Recolonization and succession of a subtidal hard-bottom epibenthic community in Smeerenburgfjorden, NW Svalbard.

Abstract

Rapid changes to the physical environment of Arctic marine systems in recent years impact the structure and function of benthic ecosystems. Exploring the resilience of these systems to perturbations requires a solid understanding of key ecological processes and must be conducted over appropriate time scales due to the slow growth and recruitment of many Arctic benthic organisms. This study addresses the successional pattern of a hard-bottom benthic community in Smeerenburgfjorden (NW Svalbard) after a perturbation and the functional traits involved in the different stages of recolonization. Spanning nearly four decades, the time series was initiated in 1980 by clearing the substrate free of organisms on a vertical rock wall at 15 meters depth, and the site was subsequently photographed annually by scuba divers. The structure of the ecosystem was investigated by estimating the abundance (solitary taxa) and percentage cover (colonial taxa and macroalgae) of the benthic organisms from the images, whereas the ecological functioning of the system was examined via functional traits analysis based on literature sources. Single taxa showed different return rates and fluctuating abundance and cover throughout the time series. Hydrozoans and mobile mollusk grazers Tonicella spp. and Margarites spp. appeared in the early recolonization stage, whereas late-successional taxa included ascidians, sponges, barnacles, and the bivalve Hiatella arctica. A climate-driven foliose macroalgae takeover was observed in the year 2000, in conjunction with a reorganization in the invertebrate community structure. Recovery rate at community level following the clearing confirms previous observations of slow recolonization in polar benthic systems. It took ten years for the cleared substrate to be covered by living organisms comparable to the control area, and the convergence of the community compositions of cleared and control transects took more than two decades. The community-weighted mean traits displayed a decrease in body size and longevity in response to the clearance manipulation, and a small increase in mobility and grazing and predatory feeding habits. This study provides insights into the succession and recolonization of Arctic hard-bottom benthic communities after a perturbation and their implications for ecosystem functioning, important knowledge at a time of rapid change and increasing borealization of high-latitude ecosystems.