The fifteenth Conference of the Parties (COP) to the United Nations Convention on Biological Diversity will conclude negotiations on a global biodiversity framework in late-2022 that will aim to slow and reverse the loss of biodiversity and establish goals for positive outcomes by 20501. Currently recognized drivers of declines in marine and coastal ecosystems include overexploitation of resources (for example, fishes, oil and gas), expansion of anthropogenic activities leading to cumulative impacts on the marine and coastal environment (for example, habitat loss, introduction of contaminants and pollution) and effects of climate change (for example, ocean warming, freshening and acidification). Within these broad categories, marine and coastal ecosystems face a wide range of emerging issues that are poorly recognized or understood, each having the potential to impact biodiversity. Researchers, conservation practitioners and marine resource managers must identify, understand and raise awareness of these relatively ‘unknown’ issues to catalyse further research into their underlying processes and impacts. Moreover, informing the public and policymakers of these issues can mitigate potentially negative impacts through precautionary principles before those effects become realized: horizon scans provide a platform to do this.

Horizon scans bring together experts from diverse disciplines to discuss issues that are (1) likely to have a positive or negative impact on biodiversity and conservation within the coming years and (2) not well known to the public or wider scientific community or face a substantial ‘step-change’ in their importance or application2. Horizon scans are an effective approach for pre-emptively identifying issues facing global conservation3. Indeed, marine issues previously identified through this approach include microplastics4, invasive lionfish4 and electric pulse trawling5. To date, however, no horizon scan of this type has focused solely on issues related to marine and coastal biodiversity, although a scan on coastal shorebirds in 2012 identified potential threats to coastal ecosystems6. This horizon scan aims to benefit our ocean and human society by stimulating research and policy development that will underpin appropriate scientific advice on prevention, mitigation, management and conservation approaches in marine and coastal ecosystems.


We present the final 15 issues below in thematic groups identified post-scoring, rather than rank order (Fig. 1).

Fig. 1: The 15 horizon issues presented in thematic groups: ecosystem impacts, resource exploitation and new technologies.
figure 1

Numbers refer to the order presented in this article, rather than final ranking. Image of brine pool courtesy of the NOAA Office of Ocean Exploration and Research, Gulf of Mexico 2014. Image of biodegradable bag courtesy of Katie Dunkley.

Ecosystem impacts

Wildfire impacts on coastal and marine ecosystems

The frequency and severity of wildfires are increasing with climate change7. Since 2017, there have been fires of unprecedented scale and duration in Australia, Brazil, Portugal, Russia and along the Pacific coast of North America. In addition to threatening human life and releasing stored carbon, wildfires release aerosols, particles and large volumes of materials containing soluble forms of nutrients including nitrogen, phosphorus and trace metals such as copper, lead and iron. Winds and rains can transport these materials over long distances to reach coastal and marine ecosystems. Australian wildfires, for example, triggered widespread phytoplankton blooms in the Southern Ocean8 along with fish and invertebrate kills in estuaries9. Predicting the magnitude and effects of these acute inputs is difficult because they vary with the size and duration of wildfires, the burning vegetation type, rainfall patterns, riparian vegetation buffers, dispersal by aerosols and currents, seasonal timing and nutrient limitation in the recipient ecosystem. Wildfires might therefore lead to beneficial, albeit temporary, increases in primary productivity, produce no effect or have deleterious consequences, such as the mortality of benthic invertebrates, including corals, from sedimentation, coastal darkening (see below), eutrophication or algal blooms10.

