Transfers of bryozoan species between the Atlantic Ocean and the Mediterranean Sea via the Strait of Gibraltar

Abstract

The deep-water benthos (115-2100 m) was sampled in the Atlantic-Mediterranean exchange area (Balgim cruise, N.O. Cryos: lberian-Moroccan Gulf, Strait of Gibraltar, Alboran Sea). The species richness of the bryozoan fauna (146 spp.) showed a marked between-site variability. Peaks of species richness occurred on both sides and in the Strait of Gibraltar under various conditions of temperature and salinity. 'rhe common features of the corresponding sites were the abundance of suitable substrates and the presence of currents facilitating supply of food and propagules. In the Alboran Sea, the species richness decreased with depth more rapidly than in the Atlantic and the depth distribution of several deep-water species showed an upward shift. There is an eastward decrease in the stock of Atlantic species: 15 % of them are not found beyond the Strait sill and only 79 % of the remaining species extend to the whole Mediterranean. Several endemic Atlantic species or morphotypes can live in conditions of typical Mediterranean water (high temperature and salinity), but do not spread beyond the eastern limits of either the Gibraltar Strait or the Alboran Sea. These occurrences are hypothesized to be signatures of local increases of the trophic resources, which could mask the barrier effect of the Mediterranean oligotrophy. The organic sedimentation originating from the high-productivity area localized in the north-west of the Alboran Sea appears to create this distribution pattern