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Abstract The field of eco-engineering has burgeoned in recent years in response to the proliferation of artificial structures. Adding water-retaining features to seawalls has been successful in increasing biodiversity relative to the surrounding structure. Artificial rock pools may not, however, completely mimic natural rock pools. Here, we compared natural colonisation, through dispersal and recruitment, of intertidal mobile species to water-retaining flowerpots on seawalls with that into rock pools. This represents the more usual 'passive' approach to eco-engineering where features are built to enhance biodiversity and are allowed to colonise naturally, as opposed to seeding or transplanting organisms to features. While flowerpots supported some mobile species not found on the seawall, other species common on natural shores did not recruit to flowerpots. Thus, in a second experiment we tested the effectiveness of an 'active' approach through transplanting mobile organisms to flowerpots to expedite the colonisation process. For the species examined, however, most individuals did not stay in the flowerpots for more than 24 h after being transplanted. Further understanding of the processes (e.g. dispersal distances, recruitment) influencing colonisation of eco-engineered habitats is needed to effectively inform management of marine infrastructure, particularly for projects targeted at restoration rather than enhancement. Highlights • Mobile invertebrates are ubiquitous in natural rock pools. • Water-retaining flowerpots were added to seawalls to enhance biodiversity. • Some mobile species found in rock pools do not colonise or recruit to flowerpots. • Seeding was tested as an active approach to increase flowerpot colonisation. • Most individuals transplanted did not stay in the flowerpots for more than 24 h. [ABSTRACT FROM AUTHOR]

Highlights • Adding coir to increase effectiveness of eco-engineered rock pools was proposed. • No consistent response of benthic, epifaunal or fish assemblage to coir. • Low proportion and abundance of non-native species in rock pools. Abstract The expansion of built infrastructure in the marine environment threatens natural ecological communities at local and regional scales. An increasing interest in incorporating heterogeneity that is reflective of natural rocky shores into artificial structures through ecological engineering seeks to mitigate negative impacts. The addition of complex surfaces and novel habitats, such as water-retaining features, has been particularly successful at increasing biodiversity of marine infrastructures to date. Importantly, key habitat-forming groups, such as the complex turfing algae Corallina officinalis found on natural shores and their associated assemblages are still lacking from these eco-engineered features. Furthermore, whether observed biodiversity increases from eco-engineering are due to native or non-indigenous species remains largely unknown. Here, we investigated whether adding small-scale complexity (artificial turf) to artificial rock-pools (‘flowerpots’) on urban seawalls enhanced their effectiveness to increase native biodiversity. Responses of benthic invertebrates, algae, epifauna and fish in flowerpots with and without artificial turf (coir) were quantified. Contrary to existing literature, which reports an increase in biodiversity with an increase in complexity, no consistent effect of coir was seen on benthic, epifaunal or fish assemblages. Native species consistently occupied more than 95% of space in flowerpots while the proportion of non-indigenous species in flowerpots was small (<75% of the assemblage) regardless of treatment, and decreased over time. This result is promising, but warrants further investigation to determine if these trends reflect seasonal patterns or if non-indigenous species colonise early, but are replaced over time by native species. These are important considerations when planning large-scale deployments of eco-engineering features on seawalls to ensure that native species are targeted without increasing opportunities for non-indigenous species. [ABSTRACT FROM AUTHOR]