Marine Protected Areas (MPAs) are a cornerstone of global ocean conservation strategies. However, they are under multiple pressures related to anthropogenic terrestrial pollution. Untreated and poorly treated domestic wastewater represents a widespread and under-recognized threat to tropical coastal ecosystems and adjacent coastal human populations. We present the world’s first assessment of total nitrogen loads (TN) from wastewater pollution within coastal LMPAs associated with tropical coastal ecosystems: coral reefs, seagrass meadows and mangrove forests. Using modeled spatial data, we quantified TN exposure on 1855 AMPs within 50 km of the coast, and evaluated both the distribution and extent of exposure in six tropical regions associated with the three types of ecosystems. The results revealed a great variability in the level of exposure to wastewater pollution according to the different regions. The regions of East Africa and the Middle East and North Africa had the highest average, median, maximum and standard pollutant loads. Overall, in all regions, average TN loads were consistently above median values, highlighting the disproportionate levels of pollution to which some MPAs are exposed compared to others. In addition, the pixel analysis revealed that in four regions, AMPs had higher median pollution than their non-AMP counterparts, suggesting that protection status does not guarantee benefits by reducing exposure to pollution. This research highlights that the reduction of wastewater pollution must be prioritized within the framework of the global biodiversity goals around effective area-based conservation, which simultaneously benefits the health and resilience of coastal ecosystems to climate change, as well as human health and well-being in adjacent local communities.
1. Introduction
Land pollution, especially untreated or poorly treated domestic wastewater, is among the most ubiquitous and poorly managed threats to coastal ecosystems such as coral reefs, seagrass and mangrove forests (Andrello et al., 2022; Tuholske et al., 2021; Wear et al., 2024). Globally, it is estimated that 55% of coral reefs and 88% of marine grass ecosystems are exposed to wastewater pollution (Tuholske et al., 2021). Despite its well-known magnitude, wastewater pollution is often overlooked in marine conservation initiatives (Wear, 2016), leading these ecosystems to a crucial unresolved challenge.The ecological impacts of wastewater pollution on tropical coastal ecosystems are well documented. Nutrient enrichment by wastewater reduces coral reproduction processes, growth rates and survival of early stages of life, while increasing the prevalence of coral diseases and bioerosion processes (De’ath and Fabricius, 2010; Fabricius, 2005; Tebbet et al., 2025). As a result, pollution pressures have led to measurable declines in the abundance and diversity of coral reef species worldwide (Wenger et al., 2020; Delevaux et al., 2018; Duprey et al., 2016; Ennis et al., 2016; Tebbett et al., 2021; Cleary et al., 2016). Wastewater pollution also inhibits light penetration, limiting photosynthetic activity in seagrass meadows, while introducing pathogens and promoting the growth of competing macroalgae and epiphytes (Cabaço et al., 2008). Mangrove forests become more vulnerable to erosion and less efficient at storing carbon when exposed to wastewater pollution (Santos-Andrade et al., 2021; Naidoo, 2009). Overall, these impacts undermine the structure, function and long-term persistence of tropical coastal ecosystems, threatening their associated biodiversity and the essential ecosystem services on which millions of people depend.Not only does wastewater pollution have significant ecological impacts on tropical coastal ecosystems, but it also synergistically aggravates the impacts of climate change that they are already experiencing. Chronic nutrient load increases the vulnerability of corals to bleaching events and slows post-disturment recovery (Wang et al., 2018; Joppien et Morgan, 2025; Claar et al., 2020; Gove et al., 2023; Donovan et al., 2020; Wagner et al., 2010; DeCarlo et al., 2020). Mangroves are also becoming more likely to die in conditions of nutrient enrichment when facing droughts caused by climate change (Lovelock et al., 2009). Marine heat waves aggravate eutrophication and hypoxic events caused by wastewater pollution, threatening the sustainability of rich fish biodiversity associated with tropical coastal ecosystems (Wear et al., 2024; Brauko et al., 2020). In addition, these ecosystems become more vulnerable to increased erosion rates and reduced light availability with rising sea levels, also aggravated by wastewater pollution (IPCC), 2022; Wear and Thurber, 2015). These generalized impacts underscore the urgent need to improve wastewater management in order to protect health and promote the resilience of coastal ecosystems.In order to protect tropical coastal ecosystems, global efforts have focused on the development of marine protected areas (MPAs). AMPs are a cornerstone of global biodiversity conservation and a key mechanism to achieve Goal 3 of the Kunming-Montreal Global