A contemporary review on restoration efforts in kelp forests

Introduction 

Kelp forests cover 25% of the world’s coastlines and contain high biodiversity rates around the world [1]. These biomes provide a myriad of services to all life on earth [2]. Kelp forests are massive carbon sinks, helping sequester over 1.48 million tons every year [2]. They also support the livelihoods of millions of people by contributing over $562 million through fisheries annually by sustaining fish populations through interconnected food webs [2]. But their decline in recent years has been largely caused by several abiotic and biotic factors, most notably rising ocean temperatures and unrestricted grazing from sea urchins respectively [3]. Sea urchins are native herbivores of kelp [4], grazing away the holdfasts that anchor them to reefs [5]. Sea urchin populations are in turn regulated by predators such as fish, lobsters, sea stars, and sea otters [6]. However, these predators are under decline due to overfishing, increasing ocean temperatures, and other anthropogenic stressors [7]. These declines have released sea urchins from predation [8]. As a result, sea urchins have decimated entire kelp forests, turning them into empty wastelands known as urchin barrens [5]. Urchin barrens are able to persist for decades, preventing kelp species from growing back [5]. The resulting deforestation results in many species without a food source, a home, and a great shift in the ecosystem [9]. 

Restoration of these forests has proved to be difficult [4]. Researchers must address the numerous causes driving deforestation while simultaneously attempting to restore kelp populations [4]. Many countries are currently investigating different methods to circumvent these barriers by reducing the deforestation of kelp, restoring kelp populations in urchin barrens, or creating new kelp beds to mimic their former counterparts [4]. Efforts to reduce deforestation include culling sea urchins and reintroducing major sea urchin predators while restoration efforts are reintroducing and transplanting kelp species to sites that were previously kelp forests [4]. However, these efforts still struggle in measuring holistic changes in the entire ecosystem because progress is only assessed as a function of kelp rehabilitation. This review will discuss recent research on methods taken to restore kelp forests in different areas of the world. By compiling existing research in novel techniques, this review sheds light on current restoration efforts to maintain a crucial ecosystem and can enhance future research to improve the efficiency and effectiveness of restoration efforts. 

Native Predator Restoration

Several factors contribute to the deforestation of kelp, including loss of urchin predators [8]. Hence, a restoration method under active research is native predator restoration. This solution has been especially of interest in the Northeastern Pacific. The best-known urchin predator in this region is the sea otter (Enhydra lutris), long recognized as a keystone species that drives top-down control on kelp forests through its diet [6]. However, their populations have greatly declined as of recent due to increased predation by killer whales, leading to large absences in their natural habitats [9] where urchin barrens can take hold. Even if otters are reintroduced to these areas, they have shown to have a preference towards urchins in kelp-dense areas due to their greater nutritional content than barren urchins, which become starved in the absence of kelp [10]. While this sustains healthy kelp forests, it also leaves urchin barren sites to worsen [10]. Instead, scientists have looked into the sunflower sea star (Pycnopodia helianthoides), a generalist predator that preys on a variety of invertebrates including sea urchins [8]. Sunflower sea stars have been greatly impacted by a 2013-16 outbreak of sea star wasting disease, which eradicated most of the population [8]. A subsequent explosion of urchin barrens that followed led scientists to investigate whether restoring them can be effective in controlling barrens [8]. Galloway et al. (2023) tested the preference of sunflower sea stars towards different nutritional states of sea urchins, specifically whether they have a preference between well-fed sea urchins characteristic of healthy kelp forests and starved sea urchins characteristic of urchin barrens. This would tell us whether sunflower sea stars are a viable option for natural predation in urchin barrens. They concluded that sunflower sea stars did not show a preference between starved or fed sea urchins [8]. Thus, restoring and utilizing sunflower sea stars as a natural predator to exhibit top-down control on sea urchins could be a viable method to combat urchin barrens in the Northeastern Pacific, as well as incentivize reintroduction efforts [8] that can extend to similar predators in other regions. In fact, efforts to recover this critically endangered species are already set in motion with captive breeding and transplanting to aid in their reintroduction [8].

