Ecological impact of single and semi-contiguous artificial rockpool installations on the assemblages and species richness of vertical seawalls

Local improvements to species diversity through the creation of microhabitat features have been adopted as an approach for “Greening Grey Infrastructure” (GGI) in urbanised coastal ecosystems. To confidently implement these enhancements asset managers and engineers need quantitative information on the value of different feature types, densities, and configurations. We compared the biodiversity benefits of horizontal arrays of semi-contiguous 3 and 5 artificial rockpools with single isolated rockpool units and unenhanced sections of seawall. Rockpools were fixed within seawall sections 2 m wide at Mean High Water Neap Tide Level. At low tide, biota was monitored inside the pools, on the side of the pool units, the sea wall adjacent to the rockpools and in sea wall zones above and below the pools. After 36 months, species richness (all zones combined) of seawall sections with five rockpools was up to four times greater than controls and included protected and non-indigenous species. Increased richness was attributable to a higher density of rockpools and not rockpool contiguity. Grazers attracted to areas between and above rockpools modified assemblages that may limit persistence of algae. At one site, recovery of brown algae following disturbance during rockpool installation remained incomplete after 36 months. Benefits of arrays of semi-contiguous pools remain unclear, and deployment of individual rockpools (or similar enhancements) over a larger habitat area, that experience a wider range of conditions, may be at least as valuable. Quantifying species richness per unit size/ area of structure should assist managers and the development of metrics designed to measure ecological benefits in GGI.