Some of the most populous African cities are situated along the coastline. They are not equally distributed, though, with many of the populous centers situated

Table 2. African coastal cities among the 50 largest African cities and the most populous African city on the Red Sea [59].

in the Gulf of Guinea, along the South African coast and along the Kenyan and Tanzanian Indian Ocean coastlines and in certain regions along the Mediterranean Sea. While fewer main cities lie on the African coast of the Red Sea, it must be remembered that on the Arabian side of the Red Sea lie the Saudi Arabian cities of Jeddah (4,000,000), Jazan (Metro: 1,400,000) and Yanbu (350,000), the Yemini city of Hodeidah (Metro: 812,000) and the Jordanian city of Aqaba (100,000). Also, while not on the list of the 50 most populous cities in the world, Hurghada (Egypt, 250,000) and Port Sudan (Sudan, Metro: 550,000) are sizable cities, too, that are situated on the African coast of the Red Sea.

The coastlines of Africa bordering the Atlantic and Indian Oceans are influenced by several major ocean currents. Along the eastern and southern African coast, the principal currents are the southward-flowing Agulhas Current and its feeder currents, the East Madagascar Current and Mozambique Current. Along the Atlantic coast, the major currents include the northward-flowing Benguela Current, the eastward-flowing Guinea Current, and the southwestward-flowing Canary Current, a wind-driven eastern boundary current associated with the North Atlantic Gyre.

4.2.1. Currents in the Indian Ocean

The Agulhas Current is the western boundary current of the southwest Indian Ocean, flowing southward along the east coast of Africa from approximately 27˚S to 40˚S. It is narrow, fast, and powerful, with an estimated transport of around 70 - 100 Sv, making it one of the largest western boundary currents in the world. The current is supplied mainly by the East Madagascar Current, the Mozambique Current, and recirculated water from the southwest Indian Ocean gyre. Its flow is strongly constrained by the continental shelf as it follows the African coast southward. Along its path, the current frequently develops meanders and large offshore bulges known as Natal Pulses. These instabilities can generate Agulhas rings, which detach from the main current and move westward into the South Atlantic. Near the southern tip of Africa, the Agulhas Current turns back toward the Indian Ocean at the Agulhas Retroflection, where it interacts with the Antarctic Circumpolar Current. Most of its water returns to the Indian Ocean through the Agulhas Return Current, while a smaller portion enters the South Atlantic through Agulhas Leakage, primarily in the form of surface filaments and large eddies. This leakage contributes to the transfer of warm, saline Indian Ocean water into the South Atlantic.

4.2.2. Currents in the Atlantic Ocean

The Benguela Current is a broad, cold, northward-flowing current along the southwest coast of Africa and forms the eastern boundary current of the South Atlantic gyre. It extends approximately from Cape Point to the Angola-Benguela Front near 16˚S and is driven primarily by the southeasterly trade winds. These winds also induce coastal upwelling, bringing cold, nutrient-rich water from depths of approximately 200 - 300 m to the surface. The resulting high nutrient availability supports substantial phytoplankton growth and makes the Benguela Upwelling System one of the most productive marine ecosystems in the world. The Benguela Current is approximately 200 - 300 km wide and contains numerous eddies and meanders. Its waters originate mainly from deep Atlantic upwelling and are supplemented by nutrient-poor South Atlantic waters and, occasionally, by warm-water eddies originating from the Agulhas Current. Around southern Africa, the interaction between the cold Benguela waters and the warm Agulhas waters produces strong temperature gradients and complex oceanographic conditions.

The Canary Current is a cold, southwestward-flowing eastern boundary current along the northwest African Atlantic coast. It branches southward from the North Atlantic Current and flows toward the coast of West Africa, reaching approximately as far south as Senegal before turning westward and contributing to the North Equatorial Current. The Canary Current is part of the North Atlantic Gyre and is strongly influenced by the prevailing trade winds. In the region off northwest Africa, it is accompanied by a poleward-flowing coastal countercurrent, commonly referred to as the Mauritania Current. The Guinea Current is a slow-moving, warm ocean current that flows eastward along the coast of Guinea in West Africa. It affects the coastlines of many West African countries such as Ghana and Nigeria.

4.2.3. Currents in the Mediterranean and the Red Sea

The Mediterranean Sea is characterized by a complex large-scale circulation of interconnected sub-basin and mesoscale gyres, bounded by currents and jets that exhibit strong seasonal and interannual variability [60]. Surface circulation is initiated by the inflow of cool, relatively low-salinity Atlantic water through the Strait of Gibraltar, which flows eastward along the North African coast. This large-scale circulation directly influences coastal and shelf dynamics, as the Mediterranean’s relatively narrow shelves are separated from deep waters by steep shelf breaks. Consequently, basin-scale currents can intrude into coastal areas, enhancing the exchange and transport of material between the coast and open ocean. This process can facilitate the redistribution of land-derived pollutants.

For the Red Sea, detailed current data are limited because the currents are weak and highly variable in both space and time. Their variability is largely wind-driven: during summer, northwesterly winds push surface waters southward for approximately four months at 15 - 20 cm/s, while winter winds reverse the flow, driving water from the Gulf of Aden into the Red Sea. Because the winter inflow predominates, the overall circulation is directed northward along the Red Sea.

