Kelp PBI-Support provides an evidence-based methodological approach for identifying potential blue NbS relying on kelp forest ecosystems that could help address the desired societal challenge(s) and be appropriate for an area. Thus, the first step involves identifying the societal challenge(s) a community aims to tackle, together with the ecosystem services that are relevant to address the specific societal challenge(s) (step 2) in a specific social-ecological context. Through expert knowledge and the literature review, five societal challenges were identified that kelp forests can help address through their related ecosystem services. Ecosystem services included provisioning (from [a]biotic sources), regulation and maintenance (transformation of biochemical or physical inputs to ecosystems), and cultural ([in]direct, in situ, or remote interactions). These results are summarised in Table 1.

When selecting interventions for kelp forest ecosystems, it is crucial to consider the specific ecological and governance context. First, defining the spatial scale for NbS is important to identify the boundaries of the ecological and social governance system within which the potential intervention(s) would be applied. This aspect has been covered in detail in Pérez et al. (2024) and is not specific to kelp ecosystems, so we will not develop this idea further here.

Secondly, assessing the ecosystem’s vulnerability is essential (Pérez et al., 2024). Vulnerability refers to the extent to which ecosystems are exposed to various stressors, their sensitivity to these stressors, and their adaptive capacity to cope with and recover from their adverse effects (IPCC, 2014), e.g., how robust or resilient is the ecosystem? This adaptive capacity is closely linked to the ecosystem’s ecological condition, which is defined as “the quality of an ecosystem measured in terms of its abiotic and biotic characteristics” (United Nations, 2024) and encompasses factors like biodiversity, functional redundance, community evenness or habitat integrity (Steneck et al., 2002). Indeed, an ecosystem in poor ecological condition tends to have a lower adaptive capacity, making it more vulnerable to stressors and more prone to collapse (trophic and/or total) or regime shift (Gann et al., 2019; United Nations, 2024). Ecological condition is further used as a relative concept based on a reference ecosystem, which can either be that same ecosystem in a past state when known or an ecosystem from a different area (but with similar characteristics) known to be in “pristine condition”. Local ecological knowledge is critical with these steps. The comparison will then determine the current ecological condition of the ecosystem, relying on pre-identified relevant ecological indicators. Our literature review, combined with expert knowledge, indicates that the ecological condition in kelp forest ecosystems is primarily determined by parameters such as biomass, density, and canopy structure (Norderhaug et al., 2020; Smale et al., 2022; Strand et al., 2020; Eger et al., 2023).

Assessing vulnerability and all its dimensions in coastal ecosystems is challenging due to the complex interplay of ecological, social, and economic dimensions (Turner et al., 2003; IPCC, 2014). As such, assessing the ecological condition of an ecosystem is often easier/simpler than assessing its adaptive capacity for instance. When developing Kelp PBI-Support, we, therefore, chose to look at ecological condition rather than adaptive capacity for ease-of-use, and we defined the vulnerability of kelp forests as the degree to which these ecosystems are likely to experience negative impacts from various stressors (i.e., a combination of exposure and sensitivity), which can be driven by climate change or human activities. In other words, the user will need to evaluate, even qualitatively, the ecological condition of its focus kelp forest(s), and then reflect on what pressures could affect it, how intense these pressures are, and how negatively they impact it.

The stressors affecting kelp forests are multifaceted and can even lead to their disappearance (Table 2). For instance, increased temperatures can lead to a dominant species shift associated with decreased biodiversity (Teagle and Smale, 2018), while sea urchins overgrazing can decimate kelp forests and cause phase shifts from structurally and biologically diverse habitats to “barrens” (Steneck et al., 2002). Furthermore, stressors, which operate from local to global scales (Smale et al., 2013; Wernberg et al., 2019), can interact, resulting in a combined cumulative effect. Although some studies have been published (e.g., Trégarot et al., 2024), more research needs to be done to better understand the single and combined effects of different stressors in kelp forest ecosystems. Indeed, while some studies report most interactions being additive (e.g., sum of their effects) (Strain et al., 2014), others suggest that the combined effects of two different stressors frequently result in synergistic and antagonistic interactions more than additive interactions (Wear et al., 2023). That means that the sum of their effects is often greater or less than the simple addition of their individual impacts.

