“Accidental” artificial reefs — lessons from Singapore for coral and seagrass restoration along urban shorelines

Abstract

Due to rapid urbanization and coastline modification, the city-state of Singapore has lost nearly three-quarters of its intertidal coral reef area and nearly 90% of its subtidal coral reef area since pre-colonial times. However, the recolonization of coral communities has occurred naturally on seawall structures despite the lack of any deliberate planning or intervention. Recent mapping of coastal habitats shows that seawall reefs now form a significant proportion of habitat occupied by hard corals in Singapore, including areas where there were historically no coral reefs, and often exist side-by-side with other regenerated marine habitats such as seagrass meadows. These “accidental” artificial reefs demonstrate that new hard coral and seagrass communities can establish along highly urbanized shorelines, albeit in a novel form, and provide ecosystem services to urban communities. Key lessons can be drawn from these case studies, particularly relating to landscape design of urban shorelines to facilitate ecological coral and seagrass restoration. These include constructing seawalls with optimal slope profiles, providing creviced berms at the lower intertidal of seawalls, co-locating seawalls with artificial intertidal lagoons, and considering coral larvae supply in choosing sites for restoration using seawalls. Most importantly, these case studies illustrate the potential of using seawalls to restore hard coral and seagrass habitat on a landscape-scale, especially in urban environments.