4.2.4. African River Discharging into the Neighboring Oceans

Table 3 summarizes the major African rivers discharging into the Atlantic and Indian Oceans and the Mediterranean Sea. No major perennial rivers discharge into the Red Sea from the African side. The distribution of major rivers along the African coastline is highly uneven, with a particularly high concentration of river systems along the Gulf of Guinea. In contrast, several extensive stretches of the African coastline lack major perennial rivers. Along the approximately 1500 km-long Namibian coast, for example, no major perennial river reaches the Atlantic between the Kunene River at the Angolan-Namibian border in the north and the Orange River at the Namibian-South African border in the south. Similarly, none of the approximately 1770 km of Libyan coastline is drained by a perennial river, while in Egypt the Nile is the only perennial river discharging into the Mediterranean. Western Sahara likewise has no perennial rivers along its approximately 1000 km-long Atlantic coastline. Epheremal rivers such as seasonal wadis can exist in these areas and contribute to the transport and release of plastics, including microplastics.

Table 3. The 20 major African rivers discharging into marine waters. Different reference periods can move rivers such as the Sassandra, Cavally, Mangoky, Ruvuma and Save considerably up or down the ranking. Other major rivers are shown in Figure 1; see also ref. [61].

Strokal et al. [13] report significant plastic transport from sub-basins by African rivers, with estimated loads of 10.1 - 50 kg/km²/year in several regions of the Gulf of Guinea, including the Cavally, Sassandra, Bandama, Volta, and Ouémé rivers, as well as in Central-Western Africa, including the Congo and Cuanza rivers. In the case of the Niger and Cross Rivers, plastic transport from the basin is estimated to exceed 50 kg/km²/year. Along the East African coast, the Limpopo and Rufiji rivers are also estimated to transport between 10.1 and 50 kg/km²/year. By weight, however, more than 70% of the transported plastic consists of macroplastics. With regard to plastic inputs, Lebreton et al. [15] identify the Cross River (Cameroon/Nigeria; ranked 5th), Imo River (Nigeria; ranked 12th), and Kwa Ibo River (Nigeria; ranked 20th) among the 20 rivers with the highest estimated contributions to global plastic pollution. Notably, these rankings do not necessarily correspond to river size or discharge; the Imo River and the Kwa Ibo River do not rank among the top 25 African rivers by average discharge.

Work by F. Andersen and colleagues [62] in the Atlantic Ocean along the African coastline indicates that microplastic (MP) concentrations are influenced by the proximity of sampling locations to the coastline. This is particularly relevant outside the North Atlantic garbage patch, which extends approximately from 22˚N to 38˚N, although its western and eastern boundaries remain poorly defined and its northern and southern boundaries shift seasonally. Despite a fivefold increase in global plastic production over a 22-year period, plastic concentrations within the North Atlantic garbage patch have remained relatively stable. In the study of Andersen et al. [62], the sampling sites in the equatorial currents were located comparatively far from land, and the authors reported low MP concentrations in these currents. The coastline of West Africa is influenced by the Guinea Current, a warm, westward-flowing current that regulates nearshore circulation, temperature, and salinity patterns [63] [64]. The current drives obliquely breaking southwesterly waves. Cross-shore sediment transport occurs through alternating swash (onshore) and backwash (offshore) processes, which are enhanced under high-energy wave conditions during the rainy season [64] [65].

The Benguela Current is an important driver of material transport along the southwestern African coastline. It extends northward toward the Angola-Benguela Front, a persistent frontal feature located between approximately 15˚ and 17˚S off the coasts of Angola and Namibia. From there, the current feeds into the South Equatorial Current, thereby circumventing the South Atlantic Gyre.

An important issue for the distribution in the Southeastern Atlantic Ocean, the Southeast Indian Ocean and the Antarctic waters (Southern Ocean) is the surface circulation around southern Africa, which is primarily governed by two major current systems: the Agulhas Current System along the east coast and the Benguela Current System along the west coast. The Agulhas Current, the principal western boundary current of the Southern Hemisphere, generally follows the narrow continental shelf in the northern region before turning offshore where the shelf widens. Near the southern tip of the African continent, most of the current retroflects and flows eastward into the South Indian Ocean as the Agulhas Return Current. During this retroflection, large anticyclonic eddies, known as Agulhas Rings, are shed and propagate into the South Atlantic, where they interact with the Benguela Current system. The Benguela Jet, a strong, narrow, equatorward-flowing shelf-edge current that extends around the Cape Peninsula toward Cape Columbine, plays an important role in transporting material from the Agulhas Bank into the southern Benguela region [66]. It also is a conduit to channel particles from the Indian Ocean into the Atlantic Ocean. Collins and Hermes [67] have now postulated based on modeling that low- and high-density polyethylene, floating at the surface of the ocean, will be mostly exported towards the accumulation zones in the South Atlantic and South Indian Ocean. PET and PVC, plastics with higher density, will more readily sink and, drifting deeper in the water column, are more likely to accumulate along the coast and continental shelf.

