The rainbow connection: The case for including substrate colour in the ‘eco‐engineering’ of marine constructions

Abstract The urbanisation of the world's coastlines is replacing diverse natural substrates with grey concrete. The changing shoreline colourscape may potentially influence colonisation of marine surfaces by influencing settlement patterns and predator–prey interactions, but is overlooked in the design of marine structures for ecological co‐benefits. This study assessed how manipulating the colour of concrete influenced benthic community development on intertidal seawalls in Sydney Harbour, Australia, when finfish predators had access to panels or were excluded by cages. Four colours—grey (the colour of concrete), red and yellow (the colours of natural Sydney sandstone rock) and green (the colour of algal biofilms)—were compared at high‐ and low‐intertidal elevations of two sites. Red substrate developed distinct ecological communities compared to yellow, green and grey substrates, with many taxa positively associated with red. Effects of colour were greater in the low intertidal than in the high intertidal, reflecting the greater number of colonising species on which colour could act. Effects were generally similar across caged and open surfaces, consistent with effects of colour on settlement preferences rather than predation effects. Contrary to the hypothesis that effects would diminish through time with declining bare space with which colonists could interact, the colour effect persisted for the duration of the 12‐month study. Synthesis and applications . Our results show that in the low intertidal, colour can shape sessile community assembly on concrete surfaces over time scales of at least 12 months. Consequently, alongside the habitat complexity and material type of marine built structures, colour warrants consideration in the development of eco‐friendly designs. The colour of marine built structures is easily manipulated, for example through the addition of non‐toxic oxides to concrete as done here.