Urchin culling

While restoration of natural predators presents as a holistic solution, some scientists have opted to take a more direct and cheaper approach on regulating sea urchin populations [4]. Manual culling and collection of sea urchins by divers has shown to be a quicker and more successful short-term method than reintroducing predators [5]. Williams et al. (2021) and Miller and Shears (2022) tested the collection and culling rates on sea urchins in Southern California [5] and Australia [11] respectively to determine the efficiency and effectiveness. Miller and Shears (2022) looked at the removal rate and proportion of sea urchins removed by both experienced and beginner to intermediate divers. They found that culling was 1.9-4.4 times faster than collecting and those with more experience were 1.5-3.3 times faster than the beginner and intermediate groups. Culling was found to be the most efficient and effective method in removing sea urchins from the selected sites [11]. Williams et al. (2021) employed culling methods and observed how the presence of sea urchins changed community structures. They examined the types of species present and their abundance before and after urchin removal [5]. The study concluded that culling sea urchins allowed for community compositions in urchin barrens to return to a state similar to those of kelp forests. The reduction in sea urchin densities allowed for the ecosystem to push past an ecological tipping point, allowing for a community shift from an urchin barren state to a kelp-dominated state, remaining stable for years following. Williams et al. (2021)’s experiment mimicked local sea urchin mortality events, such as the disease outbreak in 2013-2014, and results showed resemblances to the rates of decline from these events. A combination of natural and manual restoration methods should be used conjointly when managing sea urchin populations. However, it should be noted that culling at a large scale can come with challenges in economic feasibility, as barren urchins are too poor-quality to sell. There are also ethical and cultural concerns in regions where local urchins are valued by indigenous communities [11]. Manual collection and culling may serve short-term benefits, but these concerns make this solution unsustainable in the long run. Therefore, other restoration methods, such as restoring natural predators, should be utilized to provide a long-term solution. 

A Novel Technique to Restore Kelp Forests 

A novel technique that has been developed is called “green gravel,” where gravel is used to disperse kelp seedlings into sites of restoration [12]. Here, kelp zoospores are placed into small rocks that are then incubated until kelp sporophytes are either visible or at a certain length [12]. The rocks are subsequently dispersed into different areas of the ocean in hopes that the kelp continues to grow and reproduce, aiding reforestation in those regions [12]. This technique allows for widespread implementation that is more cost-efficient than manual transplants and urchin slaughters [12]. In Fredriksen et al.'s (2020) study, the zoospores of sugar kelp (Saccharina latissima) were seeded onto granite rocks and dispersed near research stations in the southern coasts of Norway. Their results were promising, showing more than 50% kelp retention with different dispersal methods. Alsuwaiyan et al. (2022) expanded on Fredriksen et al.'s (2020) work by testing out different types of gravel and the potential for detachment and settlement of gametophytes. By testing detachment and settlement, long term effects on green gravel can be measured through their ability to continue reproduction and increase populations after the initial dispersal [13]. The study used small basalt, large basalt, crushed lattice, and limestone [13]. The first three substrates showed varying success rates with basalt having greater sporophyte growth [13]. Gametophytes on basalt and lattice substrates were also all able to settle to their surrounding areas successfully [13]. This presents promising results as it shows the potential for renewed, sustainable kelp populations and continued growth without the requirement of human intervention. 

Since the development of green gravel, more research has been conducted in order to optimize all phases of the process. One crucial phase is the growth stage, where kelp is cultivated in gravel and incubated in light to promote growth [12, 14]. A study conducted by Chemello et al. (2023) observed the effect of different light intensities on kelp growth and contamination by other algal species. Early stages of kelp growth are sensitive to contamination due to their slow development, which gives ample opportunity for colonizing algae to take up space and compete with juvenile kelp [14]. To test the effect of light on early stages of kelp growth, Chemello et al. (2023) grew golden kelp (Laminaria ochroleuca) at high and low light levels. High light levels helped with faster growth but greatly increased the rates of contamination by other algae. In contrast, low lights showed no signs of algal contamination but resulted in lower kelp length. There may exist a fine line between the two extremes that allows light levels to aid with faster and greater growth but minimize rates of contamination, and finding it can help enhance the environment in which scientists cultivate kelp to prepare them for release [14].

These studies provide a foundational basis for green gravel. The technique has already shown great potential as a restoration tool due to the simplicity of the process and broad applicability. Continued testing of the different variables in the initial incubation stages that favor kelp growth and distribution, such as substrate type and light intensities, will further strengthen its outcomes after dispersal.

Methods of Measuring Impacts 

As kelp forests offer a myriad of benefits, researchers have different measures of success when looking into restoration efforts [4]. Fredriksen et al. (2020) measured green gravel success by examining kelp growth and detachment. Miller and Shears (2022) and Williams et al. (2021) mainly accounted for sea urchin removal as a benchmark for success. These studies measure direct relationships of the experimental variable on the presence of an organism–either kelp or sea urchins. However, it does not encompass the state of the ecosystem’s health. Measuring patterns of biodiversity and primary productivity of the ecosystem are essential to fully understand ecosystem function [9].

