Depth-Related Controls on the Quantitative Composition of Rhodolith Matrices in the High Arctic

Abstract

The calcareous matrix of rhodoliths can be composed of one or more crustose coralline algae (CCA) taxa as well as a mixed assemblage of various encrusting organisms. Studies on modern and fossil rhodoliths assume such associations to vary with water depth. Our study explores the quantitative biological composition of calcareous rhodolith matrices along a bathymetric gradient at the Arctic Svalbard archipelago. Using a methodological combination of virtual micro-CT cross-sections with a modified point counting approach, we found five different taxonomic groups: CCA, bivalves, serpulids, bryozoans and balanids. While water depth does not influence the general taxon richness as well as the abundance of bivalves, it significantly affects the proportional matrix composition of encrusting organisms by a combination of environmental factors and biological interactions. The decrease in CCA skeletal material with increasing water depth is significantly governed by impaired irradiance conditions. Regular rhodolith movement in shallow waters fosters the proportion of CCA, while decreased movement in deeper waters spurs the proportion of other encrusters. This potentially results from post-mortem fouling of dead rhodolith parts followed by a recolonization with slow-growing CCA species Boreolithothamnion glaciale. Our results highlight mechanisms controlling the biogenic composition of calcareous rhodolith matrices and underline the potential of matrices compositions for palaeogeography and palaeobathymetry. Our study contributes to an improved understanding of the composition and developmental patterns of rhodoliths. This can add to a better comprehension of the respective ecosystems on a broader scale and be beneficial for conservation purposes.