Ecological effects of coastal armouring on sedimentary shorelines

As human populations continue to grow in the coastal zone, there is need to protect both natural and man-made infrastructure from coastal erosion and inundation. The construction of seawalls is one common approach to coastal protection, which can have large impacts on the structure and function of ecosystems. Although commonly constructed along sedimentary shorelines, seawall impacts to soft sediment ecosystems remain poorly understood, particularly in the southern hemisphere. This thesis compares the structure and function of estuarine sedimentary communities between pairs of sites with and without seawalls in estuarine habitats of temperate south-eastern Australia. Sampling along largely unvegetated shorelines revealed differences in infaunal communities between armoured and unarmoured sites that varied according to environmental setting. Patterns with respect to armouring were strongest at mid-intertidal elevations, with higher abundances at armoured than unarmoured sites at sandy locations, but the reverse pattern was apparent at muddy locations. Mangrove forests with and without armouring displayed structural differences, with armoured forests narrower in width, with higher pneumatophore densities, and smaller wrack deposits including a greater proportion of terrestrial litter. Wrack on armoured mangrove forests washed away more readily, and decomposed more rapidly when containing terrestrial litter. Densities and richnesses per unit area of epifauna generally displayed idiosyncratic patterns with respect to armouring in mangrove forests, but taxa, such as anemones, that attach to pneumatophores had higher densities at sites with seawalls. Although densities of fauna did not display consistent patterns with respect to armouring, reduced habitat availability was associated with armouring. Overall the results of this thesis suggest that impacts of seawalls on sedimentary shoreline are highly context dependent and understanding how impacts vary as a function of the local species pool, abiotic conditions, and seawall design will be critical to managing and mitigating impacts.