by differences between sites in the mean size of species ( RW). In other words changes in P and P/B were not explained by removal of large size specimens. Some trawling activity occurred at S2, and it may alter the habitat of small macrofauna. In addition to natural variability, we discussed the possible influence of trawling disturbance on P and P/B, bearing in mind the feeding type, mobility, and dispersal capability of species. We suggest that amphipods of the genus Ampelisca, showing higher P/B on S1, characterized by low mobility (probably tube-dwellers), low trophic level (e.g. filter feeding strategies) and low dispersal capability (not free larvae), could be especially vulnerable to trawling impact.