Patterns of loss and persistence in kelp forests
South Puget Sound (1873-2018)
Summary
Like coral reefs and rain forests, kelp forests support rich communities of animals, including forage fish and salmon. Global concerns exist about kelp forest losses, yet information is limited.
This storymap highlights the first peer-reviewed scientific study about changes to kelp forests within Puget Sound (Washington state, USA).
South Puget Sound has lost the majority of its bull kelp forests: linear extent in 2017 declined 63% compared to the earliest 1878 baseline and by 80% compared to the cumulative total of all observations over 145 years. The remaining bull kelp is primarily restricted to one location, the Tacoma Narrows. This stark contrast - widespread loss vs pockets of persistence - provides clues about the factors impacting kelp. It also suggests that the Narrows could be a local refuge from common stressors.
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Introduction
Kelp is known as an ecosystem engineer because it creates habitat for a wide range of species. Its high growth rate also fuels local and distant food webs.
Recent news of kelp forest losses across the globe have generated widespread concern and highlighted considerable gaps in our knowledge. In Puget Sound, scientists and managers identified a pressing need for baseline information in order to understand changes, restore loses, and conserve remaining beds.
The Kelp Plan (2020) identified concerns and proposed actions for Puget Sound.
Photo: Rockfish in understory kelp, by Adam Obaza, Paua Marine Research.
In the Salish Sea, South Puget Sound is an area of particular concern for kelp losses. It is the most distant basin from oceanic influence and it naturally experiences lower flushing rates and longer water residence times. These characteristics make it particularly sensitive to stressors such as water quality degradation and climate extremes.
Many human activities have impacted natural systems in the region since the 1870s, when widespread European settlement began. Early economic activity was dominated by lumber production, fishing, agriculture and aquaculture. Puget Sound is now extensively urbanized, with a regional population of more than 4 million in 2019.
Extensive clearcutting in the region both fueled economic activity and impacted the natural environment (Mud Bay Co. logging camp in South Puget Sound watershed). Photo by C. Kinsey. Source: University of Washington Libraries Special Collections .
South Puget Sound has 452 km of complex shorelines, composed of islands, passages, deltas, and shallow inlets. Gravel, sand, and mixed fine substrates predominate in the intertidal and shallow subtidal zones, reflecting its glacial origins. It is the shallowest major basin in Puget Sound (mean depth 37 m), and has the highest diurnal tidal range (4.4 m in Olympia). The most intense currents and mixing occur at the Tacoma Narrows, a narrow 1.5 km channel with a shallow sill.
Methods
We examined the long-term patterns in the distribution of bull kelp, the only kelp species in Puget Sound that forms a floating surface canopy.
Bull kelp (Nereocystis luetkeana). Photo: DNR Nearshore Habitat Program.
Photo: drone imagery of bull kelp at Squaxin Island, by Russ McMillan, Aerial Images Northwest.
Data Sources
We reconstructed bull kelp distribution over 145 years by synthesizing 48 historical and modern data sources, including recent monitoring by DNR .
Diverse historical documents have noted the occurrence of bull kelp for navigation, scientific research, and resource management.
This 1879 map provides an example of our methods. Using GIS , we placed a digital subtidal line on the 1879 map, and then coded the line for bull kelp present and absent. You can explore this map: look for the wavy symbols that note kelp on the original map. The wavy kelp symbols are located between the pink line and the dotted line that marks low tide.
We recorded 3,352 records of bull kelp presence/absence at 1-km segments between 1873 and 2018. The bulk of the observations (93%) represented 7 synoptic snapshots that each comprehensively surveyed the study area over a limited time period (see table below).
7 synoptic snapshots of bull kelp comprehensively surveyed the study area. We refer to each survey by the reference year in which the majority of segments were surveyed. Click on image to enlarge.
After synthesizing all bull kelp observations, we compared bull kelp occurrence over time to environmental datasets in order to draw inferences about likely stressors:
- Climate patterns described by the Ensemble Oceanic El Nino index ( Webb 2019 ).
- Salinity, temperature and nutrient data from monthly long-term monitoring by the WA Department of Ecology Marine Waters Program.
- Wave height, modeled data developed by the Washington Coastal Resilience Project (VanArendonk 2019 ).
- Surface current velocity, modeled data from the from the 2014 run of a Salish Sea Circulation Model (Khangaonkar et al. 2017 , 2018 ).
Findings
Bull kelp linear extent in 2017-2018 was 80% less than the cumulative total of all observations over 145 years. Bull kelp was observed at least once along 26% of the shoreline over 145 years. In recent years, bull kelp has been limited to 5% of the shoreline.
To see the map legend, click on the icon in the lower left corner of the map. To see details in the map, zoom in.
Another method for assessing change is to compare two points in time. The 2017 survey measured a 63% decrease in bull kelp extent, compared to the earliest baseline in 1878. The 1878 baseline provides an important long-term reference point, capturing conditions early in the process of European settlement in Puget Sound.
