Ecological response and physical stability of habitat enhancements along an urban armored shoreline

Abstract

Shoreline armoring is prevalent worldwide and has resulted in substantial habitat alteration in heavily urbanized areas. The biological and physical processes associated with these shorelines have in many cases been compromised, which has led to a recent focus on how to design and implement projects to restore some of the lost or impaired functions, termed enhancement. We describe a multi-year effort testing whether an enhanced site has improved conditions in Seattle, WA, USA, along urban marine shorelines of Puget Sound. The Olympic Sculpture Park opened in January 2007 and included construction of two shallow-water features: a low-terrace habitat bench placed in front of an existing seawall, and a constructed pocket beach that replaced existing riprap. Riparian vegetation was also planted in the uplands replacing impervious surfaces and manicured lawn. We measured the functions of these sites by sampling both before and after enhancements (2005, 2007, and 2009), and comparing to adjacent armored shorelines. Although we are limited in our ability to make generalizations beyond this specific site due to only having one replicate of each shoreline type, the unique aspects of this urban enhancement make it useful as a case study that can apply to other urban systems. Fishes that are dependent on shallow water habitat were a main focus of sampling, specifically outmigrating juvenile salmon (Oncorhynchus spp.) and larvae of other species. Terrestrial and aquatic invertebrates were also assessed, both as a metric for habitat quality and as a determinant of available prey resources for juvenile salmon. Physical features of the created habitats were monitored in post-enhancement years to measure their stability. Results showed that shoreline enhancements increased densities of larval fishes and juvenile salmon and measurements of juvenile salmon feeding behavior dependent on the year, and provided habitat for invertebrate assemblages that were different from armored shorelines and had high taxa richness. Physical resilience depended on both natural processes and human activities, demonstrating the need to incorporate anthropogenic use into the management of urban shorelines.