Coastal darkening

Coastal ecosystems depend on the penetration of light for primary production by planktonic and attached algae and seagrass. However, climate change and human activities increase light attenuation through changes in dissolved materials modifying water colour and suspended particles. Increased precipitation, storms, permafrost thawing and coastal erosion have led to the ‘browning’ of freshwater ecosystems by elevated organic carbon, iron and particles, all of which are eventually discharged into the ocean11. Coastal eutrophication leading to algal blooms compounds this darkening by further blocking light penetration. Additionally, land-use change, dredging and bottom fishing can increase seafloor disturbance, resuspending sediments and increasing turbidity. Such changes could affect ocean chemistry, including photochemical degradation of dissolved organic carbon and generation of toxic chemicals. At moderate intensities, limited spatial scales and during heatwaves, coastal darkening may have some positive impacts such as limiting coral bleaching on shallow reefs12 but, at high intensities and prolonged spatial and temporal extents, lower light-regimes can contribute to cumulative stressor effects thereby profoundly altering ecosystems. This darkening may result in shifts in species composition, distribution, behaviour and phenology, as well as declines in coastal habitats and their functions (for example, carbon sequestration)13.

Increased toxicity of metal pollution due to ocean acidification

Concerns about metal toxicity in the marine environment are increasing as we learn more about the complex interactions between metals and global climate change14. Despite tight regulation of polluters and remediation efforts in some countries, the high persistence of metals in contaminated sediments results in the ongoing remobilization of existing metal pollutants by storms, trawling and coastal development, augmented by continuing release of additional contaminants into coastal waters, particularly in urban and industrial areas across the globe14. Ocean acidification increases the bioavailability, uptake and toxicity of metals in seawater and sediments, with direct toxicity effects on some marine organisms15. Not all biogeochemical changes will result in increased toxicity; in pelagic and deep-sea ecosystems, where trace metals are often deficient, increasing acidity may increase bioavailability and, in shallow waters, stimulate productivity for non-calcifying phytoplankton16. However, increased uptake of metals in wild-caught and farmed bivalves linked to ocean acidification could also affect human health, especially given that these species provide 25% of the world’s seafood. The combined effects of ocean acidification and metals could not only increase the levels of contamination in these organisms but could also impact their populations in the future14.

Equatorial marine communities are becoming depauperate due to climate migration

Climate change is causing ocean warming, resulting in a poleward shift of existing thermal zones. In response, species are tracking the changing ocean environmental conditions globally, with range shifts moving five times faster than on land17. In mid-latitudes and higher latitudes, as some species move away from current distribution ranges, other species from warmer regions can replace them18. However, the hottest climatic zones already host the most thermally tolerant species, which cannot be replaced due to their geographical position. Thus, climate change reduces equatorial species richness and has caused the formerly unimodal latitudinal diversity gradient in many communities to now become bimodal. This bimodality (dip in equatorial diversity) is projected to increase within the next 100 years if carbon dioxide emissions are not reduced19. The ecological consequences of this decline in equatorial zones are unclear, especially when combined with impacts of increasing human extraction and pollution20. Nevertheless, emerging ecological communities in equatorial systems are likely to have reduced resilience and capacity to support ecosystem services and human livelihoods.

Effects of altered nutritional content of fish due to climate change

Essential fatty acids (EFAs) are critical to maintaining human and animal health and fish consumption provides the primary source of EFAs for billions of people. In aquatic ecosystems, phytoplankton synthesize EFAs, such as docosahexaenoic acid (DHA)21, with pelagic fishes then consuming phytoplankton. However, concentrations of EFAs in fishes vary, with generally higher concentrations of omega-3 fatty acids in slower-growing species from colder waters22. Ongoing effects of climate change are impacting the production of EFAs by phytoplankton, with warming waters predicted to reduce the availability of DHA by about 10–58% by 210023; a 27.8% reduction in available DHA is associated with a 2.5 °C rise in water temperature21. Combined with geographical range shifts in response to environmental change affecting the abundance and distribution of fishes, this could lead to a reduction in sufficient quantities of EFAs for fishes, particularly in the tropics24. Changes to EFA production by phytoplankton in response to climate change, as shown for Antarctic waters25, could have cascading effects on the nutrient content of species further up the food web, with consequences for marine predators and human health26.

