Mesoscale eddies: Hotspots of prokaryotic activity and differential community structure in the ocean

[EN] To investigate the effects of mesoscale eddies on prokaryotic assemblage structure and activity, we sampled two cyclonic eddies (CEs) and two anticyclonic eddies (AEs) in the permanent eddy-field downstream the Canary Islands. The eddy stations were compared with two far-field (FF) stations located also in the Canary Current, but outside the influence of the eddy field. The distribution of prokaryotic abundance (PA), bulk prokaryotic heterotrophic activity (PHA), various indicators of single-cell activity (such as nucleic acid content, proportion of live cells, and fraction of cells actively incorporating leucine), as well as bacterial and archaeal community structure were determined from the surface to 2000m depth. In the upper epipelagic layer (0?200 m), the effect of eddies on the prokaryotic community was more apparent, as indicated by the higher PA, PHA, fraction of living cells, and percentage of active cells incorporating leucine within eddies than at FF stations. Prokaryotic community composition differed also between eddy and FF stations in the epipelagic layer. In the mesopelagic layer (200?1000 m), there were also significant differences in PA and PHA between eddy and FF stations, although in general, there were no clear differences in community composition or single-cell activity. The effects on prokaryotic activity and community structure were stronger in AE than CE, decreasing with depth in both types of eddies. Overall, both types of eddies show distinct community compositions (as compared with FF in the epipelagic), and represent oceanic ?hotspots? of prokaryotic activity (in the epi- and mesopelagic realms).

Ichthyofauna associated with Cymodocea nodosa meadows in the Canarian Archipelago (central eastern Atlantic): Community structure and nursery role

We carried out 84 trawls in 41 seagrass meadows composed of the phanerogam Cymodocea nodosa at three islands of the Canarian Archipelago, during June to September 2003, in order to describe the associated ichthyofauna (composition, richness, and abundance), to analyze the role that this habitat can play in fish recruitment, and to determine the potential relationship between the spatial structure of the seagrass meadow and the patterns of richness and abundance of the fish assemblage. A total of 8298 individuals were captured. The five most relevant species, in terms of abundance and frequency, were Spondyliosoma cantharus, Diplodus annularis, Syngnathus typhle, Mullus surmuletus, and Pagellus erythrinus. Gran Canaria had the largest species richness (36 species) and mean number of species per sample (8.69 ± 0.49; mean ± SE). Lanzarote had the largest number of individuals (64.83% of the total registered) and mean total abundance per sample (168.39 ± 30.91). High densities of individuals were registered (95.86 ± 13.5) and 92.91% of fishes were juveniles. Our data showed that the physical configuration of the seagrass meadows did not significantly affect the patterns of richness and abundance of the associated fish assemblage. In conclusion, the C. nodosa meadows exhibited a singular ichthyofauna and they contribute to the maintenance of the diversity of the coastal fish assemblages in the Canarian Archipelago. This habitat constitutes, during spring and summer, a nursery habitat for juvenile fishes of many species, several of them commercially targeted.