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Systematic Review

From Source to Sea: The Role of River Basins in Marine Pollution Under Development and Climate Change Variability in SADC

by
Alfredo Pedro Mabica
1,2,3,*,
Sérgio Mateus Chilaule
4,5 and
Isidro José Tamele
6
1
LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
2
LAQV-REQUIMTE, Department of Chemistry, NOVA School of Science and Technology, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal
3
Centro de Investigação em Didática e Tecnologia na Formação de Formadores (CIDTFF), Departamento de Educação e Psicologia, Universidade de Aveiro, 3810-193 Aveiro, Portugal
4
Instituto de Investigação em Águas, Ministério da Ciência, Tecnologia e Ensino Superior, Maputo 1100, Mozambique
5
Centre for Environmental and Marine Studies (CESAM), Department of Environment and Planning, University of Aveiro, 3810-193 Aveiro, Portugal
6
Department of Chemistry, Faculty of Sciences, Eduardo Mondlane University, Av. Julius Nyerere, n 3453, Campus Principal, Maputo 1100, Mozambique
*
Author to whom correspondence should be addressed.
Pollutants 2026, 6(2), 28; https://doi.org/10.3390/pollutants6020028
Submission received: 31 December 2025 / Revised: 11 April 2026 / Accepted: 28 April 2026 / Published: 27 May 2026
(This article belongs to the Special Issue Marine Pollutants: 3rd Edition)
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Abstract

This article presents a systematic review of land-based marine pollution in the Southern African Development Community (SADC), focusing on the river–sea interface. Using the PRISMA protocol, 30 articles published between 2015 and 2025 were selected. Key pollutants identified include microplastics, nutrients, heavy metals, and urban effluents, which travel through river basins and impact coastal ecosystems and communities. The region faces serious challenges due to limited wastewater treatment coverage, with many urban areas lacking adequate infrastructure or using outdated technologies. This leads to widespread discharge of untreated effluents into rivers. Fragmented governance, weak institutional capacity, and insufficient implementation of regional treaties further hinder pollution control. Climate change exacerbates these pressures by increasing hydrological extremes and the vulnerability of sanitation systems. Despite existing legal instruments, cooperation on monitoring and wastewater management remains limited. The integration of basin and coastal planning, improved governance, and transboundary collaboration are essential to reduce pollution and promote ecological and social resilience in the region.

1. Introduction

Marine pollution is recognized as one of the most pressing environmental challenges of the twenty-first century, with profound implications for biodiversity, food security, and human well-being [1]. Approximately 80% of marine pollution originates from land-based sources, with river basins acting as major conduits that transport contaminants from inland areas to coastal and marine ecosystems [2,3]. These hydrological systems function as ecological corridors that mobilize solid waste, nutrients, pesticides, heavy metals, and emerging contaminants such as pharmaceuticals and microplastics [4,5]. Globally, about one thousand rivers are responsible for nearly 80% of plastic inputs to the ocean, corresponding to between 0.8 and 2.7 million tons annually [5].
In the Southern African Development Community (SADC), this situation is particularly complex. The region’s large transboundary basins, including the Zambezi, Limpopo, Orange-Senqu and Shire, provide essential services related to water supply, energy generation and food production to millions of people. However, these basins face increasing environmental pressures resulting from rapid urbanization, intensive agriculture, mining activities, poor waste management and limited institutional capacity for integrated water resources management [6,7]. Estimates suggest that nearly 80% of marine pollution in the region arises from land-based sources, a scenario aggravated by weak regulation and unplanned coastal development [7]. In Mozambique, urban plastic waste is often washed by rainfall into mangrove ecosystems, threatening their regeneration and compromising coastal resilience [8]. In this context, marine pollution is defined as the introduction of physical contaminants such as microplastics and solid waste, as well as chemical substances including agricultural nutrients, heavy metals and emerging pollutants, with associated ecological impacts. Although biological and thermal effects from urban and industrial effluents are recognized, these were not the primary focus of this review.
Recent studies have confirmed the widespread presence of microplastics across the region. These particles have been detected in rivers, estuaries and sediments, often with higher concentrations during the rainy season and in densely populated areas [6,9,10]. Evidence from South Africa’s Vaal River indicates measurable microplastic loads that are likely transported to the Indian Ocean [11]. This reinforces the notion that the African continent is among the most affected globally by aquatic plastic pollution. In addition to plastics, agricultural and urban nutrients also play a decisive role. Projections for the Zambezi Basin indicate that nitrogen exports could double by 2050 due to agricultural intensification and population growth [12]. The transboundary nature of plastic and chemical pollutants demonstrates that isolated solutions are insufficient and that regional cooperation mechanisms are essential [13].
Climate change exacerbates these pressures by altering precipitation patterns, hydrological regimes and the frequency of extreme events, intensifying sediment transport and contaminant fluxes toward coastal zones [14]. Extreme rainfall enhances the leaching of plastics and agrochemicals, whereas prolonged droughts reduce the dilution capacity of rivers, increasing pollutant concentration and ecological risks [1,15]. The combination of environmental degradation and socio-economic vulnerability makes the watersheds of the SADC region particularly susceptible to the compounded effects of pollution and climate variability [16,17].
Addressing this scenario requires coordinated, interdisciplinary and cross-border approaches grounded in sustainable watershed management. The SADC water protocols already promote integrated and cooperative management of shared resources as a strategy to mitigate pollution and climate impacts [16]. However, effective governance must include social, economic and ecological dimensions, investing in innovative waste-treatment technologies and promoting sustainable agricultural and urban practices [17].
Despite the growing scientific attention to this topic, there remains a shortage of comprehensive analyses that integrate river basin dynamics, marine pollution and climate change interactions in Southern Africa. Systematic reviews are therefore a valuable methodological approach for synthesizing dispersed evidence, identifying research gaps and supporting public policy design. Guided by the PRISMA 2020 framework [18], this study systematically reviews peer-reviewed literature published between 2000 and 2025 to evaluate the role of river basins in marine pollution under scenarios of development and climate change in the SADC region. The review aims to identify the main sources of pollution, analyze environmental and climatic interactions, and highlight knowledge gaps that can inform regional governance and future research agendas.

