Shoreline armoring disrupts marine-terrestrial connectivity across the nearshore ecotone

Abstract

As the interface between land and sea, the nearshore (marine-terrestrial) ecotone converges at the intertidal zone, where the exchange of organic materials between ecosystems occurs in the form of beach wrack: piles of seaweed, seagrass, and terrestrial plant debris suspended in water and deposited on shore as the tide ebbs. The ecological significance of algal and seagrass wrack subsidies has been well-documented for exposed-coast sandy beaches but is relatively unstudied in lower-energy and mixed-sediment beaches. In the nearshore ecotone where beaches are fringed with riparian vegetation, the potential for reciprocal subsidies between marine and terrestrial ecosystems exists. Human modification also occurs within this ecotone, particularly in the form of armoring structures for bank stabilization that physically disrupt the connectivity between ecosystems. I conducted detailed surveys of beach physical parameters, wrack and log accumulations, and supralittoral invertebrates in spring and fall over 3 years at 29 armored-unarmored beach pairs, and behavioral observations of juvenile salmon (<italic>Oncorhcynchus</italic> spp.) and birds at 6 pairs in Puget Sound, WA, USA. Armoring lowered the elevation of the interface between marine and terrestrial ecosystems and narrowed the width of the intertidal transition zone. Armored beaches had substantially less wrack overall and a lower proportion of terrestrial plant material in the wrack, while marine riparian zones (especially trees overhanging the beach) were an important source of wrack to unarmored beaches. Armored beaches also had far fewer logs in this transition zone. Invertebrate assemblages were significantly different between armored and unarmored beaches. Unarmored invertebrate assemblages were characterized by talitrid amphipods and adult and larval dipteran and coleopteran insects (flies and beetles, including some types that have been shown to contribute to juvenile salmon diets in other studies) and correlated with the amount of beach wrack and logs, the proportion of terrestrial material in wrack, and the maximum elevation of the beach. Shoreline type (armored or unarmored) influenced juvenile salmon distribution, however their feeding rates were relatively high at all sites, thus decreased prey availability (i.e. fewer marine riparian and/or wrack-associated insects) or altered prey resources are likely the most detrimental effects of armoring on these fish in the nearshore ecotone. Terrestrial birds, particularly Song Sparrows (<italic>Melospiza melodia</italic>) were commonly observed foraging among beach wrack and logs at unarmored beaches, but were largely absent from armored beaches. Based on my results, I developed a conceptual model summarizing marine-terrestrial connections across the nearshore ecotone and the disruptive effects of armoring. This study demonstrates that shoreline armoring disrupts marine-terrestrial connectivity, affecting the amount and type of organic material delivered to the nearshore ecotone in the form of wrack and logs, the abundance and taxonomic composition of supralittoral invertebrates, and the distribution and behavior of secondary consumers (juvenile salmon and birds). The results of my dissertation provide new information on relationships between physical and biological variables in the nearshore ecotone and connections between marine and terrestrial ecosystems that may be useful in informing conservation, restoration, and management actions.