Engineering rock pools from seawalls

Seawalls, used to reduce the impacts of waves, are a common built feature of the costal landscape. While hard surfaces occur naturally in marine environments, past research has found that seawalls contain little of the diversity of species found on the natural shore. One possible reason for this is that seawalls lack the physical complexity of natural hard surfaces such as rock pools.

A recent study in the Journal Oecologia tested out a method of ‘ecological engineering’ to create sea walls that better resemble the physical features of rock pools. Intertidal rock pools provide important habitat for numerous specialist species. The researchers wanted to engineer the seawalls to increase local diversity of species that rely on rock pools as habitat.

Workers constructing engineered 'rock pools.' Image credit, Oecologia. On front page, sea anenomes. Image credit, Michael Zinkova.The study was conducted on a new seawall that was being constructed along a heavily urbanized portion of Sydney Harbor in New South Wales, Australia. The rock pool features were engineered into the wall by removing some sandstone blocks to create cavities and lining them with 5-cm thick sandstone shelves. A 10-cm lip was added to the front which created a feature that was submersed or flushed during high tide, but retained water during low tide. This engineering resulted in the addition of two habitat features not found on regular seawalls - 5 cm deep pools and a shaded vertical substratum. The researchers recorded species occurrence in the engineered habitat areas, the untreated portions of the seawall, and natural rock pools and compared the results.

Study Findings…
The study found that the new habitat features substantially increased the diversity of foliose algae and sessile and mobile animals, compared to the untreated seawall. Species that benefited included barnacles and tubeworms along with rarer species of bryozoans, ascidians and sponges not normally seen on seawalls. This was especially true higher on the shore where diversity was 7 x greater for the engineered habitat areas versus the untreated seawall. However, the study also found that a number of species found in natural rock pools were not found in the treated areas of the seawall – particularly mobile animals For example, sea urchin (Heliocidaris erythrogramma), starfish (Patiriella exigua), and various grazing snails were found in natural rock pools but absent from the seawall habitat areas.

According to the researchers, these results show that ecological engineering can substantially improve the biological diversity of otherwise sparse, artificial intertidal features. However more research is needed to figure out how to engineer habitat on seawalls to better enable colonization by a full range of species.

Source:	Oecologia
Title:	Engineering novel habitats on urban infrastructure to increase intertidal biodiversity
Authors:	M. G. Chapman and D. J. Blockley
	University of Sydney, Australia