Impact Statements

The pollution of water and waterways from land-based human activities has extensive impacts on both human and ecosystem health, contributing to significant global health burdens and loss of critical ecosystem services. Management of pollution is therefore a major focus of multiple sustainable development goals (SDGs) to achieve targets for: zero hunger (SDG 2); good health and well-being (SDG 3); clean water and sanitation (SDG 6); climate action (SDG 13); life below water (SDG 14); and life on land (SDG 15). Despite extensive and complex impacts of poor water quality, pollution control has been highly sectorised and under-resourced, with poor coordination of implementation, often across insufficient scales to realise benefits. This review provides a novel summary of the overlapping impacts of water pollution to downstream public and coastal ecosystem health to support planning and decision-making that benefits a wide range of stakeholders from government, civil society and the private sector. We provide evidence-based suggestions to optimise investments in holistic, integrated watershed management (IWM) to improve water quality and achieve overall systems health, which also provides co-benefits for biodiversity and climate. We also identify the key enabling factors required to coordinate and monitor IWM implementation to achieve desired outcomes. Specifically, the summary of pollution impacts and suggested management strategies provided in this review aim to provide awareness and tools to alleviate impacts to nutrition, water-related disease burdens and food poisoning that arise from poor water quality, which cause devastating economic and health costs disproportionately borne by the poorest countries.

Water quality impacts on coastal ecosystems

The quantity and quality of land-based runoff flowing into adjacent coastal ecosystems is determined by the characteristics of the watershed, such as geology, rainfall, soil type, land cover/vegetation (type and quantity) and slope (Douglas, Reference Douglas1967). There is a large body of evidence that demonstrates how human activities within watersheds alter runoff by removing native vegetation, changing the hydrology, altering microbial communities and adding/increasing pollutants within runoff (e.g., Peters and Meybeck, Reference Peters and Meybeck2000; Liao et al., Reference Liao, Yen, Guan, Ke and Liu2020).

Several broad pollutant categories are used to describe the pollutants reaching coastal waters from land-based activities. Here, we focus on the following common categories applicable to both human and coastal ecosystem health: sediments, nutrients, persistent organic pollutants (POPs), plastics and microdebris, pathogens, heavy metals, and pharmaceuticals and personal care products (Todd et al., Reference Todd, Ong and Chou2010; World Health Organization (WHO), 2016; Kroon et al., Reference Kroon, Berry, Brinkman, Kookana, Leusch, Melvin, Neale, Negri, Puotinen, Tsang and van de Merwe2020). Terrestrially derived sediments, heavy metals and nutrients are naturally transported from soils into coastal environments by ground and surface water, but due to large-scale human activities such as land-clearing (Table 1), the sources and transport into coastal waters has increased drastically, threatening over 30% of coral reefs globally (Andrello et al., Reference Andrello, Darling, Wenger, Suárez‐Castro, Gelfand and Ahmadia2021). POPs are synthetic organic chemicals that can persist in soils and water and bioaccumulate in organisms. POPs are widely produced across industries (Table 1) both intentionally, such as some insecticides, and unintentionally as by-products, such as dioxins (Weber et al., Reference Weber, de Beer, Lott, Polerecky, Kohls, Abed, Ferdelman and Fabricius2011). Other synthetic pollutants include the nonorganic plastics and microdebris, which can flow into coastal waters from numerous human sources (Table 1) such as trash, litter and weathering of materials like tires (Smith et al., Reference Smith, Love, Rochman and Neff2018; Macleod et al., Reference MacLeod, Arp, Tekman and Jahnke2021). Pathogens are disease-causing microbes and can naturally exist in coastal water and organisms but can also be introduced from land-based sources such as sewage (Table 1). Pharmaceuticals include chemicals used for personal, agricultural or animal health, such as antibiotics, while personal care products include chemicals generally used for cosmetic reasons, such as shampoos and moisturisers (Boxall et al., Reference Boxall, Rudd, Brooks, Caldwell, Choi, Hickmann, Innes, Ostapyk, Staveley, Verslycke and Ankley2012).

