Shoreline armoring disrupts marine-terrestrial connectivity in the Salish Sea, with consequences for invertebrates, fish, and birds

Within the marine-terrestrial ecotone, upper intertidal “wrack zones” accumulate organic debris from algae, seagrass, and terrestrial plant sources and provide food and shelter for many organisms. We conducted detailed surveys of wrack and log accumulations and supralittoral invertebrates in spring and fall over 3 years at 29 armored-unarmored beach pairs in Puget Sound, WA, USA. Additionally, behavioral observations of juvenile salmon (Oncorhynchus spp.) and birds were conducted at 6 pairs. Armored beaches had substantially less wrack overall, a lower proportion of terrestrial plant material in the wrack, and far fewer logs. Armored beaches had significantly fewer invertebrates and differed from unarmored beaches in their taxonomic composition. Unarmored invertebrate assemblages were dominated by talitrid amphipods and insects, and were correlated with the amount of beach wrack and logs, the proportion of terrestrial material in wrack, and the maximum elevation of the beach. Shoreline armoring influenced juvenile salmon distribution, with fewer overall observations and fish in deeper water at armored beaches, but their feeding rates were relatively high at all sites. Terrestrial birds were commonly observed foraging among beach wrack and logs at unarmored beaches, but were largely absent from armored beaches. 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 impacts of armoring on Salish Sea beaches: overview, background, and methods

Shoreline armoring is widespread in the Salish Sea, but few data have documented actual impacts on physical or biological features of local nearshore ecosystems. Armoring marine shorelines can alter natural processes at multiple spatial and temporal scales; some, such as ‘starving’ the beach of sediments, may take decades to become visible, while others such as ‘placement loss’ are immediate. Two research teams have been studying a broad set of sites to document parameters that do and do not change when a shoreline is armored, and to determine under what conditions armoring has significant effects. Our methodology has been to compare paired adjacent armored and unarmored beaches. During the years 2010 - 2013 we completed surveys at 6 pairs of beaches in South Sound, 25 pairs in Central Sound, and 36 pairs in the North. At all sites we have data on habitat and setting (e.g., overhanging vegetation, location in the drift cell); beach topography; sediment size distributions; abundance and types of logs, wrack, and invertebrates in the wrack line; and juvenile clams at Mean Low Water. At some sites we have data on forage fish spawn. Overall, demonstrating physical differences between paired beaches is difficult given the very high natural variability among beaches, although obvious effects such as reduction in beach width are evident. Limited data show that wave heights are greater where they interact with armoring, and beach slope near the armoring tends to steepen. Many biological effects are clear at the upper part of the beach, while lower elevation effects (further from armoring) are progressively harder to demonstrate. Armoring impacts are less dramatic at the pairs of beaches in the northern Salish Sea. There does not appear to be a distinct threshold in elevation of armoring that causes increased impacts, but instead a gradient. Analyses of these diverse datasets are ongoing, and results are detailed in the following linked presentations