Resource exploitation

The untapped potential of marine collagens and their impacts on marine ecosystems

Collagens are structural proteins increasingly used in cosmetics, pharmaceuticals, nutraceuticals and biomedical applications. Growing demand for collagen has fuelled recent efforts to find new sources that avoid religious constraints and alleviate risks associated with disease transmission from conventional bovine and porcine sources27. The search for alternative sources has revealed an untapped opportunity in marine organisms, such as from fisheries bycatch28. However, this new source may discourage efforts to reduce the capture of non-target species. Sponges and jellyfish offer a premium source of marine collagens. While the commercial-scale harvesting of sponges is unlikely to be widely sustainable, there may be some opportunity in sponge aquaculture and jellyfish harvesting, especially in areas where nuisance jellyfish species bloom regularly (for example, Mediterranean and Japan Seas). The use of sharks and other cartilaginous fish to supply marine collagens is of concern given the unprecedented pressure on these species. However, the use of coproducts derived from the fish-processing industry (for example, skin, bones and trims) offers a more sustainable approach to marine collagen production and could actively contribute to the blue bio-economy agenda and foster circularity29.

Impacts of expanding trade for fish swim bladders on target and non-target species

In addition to better-known luxury dried seafoods, such as shark fins, abalone and sea cucumbers, there is an increasing demand for fish swim bladders, also known as fish maw30. This demand may trigger an expansion of unsustainable harvests of target fish populations, with additional impacts on marine biodiversity through bycatch30,31. The fish swim-bladder trade has gained a high profile because the overexploitation of totoaba (Totoaba macdonaldi) has driven both the target population and the vaquita (Phocoena sinus) (which is bycaught in the Gulf of Mexico fishery) to near extinction32. By 2018, totoaba swim bladders were being sold for US$46,000 kg−1. This extremely lucrative trade disrupts efforts to encourage sustainable fisheries. However, increased demand on the totoaba was itself caused by overexploitation over the last century of the closely related traditional species of choice, the Chinese bahaba (Bahaba taipingensis). We now risk both repeating this pattern and increasing its scale of impact, where depletion of a target species causes markets to switch to species across broader taxonomic and biogeographical ranges31. Not only does this cascading effect threaten other croakers and target species, such as catfish and pufferfish but maw nets set in more diverse marine habitats are likely to create bycatch of sharks, rays, turtles and other species of conservation concern.

Impacts of fishing for mesopelagic species on the biological ocean carbon pump

Growing concerns about food security have generated interest in harvesting largely unexploited mesopelagic fishes that live at depths of 200–1,000 m (ref. 33). Small lanternfishes (Myctophidae) dominate this potentially 10 billion ton community, exceeding the mass of all other marine fishes combined34 and spanning millions of square kilometres of the open ocean. Mesopelagic fish are generally unsuitable for human consumption but could potentially provide fishmeal for aquaculture34 or be used for fertilizers. Although we know little of their biology, their diel vertical migration transfers carbon, obtained by feeding in surface waters at night, to deeper waters during the day across many hundreds and even thousands of metres depth where it is released by excretion, egestion and death. This globally important carbon transport pathway contributes to the biological pump35 and sequesters carbon to the deep sea36. Recent estimates put the contribution of all fishes to the biological ocean pump at 16.1% (± s.d. 13%) (ref. 37). The potential large-scale removal of mesopelagic fishes could disrupt a major pathway of carbon transport into the ocean depths.

Extraction of lithium from deep-sea brine pools

Global groups, such as the Deep-Ocean Stewardship Initiative, emphasize increasing concern about the ecosystem impacts from deep-sea resource extraction38. The demand for batteries, including for electric vehicles, will probably lead to a demand for lithium that is more than five times its current level by 203039. While concentrations are relatively low in seawater, some deep-sea brines and cold seeps offer higher concentrations of lithium. Furthermore, new technologies, such as solid-state electrolyte membranes, can enrich the concentration of lithium from seawater sources by 43,000 times, increasing the energy efficiency and profitability of lithium extraction from the sea39. These factors could divert extraction of lithium resources away from terrestrial to marine mining, with the potential for significant impacts to localized deep-sea brine ecosystems. These brine pools probably host many endemic and genetically distinct species that are largely undiscovered or awaiting formal description. Moreover, the extremophilic species in these environments offer potential sources of marine genetic resources that could be used in new biomedical applications including pharmaceuticals, industrial agents and biomaterials40. These concerns point to the need to better quantify and monitor biodiversity in these extreme environments to establish baselines and aid management.