Table 1. Key references documenting global/regional linkages between human activities within watersheds and elevated levels of pollutants in runoff to coastal ecosystems

The primary land-based activities creating these pollutants and driving global declines in coastal water quality are land clearing, poor food production practices, urban development, mining and poor wastewater management (domestic and industrial) (Lu et al., Reference Lu, Yuan, Lu, Su, Zhang, Wang, Cao, Li, Su, Ittekkot, Garbutt, Bush, Fletcher, Wagey, Kachur and Sweijd2018). These human activities erode or release pollutants such as sediment, metals, pathogens and nutrients into surface and groundwater, which are then transported downstream to coastal environments (Crain et al., Reference Crain, Halpern, Beck and Kappel2009; Amato et al., Reference Amato, Bishop, Glenn, Dulai and Smith2016). The flow of impacts from human activities within watersheds to coastal ecosystems is summarised below (Figure 1).

Figure 1. Diagram depicting flow of impacts from key land-based activities on water quality properties that reach coral reef ecosystems.

As outlined in Table 1, pollutants can have multiple sources that can make it difficult to pinpoint which activity in a watershed is having the greatest impact on coastal ecosystems. For example, nutrients and sediments can originate from both wastewater pollution and agricultural runoff (Figure 1). Similarly, pharmaceuticals and personal care products can originate from cosmetics and medications used domestically as well as from medications used in agriculture (Table 1). In addition to the complexity of sources and types of pollutants, synergistic impacts and interactions occur when multiple pollutants are present at elevated levels, which can exacerbate the degradation of coastal ecosystems and harm associated organisms (Lu et al., Reference Lu, Yuan, Lu, Su, Zhang, Wang, Cao, Li, Su, Ittekkot, Garbutt, Bush, Fletcher, Wagey, Kachur and Sweijd2018; Huang et al., Reference Huang, Chen, Song, Deng, Shen, Chen, Zeng and Liang2021). Synergistic impacts and interactions also occur when pollutants are present with other stressors, such as climate change, disease, invasive species and overfishing. We focus on synergistic interactions on coral reefs, given the large body of research.

Herbivory is an important ecological process within coral reef ecosystems and can have complex and synergistic interactions with poor water quality (Table 2; Mumby et al., Reference Mumby, Hastings and Edwards2007). For example, in reefs with combined exposure to poor water quality and few herbivores, macroalgae and sediment-laden turfs can replace live coral as the dominant benthos (McField et al., Reference McField, Kramer, Giró Petersen, Soto, Drysdale, Craig and Rueda-Flores2020, Reference McField, Soto, Craig, Giró, Drysdale, Rueda-Flores, Castillo, Kramer and Roth2022). Sea level rise and climate-driven ocean warming are predicted to increase the sensitivity of coral reef ecosystems to poor water quality. Land-based pollution can lower the threshold for thermal stress and increase coral sensitivity to infection, resulting in increased bleaching (Fisher et al., Reference Fisher, Bessell-Browne and Jones2019), coral mortality (Claar et al., Reference Claar, Starko, Tietjen, Epstein, Cunning, Cobb, Baker, Gates and Baum2020) and outbreaks of disease on coral reefs (Vega-Thurber et al., Reference Vega-Thurber, Mydlarz, Brandt, Harvell, Weil, Raymundo, Willis, Langevin, Tracy and Littman2020). Corals that bleach from thermal stress also have reduced capacity to cope with sediment pollution (Bessell-Browne et al., Reference Bessell-Browne, Negri, Fisher, Clode and Jones2017). Nutrient pollution can result in brittle corals that are less resilient to the impacts of climate change, such as sea level rise and the increased severity and frequency of cyclones (Table 2; Rice et al., Reference Rice, Maher, Correa, Moeller, Lemoine, Shantz, Burkepile and Silbiger2020). Improving water quality through management of human activities within watersheds can therefore improve the resilience of corals to global impacts such as climate change.

Table 2. Impacts of poor water quality on humans, coral reefs, and coral reef organisms categorised by pollutant type, with key references indicated for further information

* Contaminant dynamics are complex, with different impacts and response curves observed even between contaminants in the same group (e.g., different heavy metals generate different impacts, different types of nutrients generate different impacts). Different levels of exposure also generate different responses, with some nutrient species generating positive responses under certain exposure levels. Impacts reported here are a general summary of known impacts from the introduction of each contaminant group at harmful levels observed in the environment.