One study conducted by Gabara et al. (2020) utilized stable isotope ratios of the tissue of animals that live in kelp forests as indicators for ecosystem health. They specifically used δ¹³C and δ¹⁵N, which serve as signatures for diet and trophic position respectively. Together, the two form a profile of each animal’s dietary niche, known as an isotopic dietary niche breadth (IDNB), which when compared with other animals can reveal the consumer level. Higher INDBs indicate wider sources of carbon that support the consumer, signaling a greater diversity of species within the habitat. These measures span across the whole ecosystem and provide a more comprehensive understanding of overall biodiversity and ecosystem health. [15]. The study’s results showed a bottom-up effect, where reduction in kelp forest led to reductions in the IDNBs of herbivores and omnivores higher up in the food chain [15]. Substantial understanding of the effects of kelp forests on the organisms it supports to evaluate the progress of kelp restoration can also be obtained via examining INDB overlap between species, which can reveal how many consumer levels there are. Organisms at greater numbers of different consumer levels can be another indication of biomass and biodiversity [15], which can affect ecosystem service values and indicate overall ecosystem health. In another approach, Rossouw et al. (2024) collected environmental DNA (eDNA) at different heights of great tidal movement to draw conclusions about different taxonomic groups present in the ecosystem. With wave movement, eDNA can act as an extensive measure of biodiversity which can be associated with other ecosystem services. By assessing a more encompassing measure of success, kelp restoration efforts can provide a link between ecosystem health and biodiversity with ecosystem service values. 

Conclusion 

Kelp forests provide a multitude of benefits, from supporting numerous marine species to saving billions of dollars through ecosystem services. Slowing deforestation and restoring kelp forests has become increasingly critical in ameliorating the health of these ecosystems. Green gravel is emerging as a promising, cheap, and wide scale technique for kelp forest restoration. However, its effectiveness will benefit from continued research and development on existing challenges. The initial incubation and growth stages need to be further investigated in order to protect the juvenile stages while promoting growth and future gametophyte distribution. Refining this tool will give researchers the opportunity to execute wide-scale efforts that prompt kelp survival in dynamic environments. 

Addressing the cause of deforestation is also imperative for long-term results. Management tools, such as sea urchin culling and restoration of urchin predators, can facilitate positive changes in community composition to improve biodiversity and therefore should continue to be a focus of research and refinement. Harnessing tools collectively, along with utilizing different measures of ecosystem-wide biodiversity for a more comprehensive monitoring of kelp forests, will allow us to sustain and strengthen the global kelp forest ecosystems to ultimately restore and preserve biodiversity and the health of our planet.

About the Author: Ingrid Liu

I am a Animal Biology major with a minor in Wildlife, Fish, and Conservation. I’ve always been passionate about animals, and I’ve learned to appreciate how interconnected everything is with the environment. I’ve worked with many different species in research, rehabilitation, and husbandry and I’m hoping to pursue a career in conservation of wildlife. In my free time, I love to bake, crochet, and weightlift. 

Author's Note

As climate change became a more pressing issue, I started educating myself in different ways that we contribute, positive and negative. Some topics are more widely known, such as the state of rainforests and coral reefs; so when I learned about a new method that scientists are taking on with kelp forests, it piqued my interest and I decided to look into this more. I wrote this literature review for an assignment while taking UWP 102B but as I did more research, the more interested I became in my topic. The outcomes from this restoration method seem promising and hopefully give the audience more hope for the future, in times where conservation can seem filled with doom and gloom. I hope this piece can educate people of new efforts being taken in restoration and maybe even inspire those interested to continue the research and be part of the change the world needs. 

