Diversity of subtidal benthic and hard coral communities on sloping and vertical seawalls in Singapore

Decades of coastal urbanisation have replaced many natural shorelines with seawalls. Marine communities have been documented on these replacement habitats, but little is known about the effects of seawall inclination on the diversity and depth distribution of sessile organisms on this artificial substrate. Surveys of hard coral and other sessile communities in Singapore indicated that benthic communities at the deeper zone (- 2 m chart datum) were significantly different between three sloping (about 33 degrees inclination) and three vertical seawalls of similar age. Hard coral communities were significantly different between sloping and vertical seawalls and between 0 m chart datum and - 2 m chart datum. Hard coral communities on sloping seawalls were also significantly more diverse than on vertical seawalls. At both depths, sloping seawalls were dominated by abiota with hard coral comprising appr. 17% of the total surface, while vertical seawalls were dominated by algal assemblage with hard coral comprising appr. 10% of the total surface. Overall, the results showed that sloping seawalls were significantly better at supporting coral communities, likely due to less intensive habitat compression and buffered wave action on them, especially at 0 m chart datum. The need to understand how surface topography affects benthic and coral communities in light of growing coastal urbanisation is also highlighted. This study emphasises how sloping seawalls can support greater marine biodiversity than vertical seawalls.