Kelp forests are underwater areas with a high density ofkelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are calledkelp beds. They are recognized as one of the most productive and dynamicecosystems on Earth.[1][2] Although algal kelp forest combined withcoral reefs only cover 0.1% of Earth's total surface, they account for 0.9% of globalprimary productivity.[3] Kelp forests occur worldwide throughouttemperate andpolar coastal oceans.[1] In 2007, kelp forests were also discovered intropical waters nearEcuador.[4]
Global distribution of kelp forests
"I can only compare these great aquatic forests ... with the terrestrial ones in the intertropical regions. Yet if in any country a forest was destroyed, I do not believe so nearly so many species of animals would perish as would here, from the destruction of kelp. Amidst the leaves of this plant numerous species of fish live, which nowhere else could find food or shelter; with their destruction the many cormorants and other fishing birds, the otters, seals and porpoise, would soon perish also; and lastly, the Fuegian[s] ... would ... decrease in numbers and perhaps cease to exist.
Physically formed by brownmacroalgae, kelp forests provide a unique habitat for marine organisms[6] and are a source for understanding many ecological processes. Over the last century, they have been the focus of extensive research, particularly introphic ecology, and continue to provoke important ideas that are relevant beyond this unique ecosystem. For example, kelp forests can influence coastaloceanographic patterns[7] and provide manyecosystem services.[8]
However, the influence of humans has often contributed to kelpforest degradation. Of particular concern are the effects ofoverfishing nearshore ecosystems, which can releaseherbivores from their normal population regulation and result in theovergrazing of kelp and other algae.[9] This can rapidly result in transitions tobarren landscapes where relatively few species persist.[10][11] Already due to the combined effects ofoverfishing andclimate change,[12] kelp forests have all but disappeared in many especially vulnerable places, such asTasmania's east coast and the coast ofNorthern California.[13][14] The implementation ofmarine protected areas is one management strategy useful for addressing such issues, since it may limit the impacts of fishing and buffer the ecosystem from additive effects of other environmental stressors.
The termkelp refers tomarine algae belonging to theorder Laminariales (phylum:Ochrophyta). Though not considered a taxonomically diverse order, kelps are highly diverse structurally and functionally.[8] The most widely recognized species are the giant kelps (Macrocystis spp.), although numerous other genera such asLaminaria,Ecklonia,Lessonia,Nereocystis,Alaria, andEisenia are described.
Frequently considered anecosystem engineer, kelp provides a physical substrate and habitat for kelp forest communities.[16] In algae (kingdomProtista), the body of an individual organism is known as athallus rather than as a plant (kingdomPlantae). The morphological structure of a kelp thallus is defined by three basic structural units:[10]
Theholdfast is a root-like mass that anchors the thallus to the sea floor, though unlike true roots it is not responsible for absorbing and delivering nutrients to the rest of the thallus.
Thestipe is analogous to a plant stalk, extending vertically from the holdfast and providing a support framework for other morphological features.
Thefronds are leaf- or blade-like attachments extending from the stipe, sometimes along its full length, and are the sites of nutrient uptake and photosynthetic activity.
In addition, many kelp species havepneumatocysts, or gas-filled bladders, usually located at the base of fronds near the stipe. These structures provide the necessary buoyancy for kelp to maintain an upright position in the water column.
The environmental factors necessary for kelp to survive include hard substrate (usually rock or sand), high nutrients (e.g., nitrogen, phosphorus), and light (minimum annualirradiance dose > 50 E m−2[17]). Especially productive kelp forests tend to be associated with areas of significant oceanographicupwelling, a process that delivers cool, nutrient-rich water from depth to the ocean'smixed surface layer.[17] Water flow and turbulence facilitate nutrient assimilation across kelp fronds throughout the water column.[18] Water clarity affects the depth to which sufficient light can be transmitted. In ideal conditions, giant kelp (Macrocystis spp.) can grow as much as 30–60 cm vertically per day. Some species, such asNereocystis, areannuals, while others such asEisenia areperennials, living for more than 20 years.[19] In perennial kelp forests, maximum growth rates occur during upwelling months (typically spring and summer) and die-backs correspond to reduced nutrient availability, shorter photoperiods, and increased storm frequency.[10]
Kelps are primarily associated withtemperate andarctic waters worldwide. Of the more dominant genera,Laminaria is mainly associated with both sides of theAtlantic Ocean and the coasts ofChina andJapan;Ecklonia is found inAustralia,New Zealand, andSouth Africa; andMacrocystis occurs throughout the northeastern and southeasternPacific Ocean,Southern Ocean archipelagos, and in patches around Australia, New Zealand, and South Africa.[10] The region with the greatest diversity of kelps (>20 species) is the northeastern Pacific, from north ofSan Francisco, California, to theAleutian Islands, Alaska.