2. Study Area

The Southern African Development Community (SADC) is a regional organisation including 16 member states located in Southern Africa (Figure 1), which collectively occupy an area of approximately 10 million km2, ranging from inland countries such as Zambia, Zimbabwe and Botswana to coastal states such as Mozambique, Tanzania, South Africa and Namibia [19]. The region has a high degree of geographical and climatic diversity, marked by shared river basins of great importance, such as those of the Zambezi, Limpopo and Congo rivers [20].
The SADC region is characterised by predominantly tropical and subtropical climates, with a marked presence of arid and semi-arid zones, especially in the territories of Namibia, Botswana, Angola, Zimbabwe and southern Mozambique [21]. The Kalahari and Namib Deserts exemplify the region’s arid environments. Rainfall is highly seasonal and spatially variable, ranging from less than 200 mm annually in desert areas to over 800 mm in humid tropical zones such as parts of Tanzania and Democratic Republic of Congo [22].
Population growth in SADC has been steady: in 2010, the population was estimated at around 277 million people, and by 2023, it had exceeded 380 million [23]. This increase, coupled with rapid urbanisation, intensifies pressure on natural resources, particularly freshwater and marine ecosystems [24].
The management of river basins and marine resources is a key challenge for the region. Despite regional integrated water resources management (IWRM) policies and multilateral agreements, implementation faces barriers such as limited institutional capacity, inadequate infrastructure and unequal access to water [25].
Politically and socioeconomically, SADC faces challenges related to widespread poverty, political instability in some states, weak governance, and social inequality, factors that limit the effectiveness of environmental policies [26]. At the same time, strategic sectors such as agriculture are heavily dependent on predictable weather patterns, making the region vulnerable to droughts, floods and other extreme events [27].
Climate change exacerbates environmental risks: increased rainfall variability, more intense cyclones, coastal erosion and saltwater intrusion compromise water availability and quality [28]. In addition, aquatic and marine ecosystems suffer from increasing pollution from microplastics and other emerging pollutants. Recent studies have detected the presence of microplastics in river basins in South Africa and Namibia, particularly in urbanised areas with poor waste management [29,30]. These contaminants not only affect aquatic biodiversity, but also enter the food chain, posing risks to human health [31].

3. Materials and Methods

Systematic Review

This study was based on a systematic review conducted in accordance with the PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, ensuring transparency, rigour and reproducibility in the identification, selection and synthesis of relevant scientific literature, with details available in the Supplementary Materials. The objective was to gather and critically analyse published evidence on the role of river basins in the transfer of pollutants from source to sea in the Southern African Development Community (SADC) region, with an emphasis on microplastics and emerging contaminants, considering the impacts of development scenarios and climate change.
The literature searches were conducted mainly in the interdisciplinary database Scopus, supplemented by Web of Science, Google Scholar, PubMed, ScienceDirect and African Journals Online (AJOL), covering the period from 2000 to 2025. The last search was conducted in August 2025. Relevant institutional publications from SADC institutions were also included.
The review sought to answer the following research questions: (a) What types of pollutants, particularly microplastics and emerging contaminants, have been identified in SADC river basins? (b) How are these pollutants related to human activities and development processes? (c) What evidence exists on the impacts of climate change on the transport of these pollutants to marine systems? (d) How do future projections model the flow of pollutants under different development or climate trajectories (e.g., scenarios driven by economic growth versus sustainability)? (e) What governance mechanisms are in place in the region to address these pollution pathways?
Key terms used include: “SADC” or “Southern Africa”; “river basin pollution” or “transboundary river”; “marine pollution” or “source to sea”; “microplastics”, “emerging contaminants”, “pharmaceuticals”, “PFAS”, “EDCs”; “climate change and pollution”; “development” or “land use”; “river basin organization” or “governance”.
The inclusion criteria involved: (a) peer-reviewed articles; (b) studies focusing on river basins in the SADC region; (c) an approach to the transport of pollutants (microplastics, nutrients, agrochemicals and emerging contaminants); (d) consideration of aspects related to development, land use, urbanization or climate change; (e) publications between 2000 and 2025; and (f) texts written in English. On the other hand, the following were excluded: (a) studies outside the SADC region; (b) studies focused exclusively on water quantity, without addressing pollutants; (c) news reports, opinion articles or non-scientific sources; (d) publications prior to 2000; and (e) texts in languages other than English.
As a result, 1086 records were initially identified. After screening by title, 275 studies remained. Then, reading the abstracts reduced the number to 73. Finally, after reading the texts in full, 30 articles were selected and included in the final analysis, being used to assess the main sources of pollution, their environmental interactions and the associated impacts. The selection process is represented in Figure 2, which visually presents the methodological flow adopted, highlighting the exclusion criteria applied at each stage. This representation provides clarity and transparency to the method, allowing for the reproducibility of the review process adopted.