Water quality impacts on human health

Many of the same drivers of declines in water quality and aquatic biodiversity, such as watershed deforestation, forest fragmentation on riverbanks and poor coverage of sanitation services, are also associated with human health impacts (Table 2). Impacts to humans from poor water quality include enhanced transmission of disease through polluted water and waterways, nutrition deficits from fisheries decline and chronic illness, and food poisoning from the contamination of important aquatic foods (Shuval, Reference Shuval2003; World Health Organization (WHO), 2015; Chase and Ngure, Reference Chase and Ngure2016). Over a million people die each year from water-related diseases, and at least 50% of these deaths are children and attributable to microbial intestinal infections (Kovacs et al., Reference Kovacs, Mullholland, Bosch, Campbell, Forouzanfar, Khalil, Lim, Liu, Maley, Mathers, Matheson, Mokdad, O’Brien, Parashar, Schaafsma, Steele, Hawes and Grove2015). Water related diseases such as diarrhoea are major contributors to global disease burdens, causing 8% of all deaths in children under the age of 5 years largely due to inadequate drinking-water quality (United Nations Inter-Agency Group for Child Mortality Estimation (UN IGME), 2019; World Health Organization (WHO) and United Nations Children’s Fund (UNICEF), 2021). Persistent endemicity and explosive outbreaks of water-related disease are often fuelled by interacting environmental factors related to climate change, land use and changing social conditions (Cann et al., Reference Cann, Thomas, Salmon, Wyn-Jones and Kay2013; Prüss-Ustün et al., Reference Prüss-Ustün, Wolf, Bartram, Clasen, Cumming, Freeman, Gordon, Hunter, Medlicott and Johnston2019). Water-related illness and travel associated with accessing safe water sources also contributes to reduced socioeconomic outcomes, such as reduced school attendance and gender equity (Fisher, Reference Fisher2008; Sorenson et al., Reference Sorenson, Morssink and Campos2011).

Communities reliant on surface and groundwater sources for drinking, bathing and household cleaning water are most at risk to water-related diseases and exposure to pollutants of emerging concern, particularly in tropical environments (Ragosta et al., Reference Ragosta, Evensen, Atwill, Walker, Ticktin, Asquith and Tate2011; World Health Organization (WHO), 2016; Herrera et al., Reference Herrera, Ellis, Fisher, Golden, Johnson, Mulligan, Pfaff, Treuer and Ricketts2017). Climate change is predicted to further increase global disease burdens by altering water-related disease dynamics (Semenza, Reference Semenza2020). Changes in rainfall and temperature will threaten water security, enhance pathogen survival and virulence, and increase exposure to contaminated water through multiple pathways, including flooding (Hofstra, Reference Hofstra2011; Levy et al., Reference Levy, Smith and Carlton2018). Rates of diarrhoea are predicted to increase under warmer and/or wetter conditions, with 1°C of warming predicted to increase diarrhoeal disease by 5% in developing countries (Singh et al., Reference Singh, Hales, De Wet, Raj, Hearnden and Weinstein2001).

Although water-related diseases are more often associated with exposure on land and freshwater, polluted seawater also presents a significant risk to human health. An estimated 180 million cases of upper respiratory disease and gastroenteritis occur each year due to humans bathing in polluted ocean waters or ingesting contaminated seafood, while around 4 million cases (and 40 thousand deaths) of infectious hepatitis A and E (HAV/HEV) occur annually from contaminated seafood from polluted coastal waters (Shuval, Reference Shuval2003; World Health Organization (WHO), 2015). Additionally, seafood contaminated with methylmercury and polychlorinated biphenyls can cause cardiovascular diseases in humans as well as severe impacts to infants in utero (Landrigan et al., Reference Landrigan, Stegeman, Fleming, Allemand, Anderson, Backer, Brucker-Davis, Chevalier, Corra, Czerucka, Bottein, Demeneix, Depledge, Deheyn, Dorman, Fénichel, Fisher, Gaill, Galgani, Gaze, Giuliano, Grandjean, Hahn, Hamdoun, Hess, Judson, Laborde, McGlade, Mu, Mustapha, Neira, Noble, Pedrotti, Reddy, Rocklöv, Scharler, Shanmugam, Taghian, Van De Water, Vezzulli, Weihe, Zeka, Raps and Rampal2020). The impacts of polluted seawater create a huge social and economic cost to communities, with pathogens in ocean pollution causing an estimated $19.4 billion (2022 USD) in economic losses annually because of their direct impacts on humans alone (Shuval, Reference Shuval2003).