Although kelp forests are unknown in tropical surface waters, a few species ofLaminaria have been known to occur exclusively in tropical deep waters.[20][21] This general absence of kelp from the tropics is believed to be mostly due to insufficient nutrient levels associated with warm,oligotrophic waters.[10] One recent study spatially overlaid the requisite physical parameters for kelp with mean oceanographic conditions and produced a model predicting the existence of subsurface kelps throughout the tropics worldwide to depths of 200 m (660 ft). For a hotspot in theGalapagos Islands, the local model was improved with fine-scale data and tested; the research team found thriving kelp forests in all eight of their sampled sites, all of which had been predicted by the model, thus validating their approach. This suggests that their global model might actually be fairly accurate, and if so, kelp forests would be prolific in tropical subsurface waters worldwide.[4] The importance of this contribution has been rapidly acknowledged within the scientific community and has prompted an entirely new trajectory of kelp forest research, highlighting the potential for kelp forests to provide marine organisms spatial refuge under climate change and providing possible explanations for evolutionary patterns of kelps worldwide.[22]
Rockfish swimming aroundgiant kelpA diver in akelp forest off the coast of CaliforniaA kelp forest off of the coast of Anacapa Island, CaliforniaGiant kelp uses gas-filled floats to keep the thallus suspended, allowing the kelp blades near the ocean surface to capture light for photosynthesis.
The architecture of a kelp forest ecosystem is based on its physical structure, which influences the associated species that define its community structure. Structurally, the ecosystem includes three guilds of kelp and two guilds occupied by other algae:[10]
Canopy kelps include the largest species and often constitute floating canopies that extend to the ocean surface (e.g.,Macrocystis andAlaria).
Stipitate kelps generally extend a few meters above the sea floor and can grow in dense aggregations (e.g.,Eisenia andEcklonia).
Prostrate kelps lie near and along the sea floor (e.g.,Laminaria).
Thebenthic assemblage is composed of other algal species (e.g., filamentous and foliose functional groups, articulated corallines) and sessile organisms along the ocean bottom.
Encrusting coralline algae directly and often extensively cover geologic substrate.
Multiple kelp species often co-exist within a forest; the term understory canopy refers to the stipitate and prostrate kelps. For example, aMacrocystis canopy may extend many meters above the seafloor towards the ocean surface, while an understory of the kelpsEisenia andPterygophora reaches upward only a few meters. Beneath these kelps, a benthic assemblage of foliose red algae may occur. The dense vertical infrastructure with overlying canopy forms a system of microenvironments similar to those observed in a terrestrial forest, with a sunny canopy region, a partially shaded middle, and darkened seafloor.[10] Each guild has associated organisms, which vary in their levels of dependence on the habitat, and the assemblage of these organisms can vary with kelp morphologies.[23][24][25] For example, in California,Macrocystis pyrifera forests, thenudibranchMelibe leonina, andskeleton shrimpCaprella californica are closely associated with surface canopies; the kelp perchBrachyistius frenatus, rockfishSebastes spp., and many other fishes are found within the stipitate understory;brittle stars and turban snailsTegula spp. are closely associated with the kelp holdfast, while various herbivores, such as sea urchins and abalone, live under the prostrate canopy; many seastars, hydroids, andbenthic fishes live among the benthic assemblages; solitary corals, variousgastropods, andechinoderms live over the encrusting coralline algae.[23] In addition,pelagic fishes andmarine mammals are loosely associated with kelp forests, usually interacting near the edges as they visit to feed on resident organisms.
