Quantifying biotic and abiotic drivers of functionally key taxa abundances in intertidal rocky reefs of the English Channel.Long-term Temporal Trends

Lucas Greiner¹, Stanislas Dubois¹, Mathieu Chevalier¹

¹Ifremer, France

Documenting the relative effect of biotic and abiotic factors on the spatio-temporal dynamic of biodiversity is a major challenge requiring long-term monitoring programs. Such long-term studies are, however, relatively rare in the marine environment and science is beginning to look at historical impact studies, originally intended for other ecological issues, from the angle of global change. Here we took advantage of long-term (30 years) time-series data collected as part of the surveillance of nuclear powerplants in the English Channel to disentangle the effect of biotic and abiotic factors on five iconic and functionally structuring taxa in two intertidal rocky habitats. We focused on: barnacles (Semibalanus balanoides, Chthamalus spp., Austrominius modestus) in high and mid-intertidal exposed rocky shores, and brown macroalgae (Fucus spp.) and isopods (Idotea granulosa) in mid to low-intertidal sheltered rocky shores. Using monotonic trend analysis, we found steady declines in the total abundance and biomass of macroalgae, isopods and barnacles, with a replacement of S. balanoides by Chthamalus spp over time. To investigate the potential drivers of changes in these habitats, we ran linear mixed models integrating density-dependence and inter-taxa relationships. We considered as covariates various local abiotic variables and global climatic indices, along with biotic variables (i.e. chlorophyll a and total zooplankton abundance). Overall, we found that relationships between our target benthic structuring species and biotic variables were not significant, while inter-species interactions and abiotic parameters emerged as key drivers. Sessile taxa exhibited strong density-dependence control, likely explained by competition for space. Macroalgae Fucus spp. and isopod I. granulosa were positively related, evidencing the role of macroalgae in providing food and shelter but also – and quite remarkably so the likely role of isopods in disseminating macroalgae gametes. Barnacle abundances were mostly influenced by abiotic variables, including local but also global variables (NAO modes). The latter suggests that future studies should integrate the effect of large-scale climatic factors in their analyses to better understand the drivers of biodiversity changes in the Anthropocene.