Shoreline Armoring in an Estuary Constrains Wrack-Associated Invertebrate Communities

Abstract

Beach wrack is an organic subsidy that supports high intertidal and supralittoral invertebrate communities in many coastal systems. Beaches fringed with riparian vegetation accumulate wrack from both terrestrial leaf litter and marine algae/seagrasses, forming a reciprocal connection. Previous research has shown that shoreline armoring disrupts this marine-terrestrial connection and alters the amount and composition of beach wrack. We sampled invertebrates associated with beach wrack at 29 paired armored and unarmored beaches in Puget Sound, WA and conducted wrack decomposition experiments. Armored beaches had significantly fewer invertebrates as well as different assemblages. Unarmored assemblages were characterized by talitrid amphipods and dipteran and coleopteran insects (flies and beetles), and were correlated with the amount of beach wrack and logs, the proportion of terrestrial material in the wrack, and the maximum elevation of the beach. Experiments showed that talitrid amphipods and oligochaete worms were positively correlated with wrack decomposition rates. The substantial reduction in high-shore invertebrates at armored beaches represents a decrease in subsidies to secondary consumers in both adjacent terrestrial and nearshore ecosystems. These armoring effects may thus cascade, via altered food webs, to organisms in other environments. Our sampling of multiple armored-unarmored beach pairs allowed us to control for variability of many environmental parameters, improving our ability to identify armoring-related differences, and greatly expanding the scale of inference of previous studies showing the negative effects of armoring on beach fauna.