Biodiversity Framework, commonly known as « 30×30 », which aims to protect 30% of the ocean by 2030 (Stephens, 2023). The success of MPAs is generally measured in terms of law enforcement, size, longevity, lack of taking and location (Edgar et al., 2014). However, it is well documented that the effectiveness of AMPs in protecting biodiversity and obtaining the expected ecological results is lost when they are exposed to pollution (Lamb et al., 2016; Wenger et al., 2016; Halpern et al., 2013; Suchley and Alvarez-Filip, 2018; Bégin et al., 2016).Although pollution reduces the ability of AMPs to achieve biodiversity results, pollution in MPAs remains largely unquantified, and when efforts have been made, it is often underestimated or insufficiently monitored to support effective management (Abessa et al., 2018; Partelow et al., 2015). In addition, exposure to wastewater pollution has not been included in global assessments that quantify human impacts on marine coastal systems and in MPAs (Halpern et al., 2025; Jones et al., 2018; Williams et al., 2021), which has been identified as a serious knowledge and research gap that hinders their effective management (Abessa et al., 2018). These shortcomings underscore the critical need for a systematic assessment of exposure to pollution within MPAs to ensure that they can achieve their planned conservation and ecological objectives.In this study, we conduct a global spatial analysis of AMP exposure to domestic wastewater pollution in tropical regions with high diversity associated with coral reefs, seagrass meadows and mangrove forest ecosystems. We aim to quantify the extent, magnitude and variability of exposure to pollution through MPIs, and to assess how MPA protection compares to surrounding non-AMP areas, in order to assess whether existing protection status effectively mitigates exposure to wastewater pollution.
2. Methods
This study evaluated the exposure of MPAs, as established by the Global Protected Area Database, to domestic wastewater pollution using the total modeled nitrogen load (TN) as a proxy (Tuholske et al., 2021; UNEP-WCMC and IUCN, 2025). The TN load data is a ∼1 km resolution raster produced by Tuholske et al. (2021) that summarizes the modeled nitrogen emissions derived from wastewater based on population and housing type data at the country level, protein consumption, as well as accessibility at different levels of wastewater treatment facilities. The dataset provides spatial information on nitrogen loads from treated septic and open-air effluents in coastal areas worldwide. For this study, the TN load raster layer was used, which combines the three types of wastewater effluents. A 50 km coastal buffer was applied to both the pollution data set and the limits of the AMPs, following the approach used by Williams et al. (2021) (Williams et al., 2021), to ensure that MPAs located within or intersecting this coastal area were included. For AMPs extending beyond 50 km from the shore, only the area inside the coastal buffer was analyzed to maintain consistency with the coastal influence.The TN load data set, measured in grams (g) per pixel, was spatially intersected with each MPA polygon worldwide within the 50 km buffer limit using QGIS software version 3.44 (Soleure). The total load of TN per MPA was calculated by adding all the TN values per pixel in each MPA polygon. Using the QGIS software, the TN load per AMP was converted into kilograms (kg), and the area per polygon AMP was calculated and then converted into square kilometers (km)2) for further analysis. To compare exposure to pollution, the TN load was standardized by the surface of each polygon of the AMP in order to obtain a relative concentration of pollution per square kilometer (kg/km2).The average, median, minimum and maximum overall loads were calculated on all AMPs worldwide, as well as percentile thresholds (25th, 50th, 75th and 90th percentiles) to classify AMPs in percentile-based bins in subsequent regional analyses.Given the sensitivity of tropical coastal ecosystems to wastewater pollution, we focused on AMPs located in six regions where there is a high level of biodiversity associated with coral reefs, seagrass meadows and mangrove forests (Andrello et al., 2022; Jayathilake and Costello, 2018; Giri et al., 2011; Beyer et al., 2018): Australasia and Melanesia, the Caribbean and the Bahamas, the Coral Triangle, East Africa, the Indian Ocean, as well as the Middle East and North Africa. In each region, the average, median, minimum and maximum NP loads (kg/km2) were calculated and the AMPs were classified in the global percentile tanks to assess the distribution of TN exposure (Fig. S1)To contextualize exposure to pollution in AMPs, we compared the pixel-level TN loads in AMPs to those in non-AMP areas. Non-MPA zones were defined by the same 50 km coastal buffer used for each regional AMP, ensuring that the comparison encompassed the same coastal areas. These non-AMP zones were defined as all buffer pixels that did not cross the existing AMP polygons, providing a regional wastewater exposure base for areas without formal protection (Fig. 1).
