Succession of seawall algal communities on artificial substrates

Increasing coastal urbanisation has resulted in the extensive conversion of natural habitats with manmade hard structures, such as seawalls, which tend to support communities with low biodiversity. While seawalls are often colonised by species that can be found on natural rocky shores, some studies have shown that their community structure and dynamics are markedly different. However, relative to rocky shores, ecological research on seawalls is limited, and this is especially so in the tropics. To our knowledge, no research to date has examined, in the context of artificial coastal defences, the ecological succession of communities on substrates of varying complexity near the equator. Hence, the aim of the present study is to quantify the patterns of algal succession on ‘simple’ and ‘complex’ concrete tiles and granite controls mounted onto seawalls at two offshore sites in Singapore (Pulau Hantu and Kusu Island). Our results revealed the development of an algal assemblage that is typical of many tropical rocky shores; i.e., ephemeral green turfs succeeded by high cover of a grazer-resistant mat of erect and encrusting algae with the foliose macroalgal functional group poorly represented. All treatments developed macroalgal cover by the first month. Final mollusc assemblage structure after one year was also quantified, as molluscs are important consumers in structuring algal assemblages. While the succession trajectories were similar at both sites, the rates of succession differed. The transitions from ephemeral green turfs to the mixture of red and brown macroalgal assemblages, as well as the development of encrusting coralline and non-coralline algae, occurred two months later at Pulau Hantu (the more sheltered site). Granite controls did not support foliose or articulated calcareous algal functional groups within the sampling period, probably due to material and structural/ topographical differences. Documenting such small-scale spatial patterns of algal distribution represents the first step towards a better understanding of the processes occurring in artificial habitats—and this should ultimately aid their reconciliation.