New technologies

Colocation of marine activities

Climate change, energy needs and food security have moved to the top of global policy agendas41. Increasing energy needs, alongside the demands of fisheries and transport infrastructure, have led to the proposal of colocated and multifunctional structures to deliver economic benefits, optimize spatial planning and minimize the environmental impacts of marine activities42. These designs often bring technical, social, economic and environmental challenges. Some studies have begun to explore these multipurpose projects (for example, offshore windfarms colocated with aquaculture developments and/or Marine Protected Areas) and how to adapt these concepts to ensure they are ‘fit for purpose’, economically viable and reliable. However, environmental and ecosystem assessment, management and regulatory frameworks for colocated and multi-use structures need to be established to prevent these activities from compounding rather than mitigating the environmental impacts from climate change43.

Floating marine cities

In April 2019, the UN-HABITAT programme convened a meeting of scientists, architects, designers and entrepreneurs to discuss how floating cities might be a solution to urban challenges such as climate change and lack of housing associated with a rising human population ( The concept of floating marine cities—hubs of floating structures placed at sea—was born in the middle of the twentieth century and updated designs now aim to translate this vision into reality44. Oceanic locations provide benefits from wave and tidal renewable energy and food production supported by hydroponic agriculture45. Modular designs also offer greater flexibility than traditional static terrestrial cities, whereby accommodation and facilities could be incorporated or removed in response to changes in population or specific events. The cost of construction in harsh offshore environments, rather than technology, currently limits the development of marine cities and potential designs will need to consider the consequences of more frequent and extreme climate events. Although the artificial hard substrates created for these floating cities could act as stepping stones, facilitating species movement in response to climate change46, this could also increase the spread of invasive species. Finally, the development of offshore living will raise issues in relation to governance and land ownership that must be addressed for marine cities to be viable47.

Trace-element contamination compounded by the global transition to green technologies

The persistent environmental impacts of metal and metalloid trace-element contamination in coastal sediments are now increasing after a long decline48. However, the complex sources of contamination challenge their management. The acceleration of the global transition to green technologies, including electric vehicles, will increase demand for batteries by over 10% annually in the coming years49. Electric vehicle batteries currently depend almost exclusively on lithium-ion chemistries, with potential trace-element emissions across their life cycle from raw material extraction to recycling or end-of-life disposal. Few jurisdictions treat lithium-ion batteries as harmful waste, enabling landfill disposal with minimal recycling49. Cobalt and nickel are the primary ecotoxic elements in next-generation lithium-ion batteries50, although there is a drive to develop a cobalt-free alternative likely to contain higher nickel content50. Some battery binder and electrolyte chemicals are toxic to aquatic life or form persistent organic pollutants during incomplete burning. Increasing pollution from battery production, recycling and disposal in the next decade could substantially increase the potentially toxic trace-element contamination in marine and coastal systems worldwide.

New underwater tracking systems to study non-surfacing marine animals

The use of tracking data in science and conservation has grown exponentially in recent decades. Most trajectory data collected on marine species to date, however, has been restricted to large and near-surface species, limited by the size of the devices and reliance on radio signals that do not propagate well underwater. New battery-free technology based on acoustic telemetry, named ‘underwater backscatter localization’ (UBL), may allow high-accuracy (<1 m) tracking of animals travelling at any depth and over large distances51. Still in the early stages of development, UBL technology has significant potential to help fill knowledge gaps in the distribution and spatial ecology of small, non-surfacing marine species, as well as the early life-history stages of many species52, over the next decades. However, the potential negative impacts of this methodology on the behaviour of animals are still to be determined. Ultimately, UBL may inform spatial management both in coastal and offshore regions, as well as in the high seas and address a currently biased perspective of how marine animals use ocean space, which is largely based on near-surface or aerial marine megafauna (for example, ref. 53).