To collect both the regional average and the variability of exposure to pollution in each region, we obtained the mean, median, maximum and standard deviation of the TN load values at the pixel level in the AMP and non-AMP zones. The statistical significance of the differences between exposure to AMP and non-AMP pollution was assessed using a Mann-Whitney U (MWU) test, a non-parametric test adapted to the strongly matched distribution and the extreme values observed in the data. Tests were carried out for each region individually and for all regions combined. The overall indicators for all regions were calculated, for MPS and non-MPA areas, providing a summary that takes into account both the variability of exposure to pollution within and between regions.
3. Results
The distribution and extent of exposure to TN from wastewater through the AMPs varied according to the six focus regions (Fig. 2). In the Australasia-Melanesia region, most AMPs experienced relatively low exposure, with nearly 80% falling below the 50th percentile and a low median TN load of 6.8 kg/km2(Table 1). The Caribbean, Bahamas and Coral Triangle AMPs showed similar indicators, with 57% of AMPs below the 50th percentile for both regions, as well as similar average TN loads of 161.8 and 166.9 kg/km2, respectively. East and Middle East Africa and North Africa had similar proportions of MPA above the 50th percentile (53% and 57% respectively), while having the highest mean, median and maximum TN loads and the highest standard deviations, indicating that many MPAs experience extremely high TN pollution. In all regions, average TN loads were consistently higher than medians, reflecting the influence of a subset of AMP with extremely high nutrient pollution from wastewater. The wide range of standard deviations also illustrates the heterogeneous nature of exposure to wastewater pollution within and between regions, highlighting the disproportionate pollution experienced by some MPS.
Table 1. Distribution of TN exposure across the AMPs of six tropical coastal regions. AMPs were classified into global percentile compartments based on the load of TN (kg) per km2. The percentages indicate the proportion of AMP in each percentile compartment. The « Global » line represents the values of all MPAs, including those beyond the focus areas, for additional context. The minimum load of TN was not included in the table because it is zero (0) for all lines. SD = Standard deviation.
| Region | Area of all MPAs (km2) | <25th percentile (%) | 25-50th percentile (%) | 50-75th percentile (%) | 75-90th percentile (%) | >90th percentile ( %) | Average TN load (kg/km2) | TN median load (kg/km2) | Maximum load in TN (kg/km2) | SD |
|---|---|---|---|---|---|---|---|---|---|---|
| Australasia & Melanesia | 347.595.3 | 32 | 47 | 19 | 3 | 0 | 30.8 | 6.8 | 744.8 | 81.1 |
| Caribbean and Bahamas | 277.880.6 | 27 | 30 | 31 | 10 | 3 | 161.8 | 16.5 | 6608.0 | 533.5 |
| Coral Triangle | 288.299.5 | 30 | 27 | 30 | 10 | 3 | 166.9 | 14.2 | 9450.0 | 702.2 |
| East Africa | 97.618.8 | 17 | 31 | 34 | 13 | 6 | 363.13 | 42.3 | 10,751.1 | 1255.2 |
| Indian Ocean | 2066.7 | 48 | 25 | 17 | 7 | 4 | 182.8 | 4.2 | 6454.6 | 729.5 |
| Middle East and North Africa | 122.348.4 | 20 | 24 | 30 | 17 | 10 | 706.4 | 65,0 | 14.084.6 | 2070.3 |
| World | 1,135,809.3 | – | – | – | – | – | 934.0 | 30.7 | 247.445.9 | 6274.4 |
Exposure to wastewater pollution differed significantly between AMPs and non-AMP coastal areas in the six regions concerned (MWU test, p < 0.001; Table 2). Median pixel pollution loads in AMPs and non-AMPs revealed contrasting regional patterns. AMP pixels had a higher median exposure in four regions compared to their non-AMP counterparts: Coral Triangle (0.68 versus 0.005 kg/pixel), Indian Ocean (45.65 versus 2.6 × 10−4 kg/pixel), the Caribbean and the Bahamas (0.01 versus 2.6 × 10−4 kg/pixel), as well as the Middle East and North Africa (0.01 versus 0.001 kg/pixel). Average pollution loads were consistently higher than medians in all regions, with large standard deviations, indicating a highly unbalanced distribution due to localized pollution hotspots (Fig. 3). When all regions were combined, AMPs had a higher overall median compared to non-AMPs (0.01 versus 0.001 kg/pixel) but a lower average (42.7 vs.74.4 kg/pixel), reflecting the influence of extreme values in unprotected areas and the regional variation in the placement of AMPs relative to exposure to pollution.