Microplastics and debris found in wastewater pollution can also form a unique microbial community that is distinct from the surrounding water (Zettler et al., Reference Zettler, Mincer and Amaral-Zettler2013). The microbial community on plastic can include pathogenic microorganisms, such as Vibrio spp., that cause infections through contaminated water or seafood consumption (Zettler et al., Reference Zettler, Mincer and Amaral-Zettler2013; Kirstein et al., Reference Kirstein, Kirmizi, Wichels, Garin-Fernandez, Erler, Löder and Gerdts2016). In the case of some zoonotic parasitic microbes that cause illness in aquatic wildlife and illness in humans from shellfish consumption, counts of the microbes are higher on plastics than in surrounding water (Zhang et al., Reference Zhang, Kim, Rueda, Rochman, VanWormer, Moore and Shapiro2022). Plastics therefore potentially create a novel habitat for pathogens to be concentrated and dispersed beyond their typical range, as floating plastics can travel longer distances than natural substrates (e.g., wood and macroalgae), and sinking microplastics are readily ingested by filter-feeding shellfish (Zettler et al., Reference Zettler, Mincer and Amaral-Zettler2013; Littman et al., Reference Littman, Fiorenza, Wenger, Berry, van de Water, Nguyen, Aung, Parker, Rader, Harvell and Lamb2020; Zhang et al., Reference Zhang, Kim, Rueda, Rochman, VanWormer, Moore and Shapiro2022).

Polluted coastal ecosystems also affect the health of coastal human populations through fisheries decline (Hicks et al., Reference Hicks, Cohen, NAJ, Nash, Allison, D’Lima, Mills, Roscher, Thilsted, Thorne-Lyman and MacNeil2019; Li et al., Reference Li, Tian, Xu and Cheng2019). Millions of people depend on tropical coastal fisheries for essential protein and micro-nutrients (Kawarazuka and Béné, Reference Kawarazuka and Béné2010; Teh et al., Reference Teh, Teh and Sumaila2013). More than 10% of the global population is likely to face micronutrient and fatty acid deficiencies if the current trajectories of fisheries decline continue, especially in the developing nations at the Equator (Golden et al., Reference Golden, Allison, Cheung, Dey, Halpern, McCauley, Smith, Vaitla, Zeller and Myers2016). In addition, individuals already experiencing chronic health effects due to repeated exposure to pathogens will have nutrient absorption challenges, further exacerbating any micronutrient deficiencies from declining fisheries (Chase and Ngure, Reference Chase and Ngure2016). Better recognition of the economic and human health costs resulting from pollution impacts is critical for prioritising action and leveraging the necessary cross-sectoral partnerships and resources required for managing pollution at appropriate scales.

Systems approaches to watershed management

There are an array of site-based management interventions that can be implemented at nested scales within watersheds to improve water quality (Liu et al., Reference Liu, Engel, Flanagan, Gitau, McMillan and Chaubey2017; Richmond et al., Reference Richmond, Golbuu, Shelton, Wolanski, Day, Elliot and Ramachandran2019; Leder et al., Reference Leder, Openshaw, Allotey, Ansariadi, Barker, Burge, Clasen, Chown, Duffy, Faber, Fleming, Forbes, French, Greening, Henry, Higginson, Johnston, Lappan, Lin, Luby, McCarthy, O’Toole, Ramirez-Lovering, Reidpath, Simpson, Sinharoy, Sweeney, Taruc, Tela, Turagabeci, Wardani, Wong and Brown2021). Mitigation efforts typically include policy instruments and place-based interventions.

