Effect of depth and a latitudinal gradient in the functioning of rhodolith seabeds

Abstract

Rhodolith beds are important ecosystem engineers composed by free-living nodules, calcareous red macroalgae distributed across the Atlantic Ocean covering a large latitudinal gradient. The aim of this study was to understand how the variation in depth affected the structure (size and morphology) of rhodoliths and the abundance of floral and faunal epibionts across the Eastern Atlantic Ocean. Specifically, sampling was carried out at five locations: Brittany, Galicia, Madeira, Gran Canaria and Principe Island in three depth strata (shallow, intermediate and depth), corresponding to the bathymetric range in which the beds were found. Different size trends existed depending on the region, but generally the smallest rhodoliths were found in the shallow strata. The shape reflected the hydrodynamic context, with more spherical rhodoliths in open areas and discoidal rhodoliths in sheltered areas. Biomasses of attached macroalgae (epiphytes) varied consistently across regions but no significant differences were found between depths. The presence of epifauna was variable for the different phyla, but in general arthropods and molluscs dominated across all regions. Overall, local variability had a greater impact than the latitudinal gradient.