It’s not all black and white – the effects of substrate brightness on the intertidal communities of seawalls

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. 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. Brightness was manipulated using
grey-scale 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 was tracked across brightness treatments at all three sites
quarterly for two years. 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. 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.