The impacts of sedimentation on the spatial distribution of animal-dominated reefs in Wellington Harbour

<p><strong>Coastal ecosystems contain some of the most productive habitats worldwide, yet they are increasingly threatened by anthropogenic activities. The increased input and resuspension of sediments represents a major threat to global marine biodiversity. Elevated sediment levels can have severe impacts on marine habitats by reducing light penetration, smothering benthic organisms, and clogging feeding apparatus. Animal-dominated reefs (ADRs), which are dominated by sessile suspension feeders, are particularly vulnerable to increases in sedimentation. Recent habitat mapping in Wellington Harbour, New Zealand has revealed the existence of many shallow-water (<30m) ADRs. Currently, these habitats are under pressure from anthropogenic activities, like habitat alteration which can influence sedimentation regimes. My research aimed to increase our understanding of how sediment may influence the spatial distribution of ADRs in Wellington Harbour. My thesis aims to: 1) identify which environmental variables (if any) correlate with the distribution and community composition of ADRs in Wellington Harbour; and 2) assess the tolerance of a key habitat-forming species to elevated sediment levels.</strong></p><p>In my second chapter I collected environmental data over a one year period to examine how environmental conditions vary both spatially and temporally across four sites within Wellington Harbour. I then quantified the composition of ADRs at each site and used a modelling approach to explore the relationships between community composition and environmental variables. I found that most environmental variables (temperature, chlorophyll-a, and oxygen concentration) varied seasonally, but not spatially within the harbour, while sedimentation varied significantly between sites and seasons. Substrate composition and ADR community composition varied significantly between all sites. Statistical modelling revealed that 54% of the variation in community composition was driven by the environmental factors measured. Sedimentation and substrate composition were found to be a significant drivers of ADR community composition in Wellington Harbour.</p><p>In my third chapter I investigated how a sponge species common across all sites, Suberites australiensis, responded to elevated deposited and suspended sediment loads. Two ex-situ experimental studies were conducted. The first experiment exposed sponges to a range of sedimentation rates over the course of 16 days, while the second experiment used pulses of suspended sediment for 19 days. The main finding was that S. australiensis was highly tolerant to both forms of sediment, with high survival rates and no significant negative effects. My study provided evidence that this species may employ a combination of passive and active mechanisms to mitigate sediment accumulation and clogging. These adaptations may allow S. australiensis to survive in highly-sedimented areas where other suspension feeders may be excluded.</p><p>Overall, my results demonstrate that ADR community composition in Wellington Harbour may be vulnerable to shifts in sediment regimes. Increases in sediment deposition at low-sedimentation sites may drive shifts in ADR community structure by favouring sediment-tolerant species, such as S. australiensis, and potentially causing declines in biodiversity as sediment-intolerant species are excluded. Conversely, highly-sedimented communities may be vulnerable to decreased sediment input as it could allow species that were previously excluded from these areas to become established. My findings, alongside similar patterns in other countries, demonstrate the importance of sediment in structuring benthic communities in coastal areas. Additionally, it highlights the vulnerability of ADRs to anthropogenic activities that may alter the input or resuspension of sediment in marine environments. My research provides valuable insights that may be used to inform the management and conservation of these ecologically important habitats on a local and global scale.</p>