Small Creatures Signal Big Changes: The Role of Invertebrates in Understanding Restoration Effectiveness in the Stillaguamish River Delta

Abstract

Estuarine and river delta habitats contribute a vast array of ecosystem services that support coastal economies, livelihoods, cultural traditions and wellbeing. Human development has driven losses of these important ecosystems on a global scale, and restoration is being prioritized to address the environmental impacts of these losses. In the Pacific Northwest region of the United States, river delta restoration is often performed to support recovery of Pacific salmon (Oncorhynchus spp.) that inhabit these ecosystems as juveniles during their outmigration from natal streams to the Pacific Ocean. For Puget Sound Chinook salmon (O. tshawytscha), the period of marine entry is a critical life stage in which access to abundant and high-quality invertebrate prey can determine marine survival. Successful restoration returns dynamic ecosystem processes to delta landscapes that enable establishment of macroinvertebrate communities, and in turn, produce favorable foraging conditions for juvenile salmon as well as other estuary-dependent consumers. We investigated spatial and temporal variation in terrestrial and benthic macroinvertebrate community structure in restoration and reference areas of the Stillaguamish River delta, Puget Sound, Washington State, USA. We found that high-quality invertebrate prey, including adult dipterans and other insects, polychaetes, and amphipods were abundant in restoration areas, suggesting that restoration efforts are effectively producing energy-rich prey for juvenile Chinook salmon. The largest increase in terrestrial invertebrate abundance occurred between April and May, and benthic invertebrate abundance peaked in May. We found similar terrestrial invertebrate abundance and diversity across the delta. Restoration areas had higher benthic invertebrate abundance and greater diversity of marine-associated taxa than reference areas. Our results will provide a useful comparison as current restoration sites age and as new restoration projects are completed in the future.