Of the 58 Small Island Developing States (SIDS) recognized by the United Nations in 1992 for their unique social, environmental and development issues, the vast majority are in the Pacific (67%) and Caribbean (25%) basins (United Nations, 1993; UNCTAD, 2021). Shared features include remoteness, limited natural resources, fragile environments, susceptibility to natural disasters, and excessive dependence on international trade. Their small but increasing populations tend to be concentrated in coastal areas <5 m above mean sea level (SIDS average 30% of population) (UN-OHRLLS, 2013).

Interactions among the three major components of sustainability (water, energy, food) are conveniently conceptualized via the WEF nexus. Castillo and Crisman (2019) envisioned the nexus as a wheel, whose spokes are water, energy and food that join at the hub (health) and whose rim is economics. Economics is the overall control over the resources and their interactions, and imbalances in inter sector relationships can adversely affect human health. Application of the nexus approach to 10 Caribbean SIDS (Winters et al., 2022) and Puerto Rico (Crisman et al., 2021) concluded that none of the islands are sustainable. While infrastructure and governance are either unable to address requirements of the nexus under current or projected conditions, local communities and the natural environment are amazingly resilient even when devastated by natural disasters (Crisman et al., 2022) (Figure 1).

Although Castillo and Crisman (2019) recognized the negative impacts of the nexus on human and environmental health, most nexus research has largely focused on quantitative aspects of each component while ignoring quality issues. Water resources in Caribbean SIDS are increasingly taxed by expanding population demands, projected reduced precipitation associated with climate change and losses to evapotranspiration (Jordan and Fisher, 1977; Gohar and Cashman, 2017; IPCC, 2022). Watersheds tend to be small and steep, thus magnifying nutrient and pollutant discharges from human sources (ECLAC, 2000; Winters et al., 2022). Impacts to coastal marine areas including reefs (the principal fishing area) and beaches, where recent infestations of seaweed (Langin, 2018), have added to economic damage to the tourist industry contributed by fecal contamination (Siung-Chang, 1997; Sutherland et al., 2010; Ali et al., 2018; Compton and Forde, 2020).

Infrastructure for wastewater treatment is either mostly antiquated or non-existent, and few Caribbean SIDS have resources to improve the situation. Islands can never become sustainable as long as environmental impacts from the WEF nexus exceed balanced conditions and maximum efficiency of the nexus. In keeping with the spirit of the nexus, nature-based solutions (NBS) to waste management are the best option of Caribbean SIDS but their implementation has languished (Castillo and Crisman, 2019; Rodriguez-Dominguez et al., 2020). As demonstrated globally (Masi et al., 2010; Gorgoglione and Torretta, 2018), constructed wetlands are inexpensive and highly effective treatment alternatives for traditionally engineered infrastructure and have the added benefit of contributing nexus sustainability via water reuse for domestic and agricultural purposes, product development for the local economy and nature conservation (Mycoo et al., 2015; Peters, 2015; Corbi et al., 2021; Valero et al., 2021; Carvalho et al., 2022).

The current study revisited the 10 Caribbean SIDS for which current and projected WEF nexus sustainability was based on water quantity (Winters et al., 2022) to evaluate nexus impacts on water quality and whether these could be reduced by implementation of NBS wastewater treatment systems. Scenarios considered both human and climate projected future impacts. Particular emphasis was placed on the potential for treated water to supplement local water resources and community stability.

Ten of the 16 Caribbean SIDS were selected for this study based on data availability. Countries spanned the Caribbean basin and included: Antigua and Barbuda, the Bahamas, Barbados, Dominica, Grenada, Jamaica, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, and Trinidad and Tobago (Figure 2). Demographic and physical characteristics of islands were compiled. Most countries had a population of under 1 million; however, populations ranged from around 53,000 (St. Kitts and Nevis) to 3 million (Jamaica). GDP values varied from nearly $546,000,000 (Dominica) to approximately $21 billion (Trinidad and Tobago). Surface area ranged from 260 km² (St. Kitts and Nevis) to nearly 14,000 km² (Bahamas) and agricultural area from 60 km² (St. Kitts and Nevis) to 4,540 km² (Jamaica) (Table 1).

Since its inception in 2011, the WEF Nexus has been used increasingly as the preferred approach to predict societal and ecosystem sustainability via interactions among the three principal controlling variables: water, energy and food (Simpson and Jewitt, 2019). Most research has emphasized quantitative aspects of the WEF Nexus, but Castillo and Crisman (2019) stressed that wastewater generated from each sector has a profound impact on human health. Heal et al. (2020) stressed that both water quality and quantity are critical for facing the complex multidisciplinary challenges of WEF Nexus sustainability.

The ten SIDS of this study (Winters et al., 2022) and Puerto Rico (Crisman et al., 2021) currently fail all three components of the WEF Nexus. Both the environment and local communities in all displayed a great deal of resilience, but governance and economics clearly were not adapting to changing conditions and were a root cause of sustainability failure, as highlighted by Mycoo and Bharath (2021). In their literature review, Simpson and Jewitt (2019) identified both governance and the environment as key missing components of WEF Nexus assessments.