Classic studies in kelp forest ecology have largely focused on trophic interactions (the relationships between organisms and theirfood webs), particularly the understanding and top-down trophic processes. Bottom-up processes are generally driven by the abiotic conditions required for primary producers to grow, such as availability of light and nutrients, and the subsequent transfer of energy to consumers at higher trophic levels. For example, the occurrence of kelp is frequently correlated with oceanographic upwelling zones, which provide unusually high concentrations of nutrients to the local environment.[26][27] This allows kelp to grow and subsequently support herbivores, which in turn support consumers at highertrophic levels.[28] By contrast, in top-down processes, predators limit the biomass of species at lower trophic levels through consumption. In the absence of predation, these lower-level species flourish because resources that support their energetic requirements are not limiting. In a well-studied example from Alaskan kelp forests,[29]sea otters (Enhydra lutris) control populations of herbivoroussea urchins through predation. When sea otters are removed from the ecosystem (for example, by human exploitation), urchin populations are released from predatory control and grow dramatically. This leads to increased herbivore pressure on local kelp stands. Deterioration of the kelp itself results in the loss of physical ecosystem structure and subsequently, the loss of other species associated with this habitat. In Alaskan kelp forest ecosystems, sea otters are thekeystone species that mediates thistrophic cascade. In Southern California, kelp forests persist without sea otters and the control of herbivorous urchins is instead mediated by a suite of predators including lobsters and large fishes, such as theCalifornia sheephead. The effect of removing one predatory species in this system differs from Alaska because redundancy exists in the trophic levels and other predatory species can continue to regulate urchins.[24] However, the removal of multiple predators can effectively release urchins from predator pressure and allow the system to follow trajectories towards kelp forest degradation.[30] Similar examples exist inNova Scotia,[31] South Africa,[32] Australia,[33] and Chile.[34] The relative importance of top-down versus bottom-up control in kelp forest ecosystems and the strengths of trophic interactions continue to be the subject of considerable scientific investigation.[35][36][37]
The transition from macroalgal (i.e. kelp forest) to denuded landscapes dominated by sea urchins (or ‘urchin barrens’) is a widespread phenomenon,[8][38][39][40][41] often resulting from trophic cascades like those described above; the two phases are regarded as alternative stable states of the ecosystem.[42][43][44] The recovery of kelp forests from barren states has been documented following dramatic perturbations, such as urchin disease or large shifts in thermal conditions.[30][45][46] Recovery from intermediate states of deterioration is less predictable and depends on a combination of abiotic factors and biotic interactions in each case.
Though urchins are usually the dominant herbivores, others with significant interaction strengths includeseastars,isopods, kelpcrabs, andherbivorous fishes.[10][35] In many cases, these organisms feed on kelp that has been dislodged from substrate and drifts near the ocean floor rather than expend energy searching for intact thalli on which to feed. When sufficient drift kelp is available, herbivorous grazers do not exert pressure on attached thalli; when drift subsidies are unavailable, grazers directly impact the physical structure of the ecosystem.[47][48] Many studies in Southern California have demonstrated that the availability of drift kelp specifically influences the foraging behavior of sea urchins.[49][50] Drift kelp and kelp-derived particulate matter have also been important in subsidizing adjacent habitats, such as sandy beaches and the rocky intertidal.[51][52][53]
Another major area of kelp forest research has been directed at understanding the spatial-temporal patterns of kelp patches. Not only do such dynamics affect the physical landscape, but they also affect species that associate with kelp for refuge or foraging activities.[23][28] Large-scale environmental disturbances have offered important insights concerning mechanisms and ecosystemresilience. Examples of environmental disturbances include:
Acute and chronicpollution events have been shown to impact southern California kelp forests, though the intensity of the impact seems to depend on both the nature of the contaminants and duration of exposure.[54][55][56][57][58] Pollution can include sediment deposition andeutrophication from sewage, industrial byproducts and contaminants likePCBs and heavy metals (for example, copper, zinc), runoff oforganophosphates from agricultural areas, anti-fouling chemicals used in harbors and marinas (for example,TBT andcreosote) and land-based pathogens likefecal coliform bacteria.