4. Results and Discussion

4.1. Geographical Distribution of Scientific Output in the SADC Region

Figure 3 shows the percentage distribution of articles selected in this systematic review, according to the countries of the Southern African Development Community (SADC). The data reveals a significant imbalance in scientific output related to marine pollution and the contribution of river basins. South Africa accounts for many publications, with 64%, followed by Namibia with 12%, and Malawi and Mozambique, both with 6% of contributions. Countries such as Zambia, Zimbabwe, Lesotho, and Eswatini have a smaller share, each contributing approximately 3%. On the other hand, several island and continental nations, such as Botswana, Tanzania, the Democratic Republic of Congo, Angola, Comoros, Seychelles, Mauritius, and Madagascar, did not meet the criteria for this review, accounting for 0% of the selected studies.
This concentration of scientific production in a few countries, as shown in the map in Figure 3, suggests not only inequalities in the development of environmental research, but also institutional and technological limitations that impact the capacity to monitor, manage, and respond to aquatic pollution in a significant part of the region. This reinforces the importance of fostering regional cooperation, investing in local scientific capacity building and expanding research networks to ensure an integrated and equitable approach to the governance of SADC river basins and coastal environments.

4.2. Temporal, Geographical, and Thematic Trends in Publications

Figure 4 provides a detailed overview of the quantitative and thematic characteristics of the scientific output analysed in this review. Panel (a) shows the temporal evolution of the number of publications between 2001 and 2025. There is a general trend of slow growth until 2018, followed by a sharp increase between 2019 and 2021, a period that concentrates the largest number of published studies, signalling a recent intensification of academic interest in the topic in the SADC region.
Panel (c) reinforces the geographical concentration of scientific production. South Africa leads by a wide margin, with more than twenty publications, while other countries, such as Namibia, Zambia, and Angola, contribute much more modestly. This asymmetry highlights disparities in research infrastructure, funding, and political priority given to water and marine pollution in different national contexts.
In relation to the river basins analysed, panel (d) reveals that the Limpopo, Inkomati and Zambezi systems are among the most studied. These basins have high population density, significant agricultural and industrial activity, and are transboundary, which increases their environmental and political relevance. The analysis by type of pollutant, presented in panel (b), indicates that microplastics are the predominant focus of research, with more than a dozen publications, followed by heavy metals, pesticides, emerging pollutants, and sediments. This trend reflects a growing scientific concern with the widespread and persistent presence of microplastics in aquatic ecosystems, as well as their risks to human and environmental health.
This quantitative data reinforce the need to expand research efforts to less represented countries and basins, diversify the types of pollutants studied, and deepen the understanding of the combined impacts of pollution and climate change.

4.3. Distribution of Studies by Subject of Analysis and Type of Plastic Pollutant

Figure 5 shows the thematic distribution of studies according to the environment or object of analysis, as well as the types of plastic pollutants investigated. The left panel shows that most articles focused on marine biota (8 publications) and river environments (7 publications), reflecting a concern with the ecological impacts on aquatic organisms. Next are studies on transboundary rivers and marine debris (both with 5 publications), indicating attention to cross-border pollutant flows and coastal pollution. Studies on soil and other categories represented a smaller share, with 3 and 5 publications, respectively.
The panel on the right specifies the types of plastic-derived pollutants examined. Polychlorinated biphenyls (PCBs) lead among the associated contaminants, with three publications, followed by PAHs and PET, with two occurrences each. Other polymers identified include PE, PP, and PS, as well as microplastics in the form of fibers. Although present in smaller numbers, organochlorine pesticides (HCHs and DDTs) appear as relevant contaminants associated with plastic debris. These data suggest a growing concern about the toxicity associated with plastic additives and bioaccumulation in coastal and estuarine ecosystems. The analysis reinforces the need for integrated research on the ecological effects of microplastics and their associated contaminants in different environmental compartments in the SADC region.

