Distribution of current and future cumulative risk and contribution of threats to eelgrass and canopy kelp in Puget Sound

Executive Summary

In Puget Sound, Washington USA, native eelgrass (Zostera marina) and floating bull kelp (Nereocystis luetkeana) are less abundant compared to their historical distributions. Despite conservation and restoration efforts, recent, localized declines raise ongoing concerns. Eelgrass beds and kelp forests exist at the marine-terrestrial interface and are foundational nearshore habitats that support biodiversity, fisheries, and coastal resilience. Therefore, conservation and restoration decisions for eelgrass and kelp should be guided by a large-scale understanding of both current and future threats in order to maximize ecosystem resilience. This project assesses current and future risks to these habitats through a region-wide cumulative, spatially explicit risk analysis for eelgrass and kelp habitats using the open source InVEST Habitat Risk Assessment tool. Cumulative risk assessment is a widely used framework to identify risk from multiple, synergistic threats and assist in prioritizing management actions. For this model, we integrated spatial data on habitat extent, seventeen current and future threats, and one ecosystem resilience factor into a unified framework. The analysis encompassed eight marine subregions and used expert-based scores in the calculation of risk. Threat layers broadly described marine and terrestrial human activities and water quality. Risk model results were used to identify hotspots of risk for eelgrass and kelp, evaluate the coherence between risk and habitat monitoring trends, and examine the relative contribution of individual threats to total risk.

Risk to eelgrass was highest in Hood Canal and North Puget Sound. For kelp, the highest-risk areas included South and Central Puget Sound and the Saratoga/Whidbey Basins. While San Juan Islands had the lowest risk for both habitats, there were multiple high risk outliers. The importance of individual threats differed among subregions and between habitats, but there were commonalities: shoreline armor, river flow, industrial land use, and impervious surface coverage were commonly the top threats to eelgrass by subregion and nutrients (low and high), vessel traffic, industrial land use, and impervious surface coverage were commonly top threats to kelp. Risk estimates aligned more closely with kelp monitoring trends than eelgrass, indicating that eelgrass threats may not have been adequately captured in our analysis. Projected future water quality and sea level rise threats generally resulted in increased risk to both habitats, although eelgrass risk increased more than kelp. A detailed analysis of risk and contributing threats is presented for three high priority subregions: Eastern Strait of Juan de Fuca, San Juan Islands, and South Puget Sound.

Our analyses can inform investment in habitat restoration, conservation planning, monitoring, and regulatory decision-making from a regional perspective and complements ongoing work to identify drivers of marine vegetation and appropriate management interventions at finer scales. Future work can evaluate regions with similar risk profiles but different habitat trends to elucidate possible reasons for observed differences. Importantly, this project establishes a risk framework that can be updated for eelgrass and kelp as better information becomes available and can be adapted to support efforts to manage other vulnerable habitats in Puget Sound.

Citation

Magel, C. L., Bogue, K., Raymond, W. W., Mazzilli, S., & Kanojia, M. (2025). Distribution of current and future cumulative risk and contribution of threats to eelgrass and canopy kelp in Puget Sound [Report prepared for Puget Sound Partnership]. Puget Sound Institute.