Isolation, small size, limited resources and vulnerability to natural hazards and climate change have profound implications for sustainability of SIDS populations and environments (Scandurra et al., 2018; Nel et al., 2021). Optimized use and reuse of existing water resources and identification of alternative water resources are key for sustainability of Caribbean SIDS (Figure 9). All three components of the WEF Nexus generate wastewater of differing quantity and quality that can be collected and treated with various nature-based solutions, especially constructed wetlands, and tailored to meet the conditions of each waste stream. Key to successful treatment is avoidance of centralized grey infrastructure and implementing treatment as close to waste source as possible.

With infrastructure and populations concentrated in a narrow zone fringing the coast (Cashman and Nagdee, 2017; Pathak et al., 2021), all SIDS of this study appear to have sufficient open spaces to implement nature-based solutions until at least 2100 based on projected trends in resident populations. Future impacts from rapidly expanding tourist infrastructure and profound seasonal swelling of water resource demands and wastewater generation from tourists are likely to overwhelm any centralized water sources and wastewater treatment. Nature-based treatment options can be moved inland and tailored to meet unexpected and rapid changes in wastewater streams and should be keystone components for adaptative management plans for all Caribbean SIDS.

Treatment wetlands can be important areas for nature reserves and passive recreation (Castillo and Crisman, 2019). They are the linchpin to stop pollution of coastal resources and environmental from untreated wastewater. The near shore marine environments, including coral reefs, are facing serious problems throughout the Caribbean from bleaching and disease of coral, E. coli and total coliform contamination, elevated nutrients and contaminants and excessive growth of Sargassum and deposition nearshore (Siung-Chang, 1997; Lewsey et al., 2004; Patterson-Sutherland et al., 2010; Langin, 2018; Oviatt et al., 2019; Compton and Forde, 2020). All studies attribute these problems directly to unsustainable land use practices and discharge of raw sewage. The methodology discussed by Aide et al. (2020) provides an excellent model for incorporating native and functional species for wetland construction.

Tourism development depends on relatively intact and pristine ecosystems, but potential negative impacts on the environment from tourist activities can spell doom for the industry. Ali et al. (2018) and Mycoo (2006) stressed the critical need for sustainable ecosystem management of coastal areas of the Caribbean. Over 50% of coastal development will be lost with a 1 m of sea level rise (Scott et al., 2012), and 80% of tourists surveyed on Bonaire and Barbados stated an unwillingness to return if marine wildlife became degraded and beaches were lost from sea level rise (Uyarra et al., 2005). Sustainability practices currently implemented by the tourist industry are insufficient for environmental preservation, government intervention is needed to protect vulnerable ecosystems from tourist development (Gandoit, 2005; Charles, 2013; Mycoo, 2014).

All SIDS of this study suffer from unsustainable water resources for resident populations, and alternative water sources must be found. Although stormwater treatment wetlands are effective at capturing nutrients and many contaminants, their impact on microplastics and personal medications is unclear (Reyes et al., 2023). Still, van Houselt (2021) calculated that runoff from roads and stored in reservoirs could supply 45% of annual non-potable water demand in arid Curacao.

Rainwater harvest (RWH) brings people closer to the water source, thus encouraging water conservation and building societal resilience during climate extremes (Waite, 2012). RWH can be an essential water source for households in hilly terrain where connections to centralized systems are extremely difficult, and poor residents often spend 10% of their expendable income to meet household water demand (Waite, 2012; Rovira et al., 2020). The Grenadine islands have long relied on RWH and storage in cisterns for their water supply (Peters, 2013), and Aladenola et al. (2016) estimated that a storage tank of 2.5–4.0 m³ is sufficient to meet household needs during maximum dry season months in Jamaica. RWH currently meets 25% of household water needs in Jamaica and likely will meet 15% by 2080 after accounting for population demographics and climate change (Waite, 2012).

Development of communal RWH collection systems is often more efficient and innovative than household systems (Waite, 2012; Rovira et al., 2020). Stakeholders must take responsibility of financing, building and maintenance of RWH systems (Peters, 2016) in the absence of governmental guidelines for monitoring and maintaining systems throughout the Caribbean (Cashman and Carpenter, 2020). When implementing and expanding RWH however, it is imperative to understand and mitigate the potential negative health effects, such as growing mosquito populations, as highlighted by Moglia et al. (2016).

Tourism is two-edged sword for water sustainability in the Caribbean SIDS. Most economies promote expansion of the tourism industry despite unsustainable water demands from tourist inflated populations, especially during the dry season (Waite, 2012), and their distinct preference for high quality traditional water resources over desalinated water (Emmanuel and Spence, 2009). In Barbados, daily per capita water use by tourists is 3.6 times that of the resident population, and accounting of water demand by golf courses, cruise ships, seasonal expatriates, and short-term tourists is poorly documented (Emmanuel and Spence, 2009).

Tourist infrastructure has access to technology and funding to provide their guests with a quality and healthy experience. The demand for quantity and quality aspects of water will continue to grow as tourism increases its contribution to local economies. The key to sustainable water resources in Caribbean SIDS is a partnership between tourism development and residents to enhance water availability to local populations via potable water generation from tourism infrastructure and recycling of treated wastewater via nature-based solutions. Rather than increasing the burden on existing water resources, tourist development must develop desalination technologies and work to treat wastewaters from both their operations and local communities. Negative impacts on water quantity and quality on human consumption and the environment must be addressed by national policies developed through public-private consultation that incorporate adaptive management to address population dynamics, tourism and climate change.