4.4. Summary of Reviewed Studies: Pollutants, Climate, and Environmental Impacts

Table 1 presents a structured summary of the 30 main studies selected that address the relationships between pollution, climate change, and environmental impacts on river and marine systems in the SADC region. The analysis highlights a diversity of pollutants studied, with emphasis on microplastics, heavy metals, pesticides and emerging contaminants. Most studies associate these substances with the intensification of human activities such as agriculture, urbanization, mining and improper waste disposal.
Regarding the relationship with climate change, about half of the articles directly incorporate climate variation scenarios, whether through water flow projections, changes in rainfall patterns, or extreme events such as droughts and floods. Other studies address the relationship indirectly, linking seasonal effects on the transport and accumulation of pollutants. These data reinforce that climate variability increases the complexity of pollution management, affecting both the concentration and mobility of contaminants in aquatic systems. The alternation between long periods of drought and intense rainfall, typical of the monsoon regime, intensifies pollutant mobilization through a process known as “first flush”. This mechanism rapidly transports accumulated nutrients, metals, and microplastics into aquatic environments immediately following the first seasonal rains, causing acute impacts on ecosystems.
In terms of environmental impacts, studies highlight significant risks to aquatic organisms, including bioaccumulation, chronic toxicity, and marine fauna mortality. Several articles have reported ecological effects on birds, fish, and molluscs, as well as risks to human health via the food chain. Microplastics have been widely recognized as vectors for other pollutants, in addition to having physical and chemical impacts on biota.
Finally, the studies also offer policy contributions and suggestions for integrated water resource management. Many recommend strengthening regional protocols, such as SADC agreements, implementing public policies for environmental monitoring, and adopting technologies for waste containment and treatment. There is also a call for greater cross-border cooperation, especially in shared basins such as the Limpopo, Zambezi, and Incomati.
Table 1 critically summarizes the key findings of the systematic review, highlighting gaps in environmental monitoring, the need for data standardization, and opportunities for advancing more effective public policies to address the challenges posed by pollution and climate change in the region.
The reviewed literature indicates that hydrological processes, such as surface runoff, and geomorphological processes, such as sediment mobilization and suspended transport, have a direct influence on the movement and deposition of pollutants. The morphology of river basins, combined with rainfall intensity and land use, affects the velocity, direction and volume of contaminants reaching estuaries and coastal areas.

4.5. Discussion

4.5.1. Pollutants in SADC River Basins: Origin and Effects

The results of this study confirm that river basins play a central role as transport routes for pollutants to marine ecosystems in the SADC region, with particular emphasis on the spread of microplastics. This dynamic is consistent with global observations indicating that river systems are responsible for a substantial portion of the plastic waste that reaches the oceans [60]. This relevance is also recognised in the SADC’s marine pollution response plan, which identifies river basins as critical vectors in the transfer of terrestrial pollutants to the marine environment [7].
In southern Africa, microplastics, heavy metals, nutrients, pesticides and emerging contaminants are among the main compounds transported by river basins. Recent evidence attests to the widespread presence of microplastics in sediments and surface waters of several rivers in the region, with high concentrations in basins such as the Crocodile, Vaal and Olifants rivers, including in protected areas such as Kruger National Park [11,30,37,61]. These pollutants originate mainly from the improper disposal of solid waste, urban and industrial effluents, landfills and urban runoff [6,11]. Among the most common polymers identified are polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), often detected in the form of fibres and submillimetre fragments [10,30]. These microplastics are predominantly derived from plastic packaging, textile fibers present in domestic effluents, and urban solid waste, as reported by [10] and [30].
This scenario is aggravated by the fact that microplastics not only affect freshwater ecosystems but are also exported to coastal environments, with high concentrations already recorded in marine sediments in South Africa [6]. Globally, it is estimated that a significant fraction of marine plastic pollution is transported by rivers, reinforcing the importance of control at the watershed scale [60]. In samples analyzed in African ecosystems, more than 40% of the data reveal high ecological risks associated with microplastic contamination [10], which can carry other pollutants and be ingested by aquatic organisms, affecting biodiversity [61].
In addition to microplastics, heavy metals such as mercury, lead, cadmium, arsenic, and chromium are recurring contaminants, especially in areas with intense mining and industrial activity, such as South Africa, Zambia, DRC, Zimbabwe, and Mozambique [62,63] The mercury used in gold mining in Manica (Mozambique), for example, has caused agricultural losses of up to 30% in riparian areas [62]. These metals accumulate in the food chain, particularly highly toxic methylmercury, forming in sediments and affecting aquatic organisms and humans [2]. Evidence points to oxidative stress, neurotoxicity and biodiversity loss in areas downstream of mined areas such as the Olifants and Vaal river basins and the Copperbelt region [2].
These pollutants are also detected in coastal areas, as demonstrated in Saldanha Bay, where farmed mussels showed seasonal fluctuations in metal and pesticide levels related to river runoff and climate variability [63]. The intensification of agriculture and urbanisation have contributed to an increase in the diffuse load of nutrients (nitrogen and phosphorus) and pesticides. Modelling indicates that the export of these nutrients by African rivers grew between 10% and 80% between 1970 and 2000 and could double by 2050 with the advance of intensive agriculture and urban growth [12,64]. Studies reveal that agriculture significantly contributes nitrogen and pesticide inputs, especially in basins such as the Limpopo. Mining is responsible for high levels of heavy metals such as mercury and lead, while urban areas account for nearly 80% of land-based pollution sources in the region, including untreated domestic sewage and solid waste.
Currently, about 44% of African rivers exceed acceptable levels of total phosphorus, and 15% have excessive nitrogen concentrations, reflecting widespread nutrient pollution across the continent [65]. Although international guidelines exist, national standards and monitoring systems vary considerably, depending on each country’s regulatory capacity and institutional context. This situation contributes to the worsening of environmental problems such as eutrophication and harmful algal blooms, especially during periods of heavy rainfall and surface runoff [12].
Emerging contaminants such as pharmaceuticals and personal care products (PPCPs) have also been detected in SADC water bodies, especially in South Africa. A recent study identified more than 100 pharmaceutical compounds in sewers and rivers, with high concentrations in regions such as Gauteng and KwaZulu-Natal [66]. The inefficiency of sewage treatment plants in completely removing these substances allows painkillers, antibiotics, antiretrovirals, cosmetics, and disinfectants to enter aquatic systems. Even at trace levels, such contaminants can induce effects such as bacterial resistance and endocrine dysfunction in aquatic organisms [66]. This reality, also observed in tropical basins in Latin America [67,68], suggests a challenge shared by developing regions.
The distribution of pollutants in SADC river basins is marked by distinct spatial and temporal patterns, influenced by factors such as land use and occupation, urban and industrial growth, agricultural intensification, and climatic and hydrological variations. Spatially, the highest levels of contamination occur in the vicinity of large urban centers, industrial hubs, and areas of intensive agriculture, where the diffuse load of nutrients, heavy metals, pesticides, and microplastics is most significant [63]. Examples of this are the water bodies near cities such as Maputo, Dar es Salaam, and Cape Town, which receive constant discharges of domestic and industrial sewage, as well as urban solid waste. Studies show that industrial areas, such as the Umhlatuzana and Umbilo river basins, have significantly higher pollution profiles associated with land use [69].
Even remote areas are not immune to the presence of pollutants, especially microplastics, whose persistent and light nature allows them to be dispersed over long distances by ocean currents and winds. This helps to explain the presence of these materials on beaches and islands far from urban centres, as already recorded not only in the SADC, but also in other tropical regions such as the Caribbean, where there are an estimated 5000 floating particles per km2 [70].
In terms of time, there is evidence of a progressive worsening of river and coastal pollution in recent decades, driven by demographic and economic growth without a corresponding expansion of sanitation and waste management systems. Studies suggest that land-based marine pollution has increased over time in SADC basins because of intensified human activities and increased use of chemical inputs [63]. In addition, seasonal patterns play an important role in the dynamics of pollutant transport. During the rainy season, increased surface runoff carries accumulated pollutants, resulting in peaks of contamination in estuaries and coastal areas. In the dry season, despite lower river loads, low flow contributes to the relative concentration of certain pollutants due to limited water dilution [63,71].
This variability was also observed in reference organisms along the South African coast, which showed fluctuating concentrations of metals and pesticides between wet and dry years, highlighting the effects of hydrological changes on environmental quality [63]. In studies of reservoirs, such as the Loskop Dam, it was found that contaminants such as organochlorine pesticides and phthalates are spatially concentrated in areas close to water inlets, suggesting point sources of contamination [72].
In general, the spatial and temporal patterns observed in SADC basins reflect not only the direct impacts of local human activities, but also regional climate dynamics and natural processes, showing similarities with other tropical and subtropical regions such as Southeast Asia and South America. This complexity poses challenges for environmental management and reinforces the need for integrated monitoring and mitigation strategies adapted to local hydrological and socio-economic conditions.

