Comparison of wrack dynamics between mangrove forests with and without seawalls

The supply and fate of wrack (dead organic matter) is a critical determinant of the structure and function of shoreline ecosystems, and their role as carbon repositories. The increasingly common practise of armouring urbanised shorelines with seawalls impacts wrack deposits of unvegetated estuarine and coastal shorelines by truncating the intertidal zone and/or by modifying the physical and biological processes that deliver and remove wrack. This study tested whether such effects also extend to mangrove forests. A survey of wrack deposits in mangrove forests with and without seawalls along the Parramatta River, Sydney, Australia, revealed that at sites with seawalls placed at a mid-intertidal elevation wrack deposits were shifted from the high- to mid-intertidal but were otherwise of similar cover and composition. Experiments tracking the fate of wrack determined that, as compared to the mid-intertidal zone of unarmoured shorelines, wrack deposits at sites with seawalls were more readily mobilised. This was in some instances countered by the reduction in Casuarina glauca litter on armoured shorelines, as experiments revealed that Avicennia marina leaf decomposition was slower in the absence than the presence of C. glauca. Overall, the results suggest that effects of armouring on wrack composition and dynamics may be weaker in mangrove forests than on unvegetated shorelines. This could reflect the predominantly autochthonous source of wrack in mangrove forests, the habitat structure of forests minimizing hydrodynamic impacts of seawalls, and/or the differing reasons for which hard structures are constructed in low hydrodynamic energy vegetated as opposed to high hydrodynamic energy unvegetated settings.