Understanding substrate requirements of coastal marine taxa to produce effective eco-engineered surfaces

<p dir="ltr">Coastal development is increasing rapidly worldwide and coastal eco-systems like shellfish beds are threatened by this habitat destruction. Habitat and particularly substrate complexity is a key feature driving community composition of marine hard substrate environments. Thus, understanding how substrate complexity affects organisms and how it can be manipulated for specific aims is an important research topic for marine ecology. Eco-engineering seeks to include substrate complexity into developed coastal structures to increase local biodiversity. This thesis sought to identify and understand the habitat and substrate requirements of four coastal foundation species to improve eco-engineering design and encourage native biodiversity. Understanding the requirements of foundation species allows us to incorporate these requirements into eco-engineering design to target settlement of foundation species.</p><p dir="ltr">Reef building bivalves are important, globally distributed foundation species, that live in intertidal and subtidal benthic environments. Incorporating the life history, biological, and structural requirements of reef-building bivalves into eco-engineering design could improve the abundance and biodiversity of species on eco-engineered structures. Therefore, this thesis seeks to understand the habitat requirements of <i>Perna canaliculus</i> and reproduce these habitats at small scales to test the importance of different metrics of complexity for mussel settlement. Additionally, it investigates the effects of complexity on different species of mussel settlement at larger scales. Firstly, I investigated the abundance and size distributions of juvenile (250μm-4mm), NZ-endemic, green-lipped mussels, <i>Perna canaliculus</i> among a range of low-shore habitats (e.g., mussel beds, seaweed assemblages) to determine the habitat affinities of this species between early settlement to post-larval phases. Secondly, I used 3-D scanning and printing technologies to recreate a fine-scale habitat model for <i>Perna canaliculus</i>, which I then tested for different levels of complexity metrics in a laboratory settlement experiment. The relationship between substrate complexity (the fractal dimension, rugosity and height variation) and P. canaliculus distribution was established and assessed regarding its potential to inform eco-engineering design.</p><p dir="ltr">The second half of this thesis focuses on investigating an existing eco-engineering project with different physical complexity at large scales on urban structures in Sydney, Australia. I surveyed the population demographics of <i>Trichomya hirsuta</i> and <i>Mytilus edulis</i> between different eco-engineering designs as well as rocky reference reefs and un-engineered sandstone seawalls to investigate how the composition of foundation species differed across habitat types. I found <i>Trichomya hirsuta</i> did not occur at the eco-engineered sites, while the sandstone seawall provided habitat enhancement for higher abundances of <i>T. hirsuta</i> than nearby rocky reference reefs. This is useful to consider when installing an eco-engineered design, as exclusion of native foundation species can have ecosystem wide consequences.</p><p dir="ltr">This thesis improves our understanding of the habitat requirements of several bivalve foundation species and indicates potential ways to incorporate these measures into updated eco-engineering design. It also highlights that seawalls can enhance abundance of foundation species and highlights the need for long-term monitoring of eco-engineered structures over time. This thesis sheds light on alterations to habitat structure that could be investigated and incorporated for long-term survival of biodiverse ecosystems on eco-engineered structures.</p>