Developing a marine habitat classification scheme for mesophotic reefs in Fiordland, New Zealand

<p dir="ltr">The biodiversity, functions, and services of marine coastal ecosystems worldwide are increasingly threatened by a suite of anthropogenic stressors. Temperate mesophotic ecosystems (TMEs), found in the deeper waters (~30-150 m) of high-latitude temperate regions, are characterised by low light and generally more stable environmental conditions compared to shallower waters. Despite growing recognition of their importance and vulnerability, TMEs remain poorly understood, which limits our ability to manage and conserve these ecosystems. Habitat classification is critical to inform marine management and conservation decisions as it identifies the distribution of important and/or vulnerable areas and furthers our understanding of ecosystem and habitat structure. Habitat classification schemes, such as the Coastal and Marine Ecological Classification Standard (CMECS), provide structured frameworks for the description of biotopes, which are the combination of biotic and abiotic features of a habitat. Unfortunately, descriptions of temperate mesophotic biotopes are scarce and are concentrated in the same few geographical areas. Located in the southwest of New Zealand’s South Island, the Fiordland (Te Moana o Atawhenua) Marine Area (FMA) spans 14 fiords that support biodiverse TMEs. Recent evidence has shown that the FMA is not immune to the impacts of anthropogenic stressors, notably, climate change and fishing impacts. The FMA is economically, culturally, and ecologically significant, therefore, the successful management and conservation of its TMEs is imperative. However, Fiordland’s mesophotic reef biotopes have yet to be described using a habitat classification scheme.</p><p dir="ltr">To close this knowledge gap, the overarching aim of this thesis was to develop a habitat classification scheme for mesophotic reefs (30-130 m) in Fiordland, using CMECS. This involved characterising and determining differences in benthic community composition and CMECS substrate and geoform components of Fiordland’s mesophotic reefs (30-130 m), between sites, depths, and fiords. To achieve this, I analysed remotely operated vehicle (ROV) video data sampled from 2018-2024, across 46 sites and four depth categories (30-40, 50-60, 80-100, and 120-130 m) throughout Doubtful, Dusky, and Breaksea Sounds, in Fiordland. ROV images were analysed using Coral Point Count with Excel extensions (CPCe) to determine the percentage cover of benthic organisms and composition of communities. Multivariate analyses were conducted at three different levels of taxonomic resolution to determine differences in benthic community composition between sites and 2 depths. Video analysis was used to classify the substrate and geoform components of mesophotic habitats.</p><p dir="ltr">I found that benthic community composition varied significantly between depth categories, with a general trend of decreasing algae, increasing animal, and increasing bare rock cover. Benthic community composition was also found to vary significantly between sites, both within and between fiords, with general increases in sediment and crustose coralline algae (CCA) percentage cover from outer to inner fiords. Differences in the benthic community composition, and substrate and geoform components of habitats were used to describe and map 13 distinct temperate mesophotic biotopes within the FMA using the CMECS hierarchy. I also found that biotopes varied across depths, sites, and fiords. These results suggest the influence of strong environmental gradients across multiple spatial scales (i.e., depths, within fiords, and between fiords). My findings highlight the importance of further research to classify Fiordland’s mesophotic reef habitats to determine the full list and distribution of biotopes. More studies are needed to further understand the drivers of TME structure and the distribution of biotopes in Fiordland. Overall, this thesis provides critical baseline information that will underpin future research, management, and conservation of Fiordland’s TMEs.</p>