Preserving Puget Sound’s Estuarine Habitats

USGS Cooperative Research Unit Corner

Preserving Puget Sound’s Estuarine Habitats

The U.S. Geological Survey (USGS) Oregon Cooperative Fish and Wildlife Research Unit recently led a coordinated effort among university, agency, and Tribal partners to evaluate estuarine elevation change and resilience to sea-level rise in Puget Sound. Their findings highlight the key role that sediment plays for promoting resilience in these vital coastal ecosystems.

restoration area in the Nisqually River Delta floods at high tide

Puget Sound, Washington is the second-largest estuary in the contiguous United States and is home to 16 large river deltas. These delta ecosystems provide numerous ecosystem services to society, including foraging and rearing habitat for culturally and economically valuable fish and wildlife species.

Since the 1800s, as much as 70–80% of Puget Sound’s historical delta and estuarine habitats have been lost to human development. Climate change and sea-level rise are expected to further exacerbate habitat loss, with recent estimates of sea level rise across the region ranging 0.5–1 m by 2100. Although estuarine marshes have a built in “safety mechanism,” whereby a dynamic equilibrium between sediment delivery and tidal inundation can offset the negative effects of rising sea level, this equilibrium may be disrupted by a more rapid rise in sea level. As such, it is crucial to monitor present day rates of estuarine sedimentation and elevation change to better predict marsh response to sea level rise and inform decision-makers and resource managers.

The USGS Oregon Cooperative Fish and Wildlife Research Unit recently collaborated with the USGS Western Ecological Research Center and USGS Pacific Coastal and Marine Science Center alongside scientists and managers at Western Washington University, the National Oceanic and Atmospheric Administration, the Tulalip Tribes, Snohomish County, and Padilla Bay National Estuarine Research Reserve to evaluate whether Puget Sound’s estuarine habitats may keep pace with current and projected sea level rise.

This coordinated effort used a regional network of surface elevation tables (SETs) to assess vertical rates of elevation change in five Puget Sound estuaries: the Nisqually, Snohomish, Stillaguamish, and Skagit River estuaries, and Padilla Bay. Surface elevation tables are a technique used to track elevation change in coastal habitats whereby a portable leveling device that attaches to a permanent benchmark measures surface elevation change with millimeter precision. An extensive network of SETs throughout Puget Sound that, in some cases have been operational for several decades, provided researchers with a unique opportunity to assess elevation change and habitat resilience across a broad environmental gradient.

Findings from this network of SETs indicate that rates of elevation change are highly variable both within and among Puget Sound’s estuaries. At many sites, rates of elevation change are on track to keep pace with (or even exceed) projected sea-level rise through the end of the twenty-first century; however, this is not the case for all study systems. For instance, elevation change at sites in Padilla Bay are negative (below the rate of modern sea level rise) showing erosion, likely owing to its lack of hydrological connectivity with a major and historical sediment source of the Skagit River. Estuaries with a high relative sediment load (Stillaguamish, Skagit) are far more likely to have vertical accretion rates comparable or exceeding sea level rise, but uncertainty remains as to the extent and duration that each system can retain sediment critical to outpace sea level rise given additional stressors that can affect marsh resilience. This underscores the need for improved knowledge and importance of sediment management for maintaining resilient estuarine ecosystems in the face of climate change.

This research shows that Puget Sound’s estuarine habitats can be resilient to rising sea-levels—as long as sediment delivery is maintained. In the Skagit and Stillaguamish estuaries, sediment delivery already appears sufficient to promote resilience, while in the Nisqually and Snohomish estuaries and especially Padilla Bay, managers may have to seek out creative solutions to increase surface elevations. Modifications to current habitat management strategies that enhance the availability, distribution, and retention of sediment will be key for promoting the resilience of vital estuarine ecosystems in Puget Sound and beyond.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Melanie Davis, Assistant Unit Leader & Assistant Professor, USGS Oregon Cooperative Fish and Wildlife Research Unit at Oregon State University.

Photo Credit
Ryan Munes, USFWS
June 14, 2024