Creation of microhabitats (tidepools) in ripraps with climax communities as a way to mitigate negative effects of artificial substrate on marine biodiversity

Abstract Urbanisation and the construction of commercial structures on coasts cause a significant impact on natural marine habitats. The concept of ¨Ecological Engineering¨, recently applied to marine biology, integrates ecology, economy and society’s needs into the design of artificial marine structures. Recently, some ecological designs, such as the addition of pools, pits, and crevices, have been proposed in order to reduce habitat modification, diminish colonization by invasive species, and spread and maintain ecosystem functioning and biodiversity indexes that are more similar to those of the natural environment. Within the features that may increase artificial habitats’ diversity, tidepools are unique habitats that reduce stress in the intertidal, favouring breeding and feeding, as well as providing shelter and a greater degree of moisture to a certain group of species, which may present an advantage for them. Nevertheless, the ecological benefits that arise from the inclusion of these features on fully mature artificial structures are still poorly known. In the present study, tidepools were created on artificial substrate with an affordable method (jackhammer) in the locality of Ceuta (Strait of Gibraltar). The pools were carved on dolomitic riprap at two different intertidal levels, high and low shore (+0.75 m and +0.25 m higher than the lowest tide, respectively) with an average area of 195.2 cm 2 and 353.01 cm 3 of volume. These coastal defence structures were built more than eight years ago, thus the biota has substantially completed its colonization period and is set in terms of species composition. To test humidity effects on benthic assemblages around tidepools, the adjacent halo around the pool, with a width of 5 cm, was equally traced. Pool assemblages were compared to adjacent artificial and natural substrate and also to nearby natural pools. One year later, the results in species richness and diversity were significantly higher in the tidepools than the other two studied treatments (5 cm and control), artificial pools exceeded control species richness by 39.21% and control diversity by 30.70%. Focusing on the tidepool halo (5 cm) effect in the artificial substrate, the results show that the biological assemblages were different than tidepool and emerged rock, indicating that the newly created habitat extends its influence beyond the limits of the tidepools. Tidepools increased the number of species on ripraps and contributed to reduced fragmentation of some vagile species’ populations, as they favoured settlement of species that previously seemed excluded from artificial substrate such as Pisania striata, Fisurella sp. , Phorcus turbinatus and Lepidochitona sp. In the case of Siphonaria pectinata, it increased its presence in the tidepools and most of its egg masses were observed inside the pools. Finally, it should be mentioned that the pools have extended the upper vertical limits of infra-littoral level species such as Paracentrotus sp ., mostly young individuals, and Aiptasia sp. These results may help to modify existing artificial shores to reduce their ecological impact.