It is not all black and white—The effects of substrate brightness on the intertidal communities of seawalls

1. Substrate brightness can influence colonisation of marine surfaces, but is typically overlooked in the design of marine constructions for ecological co-benefits. Many micro-organisms, invertebrate larvae and algal spores are negatively phototactic, preferentially settling on dark surfaces. Brightness may also influence post-settlement processes in the intertidal by influencing passive warming of substrates and by influencing the crypsis of organisms susceptible to visually feeding predators. 

2. To inform eco-engineering designs, this study assessed how the brightness of settlement panels influenced biofilm establishment, passive warming of the substrate and microbial and macrobenthic community development on three seawalls in Sydney Harbour, Australia. 

3. Brightness was manipulated using greyscale customised concrete panels, spanning black, white and three intermediate brightness values matching seawalls constructed of local quarried sandstone and concrete, as well as Hawkesbury sandstone rocky shores—the most similar natural habitat analogue. Biofilms were sampled from one site after 6 weeks, and the temperature and macro-community development were tracked across brightness treatments at all three sites quarterly for 2 years. 

4. Maximum temperatures were generally greater on darker (blacker) than brighter (whiter) tiles, with this pattern strengthening with tidal elevation and sun exposure. White tiles hosted distinct microbial and macrobiotic communities, which were generally more taxonomically rich and contained greater organism abundances/covers than darker tiles. Effects of brightness were greater in summer than winter, and at sunny than shaded sites, diminishing through time as tiles became fouled. 

5. Synthesis and applications. Our results suggest that brightness should be considered when designing marine structures for ecological co-benefits. Particularly in environments exposed to high solar irradiance, bright surfaces can reduce thermal stress to organisms, resulting in more biodiverse communities. Where construction using dark materials at sun-exposed orientations is unavoidable, timing the construction to occur in cooler, winter periods may allow establishment of communities that ameliorate thermal effects before peak summer temperatures occur.