Temperate carbonate production: biodiversity of calcareous epiliths from intertidal to bathyal depths (Azores)

Abstract

The diversity of calcareous epiliths along a bathymetrical transect spanning the intertidal to bathyal (0–500 m) was investigated during a two-year carbonate cycling experiment deployed in the Faial Channel, Azores. The epilith inventory comprises 47 taxa of bryozoans, 9 serpulids, 3 bivalves, 3 cirripeds, 3 foraminiferans, 2 vermetid gastropods, and 1 scleractinian coral, along with 6 rhodophyte morphotypes. Colonised surface area, carbonate accretion rates, and biodiversity peak in the deeper euphotic zone (15 and 60 m), where mature biocoenoses were established after 2 years exposure, whereas colonisation was retarded at dysphotic and aphotic depths. Particularly in the photic zone, colonised surface area, accretion rates, as well as species richness and abundance were higher on down-facing compared to up-facing surfaces. Analyses of similarity (ANOSIM) and non-metrical multidimensional scaling (NMDS) of species abundance data revealed that water depth and substrate orientation were the dominant factors controlling the community structure as a result of direct (photosynthesis) and indirect (bioerosion pressure; nutrient supply) effects of the light regime, while exposure time and substrate type had little influence. The same hierarchy applies for the ichnodiversity of bioerosion traces, but with an inverse pattern in case of substrate orientation, reflecting the interaction of encrustation and bioerosion. Positive net carbonate production rates support the development of oyster bioherms and heterozoan-dominated carbonate/volcaniclastic sediments accumulating in the Faial Channel and adjacent slope. A comparison with biogenic sediments from other Macaronesian archipelagos and seamounts demonstrates the abundance and diversity of non-tropical heterozoan carbonates in oceanic islands.