Living on colour: influence of substrate colour on biodiversity on marine structures

Marine built structures, such as seawalls, are an increasing habitat feature of urban shorelines. Compared to rocky reefs – the most similar natural habitat - marine structures often support less biodiversity. Studies have focused on how habitat complexity and material type could drive this difference, but substrate colour may also contribute. Marine structures often differ in colour to natural habitats. Substrate colour may influence colonisation by determining larval settlement or by shaping post settlement mortality by altering prey detectability. To assess how colour influences colonisation by marine invertebrates and algae, concrete panels of three natural colours (green, red, yellow) and a grey control were attached to seawalls at two elevations of two sites in Sydney Harbour. Panels were caged or open to assess whether predation may drive any colour differences. Over 12 months, colour effects were generally greater in the low than high intertidal, reflecting the greater number of colonists, and were similar across caged and open surfaces, suggesting that colour effects largely occurred on settlement. Functional groups differed in their responses to colour over time, likely reflecting direct and indirect effects of colour. The results suggest that substrate colour is an important consideration in the eco-friendly design of marine structures.