The effects of substrate brightness and complexity on intertidal communities of seawalls

The marine environment is being increasingly urbanized at the expense of biodiversity. Increased interest in ecological engineering to mitigate this loss of biodiversity has involved introducing habitat complexity to marine infrastructure, however other factors have been largely overlooked, such as substrate brightness, which can affect species colonisation and thermal absorption. Concrete panels, with different complexities and brightness treatments, were affixed to seawalls in Sydney Harbour to investigate how habitat complexity and substrate brightness influences intertidal communities and how these effects change with respect to tidal elevation. I hypothesised that the effects of habitat complexity would strengthen across a passive warming gradient created by decreasing brightness, with greater levels of colonisation when complexity was present. Overall, effects of habitat complexity and substrate brightness were largely independent of each other, with some exceptions, and varied greatly by site. Substrate brightness influenced colonisation initially, with generally greater colonisation on white panels, but its effects diminished over time, with effects of habitat complexity instead growing stronger. Abundance and richness were greater on complex than flat panels. The strength of these effects varied across sites, and tidal elevations, indicating the need to match eco-engineering interventions to site conditions in order to achieve ecological benefit.