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

Abstract

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 < 0.05) even though within-type site variability was significant (p < 0.005). Coral species diversity and growth form abundance were also significantly differentiated by seawall type (respectively p < 0.05). Tidal height (+0.3 m CD) was another significant factor for sessile cover composition and growth form abundance (p < 0.05). In general, sloping seawalls showed higher within-site variability and less obvious tidal stratification of sessile cover composition compared to vertical seawalls. In contrast, vertical seawalls showed lower within-site variability but higher site-to-site variation, as well as significant tidal stratification of coral growth form. These differences in community structure on apparently uniform seawall surfaces suggest the need for nuanced approaches to rehabilitate and enhance biodiversity in the intertidal zone.