Ecological Responses to Habitat Enhancements of an Urban Intertidal Seawall

Marine shorelines worldwide are being increasingly modified with armoring as coastal populations grow and the risk of sea level rise increases.  Seawalls are the least complex armoring structure, typically built of smooth vertical concrete.  Compared to natural rocky intertidal shorelines they reduce surface area, lack microhabitats that provide refuge for intertidal organisms, and support fewer taxa.  Much of the Puget Sound waterfront in King County, Washington has been altered, including 7000 feet of seawall in downtown Seattle.  Despite this, Seattle’s shoreline is occupied by outmigrating juvenile salmon.  In 1999, Puget Sound Chinook were listed as threatened under the Endangered Species Act and subsequent studies found that juvenile Chinook salmon were abundant and were observed in greater density at shallow depths adjacent to the seawall than in deeper areas.  With the Seattle seawall currently scheduled for replacement, the University of Washington and City of Seattle have been collaborating to test whether the seawall can be enhanced to improve ecological function by incorporating more complex substrata. Habitat enhancement test panel designs have been installed along the downtown seawall that include varied treatments of texture and slope.  Sessile invertebrates, algae, and epibenthic meiofauna were monitored over two years to compare taxa richness and abundance among various designs and textures and to the existing seawall.  Results from the monitoring (2008-2009) indicate that enhanced seawall surfaces benefit ecologically important species such as mussels (Mytilus) and algae (Fucus). Enhanced seawall habitat also had increased taxa richness of epibenthic invertebrates and increased abundances of some juvenile salmon prey such as midge larvae and harpacticoid copepods.  The findings of this study suggest that incorporating features of slope and texture into seawalls can mediate some of the declines in biodiversity and productivity along urban shorelines where restoration opportunities are limited due to infrastructure and/or bathymetry.