Intertidal seawall communities in Singapore: Are they all one and the same?

Ecological rehabilitation of seawalls requires a good understanding of the underlying community structure, which is still poorly known in the tropics where coastal urbanization is rapidly taking place. In this study, we compared intertidal sessile biota on sloping (20°) and vertical (90°) seawalls on a small, reclaimed offshore island in the Singapore Strait and examined the pattern of heterogeneity of sessile assemblages within each seawall type. Cover of sessile organisms, abundance of hard coral species as well as their growth forms across two tidal height ranges in the low littoral region (−0.2 to +0.3 m Chart Datum (CD) and + 0.3 to +0.8 m CD) were assessed. Seawalls had high biotic cover (mean cover of 91 ± 15%), comprising a variety of sessile organisms ranging from algae and non-colonial organisms including oysters and barnacles to colonial organisms such as cnidarians and sponges. Turf algae comprising coralline and non-coralline rhodophytes dominated the intertidal zone below +0.8 m CD. A moderate negative correlation was detected between crustose coralline and non-coralline algae (Spearman coefficient = −0.56). Coralline algae were relatively conspicuous on several vertical seawall sites. Seawall type was a significant factor differentiating sessile cover composition (p