Policy instruments, such as regulations or market-based incentives, can be applied at any scale and are not necessarily spatially bound within watersheds or aimed at specific watersheds. For example, the implementation of policy instruments can control, reduce and/or prevent pollution through improved use, transport, storage and disposal of chemicals (Taylor et al., Reference Taylor, Pollard, Rocks and Angus2012; Olmstead and Zheng, Reference Olmstead and Zheng2021) and nutrients (UNEP, 2012). Policy instruments can also initiate the implementation of soil conservation and erosion/runoff control strategies, such as maintaining riparian buffer zones by legislating mangrove protection (Richmond et al., Reference Richmond, Golbuu, Shelton, Wolanski, Day, Elliot and Ramachandran2019).

Place-based interventions are specifically applied at a range of scales, from landscape, residential, down to individual and microbial scales (Figure 2). Traditionally, human health focused place-based interventions have been targeted at a residential and individual scale, through the application of water, sanitation and hygiene (WASH) infrastructure improvements or behaviour change campaigns (World Health Organization (WHO), 2016; World Health Organization (WHO) and United Nations Children’s Fund (UNICEF), 2021). However, there is now substantial evidence that landscape scale interventions could deliver significant human health outcomes, while also protecting ecosystem health. For example, a study involving 35 developing countries found that higher upstream tree cover in watersheds was associated with a lower probability of childhood diarrhoeal disease downstream (Herrera et al., Reference Herrera, Ellis, Fisher, Golden, Johnson, Mulligan, Pfaff, Treuer and Ricketts2017). In Hawai ͑i, Ragosta et al. (Reference Ragosta, Evensen, Atwill, Walker, Ticktin, Asquith and Tate2011) demonstrated that higher riparian canopy cover was associated with lower Enterococcus concentrations in stream water. New genomics research is beginning to reveal how more intact ecosystems, from the watershed to the individual organism scale, are more likely to carry lower pathogen loads (Hess et al., Reference Hess, Wenger, Ainsworth and Rummer2015; Shore-Maggio et al., Reference Shore-Maggio, Aeby and Callahan2018; Bass et al., Reference Bass, Stentiford, Wang, Koskella and Tyler2019). Coastal ecosystems also play a key role in regulating disease risk in the marine environment, with a recent study showing that when seagrass meadows are present, there are 50% fewer potentially pathogenic bacteria capable of causing disease in humans and aquatic organisms (Lamb et al., Reference Lamb, Willis, Fiorenza, Couch, Howard, Rader, True, Kelly, Ahmad, Jompa and Harvell2017). However, coastal ecosystems themselves are vulnerable to high levels of pollution (Crain et al., Reference Crain, Halpern, Beck and Kappel2009; Wear, Reference Wear2016; Turschwell et al., Reference Turschwell, Connolly, Dunic, Sievers, Buelow, Pearson, Tulloch, Côté, Unsworth, Collier and Brown2021), underscoring the importance of implementing a system-wide approach when managing watersheds.

Figure 2. Nested scales of watershed processes.

