Kelp forests

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This article is a short introduction to the kelp forest ecosystem. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems and is common on continental shelves. Kelp belongs to the macro-algae family described in the articles Seaweed (macro-algae) ecosystem services and Diversity and classification of marine benthic algae.


Fig. 1. Kelp forest (Laminaria hyperborean) [1]

Kelp forests are among the most productive and dynamic marine ecosystems. They are found worldwide in temperate and polar coastal oceans. Kelp forests consist of brown macroalgae of the orders Laminariales and Fucales[2]. They have some specific requirements, such as hard, rocky soil; cold water and a continuous supply of nutrients to support strong photosynthesis. Kelps are found to a maximum depth of 30 m, but then the water must be very clear. Kelps can reach a length of more than 30 m and a biomass of 42 kg fresh weight per individual. Some kelps have flexible stems that allow the thallus to drape over the sea floor, while others have erect stems that lift the thallus into the water column, where it can form a dense canopy several meters above the sea floor. A few species have several blades and gas-filled bladders (pneumatocysts), which allow them to create a floating canopy on the sea surface[3].

Kelps reproduce by motile zoospores released in high density from the mature sporophyte (tens to hundreds of thousands of spores). The dispersal capacity is limited because the swimming speed of the zoospores is low; zoospores and recruits are generally found close to the source population, but sometimes a few miles away[4].

Ecosystem services

Kelps are very efficient primary producers, with net primary production averaging about 500 g C m-2yr-1. Peak productivity in arctic, subarctic and temperate ecosystems occurs in late winter and early spring, when nutrient levels are high and water temperatures are low[3]. This high productivity provides an abundant food source for herbivores such as fish, urchins, crustaceans and snails that graze directly on the kelps. A large proportion of the kelp productivity is exported as detritus, fueling high secondary production and shaping the diversity and abundance patterns of species in adjacent or distant habitats such as beaches (beach wrack), temperate reefs, deep coastal areas and deep see canyons[3].

Kelp is a unique habitat for organisms and a resource for understanding many ecological processes. Kelp forests dampen waves and currents and attenuate the impact of storms. Large numbers of organisms use the kelp as food and/or shelter. Kelp is often regarded as an ecosystem engineer, creating habitat for many species. See also Seaweed (macro-algae) ecosystem services for a more extensive overview of ecosystem services.

Occurrence and threats

Kelp forests dominate along approximately one-third of the world’s coastlines in polar / subpolar and temperate latitudes in both hemispheres (Fig. 2), and cover an area of about 2 million km2.[5]. The global distribution of kelp reflects the upper temperature growth limit of 22-24 oC[6]. The temperature range for optimal growth is much lower, depending on the species. Global warming therefore presents a serious threat to kelp forests. In the warm-temperate zone, a shift from kelp to turf algae is taking place in many regions, leading to declines in biomass and productivity[7][4]. There is also evidence that global warming increases fish herbivory which can lead to the deforestation of temperate kelp communities[8]. The distribution, structure and productivity of kelp forests is further influenced by several other environmental factors (e.g., available nutrients and light for growth and photosynthesis, rocky substrate for attachment, wave exposure) and ecological factors (e.g. grazing, competition, disease). The occurrence and density of kelp forests are therefore spatially and temporally variable, with an overall trend of decline[7][3].

Fig. 2. Global distribution of kelp species. From Kelp Restoration Guidebook 2022[2].

Sea urchins and herbivorous fish are formidable kelp grazers that can strip entire kelp forests. Depleted forests are often replaced by biologically impoverished alternative ecosystems dominated by algae that inhibit kelp recruitment and prevent kelp forest restoration[9]. These so-called 'urchin barrens' which can range from a few 100 m to more than 1000 km extent, are devoid of fleshy and filamentous algae. They are primarily covered by encrusting coraline algae of low nutritional value that promotes sea urchin settlement. Kelp losses have been observed in Nova Scotia, the Gulf of Maine, North-Central California, Norway, Ireland and South Australia. One reason is that urchin predators such as cod, sea otters, crabs and lobsters have declined because of their commercial importance[3]. Commercial harvest of kelp is another factor that has contributed to the decline of kelp forests.

Related articles

Seaweed (macro-algae) ecosystem services
Diversity and classification of marine benthic algae
Blue carbon sequestration
Acoustic kelp bed mapping in shallow rocky coasts - case study Helgoland (North Sea)


  2. 2.0 2.1 Eger, A. M., Layton, C., McHugh, T. A, Gleason, M. and Eddy, N. 2022. Kelp Restoration Guidebook: Lessons Learned from Kelp Projects Around the World. The Nature Conservancy, Arlington, VA, USA
  3. 3.0 3.1 3.2 3.3 3.4 Wernberg, T., Krumhansl, K., Filbee-Dexter, K. and Pedersen, M. F. 2019. Status and trends for the world’s kelp forests, in World seas: An environmental evaluation, ed. C. Sheppard. Elsevier, pp 57–78
  4. 4.0 4.1 Smale, D. A. 2020. Impacts of ocean warming on kelp forest ecosystems. New Phytol. 225: 1447–1454
  5. Jayathilake, D.R.M. and Costello, M.J. 2021. Version 2 of the world map of laminarian kelp benefits from more Arctic data and makes it the largest marine biome. Biological Conservation 257, 109099
  6. Liesner, D., Fouqueau, L., Valero, M., Roleda, M.Y., Pearson, G.A., Bischof, K., Valentin, K. and Bartsch, I. 2020. Heat stress responses and population genetics of the kelp Laminaria digitata (Phaeophyceae) across latitudes reveal differentiation among North Atlantic populations. Ecology and evolution 10: 9144-9177
  7. 7.0 7.1 Krumhansl, K. A., Okamoto, D. K., Rassweiler, A., Novak, M., Bolton, J. J., Cavanaugh, K. C., et al. 2016. Global patterns of kelp forest change over the past half-century. Proc. Natl. Acad. Sci. 113: 13785–13790
  8. Verges, A., Doropoulos, C., Malcolm, H. A., Skye, M., Garcia-Pizá, M., Marzinelli, E. M., et al. 2016. Long-Term Empirical Evidence of Ocean Warming Leading to Tropicalization of Fish Communities, Increased Herbivory, and Loss of Kelp. Proc. Natl. Acad. Sci. 113: 13791–13796
  9. Veenhof, R.J., Dworjanyn, S.A., Champion, C. and Coleman, M.A. 2022. Grazing and Recovery of Kelp Gametophytes Under Ocean Warming. Front. Mar. Sci. 9, 866136

The main authors of this article are Töpke, Katrien and Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Töpke, Katrien; Job Dronkers; (2024): Kelp forests. Available from [accessed on 29-02-2024]