Effects of habitat-forming species on meiofauna assemblages in harbour areas

As a consequence of coastal urbanisation, the sprawl of artificial structures is increasing. These structures replace the natural habitats modifying the ecosystem. Green engineering – that aim to include ecological principles in the design of infrastructures – can help to enhance the ecological value of these “novel” coastal ecosystems creating habitat more suitable to a wider number of organisms. However, such approach could be successfully applied only after an in-depth knowledge of the associated benthic communities. Ravenna harbour is a clear example of coastal urbanisation, and it is characterised by a variety of artificial substrates. These substrates are almost entirely covered by mussels that represent the main habitat-forming species. Moreover, mussel beds provide a secondary substrate for associated communities including rich meiofauna assemblages. Despite the functional and ecological importance of meiofauna (Schratzberger and Ingels 2018), few studies have focused on the influence of artificial structures on the diversity of these assemblages. This work aims to understand how the features of the artificial structures (primary substrate) affect the spatial-temporal aggregation of mussels (secondary substrate) and, consequently, their associated meiofauna to obtain the knowledges necessary to implement the green engineering solutions in harbour environment. We sampled the mussel beds and associated meiofauna in the intertidal zone from two types of artificial substrata (rock boulders vs. concrete seawalls) at the two jetties of the harbour (North vs. South) in three different times. The analysis of the structural complexity of the mussel beds (e.g. abundance, mean length of valve, bio-volume) and the taxonomic assessment of the meiofauna showed: 1) mussels more abundant and forming a thicker bed on concrete seawalls and, reversely, meiofauna assemblages more diverse and richer on rock boulders; 2) between the two jetties, mussel beds more different, particularly in terms of mean length of valves, while the associated meiofauna resulted more similar; and 3) a negative effect of bio-volume of the mussels on meiofauna assemblages that resulted poorer and simpler at higher values of bio-volume. Finally, we observed a temporal variability related to the biological life cycle of both meiofauna organisms and mussels. This study highlighted that mussels, as habitat-forming species, and meiofauna, as associated assemblages, respond differently at spatial and temporal scales to the type of artificial structures. Moreover, the reversed pattern observed between meiofauna and mussels ‒ imputable to food and space competitions, suspension-feeding of these bivalves on infauna larvae and anoxic periods provided by the mussels ‒ probably outclasses the positive effects of increasing in niches availability and refugia provided by this habitat-forming species. Considering that nowadays most common green engineering solutions aim to increase habitat complexity incorporating topographic microhabitats or seeding habitat-forming taxa (Firth et al. 2014, Strain et al. 2018), such procedures should take into account the complexity of the substrate carefully and consider the biological characteristics of all the taxa involved.