Collins and Hermes [67] find that HD particles released off Cape Town are transported seaward across the continental shelf by a poleward-flowing undercurrent along the western margin of the Agulhas Bank. From there, the particles enter the highly variable circulation south of the Agulhas Bank, which subsequently carries them into the Indian, Atlantic, and Southern Oceans. HD particles released along the southern and eastern coasts are transported southward along the continental shelf by the Agulhas Current. Upon reaching the southern tip of the Agulhas Bank, these particles, similar to those released off Cape Town, enter the variable circulation south of the bank and are subsequently dispersed into the Indian, Atlantic, and Southern Oceans. Transport into the Atlantic Ocean occurs predominantly via the Agulhas Extension. Particles reaching the Indian Ocean appear to follow two pathways: some are transported eastward by the Agulhas Return Current, while others follow the continental shelf within the Agulhas Undercurrent. Overall, a third of the HD particles released around the South African coast is exported into the Indian Ocean and less than 10% is transported to the Atlantic and Southern Ocean.

Only a small proportion of the LD particles released around South Africa is exported to the South Indian Ocean. Particles exiting the eastern boundary of the model domain originate predominantly from the two release sites along the east coast. Approximately 10% of the LD particles released off Richards Bay are transported into the Indian Ocean, compared with only about 3% of those released off Durban [67]. In contrast, more than one-third of the LD particles released around the South African coast are exported to the South Atlantic Ocean. The majority (>80%) of particles released off Cape Town are transported into the Atlantic Ocean, while none are exported to the Indian Ocean. For particles released along the southern and eastern coasts, less than 10% are transported to the South Atlantic, with Richards Bay being the notable exception. More than 20% of the particles released off Richards Bay are exported to the South Atlantic Ocean. Only a very small fraction (<1%) of LD particles released along the South African coast reaches the Southern Ocean [67].

Interestingly, along the South African coastline, exceptionally high concentrations of plastic were observed along the west coast, particularly in areas north of St Helena Bay, offshore of Saldanha Bay, and near Cape Point. In contrast to the decline in plastic abundance with increasing distance from the coast reported by Ryan in 1988 [47], a survey of 2014 conducted along a transect between Cape Town, Tristan da Cunha, and Gough Island in the South Atlantic Ocean found that only approximately 30% of the litter collected—predominantly plastic—was located in coastal waters over the African continental shelf. The remaining 70% was recorded farther offshore [68].

MP circulation with prevailing currents in the Southern Tropics carries MPs westward across the Indian Ocean, seasonally eastward in the Northern Tropics, and southward along the African coast. Thus, islands in the Western Indian Ocean such as the Seychelles are affected by plastic litter originating from Southeast Asia. Modeling combined with physical inventories of plastic on shorelines has shown the scale of plastic transport throughout the Indian Ocean. Thus, 52.4% of Kenyan mismanaged plastic that reaches the Indian Ocean ends up on the beaches of 26 countries, with India, Myanmar and Indonesia most affected [69]. In contrast, much of the South African mismanaged plastic that has been sweptout to sea via its Indian Ocean shoreline is moved with the Agulhas current southward [69]. Much of the plastic litter found on the Seychelles Island chain stems from South East Asia [70] [71].

In the Mediterranean Sea, the Libeccio wind contributes to eastward-directed winds over the Ionian-Meridional sub-basin, potentially promoting the transport of marine and atmospheric MPs towards the eastern Mediterranean. In addition, northerly winds, particularly the Etesian winds originating from the north and northwest, strongly influence the transport and distribution of marine MPs within the Levantine-Aegean sub-basin, the southern border of which is the Egyptian coastline, especially during the summer months [72] [73].

The presence of MPs in surface seawater samples from the Red Sea has been documented even in areas located far from major urban centers. Nevertheless, MP concentrations in the Red Sea are generally relatively low and exhibit temporal variability. To date, no study has specifically investigated the influence of seasonal variability over multiple years on the occurrence and distribution of MPs. During winter, strong southerly winds drive MPs towards the western (African) coast of the Red Sea, where they can accumulate on sandy beaches [74]. This pattern is supported by sediment analyses along the Red Sea coastline, which have reported higher MP concentrations along the eastern coast of Egypt compared with the eastern coast of the Red Sea [74].

Kanhai et al. [75] investigated the abundance, distribution, and composition of microplastics in the Atlantic Ocean along a transect extending from the Bay of Biscay to Cape Town, South Africa. MPs were collected from subsurface waters using the underway sampling system of the RV Polarstern. Potential MPs were isolated from the samples and analysed by FT-IR spectroscopy to determine their polymer composition. Of the particles analysed, 63% were identified as cellulose and 37% as other synthetic polymers. Among the synthetic polymers, polyesters accounted for 49%, while blends of polyamide or acrylic with polyester represented 43%. Fibers were the predominant particle type, accounting for 94% of all particles. The mean MP abundance across the Atlantic Ocean was 1.15 ± 1.45 particles m⁻³. Of the 76 samples collected, 14 originated from the Benguela upwelling region. No statistically significant difference in microplastic abundance was found between upwelled and non-upwelled waters.

Table 4. Studies on MP presence in offshore waters in the Atlantic and Indian Ocean and the Mediterranean and Red Sea.

Suaria et al. [76] reported on the distribution of floating plastics in the eastern and southern sectors of the Atlantic Ocean based on 35 neuston net trawl samples collected during two research cruises in 2016 and 2017.A transect between 0 and 18˚E along 35˚S in the South Atlantic Ocean revealed higher densities farther west, supporting the accumulation of floating plastics in the South Atlantic gyre, mainly west of 10˚.

