dc.description.abstract |
Coastal development can place increased pressure on adjacent marine ecosystems through increased sedimentation derived from terrestrial sources. Carpophyllum flexuosum forests characterise rocky reefs in sheltered embayments in many parts of New Zealand. These sheltered habitats are often subjected to high levels of sedimentation yet little is known about how sedimentation affects the resilience of these algal canopies and their associated communities. In this thesis the structure, dynamics, growth rate, and resilience of Carpophyllum flexuosum stands and associated understorey communities were investigated to see how they vary across a sedimentation gradient within the Hauraki Gulf; and additionally, what ecological role C. flexuosum plays as a habitat-forming species. This was investigated by carrying out surveys and experimental manipulations, in the form of canopy removal and transplant experiments, at sheltered sites where large C. flexuosum stands naturally occur, across the Hauraki Gulf. This meant that the effects of sedimentation could be examined without the often confounding effects of wave exposure. The sedimentation gradient, which exists in the Hauraki Gulf, was quantitatively described, with sites closer to Auckland having higher sedimentation regimes, which diminish in a northerly direction. The two variables that consistently explained the most variation in understorey communities among the sites were water clarity and percentage of fine sediments (<63 μm) trapped in sediment traps, suggesting that the communities associated with C. flexuosum forests were strongly influenced by sedimentation levels. There was generally lower diversity at high sediment sites, with lower covers and abundances of encrusting sponges, compound and stalked ascidians, bryozoans, crustose coralline algae (CCA) and a number of gastropod species compared to low sedimentation sites. The removal of C. flexuosum canopies had large affects on the understorey communities indicating that these habitats play an important role in facilitating associated communities, most likely by regulating light levels and also influencing the amount of sediment deposition on the substratum. In high sediment areas, the recovery of C. flexuosum following disturbance appeared to be slower; however, growth experiments observed greatest growth rates at the more urban high sediment impacted sites. This suggests that while C. flexuosum exhibits a tolerance to sedimentation in relation to growth, the overall resilience of C. flexuosum stands may be limited in high sediment environments by other mechanisms. Reduced resilience of C. flexuosum forests and the vulnerability of understorey communities to elevated sedimentation, emphasises to policy makers the importance of ensuring that land management practices minimise the input of sediments into the coastal environment in order to help maintain this important habitat. |
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