Soft robotics for marine research

The application and utility of soft robotics in marine environments is expected to accelerate in the next decade. Soft robotics, using compliant materials inspired by living organisms, could eventually offer increased flexibility at depth because they do not face the same constraints as rigid robots that need pressurized systems to function54. This technology could increase our ability to monitor and map the deep sea, with both positive and negative consequences for deep-sea fauna. Soft-grab robots could facilitate collection of delicate samples for biodiversity monitoring but, without careful management, could also add pollutants and waste to these previously unexplored and poorly understood environments55. With advancing technology, potential deployment of swarms of small robots could collect basic environmental data to facilitate mapping of the seabed. Currently limited by power supply, energy-harvesting modules are in development that enable soft robots to ‘swallow’ organic material and convert it into power56, although this could result in inadvertently harvesting rare deep-sea organisms. Soft robots themselves may also be ingested by predatory species mistaking them for prey. Deployment of soft robotics will require careful monitoring of both its benefits and risks to marine biodiversity.

The effects of new biodegradable materials in the marine environment

Mounting public pressure to address marine plastic pollution has prompted the replacement of some fossil fuel-based plastics with bio-based biodegradable polymers. This consumer pressure is creating an economic incentive to adopt such products rapidly and some companies are promoting their environmental benefits without rigorous toxicity testing and/or life-cycle assessments. Materials such as polybutylene succinate (PBS), polylactic acid (PLA) or cellulose and starch-based materials may become marine litter and cause harmful effects akin to conventional plastics57. The long-term and large-scale effect of the use of biodegradable polymers in products (for example, clothing) and the unintended release of byproducts, such as microfibres, into the environment remain unknown. However, some natural microfibres have greater toxicity than plastic microfibres when consumed by aquatic invertebrates58. Jurisdictions should enact and enforce suitable regulations to require the individual assessment of all new materials intended to biodegrade in a full range of marine environmental conditions. In addition, testing should include studies on the toxicity of major transition chemicals created during the breakdown process59, ideally considering the different trophic levels of marine food webs.


This scan identified three categories of horizon issues: impacts on, and alterations to, ecosystems; changes to resource use and extraction; and the emergence of technologies. While some of the issues discussed, such as improved monitoring of species (underwater tracking and soft robotics) and more sustainable resource use (marine collagens), may have some positive outcomes for marine and coastal biodiversity, most identified issues are expected to have substantial negative impacts if not managed or mitigated appropriately. This imbalance highlights the considerable emerging pressures facing marine ecosystems that are often a byproduct of human activities.

Four issues identified in this scan related to ongoing large-scale (hundreds to many thousands of square kilometres) alterations to marine ecosystems (wildfires, coastal darkening, depauperate equatorial communities and altered nutritional fish content), either through the impacts of global climate change or other human activities. There are already clear impacts of climate change, for example, on stores of blue carbon (for example, ref. 60) and small-scale fisheries (for example, ref. 61) but the identification of these issues highlights the need for global action that reverses such trends. The United Nations Decade of Ocean Science for Sustainable Development (2021–2030) is now underway, aligning with other decadal policy priorities, including the Sustainable Development Goals (, the 2030 targets for biodiversity to be agreed in 2022, the conclusion of the ongoing negotiations on biodiversity beyond national jurisdictions (BBNJ) (, the UN Conference on Biodiversity (COP15) ( and the UN Climate Change Conference 2021 (COP26) ( While some campaigns to allocate 30% of the ocean to Marine Protected Areas by 2030 are prominently aired62, the unintended future consequences of such protection and how to monitor and manage these areas, remain unclear63,64,65.

Another set of issues related to anticipated increases in marine resource use and extraction (swim bladders, marine collagens, lithium extraction and mesopelagic fisheries). The complex issue of mitigating the impacts on marine conservation and biodiversity of exploiting and using newly discovered resources must consider public perceptions of the ocean66,67, market forces and the sustainable blue economy68,69.