The rate at which kelp species can respond to climate change drivers and other localised stressors is unclear (Smale et al., 2013). Some studies suggest that their seasonal and interannual variability reflects kelp forests’ high reactivity to environmental factors and their varying capacity to resist perturbations and recover from both minor and large-scale disturbances (e.g., Ghedini et al., 2015; Scheibling et al., 2013). Rapid recovery of kelp populations following catastrophic losses may result from frequent recruitment and fast individual growth rates (Mann, 1973). Some studies also indicate that controlling local stressors can potentially affect/impact the resilience of kelp forests to other stressors (Strain et al., 2014; Morris et al., 2020a; Wear et al., 2023). Still, more research is needed to understand these processes as the tolerance to stressors and their interactions can differ depending on life stages (sporophyte vs. gametophyte), species and locations (Wear et al., 2023).

In summary, the ability of kelp forests to address the defined societal challenge(s) will depend on the ecological condition of the ecosystem as well as its exposure and sensitivity to human and natural stressors. The spatial scale together with the ecological condition and the vulnerability of the ecosystem, helps understand the environmental context (Step 3) that will define the most suitable intervention approach (es) (Step 4).

Blue NbS build on existing ecosystem-based approaches to management, namely, protection, restorative activities, and other sustainable management measures. They aim to improve the effectiveness of such approaches by requiring a comprehensive and integrated perspective to achieve greater outcomes for cross-cutting issues (O’Leary et al., 2023; IUCN, 2020). Kelp PBI-Support directs users towards implementation of such approaches, starting, for instance, with Marine Protected Areas (MPAs) with varying levels of protection based on local pressures. When the ecosystem in an area is in good ecological condition, it can provide its full complement of services. MPAs help maintain ecosystems in good ecological condition by managing potential stressors to encourage biodiversity net gain. Such interventions appear to be an effective solution for maintaining kelp forests already in a good ecological condition while managing human activities according to the local governance objectives. For example, the Cape Rodney-Okakari Point Marine Reserve was established in 1977 in northern New Zealand, where overfishing led to historical kelp declines (Peleg et al., 2023). The MPA, a no-take zone, enabled the recovery of predatory fish populations, which helped control sea urchin numbers and reduce their destructive grazing over kelp forests. The reserve sites showed a clear successional trajectory towards stable kelp forests, unlike the fished sites (Peleg et al., 2023). Another example in the East Atlantic would be the Iroise Marine Natural Park (https://parc-marin-iroise.fr/), a MPA in western Brittany, France, established in 2007. This MPA was created to promote a sustainable use of the regional marine resources, including kelp species such as Laminaria digitata (Couceiro et al., 2013). More than 50% of the yield of Laminaria digitata in France comes from the area of Molène located within this MPA (Alban et al., 2011). Efforts to protect and restore kelp forests in the MPA involve collaborative governance with local stakeholders, government agencies, marine professionals, and the community (Mazé et al., 2022). This collaborative approach ensures that conservation measures are well-integrated with sustainable local economic activities such as traditional fishing and seaweed harvesting.

Alternatively, when an ecosystem is in poor ecological condition, Kelp PBI-Support directs the user towards restorative activities, the type of restoration depending on the capacity to stop or reduce the pressure(s) and the level of restoration possible. The spectrum of restorative activities aims to restore a degraded ecosystem for one specific function through restoring it to a good ecological condition with all associated ecosystem services, depending on the project goal. Restorative activities are ideally based on a reference ecosystem that provides the specific condition an intervention aims to achieve (McDonald et al., 2016). These activities can be effective for recovering damaged kelp forests to good ecological condition, as these ecosystems show strong resilience once the main pressure(s) is/are halted (e.g., Filbee-Dexter and Scheibling, 2014).

