Effect of depth and seasonality on the functioning of rhodolith seabeds

Similar to altitudinal gradients in terrestrial habitats, subtidal habitats experience abrupt environmental gradients across depth. The objective of this study was to understand how variation with depth of environmental factors (water temperature, light availability, water motion and sedimentation) affected the structure (size and morphology) of rhodoliths and the abundances of attached floral and faunal epibionts in a rhodolith bed at Gran Canaria Island (central-eastern Atlantic). Specifically, sampling took place seasonally at three depth strata: 18, 25 and 40 m throughout two successive years. Depth affected the size and morphology of rhodoliths, with bigger and mainly spherical nodules at 25, relative to those at 18 and 40 m depth. Larger biomasses of attached (epiphytic) macroalgae were observed at 18 and 25 m than at 40 m. The presence of hydrozoans living over rhodoliths also changed with depth, including higher abundances at 25 m than at 40 and 18 m, respectively. Wave-induced turbulence in the upper depth layer, and light intensity and sedimentation, in the lower depth layer, are the main environmental drivers regulating the presence, structure and functioning of rhodolith habitats.