Catastrophic storms can remove surface kelp canopies through wave activity, but usually leave understory kelps intact; they can also remove urchins when little spatial refuge is available.[42][48] Interspersed canopy clearings create a seascape mosaic where sunlight penetrates deeper into the kelp forest and species that are normally light-limited in the understory can flourish. Similarly, substrate cleared of kelp holdfasts can provide space for other sessile species to establish themselves and occupy the seafloor, sometimes directly competing with juvenile kelp and even inhibiting their settlement.[59]
El Niño-Southern Oscillation (ENSO) events involve the depression of oceanographic thermoclines, severe reductions of nutrient input, and changes in storm patterns.[42][60] Stress due to warm water andnutrient depletion can increase the susceptibility of kelp to storm damage and herbivorous grazing, sometimes even prompting phase shifts to urchin-dominated landscapes.[46][49][61] In general, oceanographic conditions (that is, water temperature, currents) influence the recruitment success of kelp and its competitors, which clearly affect subsequent species interactions and kelp forest dynamics.[42][62]
Overfishing higher trophic levels that naturally regulate herbivore populations is also recognized as an important stressor in kelp forests.[9][37][63] As described in the previous section, the drivers and outcomes of trophic cascades are important for understanding spatial-temporal patterns of kelp forests.[29][30][35]
In addition to ecological monitoring of kelp forests before, during, and after such disturbances, scientists try to tease apart the intricacies of kelp forest dynamics using experimental manipulations. By working on smaller spatial-temporal scales, they can control for the presence or absence of specific biotic and abiotic factors to discover the operative mechanisms. For example, in southern Australia, manipulations of kelp canopy types demonstrated that the relative amount ofEcklonia radiata in a canopy could be used to predict understory species assemblages; consequently, the proportion ofE. radiata can be used as an indicator of other species occurring in the environment.[64]
Kelp forests have been important to human existence for thousands of years.[65] Indeed, many now theorise that the first colonisation of the Americas was due to fishing communities following the Pacific kelp forests during the last ice age. One theory contends that the kelp forests that would have stretched from northeast Asia to the American Pacific coast would have provided many benefits to ancient boaters[66] The kelp forests would have provided many sustenance opportunities, as well as acting as a type of buffer from rough water. Besides these benefits, researchers believe that the kelp forests might have helped early boaters navigate, acting as a type of "kelp highway". Theorists also suggest that the kelp forests would have helped these ancient colonists by providing a stable way of life and preventing them from having to adapt to new ecosystems and develop new survival methods even as they traveled thousands of miles.[67]
Modern economies are based onfisheries of kelp-associated species such aslobster and rockfish. Humans can also harvest kelp directly to feed aquaculture species such asabalone and to extract the compoundalginic acid, which is used in products like toothpaste and antacids.[68][69] Kelp forests are valued for recreational activities such asSCUBA diving andkayaking; the industries that support these sports represent one benefit related to the ecosystem and the enjoyment derived from these activities represents another. All of these are examples ofecosystem services provided specifically by kelp forests. The Monterey Bay aquarium was the first aquarium[70] to exhibit an alive kelp forest.
Kelp forests grow in rocky places along the shore that are constantly eroding carrying material out to the deep sea. The kelp then sinks to the ocean floor and store the carbon where is it unlikely to be disturbed by human activity.[71] Researchers from theUniversity of Western Australia estimated kelp forest around Australia sequestered 1.3-2.8 teragrams of carbon per year which is 27–34% of the total annualblue carbon sequestered in the Australian continent bytidal marshes,mangrove forests andseagrass beds.[72] Every year 200 million tons of carbon dioxide are being sequestered by macroalgae such as kelp.[73]
The nudibranchMelibe leonina on aMacrocystis frond (California): Marine protected areas are one way to guard kelp forests as an ecosystem.