References

1. Duarte, Carlos M., Jean‐Pierre Gattuso, Kasper Hancke, Hege Gundersen, Karen Filbee‐Dexter, Morten F. Pedersen, Jack J. Middelburg, et al. 2022. “Global Estimates of the Extent and Production of Macroalgal Forests.” Global Ecology and Biogeography 31 (7). https://doi.org/10.1111/geb.13515.
2. Eger, Aaron M., Ezequiel M. Marzinelli, Rodrigo Beas-Luna, Caitlin O. Blain, Laura K. Blamey, Jarrett E. K. Byrnes, Paul E. Carnell, et al. 2023. “The Value of Ecosystem Services in Global Marine Kelp Forests.” Nature Communications 14 (1): 1894. https://doi.org/10.1038/s41467-023-37385-0.
3. Krumhansl, Kira A., Daniel K. Okamoto, Andrew Rassweiler, Mark Novak, John J. Bolton, Kyle C. Cavanaugh, Sean D. Connell, et al. 2016. “Global Patterns of Kelp Forest Change over the Past Half-Century.” Proceedings of the National Academy of Sciences 113 (48): 13785–90. https://doi.org/10.1073/pnas.1606102113.
4. Eger, Aaron M., Ezequiel M. Marzinelli, Hartvig Christie, Camilla W. Fagerli, Daisuke Fujita, Alejandra P. Gonzalez, Seok Woo Hong, et al. 2022. “Global Kelp Forest Restoration: Past Lessons, Present Status, and Future Directions.” Biological Reviews 97 (4). https://doi.org/10.1111/brv.12850.
5. Williams, Jonathan P., Jeremy T. Claisse, Daniel J. Pondella II, Chelsea M. Williams, Matthew J. Robart, Zoe Scholz, Erin M. Jaco, Tom Ford, Heather Burdick, and David Witting. 2021. “Sea Urchin Mass Mortality Rapidly Restores Kelp Forest Communities.” Marine Ecology Progress Series 664 (April): 117–31. https://doi.org/10.3354/meps13680.
6. Eger, Aaron M., Ezequiel Marzinelli, Paul Gribben, Craig R. Johnson, Cayne Layton, Peter D. Steinberg, Georgina Wood, Brian R. Silliman, and Adriana Vergés. 2020. “Playing to the Positives: Using Synergies to Enhance Kelp Forest Restoration.” Frontiers in Marine Science 7 (July). https://doi.org/10.3389/fmars.2020.00544.
7. Ling, S. D., C. R. Johnson, S. D. Frusher, and K. R. Ridgway. 2009. “Overfishing Reduces Resilience of Kelp Beds to Climate-Driven Catastrophic Phase Shift.” Proceedings of the National Academy of Sciences 106 (52): 22341–45. https://doi.org/10.1073/pnas.0907529106.
8. Galloway, A. W. E., S. A. Gravem, J. N. Kobelt, W. N. Heady, D. K. Okamoto, D. M. Sivitilli, V. R. Saccomanno, J. Hodin, and R. Whippo. 2023. “Sunflower Sea Star Predation on Urchins Can Facilitate Kelp Forest Recovery.” Proceedings of the Royal Society B: Biological Sciences 290 (1993). https://doi.org/10.1098/rspb.2022.1897.
9. Edwards, Matthew, Brenda Konar, Ju-Hyoung Kim, Scott Gabara, Genoa Sullaway, Tristin McHugh, Michael Spector, and Sadie Small. 2020. “Marine Deforestation Leads to Widespread Loss of Ecosystem Function.” Edited by Maura (Gee) Geraldine Chapman. PLOS ONE 15 (3). https://doi.org/10.1371/journal.pone.0226173.
10. Smith, Joshua G., Joseph Tomoleoni, Michelle Staedler, Sophia Lyon, Jessica Fujii, and M. Tim Tinker. 2021. “Behavioral Responses across a Mosaic of Ecosystem States Restructure a Sea Otter–Urchin Trophic Cascade.” Proceedings of the National Academy of Sciences 118 (11): e2012493118. https://doi.org/10.1073/pnas.2012493118.
11. Miller, Kelsey I., and Nick T. Shears. 2022. “The Efficiency and Effectiveness of Different Sea Urchin Removal Methods for Kelp Forest Restoration.” Restoration Ecology 31 (1). https://doi.org/10.1111/rec.13754.
12. Fredriksen, Stein, Karen Filbee-Dexter, Kjell Magnus Norderhaug, Henning Steen, Torjan Bodvin, Melinda A. Coleman, Frithjof Moy, and Thomas Wernberg. 2020. “Green Gravel: A Novel Restoration Tool to Combat Kelp Forest Decline.” Scientific Reports 10 (1): 3983. https://doi.org/10.1038/s41598-020-60553-x.
13. Alsuwaiyan, Nahlah A., Karen Filbee-Dexter, Sofie Vranken, Celina Burkholz, Marion Cambridge, Melinda A. Coleman, and Thomas Wernberg. 2022. “Green Gravel as a Vector of Dispersal for Kelp Restoration.” Frontiers in Marine Science 9 (September). https://doi.org/10.3389/fmars.2022.910417.
14. Chemello, Silvia, Isabel Sousa Pinto, and Tania R. Pereira. 2023. “Optimising Kelp Cultivation to Scale up Habitat Restoration Efforts: Effect of Light Intensity on ‘Green Gravel’ Production.” Hydrobiology 2 (2): 347–53. https://doi.org/10.3390/hydrobiology2020022.
15. Gabara, Scott Stanley, Brenda H. Konar, and Matthew S. Edwards. 2020. “Causes and Consequences of Kelp Forest Foundation Species Loss - ProQuest.” Www.proquest.com. San Diego State University. 2020. https://www.proquest.com/dissertations-theses/causes-consequences-kelp-forest-foundation/docview/2572569633/se-2.
16. Rossouw, Emma I., Jannes Landschoff, Andrew Ndhlovu, Götz Neef, Masaki Miya, Kira-Lee Courtaillac, Rouane Brokensha, and Sophie von der Heyden. 2024. “Detecting Kelp-Forest Associated Metazoan Biodiversity with EDNA Metabarcoding.” Npj Biodiversity 3 (1). https://doi.org/10.1038/s44185-023-00033-3.