This month’s Tide Bite highlights another research effort that spans the divide between “basic” research into understanding the biology of a marine organism and “applied” research that helps us determine how best to ensure the long-term survival of that organism – in this case, bull kelp.  Currently there are multiple, diverse efforts in Washington and British Columbia to document and understand localized kelp losses.  We are proud that FHL is in the middle of these efforts, both with postdoctoral researcher Brooke Weigel’s work and that of FHL graduate student Robin Fales, and they build on kelp-related work done by FHL researchers ever since the 1970s!  We hope that this accumulated knowledge can be used to minimize future losses and find a path to recover populations of this critical species.  Brooke’s first introduction to FHL was as a student in the 2015 Marine Invertebrates course, with support from the FHL Adopt-a-Student Program.

Best,
Dr. Megan Dethier
FHL Director

Helping the Kelp: Effects of Ocean Warming on Bull Kelp Forests

by Brooke Weigel

Brooke Weigel is a marine ecologist who became fascinated by the ocean while growing up near a small lake in Wisconsin.  She graduated from St. Olaf College, where she occasionally went ice diving in frigid lakes…preparing for her future as a cold-water science diver?  Brooke earned her Master’s degree in Marine Biology from the University of North Carolina-Wilmington, where she studied her favorite invertebrate taxa: marine sponges.  She received her PhD from the University of Chicago.  Her dissertation research focused on kelp-microbe interactions and carbon cycling in kelp forests on Tatoosh Island, Washington.  Brooke’s postdoctoral research at FHL aims to improve our understanding of how ocean warming impacts bull kelp forests.  In addition to studying kelp, Brooke is passionate about creating artwork (currently printmaking), mentoring student research, making science more inclusive, and communicating her research to the public.

Many of you are familiar with the kelp forests that flourish throughout the Salish Sea and the outer coastal waters of Washington.  These beautiful, giant brown algae are essential habitat-forming species – some grow from the bottom of the ocean to the surface, forming underwater forests that can extend more than 80 feet in height.  Kelp forests support a myriad of ecologically, culturally, and economically important species such as salmon, rockfish, herring, bivalves, crustaceans, and even orca whales.  Kelp forests grow in temperate and arctic coastal marine ecosystems worldwide, and Washington is home to 22 species of kelp.  The most conspicuous of these are the canopy-forming kelps, which include giant kelp (Macrocystis pyrifera) and bull kelp (Nereocystis luetkeana).

Bull kelp forests have been declining over the last century in some areas of the Salish Sea, particularly in South Puget Sound (Berry et al. 2021).  One major environmental stressor for bull kelp is high seawater temperatures, as this species thrives in very cold waters from central California through Alaska.  Sea surface temperatures are increasing globally, yet we are just beginning to understand the consequences of ocean warming for kelp forest ecosystems.  In South Puget Sound, we have lost up to 80% of bull kelp forests over the last century (Berry et al. 2021, GIS story map).  Kelp forest losses have been most severe at sites with the warmest summer water temperatures and lowest nutrient concentrations.  Because of these declines, Washington state has initiated many research and conservation actions to understand, manage, and conserve kelp forests (Calloway et al. 2020).  As a postdoctoral researcher at Friday Harbor Labs funded by the Washington State Legislature, my goal is to improve our understanding of how environmental stressors – such as high temperatures – impact the growth, survival, and reproduction of bull kelp.

While you may be familiar with adult bull kelp, with its uniquely rounded float and mermaid-hair-like blades waving in the current or tangled up on the beach, many are unfamiliar with the tiny, microscopic life stages of bull kelp that live at the bottom of the ocean (Figure 1).  Bull kelp have a life cycle with two very distinct stages: the large adult kelp (called sporophytes) and microscopic kelp (called gametophytes), which live on the bottom of the ocean.  Think of it like this: if humans had a life cycle similar to that of kelp, your eggs and sperm would live outside of your body – for up to a year or more – independently.  Weird, right?  To fully understand how temperature stress impacts bull kelp across the life cycle, I am studying both the large adult kelp as well as the microscopic gametophytes.

