Ecosystem engineer morphological traits and taxon identity shape biodiversity across the euphotic–mesophotic transition

Abstract

The euphotic–mesophotic transition is characterized by dramatic changes in environmental conditions, which can significantly alter the functioning of ecosystem engineers and the structure of their associated communities. However, the drivers of biodiversity change across the euphotic–mesophotic transition remain unclear. Here, we investigated the mechanisms affecting the biodiversity-supporting potential of free-living red coralline algae—globally important habitat creators—towards mesophotic depths. Across a 73 m depth gradient, we observed a general decline in macrofaunal biodiversity (fauna abundance, taxon richness and alpha diversity), but an increase in beta-diversity (i.e. variation between assemblages) at the deepest site (86 m depth, where light levels were less than 1% surface irradiance). We identified a gradient in abundance decline rather than distinct ecological shifts, driven by a complex interaction between declining light availability, declining size of the coralline algal host individuals and a changing host taxonomy. However, despite abundance declines, high between-assemblage variability at deeper depths allowed biodiversity-supporting potential to be maintained, highlighting their importance as coastal refugia.

In situ decrease in rhodolith growth associated with Arctic climate change

“…Moreover, rhodolith beds provide important ecosystem services worldwide. Among other things, they may act as refugia for other species against acute (Fredericq et al, 2019) or chronic (Voerman et al, 2022) environmental stress, promoting ecosystem resilience, and provide ecological and genetic connectivity with other habitats (Tuya et al, 2023). Also, non-rhodolith CCA can facilitate the formation of coral reefs (Morse & Morse, 1996;Teichert, Steinbauer, et al, 2020), which are also major providers of carbonate habitat that are threatened by anthropogenic climate change (e.g., Cornwall et al, 2021).…”

Dominance of photo over chromatic acclimation strategies by habitat-forming mesophotic red algae

“…B 290: 20231329 differences in energy provision with depth as a limitation to growth. However, in contrast to the flattening of mesophotic corals with depth [3], variability in coralline algal thallus sphericity appears to be retained through the euphotic-mesophotic transition, enabling continuity of habitat provision [7].…”

“…This can alter the quantity and quality of biogenic habitat, with follow-on impacts for the diversity and composition of associated communities and the higher ecosystem services [e.g. 7 ]. Changes in the light field are therefore crucial for the functioning of benthic photoautotrophs in the mesophotic [e.g.…”

“…In their free-living form, coralline algae create diverse reef ecosystems (called maerl/rhodolith beds) with a cosmopolitan distribution from the poles to the tropics and from intertidal to mesophotic depths [15] that have globally significant roles in biodiversity provision [7] and biogeochemical cycling [16][17][18]. Despite their ecological importance, understanding of the photosynthetic processes that underpin their survival is only recently emerging.…”

Red algae acclimate to low light by modifying phycobilisome composition to maintain efficient light harvesting

“…Recent developments in ocean exploration have begun to reveal the extent of mesophotic benthic photosynthetic organisms (such as corals and algae) [2]. Despite low light levels, these organisms create complex habitats of significant ecological importance, supporting high biodiversity [3][4][5] and efficient biogeochemical cycling [6][7][8].…”