The highest level of recovery possible should always be pursued (Nelson et al., 2024). This level must be determined according to local specifics, balancing social, economic, and environmental goals. Restoring a kelp forest to its former state involves halting all pressures responsible for its loss and damage, although this is not always the objective since certain activities like fishing may be necessary to continue and only some kelp forest functions may be desired to be recovered. For instance, South Korea developed an extensive Marine Restoration Program running from 2009 to 2030 to stop the decline of kelp forests caused by sea urchin overgrazing on the east coast and coastal development and habitat loss on the south coast and Jeju Island. They employed a combination of methods, including active restoration (transplanting juvenile kelp, seeding, and urchin removal) to allow the ecosystem to recover almost all its ecosystem services, and rehabilitation (deploying concrete artificial reefs in areas with low sea urchin density) to regain habitat function (Eger et al., 2020). Another example is the Sussex Kelp Restoration Project that aims to rewild ca. 200 km² of lost kelp forest along the coast of Sussex in an ongoing effort following the disappearance of ca. 96% of kelp forests in the region by the end of the century due to a combination of increased inshore trawling, advances in fishing technology, and the “Great Storm” of 1987 (Sussex Kelp Restoration Project, 2023). The project came together after a successful campaign that saw the adoption of the Sussex Nearshore Trawling Byelaw in 2021, banning all trawling activities within 0.75–4 km of the coast, while still allowing sustainable inshore fisheries using only static fishing methods, to monitor and further support the kelp forests recovery in the area. As such, when the pressure can be reduced, passive and active restoration and ecosystem creation can be considered, while or if the pressure cannot be halted or reduced, Kelp PBI-Support recommends partial restoration or rehabilitation. Finally, Kelp PBI-Support directs the user towards the implementation of other sustainable management measures to regulate a specific pressure outside any MPA or restorative activities to gain biodiversity and ecosystem services. Such implementation can take various forms, e.g., anchoring or pollution regulations, but always seek to halt one or multiple pressures (CBD, 2018). For example, in Norway, sea urchin population regulation within kelp forests has shown remarkable results in avoiding overgrazing and letting the algae grow to restore the canopy density (Eger et al., 2022; Miller et al., 2022; Norderhaug et al., 2020). Several studies have also found that the improvement of water quality (e.g., reducing sediment load and nutrient concentrations) could be effective to prevent habitat shifts from canopy to mat-forming algae (Falkenberg et al., 2013; Strain et al., 2015). Experimental restoration projects include reseeding kelp through cultivation techniques, with juvenile plants being transplanted to degraded areas, such as the green gravel trials in Norway (Filbee-Dexter et al., 2022).

The concepts underpinning Kelp PBI-Support are summarised in Figure 2 and the resulting decision tree itself can be accessed here: https://macobios.vercel.app/en/tree.

While all societal challenges identified in Kelp PBI-Support could apply to the IOW, three stand out as particularly significant and of greater concern to its inhabitants: disaster risk reduction, water security, and economic and social development (Figure 3).

The IOW coastline is ca. 168 km long (estuaries included) and varies greatly in morphology, weathering and landslide activity (Isle of Wight Council and Royal Haskoning, 2010). More particularly, the southern coastline is often exposed to storms from the Atlantic and the English Channel, hence experiencing rapid coastal erosion compared to the more sheltered northern coastline (Isle of Wight Council and Royal Haskoning, 2010). The southern cliffs’ erosion often deposits into the littoral system, offering limited protection to the base of the cliffs (Isle of Wight Council and Royal Haskoning, 2010). This protection is often short-lived, however, with sediment deposits continuously being removed and transported away from the coast (Isle of Wight Council and Royal Haskoning, 2010). This has had major consequences for life on the island and, notably, for its road network (Isle of Wight Council and Royal Haskoning, 2010), which could prove very costly to protect and maintain ¹ .

The IOW is associated with three coastal water bodies, two in the south (Dorset/Hampshire and IOW East) and one in the north (Solent), and five transitional water bodies, all on the northern coast of the island (Western Yar, Newton River, Medina, Wootton Creek, and Eastern Yar) (Figure 1). While Dorset/Hampshire and IOW East are assessed as having good ecological condition, all others are classified as being in moderate ecological condition (United Kingdom Environment Agency) ² . All are impacted by high chemical pollution from polybrominated diphenyl ethers (PBDEs) and mercury and its compounds. Other notable indicators of moderate ecological conditions are related to biological quality elements (salt marsh, infaunal quality index and opportunistic macroalgae) and dissolved inorganic nitrogen, the later one at least being associated with poor nutrient management from agriculture and sewage discharge, and potentially from urban and transport pollution. Furthermore, while all designated bathing waters on the IOW were classified as either “Good” or “Excellent” in the last assessment in 2024, it is worth noting that some experienced a degradation, being retrograded from “Excellent” to “Good” (Ryde on the north coast, and Sandown and Bembridge Beach on the southeast coast) since the beginning of the monitoring. Despite this positive overview, NGOs such as Surfers Against Sewage regularly report on sewage pollution alerts for the United Kingdom coastline, as well as collect reports of sickness after swimming from citizens. In their 2023 Water Quality Report, for instance, they stated that the Bathing Water at Gurnard in Cowes had 649 spills, ranking 9th in the 20 most polluted United Kingdom bathing waters that year (Ross et al., 2023).