4.5.2. Environmental and Health Consequences of Pollution in SADC Basins

The environmental impacts of pollution carried by SADC river basins are diverse and interconnected, directly affecting aquatic ecosystems, human health, and the regional economy. The bioaccumulation and biomagnification of contaminants such as heavy metals and persistent organic pollutants (POPs) are particularly concerning, with records in filter-feeding organisms such as mussels, which accumulate Cd, Pb, Hg, and pesticides at levels higher than those in the water column [63]. These organisms, widely used as bioindicators, show levels that in some cases exceed the recommended standards for human consumption, as observed in coastal urban areas of South Africa [63]. Studies also show that the consumption of contaminated fish and shellfish can pose systemic risks to human health, such as mercury poisoning and endocrine effects [73,74].
Pollution from mining and industrial activities introduces heavy metals such as mercury, lead, and cadmium into aquatic environments, resulting in biodiversity degradation and changes in the structure of biological communities. These metals interfere with food chains, causing declines in fish populations and significant changes in macroinvertebrates [2,75]. Urban effluent inputs, in turn, contribute nutrients, antibiotics, and other contaminants that accumulate in sediments, affecting water quality and causing fish mortality, changes in microbial communities, and the spread of antimicrobial resistance [75].
Among emerging pollutants, microplastics stand out for their ubiquity and persistence. Although they are not conventional toxins, these fragments carry adsorbed contaminants and are ingested by marine organisms, from plankton to fish and aquatic mammals, causing intestinal blockages, oxidative stress and possible cumulative sublethal effects [61]. They have been identified in fish tissues and in human consumption products such as salt and drinking water, evidencing multiple routes of exposure [61]. These risks are amplified by the absence of systematic monitoring and specific guidelines for microcontaminants in the region.
The introduction of excess nutrients, especially nitrogen and phosphorus from agricultural fertilisers and domestic sewage, drives eutrophication in coastal waters, promoting algal blooms and episodes of hypoxia. This process leads to the loss of sensitive habitats and changes in species composition, favouring opportunistic organisms at the expense of biodiverse communities [63]. Agricultural pesticides also affect non-target organisms, with lethal or sublethal effects, such as reproductive dysfunction and behavioural changes, which can lead to the collapse of sensitive aquatic populations [67].
The degradation of critical habitats, such as mangroves and coral reefs, is another reflection of the pollutant load. Mangroves act as sinks for sediments and contaminants, accumulating metals and plastics that affect their structure and ecological functioning [76]. Reefs exposed to plastics have a higher incidence of infectious diseases, with negative implications for their resilience and diversity, a phenomenon already documented in comparable tropical environments such as the Indo-Pacific [77].
In addition to the ecological impacts, the social and economic implications are substantial. Coastal and riverside communities that depend on artisanal fishing face food and health risks due to the consumption of contaminated fish. The economic losses extend to commercial fishing and tourism, sectors that are vulnerable to the deterioration of environmental quality [67]. Given this scenario, more effective environmental management measures and ecological rehabilitation strategies are essential to restore impacted ecosystems and mitigate risks to human health and well-being [78].

