Settlement and Growth of Marine Organisms on Artificial Habitats

<p dir="ltr"><b>Coastal marine habitats are some of the most productive environments in the world. These habitats are also highly important for the economic, social, and cultural functioning of numerous countries, and are therefore vulnerable to stressors from the increasing level of human activity at the interface between land and sea. Many major urbanised coastal environments, including Wellington, have manipulated over half of their shorelines to support and protect coastal cities and towns, and their continuing growth. Temperate rocky reefs are some of the most affected habitats facing the highest levels of cumulative human impacts.</b></p><p dir="ltr">Artificial marine habitats are becoming a more frequent feature in coastal environments, contributing to numerous recent ecosystem management strategies around the world. It is crucial to understand factors that influence their performance as viable habitats for a range of species in order to effectively implement them as environmental mitigation or restoration strategies. Marine reef organisms typically exhibit complex life cycles defined by a period of pelagic larval development before recruiting to benthic habitat where they will continue to develop until they reach adulthood. In this thesis, I evaluated the effects of additional-three dimensional structure, proximity of an artificial reef to a natural reef, and time on the communities recruiting to artificial habitats. </p><p dir="ltr">In Chapter Two, I conduct a Before-After-Control-Impact-Paired Series (BACIPS) design to study the establishment of subtidal communities on two small artificial islands. I addressed the questions: </p><p dir="ltr">1) Does the addition of three-dimensional structure increase the densities of fish and invertebrate species across trophic groups on an artificial marine habitat? 2) Does the addition of three-dimensional structure on an artificial marine habitat shape community assemblages?</p><p dir="ltr">3) How does size structure of fish and invertebrate communities vary between an artificial habitat with additional three-dimensional structure and an artificial habitat without additional structure?</p><p dir="ltr">My results, from surveys conducted across a nine-month period, revealed no relationship between either treatment and any of the metrics studied, elucidating the lack of effect that additional structure seemed to have on the communities on these artificial habitats. However, I highlight the potential for alternative factors to be driving changes in these communities. In Chapter Three, I evaluated the recruitment dynamics and community assemblages of the 24 artificial reef units I constructed under varying treatments. I investigated the questions: </p><p dir="ltr">1) Does the presence of supplemental structure and the location of an artificial marine habitat increase the abundance of fish and invertebrate species?</p><p dir="ltr">2) Does the placement of an artificial reef and the presence/absence of additional structure shape community composition?</p><p dir="ltr">3) Does the placement and design of artificial reefs influence the growth of a common species occupying them?</p><p dir="ltr">The effects of either structure or reef proximity were significant in shaping recruitment dynamics for a range of species including spotty wrasses (<i>N. celidotus</i>), variable triplefins (<i>Forsterygion varium</i>), kina (<i>Evechinus chloroticus</i>), cat’s eye snails (<i>L. smaragda</i>), and top-shells (<i>Trochidae spp.</i>). Species diversity on artificial marine habitats were not significantly influenced by any treatment. Despite this, communities on bare substrate units were found to be significantly dissimilar to units on the natural reef edge by principal coordinates analysis. Size distributions of five focal species exhibited varying responses to each treatment, highlighting the species-specific nature of responses to structural complexity. I found no obvious relationship between structure or reef proximity on the growth of the <i>F. lapillum</i> individuals collected. This could demonstrate the potential of artificial reefs to provide viable habitats that support healthy populations of a common rocky reef species. However, given the limited sample size, further study is required in this area on a broader range of species to understand ecosystem-wide effects.</p><p dir="ltr">Overall, this thesis emphasizes the array of responses that marine reef organisms can have to artificial habitats based on their life history traits. It highlights the species- and spatial scale-specific nature of such responses, and the importance of acknowledging these factors when implementing environmental mitigation and/or restoration strategies that include the use of artificial reefs. Recruitment dynamics and community assembly are highly complex processes, and my research contributes to a meaningful increase in understanding of these procedures in the context of artificial marine habitats within temperate environments.</p>