Despite the recognition that pollution is one of the greatest threats facing coral reef ecosystems (Burke et al., Reference Burke, Reytar, Spalding and Perry2011; Andrello et al., Reference Andrello, Darling, Wenger, Suárez‐Castro, Gelfand and Ahmadia2021), there are limited examples of water quality management associated with successful recovery of coral reef ecosystems, and of those, the management interventions have primarily only tackled pollution arising from point-source pollution (Birkeland et al., Reference Birkeland, Green, Fenner, Squair and Dahl2013; Reef Resilience Network, 2021). Designing and measuring the effectiveness of policy instruments for water quality management is difficult due to lack of compliance and information on contaminant thresholds and monitoring (Taylor et al., Reference Taylor, Pollard, Rocks and Angus2012; Olmstead and Zheng, Reference Olmstead and Zheng2021). Place-based interventions are often impeded by difficulties in engaging stakeholders, lack of systematic/transparent planning, and funding shortfalls (Jupiter et al., Reference Jupiter, Wenger, Klein, Albert, Mangubhai, Nelson, Teneva, Tulloch, White and Watson2017; Ayala-Orozco et al., Reference Ayala-Orozco, Rosell, Merçon, Bueno, Alatorre-Frenk, Langle-Flores and Lobato2018). For example, where stakeholders are not effectively engaged, interventions can be hindered by divergent visions, interests, and tensions within and between sectors (Ayala-Orozco et al., Reference Ayala-Orozco, Rosell, Merçon, Bueno, Alatorre-Frenk, Langle-Flores and Lobato2018). Lack of engagement can also limit buy-in and uptake of interventions by groups (Oteros-Rozas et al., Reference Oteros-Rozas, Martin-Lopez, Daw, Bohensky, Butler, Hill, Martin-Ortega, Quinlan, Ravera, Ruiz-Mallen and Thyresson2015; Mitchell et al., Reference Mitchell, Mitchell, Hunt, Townsend and Lee2022). Lack of systematic/transparent planning and evaluation can generate a lack of trust, accountability and credibility from the perspective of stakeholders (Ayala-Orozco et al., Reference Ayala-Orozco, Rosell, Merçon, Bueno, Alatorre-Frenk, Langle-Flores and Lobato2018), and lead to missed opportunities for effective action (Jupiter et al., Reference Jupiter, Wenger, Klein, Albert, Mangubhai, Nelson, Teneva, Tulloch, White and Watson2017; Beer et al., Reference Beer, McKenzie, Blažek, Sotarauta and Ayres2020). Funding shortfalls and lack of personnel prohibit action at the scale and duration required (Ayala-Orozco et al., Reference Ayala-Orozco, Rosell, Merçon, Bueno, Alatorre-Frenk, Langle-Flores and Lobato2018; Beer et al., Reference Beer, McKenzie, Blažek, Sotarauta and Ayres2020). Interventions for nonpoint source pollution can be particularly challenging as pollution loading is difficult to estimate and is often attributable to many stakeholders and sectors beholden to different regulations (Shortle and Horan, Reference Shortle and Horan2001).

Kāneʻohe Bay in Hawaii is a commonly cited case-study of point-source pollution (sewage) management for coral reef ecosystems resulting in a rarely seen recovery from an algal dominated back to a coral dominated state (Bahr et al., Reference Bahr, Jokiel and Toonen2015). More recent successes include recovery of coral reef ecosystems within Faga’alu Bay in American Samoa and Molokaʻi in Hawai‘i, where harmful runoff from the upstream quarry activities (Samoa) and invasive ungulate species (Hawaiʻi) were managed through targeted watershed interventions (Vargas-Ángel and Huntington, Reference Vargas-Ángel and Huntington2020). Both regions’ intervention strategies required large and costly monitoring efforts to observe success, and both observed setbacks in recovery trajectories due to external disturbances (e.g., storm waves and bleaching; Bahr et al., Reference Bahr, Jokiel and Toonen2015; Vargas-Ángel and Huntington, Reference Vargas-Ángel and Huntington2020).

Key enabling factors

Conclusions/recommendations

The latest science makes it clear that unplanned development, poor land use, unsustainable agricultural practices and poor wastewater management within watersheds are significant threats to coastal populations and ecosystems. Despite the threats, incentivising improved watershed management practices for the sake of improving water quality for downstream environmental benefits has remained a challenge. In the future, it is recommended that policies and management are designed using systems health approaches that aim to restore water quality to achieve multiple benefits for human and coastal ecosystem health, while facilitating sustainable social and economic development.

Through our review, we identified a series of actionable recommendations to promote holistic approaches to watershed management for systems health (Table 3). These include best practice lessons from existing, IWM programs on: inclusive planning; implementation through cross-sectoral coordination; participatory management; monitoring to identify risks and measure progress of interventions; mobilising resources to sustain long-term action; and sharing information to promote replication and scaling of integrated approaches. To achieve SDG targets by 2030, there is increasing urgency to prioritise these types of management approaches that simultaneously deliver on benefits for nature, people and climate.

Table 3. Recommendations for planning, coordinating, monitoring, resourcing and scaling sustained investment in integrated watershed management for systems health