Depth determines structure and functioning of rhodolith habitats in Central-Eastern Atlantic

Abstract

Similar to altitude in terrestrial habitats, subtidal habitats experience abrupt environmental gradients (e.g. light, temperature, water motion, etc.) across narrow vertical (depth) scales. The spatial configuration of rhodolith seabeds in the Canary Islands provides an ideal system to assess whether environmental drivers across depth regulate the structure and functioning of rhodolith seabeds. In this study we characterized across depth through temporal (seasonal) scales the structure of rhodolith habitats (size, shape and living surface of nodules) and their associated communities (fauna and flora). A two-years (winter, spring, summer and autumn) monitoring program was performed at three different depths (18, 25 and 40 m) in a rhodolith bed at Gran Canaria Island, Central Eastern Atlantic. Rhodolith nodules were mainly spherical with bigger sizes at 25 m relative to the other depths; the percentage of living surface increased with depth. The structure and functioning of the community assemblages changed consistently with bathymetry but overall with the additional substrate provided by the seasonal epiphytic associated biota (mainly macroalgae) during summer and autumn. Our data also suggested that wave induced turbulence (in upper layer) and irradiance attenuation (in the lower layer) can be the main environmental drivers regulating the structure and functioning of rhodolith habitats across depth gradients.