Table 2. Statistical comparison of pixel-level wastewater pollution exposure between AMP and non-AMP zones in the six target regions of the world. All differences were statistically significant. SD = standard deviation. MWU = Mann-Whitney U test.
| Region | Pixels MPA | Average MPA pixel value (kg/pixel) | Median APM (kg/pixel) | MPA SD | Non-MPA pixels | Average non-MPA pixel value (kg/pixel) | Non-MPA median (kg/pixel) | SD not MPA | Value p MWU |
|---|---|---|---|---|---|---|---|---|---|
| Australasia & Melanesia | 225.887 | 2.9 | 9.4 × 10−6 | 24.4 | 951.313 | 8.8 | 7.6 × 10−5 | 124.4 | <0.001 |
| Caribbean and Bahamas | 247.546 | 91.3 | 0.01 | 1102.5 | 1,055.632 | 72.0 | 2.6 × 10−4 | 1434.4 | <0.001 |
| Coral Triangle | 308.438 | 39.0 | 0.68 | 168.9 | 3,063.319 | 59.9 | 0.005 | 605.5 | <0.001 |
| East Africa | 109.811 | 46.2 | 6.8 × 10−4 | 355.2 | 510.185 | 168.4 | 0.004 | 2373.1 | <0.001 |
| Indian Ocean | 2441 | 128.6 | 45.65 | 224.4 | 210.483 | 130.0 | 2.6 × 10−4 | 692.2 | <0.001 |
| Middle East and North Africa | 145.544 | 25.6 | 0.01 | 194.4 | 568.921 | 161.3 | 0.001 | 3726.2 | <0.001 |
| Total | 1,039.667 | 42.7 | 0.01 | 563.6 | 6,359.853 | 74.4 | 0.001 | 1494.1 | <0.001 |
4. Discussion
Widespread and disproportionate exposure through AMPs underscores the pressing concern that, despite their protected status, many MPAs are vulnerable to wastewater pollution, which slows down their ability to provide ecological results. Our results highlight the urgent need, such as targeted domestic wastewater management and policy interventions, as a necessary complement to the design and management of MPAs.Regional contrasts underline the importance of context-specific information for management decision-making, in particular on the state of sanitation systems, the ecological impacts of domestic wastewater pollution and the environment conducive to sanitation (Wenger et al., 2024). For example, targeted investments in infrastructure, such as the construction and modernization of centralized wastewater treatment systems, may be crucial and more appropriate in some places (Brauman et al., 2007; Gray et al., 2015), while a circular economy approach or nature-based solutions (e.g., wetland restoration), or community wastewater management may be more effective for other regions (Faragò et al., 2021; Smol, 2023; Vigerstol et al., 2021). In any case, aiming to increase the budget and strengthen the capacity of local communities is vital to strengthen the effective management of MPAs. Global monitoring programs, adapted to local pressures and aligned with local governance capabilities and structures, are essential to guide adaptive and effective management and policies.It is important to note that the most exposed areas are often also the least equipped with infrastructure or wastewater monitoring capabilities. This is particularly pressing for developing tropical regions, where infrastructure gaps and underfunded conservation efforts make MPAs particularly vulnerable (Wenger et al., 2024; Wakwella et al., 2023). This reflects broader environmental justice challenges, where many regions of the most affected tropical coastal ecosystems are located in low- and middle-income countries, where local coastal communities are heavily dependent on coastal ecosystems for food security (Nasim et al., 2023; International Water Association (IWA), 2014). Combating pollution in these contexts requires governance reform, international cooperation and targeted investments that integrate the management of land pollution into the design of MPAs and coastal conservation strategies. Together, these trends underscore that the effective protection of coastal ecosystems requires integrated land-based management and investments in improving health systems, especially in developing tropical regions.Managing wastewater pollution and improving access to secure sanitation are not only beneficial for coastal ecosystems, but they also bring benefits to the public health of adjacent local communities (Wenger et al., 2024; International Water Association (IWA), 2014). Untreated and poorly treated wastewater can contain pathogens and contaminants that pose a threat to human health, such as waterborne