4.5.3. Influence of Climate Change on Transport and Pollution Intensity

Climate change significantly intensifies pressures on water systems in the SADC region, acting as an amplifying factor on existing pollution processes. In a regional context marked by strong seasonality, extreme weather events projected for the coming decades, such as heavy rains, flash floods and prolonged droughts, are likely to substantially alter patterns of transport and discharge of pollutants from river basins to coastal environments [79,80]. Studies show that episodes of extreme precipitation promote the abrupt mobilization of sediments, urban waste and accumulated contaminants, intensifying the input of pollutants into marine ecosystems in short periods of time [63]. Urban flooding resulting from severe storms, for example, can carry significant volumes of industrial waste, agricultural effluents, and domestic sewage directly to estuaries [79,80].
Global evidence indicates that a single urban storm can release between 1.9 million and 9.6 billion microplastic particles, demonstrating the potential for sudden and intense transport of these contaminants [81]. This pattern is supported by regional studies, such as those conducted on the South African coast, which point to high concentrations of heavy metals and persistent organic compounds in bivalves during the winter rainy season, a direct result of increased surface runoff and riverine pollutant load [63].
Climate models suggest that, although total annual precipitation may decrease in some areas, the frequency and intensity of extreme events are likely to increase [33,35]. The impact of Cyclone Idai in 2019 is a prime example: when it hit Mozambique, Zimbabwe and Malawi, the resulting floods dispersed urban waste, stored toxic substances and effluents into watercourses, compromising the environmental integrity of the basins [82]. Given the trend towards more intense tropical cyclones in the south-western Indian Ocean, experts advocate the incorporation of climate adaptation measures and resilient infrastructure into hydrographic management plans [42].
The alternation between prolonged periods of drought and intense rainfall also contributes to the intensification of pollutant concentration and discharge cycles. During droughts, toxic waste accumulates in riverbeds and reservoirs. These contaminants are remobilised in large quantities in the first subsequent floods, a phenomenon known as ‘first flush’, with impacts such as increased biochemical oxygen demand and episodes of aquatic fauna mortality [14,42,53].
The warming of surface waters and oceanographic changes induced by climate change also modify the dynamics of contamination. Higher temperatures favour the occurrence of harmful algal blooms in eutrophicated environments and reduce the physiological resilience of aquatic organisms exposed to contaminants [63]. Although some studies suggest that the accelerated metabolism of bivalves in warmer waters may reduce metal accumulation through excretion, this possible beneficial effect does not compensate for the ecological losses associated with ocean acidification, increased disease incidence, and changes in reproductive cycles [63].
Given this scenario, the literature advocates the adoption of integrated strategies that combine pollution control and climate adaptation. Fragmented environmental governance in the SADC region compromises responsiveness and hinders the effective management of shared water resources [42]. Recommended actions include strengthening sanitation infrastructure to withstand extreme events, protecting natural buffer ecosystems such as mangroves and floodplains, and incorporating updated climate data into hydrographic planning tools.
In short, climate change acts simultaneously as an additional stress factor and a catalyst that exacerbates the impacts of existing water pollution. This synergy is already recognized globally, with studies showing that climate change has generated, amplified and aggravated existing problems and created vulnerabilities, intensifying the degradation of water quality [83].

