Disturbance, productivity and diversity in deep-sea canyons: a worm’s eye view

The abundance, diversity and assemblage structure of polychaetes from the Nazaré, Setúbal and Cascais Canyons along the Iberian Margin were studied as part of the EU project HERMES. A Dynamic Equilibrium Model (DEM) was used to identify the main environmental factors structuring the assemblages. Box corer and megacorer samples from upper (1000 m), middle (3400 m) and lower canyon (4300 m) settings were taken in each canyon. Polychaete abundances in the Nazaré and Setúbal Canyons were highest at 3400 m while in Cascais there were only slight differences between the various depths. Most of the polychaetes occurred in the top 5 cm of the sediment. Sample diversity both within and among the canyon sites did not differ statistically despite differences in the environmental settings, suggesting that small-scale heterogeneity at the scale of the sampler was similar at all sites. Species richness at the level of site was lowest at 3400 m sites in both the Nazaré and Setúbal Canyons. In contrast, species richness increased from 1000 m to ca. 3400 m and then again to 4300 m in the Cascais Canyon. The differences were linked to the physico-biogeochemical environment of each canyon. Analyses of physico-environmental variables indicated that the Nazaré mid-canyon sites were subject to high levels of disturbance and intermediate levels of productivity, accounting for high abundances and low species richness. Low disturbance and low productivity characterised the lower Nazaré Canyon site at 4300 m. Diversity results agreed with predictions of the DEM. However, the 4300 m site in the Setúbal Canyon did not conform to model predictions. Overall, while the Iberian Margin canyons demonstrated higher abundance and biomass than ‘open slope’ polychaete assemblages, they had lower species richness.