E. Andersen et al. [62] investigated the abundance, distribution, and characteristics of MPs in surface waters along a transect spanning six major ocean current systems in the Atlantic Ocean, extending from Scheveningen (the Netherlands) to Montevideo (Uruguay). Along the west African coast, sampling sites were located within the Canary Current, the North Equatorial Current, and the Equatorial Counter Current, all of which are situated in the North Atlantic Ocean. Except for the sampling in the Canary Current off the Moroccan coast and the North Atlantic current near Cabo Verde, the sampling sites were distant from the African coastline. A Manta trawl (mouth area: 40.5 cm², net mesh size: 500 μm; cod end mesh size: 333 μm) was used to sample surface water for MPs.

Suspended particulate matter was collected by Hildebrandt et al. [77] from subsurface waters at a depth of 6 m along an east-west transect across the tropical Indian Ocean using a specialized, inert, plastic-free fractionated filtration system. The sampling point closest to the African coast is about 1000 km off the North Madagascan coast. Here, 43 MP m⁻³ were found with acrylates/PU/varnish and PET particles making up the vast majority.

Al Hashmi et al. [78] published data gained from the collection of MPs along shipping routes off the African Mediterranean coast and off the African coast in the Red Sea. Here, the collection was performed with steel buckets while taking utmost precautions against aerial MP deposition and against contamination from the vessel itself. The found concentrations were quite high, with 15.5 ± 9.6 MPs L⁻¹ in the Mediterranean Sea and 24.5 ± 11.0 MPs L⁻¹ in the Red Sea (Table 4).

6.1.1. Egypt

MP contamination has been documented extensively along both the Mediterranean and Red Sea coasts of Egypt. Sayed et al. [74] assessed MP contamination in surface waters and sediments at eight sites, comprising four Mediterranean locations—Marsa Matruh, Alexandria, Damietta, and Port Said—and four Red Sea locations—Ras Gharib, Ain Sukhna, Hurghada, and Marsa Alam. MPs were detected in both environmental compartments at all sites, confirming their widespread distribution along the Egyptian coastline.

In surface waters, MP abundances ranged from 46.66 ± 2.27 particles 100 mL⁻¹ at Marsa Matruh to 69.3 ± 1.08 particles 100 mL⁻¹ at Damietta in the Mediterranean, and from 50.66 ± 0.57 particles 100 mL⁻¹ at Ain Sukhna to 66 ± 2 particles 100 mL⁻¹ at Ras Gharib in the Red Sea. Sediment concentrations showed comparable spatial variability. Mediterranean sites ranged from 48 ± 5 particles 100 g⁻¹ at Marsa Matruh to 76.6 ± 4 particles 100 g⁻¹ at Damietta and Port Said, whereas Red Sea sites ranged from 38.6 ± 3 particles 100 g⁻¹ at Ain Sukhna and Hurghada to 50.6 ± 4 particles 100 g⁻¹ at Ras Gharib. The relatively high abundances at Damietta, Port Said, and Ras Gharib were associated with intense urban, industrial, shipping, and other anthropogenic pressures, although local hydrodynamic and sedimentary processes likely also contributed to MP accumulation. Overall, the study demonstrates that MP contamination is widespread along both Egyptian coastlines and that local sources interact with environmental processes to control MP transport and deposition.

In the Eastern Harbor, Shabaka et al. [79] conducted a preliminary assessment at ten stations, examining both harbour water and sediments. Sediments contained 242 particles kg⁻¹ dry weight in the 0.5 - 5 mm fraction, while smaller MPs (<0.5 mm) occurred at concentrations of 33 - 108 transparent particles and 107 - 174 coloured particles 100 g⁻¹ dry weight. A diverse range of polymers was identified, including PET, ABS, PP, PS, PEVA, PTFE, LDPE, LLDPE, HDPE, and nylon, with PET and PP predominating. The diversity of particle sizes, morphologies, and polymer types indicates heterogeneous contamination associated with consumer products, packaging, industrial activities, and other anthropogenic sources.

6.1.2. Libya

Kateesh et al. [80] surveyed the MP contamination in nearshore waters and sediments at two sites along the northeastern Mediterranean coast of Libya: the urbanized Sousa site, approximately 30 km northeast of Al Bayda, and the semi-urban Alhaniyah site, approximately 25 km northwest of Al Bayda. Mean MP abundances in surface waters were 2.5 ± 2.6 particles L⁻¹ at Sousa and 0.7 ± 1.3 particles L⁻¹ at Alhaniyah. Sediment concentrations were similarly higher at Sousa, with mean abundances of 254 ± 57 particles kg⁻¹ dry weight compared with 212 ± 23 particles kg⁻¹ at Alhaniyah. The higher concentrations at the more urbanized Sousa site suggest an important influence of local anthropogenic activities on MP contamination in both coastal waters and sediments.

6.1.3. Tunisia

Tunisia provides some of the strongest evidence for substantial spatial variability in coastal MP contamination in North Africa. Abidli et al. [81] reported particularly high sediment concentrations in the northern Bizerte lagoon-channel complex, ranging from 3000 to 18,000 items kg⁻¹ dry sediment. The highest abundance occurred at Menzel Abderrahmane, followed by Carrier Bay, Menzel Jemil, and the Bizerte Channel. These elevated concentrations were associated with intensive textile, plastic, and electronic industries, as well as fishing and shellfish production.