4.5.4. Integrated Management of Basins and Coastal Zones in SADC

Environmental governance in SADC faces serious weaknesses in the integrated management of river basins and coastal zones, especially in the face of land-based pollution. Although there are regional initiatives and agreements, such as ZAMCOM (Zambezi Watercourse Commission) and OKACOM (Permanent Okavango River Basin Water Commission), many of these instruments suffer from partial adherence and limited implementation. Integration between sectoral policies remains in its infancy, hindering coordinated action between countries with shared basins, as in the case of the Zambezi River, where only four of the eight signatory countries have fully ratified the protocol [7].
In the marine-coastal sphere, instruments such as the Abidjan and Nairobi Conventions provide legal frameworks to mitigate environmental impacts, recognizing terrestrial sources as responsible for much of the pollution. However, their effectiveness is hampered by a lack of consistent enforcement of standards, funding shortfalls, a shortage of technical personnel, and low political prioritization [7]. The mismatch between freshwater management and coastal planning results in fragmented responses, exacerbated by the lack of integrated management plans and marine spatial planning in many countries [82].
The lack of integration hinders efforts to tackle diffuse pollution, which is often diluted among different agencies without central coordination. In addition, financial constraints compromise investments in sanitation, waste collection, and environmental monitoring, with many Member States dependent on external support [82]. Data gaps in water quality and emerging contaminants also compromise evidence-based policymaking.
Institutional problems, such as overlapping competences and low capacity of local governments, accentuate the fragmentation of governance [84,85]. Financial constraints are exacerbated by inefficient resource allocation [86], while conflicts between diverse interests and low participation by local communities hinder the development of effective strategies [86].
Despite this, collaborative initiatives and adaptive governance mechanisms show potential. Programs run by institutions such as the Southern African Scientific Services Centre strengthen capacities and promote regional cooperation [87]. The representation of multiple actors in decision-making is considered essential [88], as is the active involvement of communities, as exemplified by the participatory approaches of ICM and MSP [89,90]. Platforms such as OCIMS demonstrate the value of collaborative systems [91], and knowledge sharing among technical networks contributes to trust and joint action.
At the global level, similar challenges are observed in regions such as Southeast Asia and the Caribbean, where cooperation structures also face political and operational limitations. Sustainable strategies in SADC should include integration between basin and coastal zone management, strengthening sanitation infrastructure, cross-border cooperation with harmonized standards, and international technical and financial support based on common but differentiated responsibilities. This will require investment in scientific data, robust legal frameworks, and cooperation between multiple sectors and countries, ensuring socio-environmental resilience and protection of aquatic ecosystems [7,82].

5. Conclusions

Land-based marine pollution in the SADC region represents a complex environmental challenge. This challenge is exacerbated by climate change, institutional fragmentation and capacity inequalities among Member States. Despite the existence of regional agreements and legal frameworks, such as the Abidjan and Nairobi conventions, their implementation has been limited. This is partly due to the lack of effective integration between river basin management and marine-coastal planning. A review of 30 studies, conducted based on the PRISMA protocol, showed that the main pollutants, such as agricultural nutrients, heavy metals, urban effluents, and microplastics, travel from source to sea, compromising sensitive ecosystems and human health.
However, the studies analyzed have important limitations. There is a scarcity of systematic and long-term data in several SADC countries, with a predominance of research focused on South Africa. In addition, there is a low level of methodological standardization, which makes it difficult to compare basins. Although nutrients and metals are among the most studied contaminants, microplastics and emerging pollutants remain under-explored. Few studies integrate environmental, climatic, and socioeconomic variables, which limits a holistic understanding of the processes that occur between pollution sources and the marine environment. These gaps reinforce the need to expand regional monitoring programs, strengthen cross-border research networks, and encourage more effective coordination between scientific production and public policy formulation.
Against this backdrop, it is recommended that regional cooperation mechanisms be strengthened, with an emphasis on harmonizing regulatory frameworks and adopting integrated ‘source-to-sea’ management plans that link river basins and coastal zones. Equally important is the standardization of environmental quality indicators, as well as increased investment in sanitation and wastewater treatment infrastructure. The promotion of ongoing environmental monitoring programs, supported by systematic and comparable data, forms a fundamental basis for the formulation of effective public policies. Cross-border cooperation and adaptive governance, sensitive to the impacts of climate change, must guide coordinated, multi-sectoral and sustainable responses capable of ensuring the ecological and social resilience of SADC ecosystems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pollutants6020028/s1.

Author Contributions

A.P.M.: Conceptualization, methodology, validation, formal analysis, software, investigation, data curation, writing—original draft preparation, writing—review and editing, visualization. S.M.C.: Conceptualization, methodology, writing—original draft preparation, formal analysis, investigation, writing—review and editing, visualization, validation, formal analysis, writing—review and editing, supervision. I.J.T.: Conceptualization, methodology, writing—original draft preparation, formal analysis, investigation, visualization, validation, formal analysis, writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Foundation for Science and Technology (FCT) through PhD grant no. 2023.18550.PRT awarded to A.P.M., and by project UID/50006/2025 (DOI: 10.54499/UID/50006/2025).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interests.

