Mitigating against the loss of species by adding artificial intertidal pools to existing seawalls

Over 80% of the money allocated to protect coastlines from climatic change is spent building new seawalls or increasing the stability, height and length of existing seawalls. Although this protects important infrastructure, it has serious consequences for intertidal biodiversity, because walls are built and maintained according to engineering and financial criteria, despite the fact that they are known to adversely affect intertidal biodiversity. We tested the predictability and reliability of adding pots, designed to simulate rockpools, to seawalls as an artificial habitat to mitigate against loss of intertidal species. Two sizes of concrete pots were attached at mid- and highshore tidal levels to sandstone seawalls at 2 locations in Sydney Harbour, Australia. After 7 mo, pots increased intertidal biodiversity on the seawalls by adding additional species. They also supported greater covers and densities of algae and many invertebrates compared with established assemblages on the seawalls. The size of the pot, its height on the wall and its location affected the assemblages that developed, with greater abundances and diversity of organisms in shallower pots and those at midshore levels. Although there were slight differences between locations for some taxa, these results show an easy cost-effective method that authorities can use to try to mitigate the adverse effects on intertidal assemblages of armouring shorelines with featureless, vertical walls.