Despite the importance of knowing the thermal tolerance limits of bull kelp, few temperature-controlled experiments have been conducted.  This spring and summer, with the help of kelp biologist Sadie Small, we grew bull kelp from 7 different populations from around the Salish Sea at 7 different water temperatures in the lab, spanning 10 to 22°C (50 to 72°F).  Preliminary results suggest that there is a critical upper temperature limit for bull kelp reproduction.  In multiple experiments with bull kelp from different populations, the gametophytes grew quite well from 10 to 16°C (50 to 60°F), but the production of adult bull kelp was severely limited at temperatures of 18°C (64°F) and above (Figure 2).  This suggests that the maximum temperature at which bull kelp can successfully undergo sexual reproduction and complete their life cycle is around 18°C (64°F).  At temperatures above 20°C (68°F), none of the kelp gametophytes survived.

In addition to studying the effects of temperature on microscopic kelp gametophytes, I am collaborating with PhD candidate Robin Fales to study the effects of temperature and nitrogen concentrations on the growth and physiology of adult bull kelp.  Adult bull kelp are enormous, so we conducted our experiments with their photosynthetic blades in large temperature-controlled lab tanks (Figure 3), where they continue to grow multiple centimeters per day!  Over the course of 2 weeks, we tracked the growth rates, photosynthesis and respiration rates, and overall health of bull kelp blades held at 12, 16 and 20°C (54, 60 and 68°F), under both high and low nitrogen levels.  Preliminary results suggest that for adult bull kelp, growth and survival were similar at 12 and 16°C, while kelp incubated at 20°C grew very little and displayed physiological signs of stress.

To inform management and conservation actions based on these results, we need to know how warm the ocean temperatures are getting in Puget Sound kelp forests.  In addition to running lab experiments, I am tracking seawater temperatures at sites across the Salish Sea to understand when and where the environment may exceed the thermal tolerance limits for bull kelp reproduction.  This involves scientific SCUBA diving and collaboration across groups, including the Washington Department of Natural Resources.  The warmest sites in South Puget Sound have bottom temperatures that exceed 16°C (60°F) for much of the summer, while sites in the San Juan Islands usually remain below 12 to 14°C.  These environmental data will be paired with experimental results to help inform management of declining bull kelp populations and restoration efforts in the Salish Sea.

By working with multiple populations across the Salish Sea that span a natural gradient in seawater temperatures, this research will allow managers to determine whether there are sites that are better suited for bull kelp conservation and restoration, as well as whether certain kelp populations are more heat tolerant than others.  Scientists at the Puget Sound Restoration Fund are experimentally growing and out-planting bull kelp to facilitate the recovery of their forests in Puget Sound.  While studying the negative impacts of increasing ocean temperatures on bull kelp in the face of climate change can be disheartening at times, I am encouraged by the many people working together throughout the Salish Sea and around the world to rapidly enrich our knowledge of kelp forest ecology and conservation biology.  I hope to be a part of this large effort into the future as we continue to help the kelp in Puget Sound and beyond.

References:

Berry H.D., Mumford T.F., Christiaen B., Dowty P., Calloway M., Ferrier L., Grossman E.E., and N.R. VanArendonk.  2021.  Long-term changes in kelp forests in an inner basin of the Salish Sea.  PLoS One: 16(2), e0229703.  https://doi.org/10. 1371/journal.pone.0229703.

Berry H.D., Mumford T.F., Christiaen B., Dowty P., Calloway M., Ferrier L., Grossman E.E., and N.R. VanArendonk.  2021.  Patterns of loss and persistence in kelp forests.  GIS story map: https://storymaps.arcgis.com/stories/636b37a1b5f44105a485237e52adb651

Calloway M., Oster D., Berry H., Mumford T., Naar N., Peabody B., Hart L., Tonnes D., Copps S., Selleck J., Allen B., and J. Toft.  2020.  Puget Sound kelp conservation and recovery plan.  Prepared for NOAA-NMFS, Seattle, WA.  https://nwstraits.org/our-work/kelp/

Puget Sound Restoration Fund, Bull Kelp Recovery: https://restorationfund.org/programs/bullkelp/