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Figure 1. Study area map with SADC population distribution and density (proportion of urban population).
Figure 1. Study area map with SADC population distribution and density (proportion of urban population).
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Figure 2. Flowchart of the article and report selection process [18].
Figure 2. Flowchart of the article and report selection process [18].
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Figure 3. Spatial distribution of publication per country.
Figure 3. Spatial distribution of publication per country.
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Figure 4. Number of publications per year (a), type and pollutant source (b) country (c), river basin (d). Note: The analyses presented refer exclusively to the areas and countries for which data was available in the 30 publications selected through the systematic review protocol.
Figure 4. Number of publications per year (a), type and pollutant source (b) country (c), river basin (d). Note: The analyses presented refer exclusively to the areas and countries for which data was available in the 30 publications selected through the systematic review protocol.
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Figure 5. Distribution of publications by object of study (a) and by plastic-derived pollutant (b).
Figure 5. Distribution of publications by object of study (a) and by plastic-derived pollutant (b).
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Table 1. Summary of studies on pollutants, climate and environmental impacts.
Table 1. Summary of studies on pollutants, climate and environmental impacts.
No.Climate Change RelationMain Results (Basins/Pollution)Impacts on Aquatic/Marine EnvironmentPolitical Contribution/Integrated ManagementReference
1IndirectSocial factors promote conservation practices in watersheds.Reduced erosion improves basin health and water quality.Suggests rural extension, education and community policies.[32].
2Yes (RCP4.5/8.5)Projected reduction in flood frequency and magnitude.Supports disaster risk planning and climate adaptation.Provides data for infrastructure and flood management.[33].
3Yes (land use and climate)Small increase in average flow linked to reforestation.Enhances basin resilience and hydrological balance.Promotes land use and reforestation policies.[34].
4Yes (projections)Continuous flow reduction and more dry years expected.Affects agriculture and water availability.Supports regional climate adaptation policies.[35].
5NoMetals above safe limits in mussels from polluted areas.Indicates marine contamination and food safety risks.Guides industrial and agricultural pollution control.[36].
6IndirectHigh loads without downstream reduction, showing continuous sources.Cross-border transport and persistent contamination.Reinforces waste management and cooperation policies.[37].
7SeasonalCharacterized origin of seasonal flows and aquifer recharge.Provides baseline for water management and allocation.Supports IWRM and SDG 6 monitoring.[38].
8IndirectDetected 53 pesticides, many above quality limits.High toxicity and runoff pollution in catchments.Urges regulation and continuous monitoring.[39].
9NoSpecies-specific accumulation among seabirds.Bioaccumulation and trophic risk.Highlights need for regional monitoring.[40].
10IndirectHigher summer concentrations; high toxicity to estuarine fauna.Acute and chronic toxicity; bioaccumulation.Encourages integrated risk assessment and toxicity data.[41].
11YesGovernance fragmentation reduces adaptive capacity.Lower resilience and water security.Advocates harmonized SADC transboundary frameworks.[42].
12Growing pressureHg and As exceed safe intake limits.Potential human health risk via fish consumption.Reinforces fisheries regulation and traceability.[43].
13IndirectDetected in all systems studied.Ecological risk and contaminant vectors.Need for waste management and transboundary policies.[30].
14SeasonalLocal guidelines underestimate risks.Public health threat and tourism risk.Suggests update of sanitary standards.[44].
15Yes‘Oceans Without Borders’ model fosters conservation.Protects reefs and coastal communities.Aligned with SDGs 12, 13, 14 and 17.[45].
16NoSignificant marine deposits in several bays.Habitat destruction and ecological risk.Requires EIA and multisectoral management.[46].
17IndirectHigh densities in coastal bays.Widespread contamination and ecological risk.Supports sewage treatment and microfiber interception.[47].
18IndirectModerate enrichment and potential toxicity.Local ecological alterations.Calls for local sediment quality guidelines.[48].
19IndirectHigher concentrations in Durban Bay.Bioaccumulation and mangrove degradation.Recommends monitoring and restoration.[49].
20SeasonalElevated concentrations; mixed origins.High toxicity and deformities in fish embryos.Supports continuous pollution monitoring.[14].
21Yes>95% dumped or burned; 2% recycled.Severe CO2 and ecotoxic emissions.Promotes EPR and bans on open burning.[50].
22IndirectHigh loads in Durban and Richard’s Bay.Risk to fauna and protected areas.Supports integrated coastal zone management.[51].
23IndirectHigh bioaccumulation in estuarine species.Carcinogenic and ecological risks.Reinforces biomonitoring and seafood safety.[52].
24SeasonalDry season accumulation; wet season dilution.Moderate sediment enrichment.Supports catchment and land-use planning.[53].
25No5% feces and 10% feathers contaminated.Bioaccumulation and reproductive effects.Highlights need for wetland pollution monitoring.[54].
26IndirectDecline over 20 years; DDT remains high.Bioaccumulation and risk to marine fauna.Supports Stockholm Convention enforcement.[55].
27Indirect60% of turtles ingested plastics.Mortality and gastrointestinal blockage.Encourages regional mitigation actions.[56].
28SeasonalAltered benthic communities and recovery patterns.Sediment plumes affect adjacent areas.Calls for tailored impact assessment frameworks.[57].
29Indirect98% of mussels contaminated.Bioaccumulation and food chain risk.Supports mussels as bioindicators.[58].
30IndirectHigher occurrence in adult females.Chemical and physical ingestion risks.Reinforces coastal waste monitoring.[59].
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MDPI and ACS Style

Mabica, A.P.; Chilaule, S.M.; Tamele, I.J. From Source to Sea: The Role of River Basins in Marine Pollution Under Development and Climate Change Variability in SADC. Pollutants 2026, 6, 28. https://doi.org/10.3390/pollutants6020028

AMA Style

Mabica AP, Chilaule SM, Tamele IJ. From Source to Sea: The Role of River Basins in Marine Pollution Under Development and Climate Change Variability in SADC. Pollutants. 2026; 6(2):28. https://doi.org/10.3390/pollutants6020028

Chicago/Turabian Style

Mabica, Alfredo Pedro, Sérgio Mateus Chilaule, and Isidro José Tamele. 2026. "From Source to Sea: The Role of River Basins in Marine Pollution Under Development and Climate Change Variability in SADC" Pollutants 6, no. 2: 28. https://doi.org/10.3390/pollutants6020028

APA Style

Mabica, A. P., Chilaule, S. M., & Tamele, I. J. (2026). From Source to Sea: The Role of River Basins in Marine Pollution Under Development and Climate Change Variability in SADC. Pollutants, 6(2), 28. https://doi.org/10.3390/pollutants6020028

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