The same research group [82] subsequently investigated the South Lake of Tunis, North Lake of Tunis, Carthage, La Goulette, and Menzel Bourguiba. MP concentrations ranged from 141.2 ± 26.0 to 461.3 ± 29.7 items kg⁻¹ dry weight. Fibers generally dominated, followed by fragments, expanded polystyrene, films, and pellets. Higher concentrations at Menzel Bourguiba, the South Lake of Tunis, and La Goulette were associated with industrial activities, commercial and fishing harbours, and urban wastewater discharges. In contrast, lower concentrations at the North Lake of Tunis and Carthage were attributed to reduced anthropogenic pressure and environmental rehabilitation. The substantially lower concentrations relative to the northern Bizerte lagoon-channel complex emphasize the importance of local pollution sources and hydrodynamic conditions in determining MP accumulation.

Missawi et al. [83] extended the assessment to sediments at eight coastal sites spanning the Bizerte Lagoon, Gulf of Tunis, Gulf of Hammamet, and Gulf of Gabes. MPs ranging from 1.2 μm to 1 mm were detected at all locations, with concentrations of 129 - 606 items kg⁻¹. Although these concentrations were lower than those reported for the heavily impacted Bizerte lagoon-channel complex, their ubiquitous occurrence confirms the widespread distribution of MPs along the Tunisian coastline and the importance of local anthropogenic activities in controlling sediment contamination.

The contribution of land-based inputs was further demonstrated by Toumi et al. [84], who investigated sediments of seven streams draining into the Bizerte Lagoon. MP abundances ranged from 2340 ± 227.15 particles kg⁻¹ dry weight in the Khima Stream to 6920 ± 395.98 particles kg⁻¹ in the Jedara Stream. Higher concentrations occurred in streams draining densely populated areas and receiving municipal and industrial wastewater. All recovered particles were secondary MPs, with fibers predominating and PE and PP representing the major polymers. These findings identify urban runoff, wastewater, and industrial discharges as important pathways transporting MPs from terrestrial environments into the lagoon.

Chouchene et al. [85] investigated MP contamination in sediments at 35 sites along a 4.2-km coastal section of Sidi Mansour, Sfax, southeastern Tunisia, during ongoing dredging activities. MP abundances ranged from 252 to 5332 particles m⁻², demonstrating substantial spatial variability over a relatively short distance. Fragments were the most abundant morphology, reaching up to 5332 particles m⁻², followed by granules/pellets (3516 particles m⁻²), fibers/lines (1553 particles m⁻²), foams (589 particles m⁻²), and tubular structures (252 particles m⁻²). The diversity of particle shapes and sizes indicates contributions from multiple sources and degradation processes.

The relatively high MP abundance recorded at Sidi Mansour Harbour suggests considerable plastic contamination within this semi-enclosed environment. Local sediment characteristics and hydrodynamic conditions may strongly influence MP retention, with sandy sediments potentially promoting accumulation through interactions among particle properties, sediment dynamics, and local water circulation. The semi-enclosed nature of the Gulf of Gabes may further favour retention and accumulation. Regional circulation, potentially influenced by the Kerkennah Islands and restricted water exchange, may facilitate both the transport and deposition of MPs from local and more distant sources. Thus, MP contamination in Tunisian sediments appears to result from the interaction of anthropogenic inputs, sediment properties, and regional hydrodynamic processes rather than from local emissions alone.

6.1.4. Algeria

Moving westward along the North African Mediterranean coast, Setiti et al. [86] investigated the occurrence, composition, and seasonal variability of MPs in surface seawater in Bou-Ismail Bay, Algeria. Seasonal samples collected in 2018 from six stations approximately 2 - 4 km offshore contained MPs in every sample. Mean concentrations ranged from 0.36 items m⁻³ in summer to 1.26 items m⁻³ in spring, with an overall mean of 0.86 ± 0.35 items m⁻³, equivalent to approximately 101,146 ± 38,580 items km⁻².

Fibers were the predominant morphology (32%), followed by fragments (27%), films (16%), foams (13%), and granules (12%). PE and PP dominated the identified polymers, accounting for 68.2% and 24.7%, respectively, while PS represented 4.1%. The predominance of PE and PP is consistent with their extensive use in packaging, fishing-related materials, and other consumer and maritime applications.

The highest concentration occurred in spring, when surface currents were relatively strong (0.052 - 0.20 m·s⁻¹) and predominantly directed west-southwest. These currents may facilitate transport and redistribution of floating debris within the bay. Seasonal changes in the spatial distribution of MPs further suggest that shifts in current direction influence particle retention and transport. The lowest concentrations occurred in summer, when circulation was predominantly directed from the northwest towards the coast, potentially promoting shoreline deposition or stranding and thereby reducing offshore concentrations. Overall, the study demonstrates that MP abundance in Algerian coastal waters is strongly influenced by seasonal hydrodynamic conditions and cannot necessarily be attributed solely to local sources.

Taïbi et al. [87] complemented these observations by investigating plastic debris in beach sediments along 124 km of the western Algerian coast, from Sidi Mansour to Bahara. Plasticdebris ≥ 1 mm was surveyed at nine beaches, with abundances ranging from 7.6 ± 18.8 to 66 ± 107.28 items m⁻². Fragments represented 83.27% of recovered items, pellets 14.93%, and films 1.03%. The highest abundances generally occurred near coastal villages, indicating an important contribution from local anthropogenic activities. Micro- and microplastic fragments co-occurred at most sites, although MPs predominated at most beaches. Bahara was an exception, where macroplastics were dominant.

