South Puget Sound’s shorelines have lost 2/3 of their bull kelp forests since the 1870s, according to a just-published DNR study of long-term trends in the iconic seaweed that grows along Washington’s saltwater shorelines.

Like coral reefs and rain forests, kelp forests serve as the foundation of ecosystems that support a diverse community of animals, including forage fish, salmon and southern resident killer whales.

Kelp is especially critical to Washington’s waters, where 22 species of kelp make our state a global hotspot for kelp diversity.

The best known species is bull kelp (Nereocystis luetkeana), which forms a canopy that floats on the water surface and can often be found on Puget Sound beaches.

Yet despite its importance to our ecosystem, there here have been no peer-reviewed scientific studies of long-term trends in kelp in Puget Sound. To fill in this  gap in knowledge, scientists with the Washington Department of Natural Resources teamed up with scientists from Marine Agronomics and the U.S. Geological Survey to look at long-term trends in kelp forests within Puget Sound. They chose South Puget Sound because it is particularly sensitive to both natural conditions and human activities that impact kelp.

South Puget Sound has lost the majority of its bull kelp forests: the linear extent in 2017 declined 63% compared to the earliest 1878 baseline and 80% compared to its maximum extent over 145 years.

This study can be found on PLOS ONE and the below storymap prepared by DNR.

Using historical maps to understand ecological changes

The team of scientists reconstructed bull kelp presence over 145 years from navigation charts, government surveys, ecological studies and other historical documents. They defined a kelp baseline early in the process of non-native settlement, from topographic maps made in the 1870s.

The current extent of bull kelp in South Puget Sound has shrunk to a fraction of its historical extent.

These maps provided scientists with a long-term view of kelp coverage in Puget Sound prior to the region’s industrialization.

Compared to the 1870s maps, bull kelp extent along shorelines decreased 63%. Losses have persisted for decades, across a range of climate conditions.

Connecting losses and persistence to environmental conditions

In recent decades, bull kelp has predominantly grown along shorelines with intense currents and mixing, where temperature and nutrient concentrations did not reach thresholds for impacts to bull kelp performance, and high current speeds likely excluded grazers. The greatest losses of kelp were found in areas with elevated temperature, lower nutrient concentrations, and relatively low current velocities.

Comparing South Puget Sound to other areas

The pattern of long-term losses in South Puget Sound kelp coverage over 145 years contrasts starkly with findings from other areas in Washington state. Previous DNR research found that kelp forests have been stable over the last century along the shorelines of the Strait of Juan de Fuca, which have greater wave exposure, proximity to oceanic conditions, and fewer human activities.

Overall, these findings suggest that kelp beds along shorelines in the Salish Sea that are sheltered from waves and currents are more sensitive to water quality, temperature, pollution and climate change. In contrast, shorelines with strong currents and deep-water mixing, such as the Tacoma Narrows in South Puget Sound, appear to provide a refuge for kelp beds from common natural and human stressors within Puget Sound.

DNR is now developing strategies and policies to reduce the agency’s impact on aquatic vegetation coverage. Other agency efforts like increasing renewable energy on state lands and preventing wildfires should also help reverse the declining trend of kelp coverage to provide more suitable habitat for the Salish Sea’s endangered salmon and orca populations.

More information

Find out how DNR is working to study Washington’s nearshore environments to strengthen its management of 2.6 million acres of state-owned aquatic lands at