Eco-engineering of seawalls to enhance their ecological value

Coastal urbanisation is changing the intertidal seascape by introducing artificial structures such as seawalls, jetties, revetments, and breakwaters. As the global human population continues to increase and sea-levels continue to rise, the proliferation of artificial structures will increase. Artificial structures generally have hard, vertical surfaces that differ in habitat complexity, material type and colour compared to natural hard substrates. Consequently, compared to natural intertidal habitats; artificial structures support distinct communities of microbes to fish, often of lower biodiversity and abundance. Recent advances in marine ecological engineering have focussed on creating urban structures that mimic some of the conditions of natural habitats, thereby reducing changes to substrate physico-chemical conditions and allowing modified artificial structures to be multifunctional, simultaneously benefiting both people and nature. Successful approaches, achieving increases in abundance and diversity of settling organisms while maintaining anthropogenic engineering function, have included adding complexity, shading and water retaining features to marine urban structures. There are shortcomings in these solutions however, that need to be addressed in order to increase the success and large-scale adoption of these approaches across the globe. In this thesis I considered the public perception of marine eco-engineering projects and what the potential benefits are to stakeholders. I found strong support for “eco-engineering projects” and a desire to increase diversity and ecosystem services. However, willingness to pay was less than actual costs. Stakeholders identified the lack of a publicly accessible evidence base and dedicated funding for eco-engineering projects as factors that limit eco-engineering uptake and will need to be overcome. Next, using a meta-analysis, I determined how material type influences colonisation, and hence should be considered in eco-engineering. I found that although effects of material type varied between functional groups of marine organisms, polymers and metals generally supported lower abundances than biogenic materials and rock and concrete supported greater abundances than wood. Among concretes, eco-friendly mixtures by contrast had little effect on community metrics. Hence where other constraints allow biogenic materials or concrete should be favoured for construction of marine structures over polymers and metals. I then considered whether altering substrate colour (using recruitment tiles) can improve biodiversity outcomes and ultimately ecosystem service provision at both micro and macroscales. I found brightness interacted with thermal conditions and tidal elevation to influence colonisation and community development. Darker tiles attracted greater abundances and diversity of species, though this was limited to cooler sites and time periods. In summer, at warmer sites, the hotter temperatures reached by dark substrates negated this effect. Hence, substrate colour, as well as material type, should be a key consideration for eco-engineering. Finally, I investigated whether the positive ecosystem service and diversity benefits identified in a large scale eco-engineering experiment at the macroscale, are established at the time of biofilm development. I found that moisture retaining features influenced both bacterial and eukaryote microbial communities and these effects were mirrored in the macro community at later successional stages. This finding reinforces the need to incorporate complex habitats into marine artificial structures. Overall, these results highlight the need for eco-engineering strategies to consider not only habitat complexity, but also material type and colour. The spatially variable results highlight the importance of determining site specific conditions and matching eco-engineering approaches to these, and the goals of stakeholders. Establishing a solid evidence base for eco-engineering will assist in enhancing uptake and achieving truly multifunctional marine artificial structures.