The predominance of fragments indicates that secondary plastics generated through the degradation and weathering of larger items constitute an important component of beach contamination. Their accumulation near coastal settlements further highlights the role of land-based sources. The coexistence of micro- and macroplastics also suggests an ongoing fragmentation process in which larger debris generates smaller particles that can subsequently be redistributed by waves, currents, wind, and sediment dynamics.

Further evidence was provided by Tata et al. [88], who investigated macroplastic and MP contamination in the Gulf of Annaba, an area characterized by high population density and intensive industrial activity. Surface sediments contained mean concentrations of 182.66 ± 27.32 macroplastic items kg⁻¹ and 649.33 ± 184.02 MP items kg⁻¹ dry sediment. MP abundance varied considerably among sites and was strongly influenced by proximity to densely populated areas, wastewater discharge points, and industrial facilities. Shoreline exposure and complex tidal circulation also appeared to influence transport, redistribution, and accumulation.

Fibers represented 70% of MPs, followed by fragments (21%), pellets (5%), films (2%), and foams (2%). Polymer composition was dominated by PE (48%) and PP (16%), followed by PET (14%), PS (9%), butylbranham (7%), polythenepropylene (3%), and cellulose triacetate (3%). The dominance of PE and PP is consistent with their widespread use in packaging, consumer products, fishing materials, and industrial applications. Collectively, the Algerian studies demonstrate that MP contamination occurs across surface waters, beach sediments, and subtidal environments and is governed by the interaction of land-based sources, wastewater, industrial activity, hydrodynamic transport, and sedimentary processes.

6.1.5. Morocco

Bouadil et al. [89] looked at MP contamination in surface waters of Al Hoceima Bay on Morocco’s Mediterranean coast between September 2019 and 2021. Samples were collected annually at nine stations extending from nearshore waters to approximately 6 km offshore. MP abundances ranged from 1 to 15 items m⁻³, with a mean of 4.70 ± 4.50 items m⁻³. PE was the dominant polymer (67.4%), followed by PP (23.2%), PS (7.1%), and PET (2.3%).

Spatial variability reflected the influence of hydrodynamic conditions and multiple land- and sea-based sources. The Oued Ghiss and Oued Nekor rivers have highly irregular flow regimes, with episodic floods capable of transporting substantial quantities of sediment and associated plastic debris into the bay. Other potential sources included the regulated landfill, wastewater-treatment facilities, fishing activities, rural household waste, and tourism. Overall, MP contamination in Al Hoceima Bay appears to result from the combined influence of episodic riverine transport, waste-management practices, wastewater, fishing, and tourism.

Abelouah et al. [90] investigated anthropogenic macro- and mesolitter on three beaches along the northern coastline of Agadir. Plastics represented 79% of all collected items, including caps and lids, food containers, cigarette butts and filters, polystyrene fragments, small plastic bags, and beverage bottles. Taghazout and Aourir beaches were classified as clean to moderately clean according to the Clean Coast Index, whereas Anza Beach exhibited substantially greater litter pollution.

Ben-Haddad et al. [91] assessed MP contamination in sediments from a highly urbanized beach along the Taghazout coast. Mean MP abundances increased from 915 particles kg⁻¹ in 2018 to 1448 particles kg⁻¹ in 2019. The highest concentrations occurred in the southern part of the Taghazout coast, particularly near facilities and areas heavily used by beachgoers, suggesting a contribution from recreational activities and associated anthropogenic pressures.

Abelouah et al. [92] further examined surface waters off Agadir-Ida-Outanane Province during March-May 2021. MPs were detected at all 25 sampling sites, including locations up to 15 km offshore. Concentrations ranged from 0.048 to 3.305 items m⁻³, with a mean of 0.987 ± 1.081 items m⁻³. Concentrations were highest in urban areas relative to village and rural sites, indicating an influence of local anthropogenic activities. Polyester (PET) was the predominant polymer, followed by PP, PA, PS, PVC, EVA, PUR, and polyacrylates.

Haddout et al. [93] investigated MPs in surface waters and sediments of the Sebou Estuary and adjacent Atlantic coastline near Kenitra. Water concentrations ranged from 10 to 168 particles m⁻³, while sediment concentrations ranged from 10 to 300 particles kg⁻¹. Sites near Kenitra City exhibited significantly higher abundances than those along the more exposed Atlantic coast, probably reflecting greater population density and anthropogenic pressure. The Sebou Estuary was consequently identified as a potential MP hotspot, emphasizing the importance of urban and estuarine environments as pathways and accumulation zones for plastic pollution.

6.2.1. Mauritania

Lourenço et al. [94] investigated MP contamination in sediments at Ebelk Aznaya and Baie d’Aouatif within the Bancd’Arguin, Mauritania, reporting a mean concentration of 4.3 ± 4.90 MP mL⁻¹. The Bancd’Arguin consists of extensive shallow waters and tidal flats along the tropical Saharan coast and covers more than 500 km². It is a fossil estuary without freshwater inputs and is located within a sparsely populated semi-arid environment. Approximately 1500 people live in a small number of fishing villages. Given the limited population and absence of riverine freshwater inputs, MP contamination is likely influenced primarily by marine transport, local fishing activities, and retention within the extensive tidal-flat and shallow-water environments.