Click on this Birds Eye View to explore Olympia in 1879, at the time of the first bull kelp baseline. Olympia was initially the largest settlement north of the Columbia River. Source: WA State Archives
Bull kelp losses began early and continued throughout the study period. At 23% of the shoreline segments, bull kelp was most recently observed during 1860-1880. No single time period stands out for losses.
Losses were concentrated in the West and Central sub-basins and have persisted for decades. At the majority of segments in the Central and West sub-basins, the most recent bull kelp occurrence was 4 decades ago or longer. In contrast, bull kelp has been found at 72% of the segments in the East sub-basin (Tacoma Narrows) since 2000 and at all segments since 1960.
The most recent year that bull kelp was observed in the 3 sub-basins.
The observed trend of bull kelp decrease in SPS over 145 years contrasts sharply with findings along the Strait of Juan de Fuca, at the entrance to the Salish Sea. There, kelp forest area generally remained stable over the last century, except along the eastern boundary—the area farthest from oceanic influence and closest to anthropogenic development
In recent years, bull kelp predominatly occurred in high current areas. 1 m/s emerged as an approximate threshold for mean maximum daily current speed. Segments where bull kelp disappeared before 1980 fell below 1 m/s. In contrast, the majority of segments where bull kelp persisted were above 1 m/s.
Wave and current data compared to most recent year that bull kelp was present at 1-km segments. Bull kelp has been lost primarily from sites with moderate to low current speeds.
There isn't a clear pattern in bull kelp disappearance vs persistence when compared to wave height. South Puget Sound has a relatively protected wave environment.
Click to expand map and view wave height data.
Summer water temperatures at some stations reached thresholds associated with physiological damage. At the nearshore stations, peak summer temperatures were consistently lowest in the Tacoma Narrows (13.9 °C), and increased gradually, with the highest measures at Squaxin Island (16.2 °C). Winter temperatures were similar at all stations.
Temperature patterns were similar at mid-channel and nearshore stations. Measurements at nearshore stations were generally higher, which could be associated with the warm climate conditions during 2017-2018. The long-term data spanned diverse climate conditions.
We do not know how temperatures have changed over time. These data only summarize recent patterns. Critical unanswered questions: have temperatures changed over time? In the future, will temperatures increase with climate change?
Low summer Nitrogen concentrations in recent decades may be affecting bull kelp performance in some areas. A strong spatial gradient in summer Dissolved Inorganic Nitrogen (DIN) was evident. The lowest nearshore concentration occurred at Squaxin Island in August (0.4 µM at 0.25 m depth). In contrast, nearshore DIN concentrations at the other stations never dropped below 10 µM.
Patterns in Nitrogen concentrations were similar at mid-channel and nearshore stations. Measurements at nearshore stations were generally lower, which could be associated with the warm climate conditions during 2017-2018. The long-term data spanned diverse climate conditions.
Mid-channel Nitrogen concentrations were consistently slightly higher at 10 m depth (dotted line) than at the surface (solid line).
Some people wonder if human inputs of Nitrogen - through sewage treatment plants and other activities - could help kelp productivity. The answer is generally 'no'. Worldwide research has shown that human Nitrogen inputs damage kelp by stimulating growth of phytoplankton and nuisance algae. Nitrogen inputs are often accompanied by particulates and other pollutants and contaminants.
Conclusions
This study established a historical baseline for bull kelp in South Puget Sound, early in the period of European settlement. It described major losses over 145 years. Bull kelp is now predominantly limited to the Tacoma Narrows area, with one notable exception - Squaxin Island - the southernmost recorded location.
Bull kelp status (1873-2018). Historical vs recent presence.
This pattern of bull kelp loss vs persistence enables us to draw inferences about factors impacting kelp (conceptual model, below).
Improved understanding of the historical extent of kelp and patterns of change supports further research into stressors, restoration, and conservation. It could also increase our understanding of dynamics in the organisms that rely on these habitats, including forage fish, salmon, Orca whales, and birds.
Research from other regions, along with our understanding of environmental conditions in South Puget Sound, suggest a short list of candidate stressors of prime concern .
Explore the Data
Explore all the data in one map
SPS Historical Kelp Web App
Acknowledgements
Study authors:
- Helen Berry, Bart Christiaen, Pete Dowty, Max Calloway, and Lisa Ferrier, Washington Department of Natural Resources
- Tom Mumford, Marine Agronomics
- Eric Grossman and Nathan VanArendonk, US Geological Survey
Find detailed methods, findings and references in the scientific publication .
We thank many scientists who contributed to field work and data synthesis, including Jeff Gaeckle, Julia Ledbetter, Betty Bookheim, Kate Sherman, Olivia Hannah, Melissa Sanchez, and Lauren Johnson. Tarang Khangaonkar, Christopher Krembs and Julia Bos provided environmental data. We are grateful to Ron Thom for important work on this topic. Cathy Pfister, Megan Dethier and three anonymous reviewers provided thoughtful comments. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government or the Washington State Department of Natural Resources.
