Role of pockmarks in diversity and species assemblages of coastal macrobenthic communities
Résumé
We used existing bathymetric data to study the macrofauna of a geophysical pockmark field restricted to a benthic habitat engineered by the tubiculous amphipod Haploops nirae in South Brittany (France). Stations inside and outside pockmarks of different morphometric characteristics (location, size, depression depth) were sampled for macrofauna and environmental parameters (sediment characteristics, organic matter, chl a, hydrogen sulfide and methane concentrations). Diversity indices showed higher species richness inside pockmarks, especially for species with medium to high abundances. Most sediment cores showed low methane but high hydrogen sulfide concentrations. We hypothesised that after eruption, the remaining residual methane from pockmark sediments is oxidised by seawater sulfate and accounts for the high sulfide concentrations found at increasing depth in our samples and the low methane concentrations. We found no relationship between sediment profiles and morphometric features of the pockmarks. Macrofauna assemblages inside vs. outside pockmarks appeared to be different. Pockmarks appear to increase connectivity among habitats and heterogeneity within habitats, thereby creating local hotspots that allow the settlement of species that cannot otherwise develop in Haploops tube mats. Multivariate analyses distinguished 4 groups of pockmarks and control stations. We assumed that deeper pockmarks were created more recently than shallow pockmarks and that each pockmark is at a different stage of evolution, hence explaining the large variability in the characteristics of pockmark groups. This explains why previous investigations have found contradictory results when comparing macrofauna species diversity and composition between areas inside and outside pockmarks. Finally, we propose and discuss a successional stage model for pockmarks.