6.2.2. Senegal

Sonko et al. [95] examined macroplastics and MPs in nearshore waters around Dakar, generally within 500 m of the coast. Six sites were sampled around the Cap-Vert Peninsula. Cambérène was by far the most contaminated site, with approximately 945,607 MPs km⁻² and 189,995 macroplastic items km⁻². In contrast, MP abundances at the Îles de la Madeleine and Ouakamwere 12,641 and 34,236 MPs km⁻², respectively, and no macroplastics were recovered at either site. The exceptionally high contamination at Cambérène may be associated with its proximity to a wastewater-treatment plant that discharges effluent directly into the sea. Fibers predominated, supporting the potential contribution of wastewater and domestic sources.

These findings complement those of Tavares et al. [96], who examined meso- and microplastic debris on beaches south of Dakar. Plastic debris was considerably more abundant at an urbanized beach than at a semi-urbanized site. Surface densities were 3.63 items m⁻² and 0.20 items m⁻², respectively, while sediment densities at 10 cm depth reached 207.29 and 36.46 items m⁻². Plastic accounted for 97.62% of recorded debris, with plastic bags and clothing-derived plastics among the dominant categories.

Together, the two studies demonstrate a strong association between urbanization, land-based inputs, wastewater discharge, and plastic accumulation in Dakar’s coastal environment. However, the strong spatial differences among sites also show that source intensity alone does not determine MP distribution. Hydrodynamic processes, coastal transport, and distance from direct inputs substantially influence where plastics ultimately accumulate.

6.2.3. Ghana

In Ghana, early evidence of plastic pollution came from studies of resin pellets on beaches. Surveys conducted between 2009 and 2013 examined pellets at 11 beaches around Accra and Tema, including Prampram, Sakumono, Korle Lagoon, Winneba, and Cape Coast [97]. Pellets were predominantly composed of polythene and polypropylene and contained 1 - 69 ng PCB g⁻¹ pellet. Dumping and recycling sites, particularly e-waste recycling sites, were identified as likely sources of PCB contamination.

Nuamah et al. [98] subsequently investigated coastal surface waters at Sekondi, Cape Coast, Tema, and Denu between January and February 2021. The overall mean MP concentration was 1.9 ± 1.7 particles m⁻³. Concentrations generally decreased with increasing distance from shore, particularly at Cape Coast, Tema, and Denu. Population size alone did not explain the observed distribution, as Denu, despite having the smallest population, recorded relatively high MP abundance. Industrialization appeared to be an important factor, particularly at Sekondi and Tema, where extensive industrial activity may contribute to coastal plastic contamination. Fragments represented 46.47% of MPs, followed by pellets (22.55%) and fibers (13.03%). PP was the dominant polymer (47%), followed by PE (37%) and PS (16%).

A subsequent sediment study [99] reported a mean MP concentration in sediments of 0.144 ± 0.061 MP g⁻¹ dry weight, with pellets and transparent particles predominating. Concentrations ranged from 0.078 ± 0.020 MP g⁻¹ at Sekondi to 0.217 ± 0.012 MP g⁻¹ at Denu.

Faseyi et al. [100] investigated MPs in sediments from the Pra and Ankobra estuaries. MPs occurred at all sampling stations. Fragments predominated in the Pra Estuary, with 38.00 ± 25.47 MP 10 g⁻¹ dry weight, whereas microsheets were most abundant in the Ankobra Estuary, at 20.60 ± 12.59 MP 10 g⁻¹ dry weight.

Chico-Ortiz et al. [101] analyzed MPs in the Mukwei and Kpeshie lagoons between Accra and Tema. MP abundances in Kpeshie Lagoon werebetween16.39 ± 3.26 MP 10 cm⁻³ and 25.94 ± 3.13 MP 10 cm⁻³, the latter abundance recorded within a remnant mangrove forest. The higher abundance there is consistent with the role of mangrove systems as sinks for plastic debris. Complex root structures can trap plastics transported by runoff and tidal waters, while sandbars and other physical barriers can further restrict particle movement within lagoonal environments.

6.2.4. Togo

Microplastic contamination along the Togolese coast remains comparatively understudied. Aziade et al. [102] provided a baseline assessment using 30 samples collected from sediments at six sites representing different environmental conditions and levels of anthropogenic activity. MPs ranging from 0.3 to 5 mm were detected at all sites using ATR-FTIR analysis. Mean abundances ranged from 16 ± 20 to 61 ± 51 particles m⁻², equivalent to 2 ± 3 to 11 ± 13 particles kg⁻¹. Fragments predominated (67%), while PE was the dominant polymer (48%), followed by PP (34%) and PS (14%).

6.2.5. Nigeria

Nigeria, particularly the Lagos coastal and lagoon systems, represents another important West African region where widespread MP contamination has been documented. Fred-Ahmadu et al. [103] investigated 1 - 5 mm MPs in psammitic sediments from five beaches: Badagry, Oniru, Elegushi, Atican, and Eleko. MP abundance varied significantly among depositional zones, with the high-waterline accounting for 58.84% of recovered MPs. No MPs were detected at the current-waterline. This distribution indicates that waves, currents, wind, and other hydrodynamic processes strongly influence particle transport and deposition. Fragments predominated, followed by fibers and pellets, suggesting an important contribution from the fragmentation of larger debris.