The final set of issues related to new technological advancements, with many offering more sustainable opportunities, albeit some having potentially unintended negative consequences on marine and coastal biodiversity. For example, trace-element contamination from green technologies and harmful effects of biodegradable products highlights the need to assess the step-changes in impacts from their increased use and avoid the paradox of technologies designed to mitigate the damaging effects of climate change on biodiversity themselves damaging biodiversity. Indeed, the impacts on marine and coastal biodiversity from emerging technologies currently in development (such as underwater tracking or soft robotics) need to be assessed before deployment at scale.

There are limitations to any horizon scanning process that aims to identify global issues and a different group of experts may have identified a different set of issues. By inviting participants from a range of subject backgrounds and global regions and asking them to canvass their network of colleagues and collaborators, we aimed to identify as broad a set of issues as possible. We acknowledge, however, that only about one-quarter of the participants were from non-academic organizations, which may have skewed the submitted issues and how they were voted on. However, others3 reported no significant correlation between participants’ areas of research expertise and the top issues selected in the horizon scan conducted in 2009. Therefore, horizon scans do not necessarily simply represent issues that reflect the expertise of participants. We also sought to achieve diversity by inviting participants from 22 countries and actively seeking representatives from the global south. However, the final panel of 30 participants spanned only 11 countries, most in the global north. We were forced by the COVID-19 pandemic to hold the scan online and while we hoped that this would enable participants to engage from around the world alleviating broader global inequalities in science63, digital inequality was in fact enhanced during the pandemic70. Our experience highlights the need for other mechanisms that can promote global representation in these scans.

This Marine and Coastal Horizon Scan seeks to raise awareness of issues that may impact marine and coastal biodiversity conservation in the next 5–10 years. Our aim is to bring these issues to the attention of scientists, policymakers, practitioners and the wider community, either directly, through social networks or the mainstream media. Whilst it is almost impossible to determine whether issues gained prominence as a direct result of a horizon scan, some issues featured in previous scans have seen growth in reporting and awareness. Others3 found that 71% of topics identified in the Horizon Scan in 2009 had seen an increase in their importance over the next 10 years. Issues such as microplastics and invasive lionfish had received increased research and investment from scientists, funders, managers and policymakers to understand their impacts and the horizon scans may have helped motivate this increase. Horizon scans, therefore, should primarily act as signposts, putting focus onto particular issues and providing support for researchers and practitioners to seek investment in these areas.

Whilst recognizing that marine and coastal environments are complex social-ecological systems, the role of governance, policy and litigation on all areas of marine science needs to be developed, as it is yet to be established to the same extent as in terrestrial ecosystems71. Indeed, tackling many of the issues presented in this scan will require an understanding of the human dimensions relating to these issues, through fields of research including but not limited to ocean literacy72,73, social justice, equity74 and human health75. Importantly, however, horizon scanning has proved an efficient tool in identifying issues that have subsequently come to the forefront of public knowledge and policy decisions, while also helping to focus future research. The scale of the issues facing marine and coastal areas emphasizes the need to identify and prioritize, at an early stage, those issues specifically facing marine ecosystems, especially within this UN Decade of Ocean Science for Sustainable Development.


Identification of issues

In March 2021, we brought together a core team of 11 participants from a broad range of marine and coastal disciplines. The core team suggested names of individuals outside their subject area who were also invited to participate in the horizon scan. To ensure we included as many different subject areas as possible within marine and coastal conservation, we selected one individual from each discipline. Our panel of experts comprised 30 (37% female) marine and coastal scientists, policymakers and practitioners (27% from non-academic institutions), with cross-disciplinary expertise in ecology (including tropical, temperate, polar and deep-sea ecosystems), palaeoecology, conservation, oceanography, climate change, ecotoxicology, technology, engineering and marine social sciences (including governance, blue economy and ocean literacy). Participants were invited from 22 countries across six continents, resulting in a final panel of 30 experts from 11 countries (Europe n = 17 (including the three organizers); North America and Caribbean n = 4; South America n = 3; Australasia n = 3; Asia n = 1; Africa n = 2). All experts co-authored this paper.