Given the complexity of kelp forests – their variable structure, geography, and interactions – they pose a considerable challenge to environmental managers. Extrapolating even well-studied trends to the future is difficult because interactions within the ecosystem will change under variable conditions, not all relationships in the ecosystem are understood, and the nonlinear thresholds to transitions are not yet recognized.[74]
Major issues of concern include marine pollution andwater quality, kelp harvesting and fisheries,invasive species,[8] andclimate change.[75] The most pressing threat to kelp forest preservation may be the overfishing of coastal ecosystems, which by removing higher trophic levels facilitates their shift to depauperate urchin barrens.[9] The maintenance ofbiodiversity is recognized as a way of generally stabilizing ecosystems and their services through mechanisms such as functional compensation and reduced susceptibility to foreign species invasions.[76][77][78][79] More recently, the 2022 IPCC report states that kelp and other seaweeds in most regions are undergoing mass mortalities from high temperature extremes and range shifts from warming, as they are stationary and cannot adapt quick enough to deal with the rapidly increasing temperature of the Earth and thus, the ocean.[80]
In many places, managers have opted to regulate the harvest of kelp[27][81] and/or the taking of kelp forest species by fisheries.[8][63] While these may be effective in one sense, they do not necessarily protect the entirety of the ecosystem.Marine protected areas (MPAs) offer a unique solution that encompasses not only target species for harvesting, but also the interactions surrounding them and the local environment as a whole.[82][83] Direct benefits of MPAs to fisheries (for example, spillover effects) have been well documented around the world.[9][84][85][86] Indirect benefits have also been shown for several cases among species such as abalone and fishes in Central California.[87][88] Most importantly, MPAs can be effective at protecting existing kelp forest ecosystems and may also allow for the regeneration of those that have been affected.[42][89][90]
Researchers at the Bodega Marine Laboratory of UC Davis are developing replanting strategies, and volunteers of the Orange County Coastkeeper group are replanting giant kelp.[105][106] Humboldt State University began cultivating bull kelp in its research farm in 2021.[107]
Research efforts at the state level to prevent kelp forest collapse in California were announced in July 2020.[108]
At the federal level, H.R. 4458, the Keeping Ecosystems Living and Productive (KELP) Act, introduced July 29, 2021, seeks to establish a new grant program withinNOAA for kelp forest restoration.[109]
Ocean Rainforest, aFaroe Islands-based company, secured $4.5 million in U.S. government funding to grow giant kelp on an 86-acre farm off the coast of Santa Barbara, California.[110]
TheUnited Nations Environment Programme Norwegian Blue Forests Network 2023 report titled 'Into the Blue: Securing a Sustainable Future for Kelp Forests' documents a global decline in kelp forests, with an annual reduction rate of 1.8%. Over the past 50 years, 40–60% of these ecosystems have degraded due to factors such as climate change, poor water quality, andoverfishing. The report underscores the urgency of implementing global conservation efforts and emphasizes the need for international cooperation to adopt area-based management strategies. These strategies aim to mitigate the aforementioned impacts and enhance the resilience and sustainability of kelp forests.[111]
Kelp forest restoration, practiced in 16 countries over 300 years, has gained momentum, particularly from 2009 to 2019, involving diverse societal sectors such as academia, governments, and businesses. Large-scale restoration successes demonstrate its feasibility, with the best outcomes often near existing kelp forests, emphasizing the importance of preventing their decline. However, challenges persist, including the need for cost-effective methods, funding mechanisms, and adaptations to climate change. This restoration work not only supportsecological recovery but also offers significant social and economic benefits, aligning with theUnited Nations Sustainable Development Goals (SDGs), and underscores the importance of multi-sector collaboration.[111]: 135
^Joly, Aylthon Brandão (1967).Two Brazilian Laminarias. Rio de Janeiro: Instituto de Pesquisas da Marinha. pp. 1–7.OL4960200M.Archived from the original on 2024-10-07. Retrieved2024-10-06.
^Petrov, J.E., M.V. Suchovejeva and G.V. Avdejev. 1973. New species of the genusLaminaria from the Philippine Sea. Nov Sistem. Nizch. Rast. 10: 59–61.
"Kelp Watch".tas.gov.au.Tasmania,Australia: Department of Primary Industries, Water & Environment. Archived fromthe original on 2004-12-04. Excellent general information on kelp forests, as well as specific information on Tasmanian kelp forests.
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