Finally, the IOW is one of the UK’s smallest local authorities, with limited financial resources, making large-scale economic development challenging (McInnes et al., 2003). Its economy primarily depends on micro-businesses serving the tourism and construction sectors, where employment is often seasonal with much lower wages than in neighbouring counties (Cox et al., 2016). The island’s separation from the mainland, combined with the seasonal nature of tourism, influences the cost of goods and services, affecting affordability for the local community (Cox et al., 2016). This situation is further exacerbated by high levels of second-home ownership and private rentals on the island (Cox et al., 2016). The IOW also lacks a university or higher education institution, meaning many young people must either relocate to pursue further education or regularly commute to the mainland (Cox et al., 2016). These financial and educational barriers are cited as major factors driving the outmigration of 15–29-year-olds from the island (Cox et al., 2016).

As the IOW encompasses various parishes and ecosystems affected by shared ecological drivers, though with varying pressure levels depending on local factors such as substrate type and wave energy, the case study can be classified at the “meso” scale.

Several kelp species are distributed along the IOW’s coastline, particularly along the southern shores where rocky and mixed sediment habitats are common. The four dominant species are Laminaria digitata, Laminaria hyperborea, Saccharina latissima and Saccorhiza polyschides (Collins et al., 1990), all of which are characteristic canopy-forming species of the Northeast Atlantic. Laminaria ochroleuca, a warmer water kelp species, has also been documented on the Island’s coast (Smale et al., 2015) showing the effects of warming sea temperatures on the IOW kelp ecosystems.

However, the ecological state of kelp forests around the IOW is considered to be poor in comparison to forests found further west on England’s south coasts. Surveys conducted by the University of Portsmouth in 2023 revealed that many areas of kelp forests on the Island’s southern coast are fragmented and stressed by several environmental factors (personal communication) arising from a range of local and regional pressures.

The broader trend of rising sea temperatures in the Northeast Atlantic (Venegas et al., 2023) is driving thermal conditions around the island beyond the functional range of both Laminaria hyperborea and Laminaria digitata (Smale et al., 2019). Local episodes of mass coastal erosion (Isle of Wight Council and Royal Haskoning, 2010) also threaten kelp communities, with large amounts of clay and other soft sediments regularly deposited into the shallow waters along the island’s southern coastline. These sediment deposits, particularly severe near sites of large landslides, greatly increase water turbidity, thereby reducing sunlight availability essential for kelp photosynthesis. Frequent landslides also alter intertidal topography, potentially displacing and translocating kelp populations. Furthermore, repeated incidents of sewage discharge and agricultural runoff into coastal waterways (Environment Agency ² ) also endanger these communities. Increased nutrients in the water column can promote the growth of epiphytic and smothering algae, such as Ulva intestinalis, which can overgrow seabed areas and diminish suitable habitats for kelp sporophyte settlement.

Given the poor ecological condition and high pressures on kelp forests around the IOW, Kelp PBI-Support offers the following interventions as potential blue NbS to help address local societal challenges: Fully or Highly protected MPAs, restoration activities ranging from passive recovery to ecosystem creation, and other sustainable management measures (Figure 3; Table 3).

While MPAs already exist around the IOW, their current level of protection falls short of Kelp PBI-Support’s recommendations, as they are classified as “lightly” or “minimally” protected. However, activities typically regulated through MPAs, such as fishing and dredging, known threats to kelps elsewhere (e.g., trawling impacts in Sussex’s kelp forests), do not appear to be the primary drivers of kelp degradation around the IOW. Instead, pressures such as sedimentation and anthropogenic nutrient inputs have been identified as the main causes. This situation may be partly due to the regulation of fishing and dredging through other mechanisms, such as the Southern IFCA fishing bylaws and the MMO regulations controlling dredging and other potentially damaging activities. As a result, the combination of existing MPAs and other management measures may be achieving a similar objective as the “fully” and “highly” protected MPAs recommended by Kelp PBI-Support.

Kelp PBI-Support further identified restoration activities as potential intervention options, though none have yet been implemented around the IOW. Feasibility studies are underway, exploring options such as deploying hard substrates, with or without kelp alginates from genetically diverse spore pools, to promote kelp recruitment, or installing artificial structures to restore nursery habitats for some fish species such as cuttlefish. Research projects are also investigating the ecosystem services provided by other seaweed species, such as the extensive Ascophyllum nodosum and Halidrys siliquosa forests, which naturally co-occur with kelp and appear more resilient, as well as by other marine and coastal ecosystems, in order to adopt a more holistic approach to addressing societal challenges on the IOW. For these restoration efforts to succeed, they must be coupled with measures to halt existing pressures on kelps, particularly the regulation of wastewater discharges and agricultural run-offs, which are well-recognised stressors impacting not only kelp forests but many other critical ecosystems.