To reduce the potential for bias in the identification of suitable issues, each participant was invited to consult their own network and required to submit two to five issues that they considered new and likely to have a positive or negative impact on marine and coastal biodiversity conservation in the next 5–10 years (Supplementary Information text describes instructions given to participants). Each issue was described in paragraphs of ~200 words (plus references). Due to the COVID-19 pandemic, participants relied mainly on virtual meetings and online communication using email, social-media platforms, online conferences and networking events. Through these channels ~680 people were canvassed by the participants, counting all direct in-person or online discussions as individual contacts but treating social-media posts or generic emails as a single contact. This process resulted in a long list of 75 issues that were considered in the first round of scoring (see Supplementary Table 1 for the full list of initially submitted issues).

Round 1 scoring

The initial list of proposed issues was then shortened through a scoring process. We used a modified Delphi-style76 voting process, which has been consistently applied in horizon scans since 2009 (refs. 4,77) (see Fig. 2 for the stepwise process). This process ensured that consideration and selection of issues remained repeatable, transparent and inclusive. Panel members were asked to confidentially and independently score the long list of 75 issues from 1 (low) to 1,000 (high) on the basis of the following criteria:

  • Whether the issue is new (with ‘new’ issues scoring higher) or is a well-known issue likely to exhibit a significant step-change in impact

  • Whether the issue is likely to be important and impactful over the next 5–10 years

  • Whether the issue specifically impacts marine and coastal biodiversity

Fig. 2: Stepwise process used to identify, score and present the 15 horizon issues likely to impact marine and coastal biodiversity conservation in the next 5–10 years.
figure 2

Left and right columns show the process for the first and second rounds of scoring, respectively.

Participants were also asked whether they had heard of the issue or not.

‘Voter fatigue’ can result in issues at the end of a lengthy list not receiving the same consideration as those at the beginning76. We counteracted this potential bias by randomly assigning participants to one of three differently ordered long-lists of issues. Participants’ scores were converted to ranks (1–75). We had aimed to retain the top 30 issues with the highest median ranks for the second round of assessment at the workshop but kept 31 issues because two issues achieved equal median ranks. In addition, we identified one issue that had been incorrectly grouped with three others and presented this as a separate issue. The subsequent online workshop to discuss this shortlist, therefore, considered the top-ranked 32 issues (Fig. 3a) (see Supplementary Table 2 for the full list).

Fig. 3: Median rank of each issue versus proportion of issues participants had previously heard of.
figure 3

a, Round 1. Each point represents an individual issue. For all issue titles, see Supplementary Table 1. Issues in dark blue were retained for the second round. Issues that were ranked higher were generally those that participants had not heard of (Spearman rank correlation = 0.38, P < 0.001). b, Round 2. Scores as in round 1. For titles of the second round of 32 issues, see Supplementary Table 2. The 15 final issues (marked in red) achieved the top ranks (horizontal dashed line) and had only been heard of by 50% of participants (vertical dashed line). Red circles, squares and triangles denote issues relating to ecosystem impacts, resource exploitation and new technologies, respectively. The two grey issues marked with crosses were discounted during final discussions because participants could not identify the horizon component of these issues.

Source data

Workshop and round 2 scoring

Before the workshop, each participant was assigned up to four of the 32 issues to research in more detail and contribute further information to the discussion. We convened a one-day workshop online in September 2021. The geographic spread of participants meant that time zones spanned 17 h. Despite these constraints, discussions remained detailed, focused, varied and lively. In addition, participants made use of the chat function on the platform to add notes, links to articles and comments to the discussion. After discussing each issue, participants re-scored the topic (1–1,000, low to high) based on novelty and the issue’s importance for, and probable impact on, marine and coastal biodiversity (3 participants out of 30 did not score all issues and therefore their scores were discounted). At the end of the selection process, scores were again converted to ranks and collated. Highest-ranked issues were then discussed by correspondence focusing on the same three criteria as outlined above, after which the top 15 horizon issues were selected (Fig. 3b).

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this article.