Ecological enhancement of breakwater upgrades: size and type of rocks used influence benthic communities

Abstract

Existing coastal breakwaters are ageing and will need to be upgraded to withstand additional forces associated with rising sea levels and storms. Structural upgrades can affect taxa living on or adjacent to breakwaters. These impacts can be mitigated by ecological engineering of breakwaters, which can enhance habitat quality without losing their primary purpose of protection. A recently upgraded breakwater at Coffs Harbour, NSW, Australia was eco-engineered to use boulder fields to mitigate impacts on a critically endangered alga (Nereia lophocladia) living on and adjacent to the infrastructure. Over a year, we assessed the effect of different rock sizes (small versus large), types (greywacke versus granite) and orientations (top versus bottom) on the composition and diversity of benthic taxa. N. lophocladia has yet to recruit to the eco-engineered habitat; however, we found rock size, type and orientation significantly influenced overall benthic assemblage composition, at least at one of the sites. Furthermore, the bottom of the rocks had a higher taxonomic diversity than the top side, and assemblages on native greywacke rocks were more diverse than those on granite, but only at one of the two sites. Overall, the magnitude of differences in benthic assemblage structure and diversity showed substantial temporal and spatial variation, with no clear temporal trends or successional patterns. Our results indicate that the ecological outcomes of coastal protection infrastructure upgrades could be improved by including native rocks of a range of different sizes in multiple patches and layers.