diseases (e.g., cholera, hepatitis, etc.), especially for communities that depend on coastal waters for fishing and domestic use (Jenkins et al., 2018, 2019). In addition, food security for local communities can become a challenge, as pollutants such as heavy metals, excess nutrients and organic contaminants transported in domestic wastewater can bioaccumulate in fish and shellfish populations on which communities depend for their livelihoods and income (Wardrop et al., 2016; Dehm et al., 2021; Madikizela and Ncube). Thus, as the health of coastal ecosystems improves, local communities can continue to benefit from their respective ecosystem services such as coastal protection, fisheries productivity and carbon storage. Overall, combating wastewater pollution is a win-win situation both in terms of ecosystem health and human well-being.Despite the well-established links between domestic wastewater pollution and the health of coastal ecosystems and humans, wastewater pollution remains severely underfunded and deprioritized in global conservation agendas (Wear, 2016; Loiseau et al., 2021). Between 2010 and 2022, only 3.9% of ocean philanthropic funding aimed at reducing pollution (Lewis et al., 2023). In addition, the water sector faces an estimated annual deficit of US$130 billion in funding for adequate wastewater infrastructure (Joseph et al., 2024). The global desire to reach 30×30 will not keep its promises unless the effectiveness of pollution protection is prioritized at the same time as the extent. In order for MPAs to preserve biodiversity, it is necessary to tackle polluting sources that compromise their ecological integrity, such as wastewater.While our analysis provides a first global assessment of exposure to wastewater pollution in LMPAs across tropical coastal ecosystem regions, several limitations must be recognized. Although quantifying the distribution of exposure to wastewater pollution is crucial for effective management, this analysis probably underestimates the total extent of total exposure to pollution, because this approach does not take into account other key processes such as hydrodynamics or biological absorption, which influence the transport and fate of pollution (MacNeil et al., 2019). The analysis focuses only on the modeled TN loads, and therefore does not include the threats of other pollutants commonly found in wastewater (e.g. phosphorus, pathogens, pharmaceuticals and heavy metals) (van Dam et al., 2011). In addition, other mechanisms of terrestrial pollution are not included here, such as agricultural and urban sedimentation as well as nutrient runoff, widely recognized as growing threats to the health and resilience of coastal ecosystems (Brown et al., 2017, 2019; Carlson et al., 2019), noting that the results still underestimate the cumulative total exposure to terrestrial pollution. Finally, the analysis does not identify or quantify direct impacts on coastal ecosystems in the regions, but it provides evidence suggesting that there could be impacts, highlighting the need for future studies to study and quantify these impacts (Nalley et al., 2023).To achieve the ambitions of the 30×30 objective, protection must go beyond simple spatial coverage and ensure that MPAs offer concrete ecological results. This will not be possible until terrestrial pollution is systematically evaluated and treated respectively. Therefore, a more comprehensive set of parameters must be established to ensure the success of MPAs, where terrestrial pollution, including wastewater pollution, is integrated into the efficiency indicators of MPAs (Bennett and Dearden, 2014; Green et al., 2014).This requires integrating wastewater management into coastal conservation interventions, including in-depth spatially explicit tools, policy reforms and targeted funding mechanisms. Prioritizes investment in sanitation, pollution monitoring and contextual management will not only strengthen the ecological outcomes of MPAs, but also bring social, cultural and health benefits. Combating wastewater pollution in tropical coastal ecosystems is therefore a crucial and concrete step towards more global and efficient ocean management.
source : sciencedirect
