The benefits of adding complexity to seawalls vary across environmental gradients

Artificial structures such as seawalls are an increasingly dominant habitat of marine urban environments. As compared to natural habitats, seawalls are usually flat, featureless, vertical surfaces that support reduced biodiversity. Eco-engineering aims to mitigate negative ecological effects of seawalls by incorporating ecological principles into their design. One approach is to add complexity that increases microhabitat diversity and surface area. While initial tests have provided positive results, it is unclear the range of environmental conditions across which eco-engineering interventions provide ecological benefits. This study assessed how the effects of complexity on biodiversity vary across pollution and estuarine gradients. Colonisation of flat and complex (creviced / ridged) tiles, affixed to seawalls, was compared between sites close to and away from stormdrains, in inner and outer Sydney Harbour. Effects of complexity on the biodiversity colonising seawall tiles varied from neutral to highly positive. Whereas complexity effects were independent of proximity to stormdrain, having generally greater effects in the outer than inner harbour. This latter result may reflect the greater species pool of colonists in the outer harbour. These results suggest that eco-engineering interventions based on complexity will not have universally positive effects and are dependent on the species pool and the environmental conditions -- abstract.