Olarinmoye et al. [104] conducted the first assessment of MPs in both water and sediments of the Lagos Lagoon, sampling at Liverpool, Agbowa, Ojo, and Makoko. MPs occurred throughout the study area at 139 - 303 particles L⁻¹ in water and 310 - 2319 particles kg⁻¹ in sediments. Higher sediment concentrations at Makoko were partly associated with fine-grained sediments that favour MP retention. Fibers dominated the water column (92.6%) and represented 32.5% of sedimentary MPs. PE and PP were the principal polymers.

Akinhanmi et al. [105] subsequently sampled Epe, Makoko, SagboKoji, and Badagry and reported mean concentrations of 65.2 particles L⁻¹ in surface water and 3906 particles kg⁻¹ in sediments. Fibers again dominated. The substantially higher mean sediment concentration relative to that reported by Olarinmoye et al. [104] illustrates the considerable spatial variability within the lagoon but also the methodological variability and highlights the importance of sampling location, sediment characteristics, and analytical procedures, when comparing MP datasets.

Dada and Bello [106] similarly reported fiber dominance in Lagos Lagoon water and fragment dominance in sediments, implicating recreational, industrial, and domestic plastic waste as important contributors. Collectively, these studies demonstrate extensive contamination across Lagos beaches, lagoon waters, and sediments, with fibers particularly important in the water column and fragments more prominent in sediments. The lagoon functions both as a transport pathway for suspended MPs and as a reservoir for longer-term accumulation.

Yakub et al. [107] determined the MP contamination along three regions of the Nigerian coast in 2024: the Eastern Zone (EZ) along Akwa Ibom State, the Central Zone (CZ) encompassing the Niger Delta, and the Western Zone (WZ) along the Benin-Owena coast. Surface-water concentrations were highest in the EZ (705 particles L⁻¹), followed by the WZ (223 particles L⁻¹) and CZ (212 particles L⁻¹). Sediment concentrations showed the same pattern, reaching 1033 particles kg⁻¹ in the EZ, 338 particles kg⁻¹ in the CZ, and 218 particles kg⁻¹ in the WZ. Polymer composition differed among regions and compartments. CR dominated sediments in the EZ and CZ, whereas PVA dominated in the WZ. In surface waters, PVA was the predominant polymer across all three zones. The pronounced spatial differences suggest that regional anthropogenic activities and environmental conditions strongly influence MP distribution along the Nigerian coast.

Yahaya et al. [108] examined MPs at three sites along the Badagry Lagoon: Ibeshe, Amuwo Odofin, and Ojo. Sediment concentrations ranged from 283 - 315 particles kg⁻¹, compared with 108 - 199 particles L⁻¹ in surface waters. Fibers represented 52% - 90% of MPs in both compartments, followed by fragments and films. Identified polymers included PP, PE, PA, and PS. Fiber dominance indicates persistent inputs from land-based and potentially wastewater-related sources.

Cameroon

Ndumbe et al. [109] investigated plastic contamination across five beaches along the 50.6-km Limbe-Idenau coastline in southwestern Cameroon. Plastic abundance varied strongly among locations and seasons, ranging from 3.13 items m⁻² at Seme Beach to 163.88 ± 17.15 items m⁻² at Limbe Down Beach. The highest contamination occurred at Limbe Down Beach during August, coinciding with the rainy season. Intense rainfall, flooding, and surface runoff likely facilitated the transport of land-based plastic waste to the coast.

Fragments were the predominant microplastic category, reaching 37.27 ± 10.05 items m⁻², while pellets reached 0.33 ± 0.02 g m⁻². The predominance of fragments suggests substantial contributions from weathering and fragmentation of larger plastic items, whereas pellets may reflect industrial or commercial sources. The strong seasonal variability highlights the importance of rainfall and hydrological processes as transport mechanisms in tropical coastal environments.

South Africa

South Africa has one of the most extensive bodies of evidence for coastal plastic contamination in Africa. Early surveys by Ryan and Moloney [48] showed that densities of plastic debris had increased significantly between surveys of 50 beaches conducted in 1984 and 1989. Packaging and fishing equipment constituted the majority of the debris. Ryan et al. [110] subsequently examined mesodebris (approximately 2 - 25 mm) on 82 beaches in 1994, 2005, and 2015. Plastics represented 99% of items by number and 95% by mass. Industrial pellets were the most abundant items, whereas rigid plastic fragments contributed most of the mass. Mesodebris abundance was associated with urban-industrial centres, while correlations with population density were weak and no relationship with local runoff was detected. The decline in pellet size within creasing distance from urban centres further supports the importance of local land-based sources.

Chitaka and von Blottnitz [111] documented substantial plastic accumulation along five beaches in the Cape Town area, with accumulation rates ranging from 36 to 2961 particles day⁻¹ per 100 m of beach. Plastic represented 94.5% - 98.9% of collected particles. Lamprecht [112] reported approximately 30,900 MP m⁻³ of sediment at Milnerton Beach in Table Bay, further demonstrating substantial contamination of South African beach sediments.

Nel and Froneman [113] investigated approximately 480 km of southeastern coastline, sampling 21 beaches in sheltered bays and exposed coastal areas. Sediment concentrations ranged from 688.9 ± 348.2 to 3308 ± 1449 particles m⁻², while water-column concentrations ranged from 257.9 ± 53.36 to 1215 ± 276.7 particles m⁻³. More than 90% of recovered MPs were microfibers. Contrary to expectations, sheltered bays did not consistently exhibit higher concentrations than exposed beaches, indicating that local sources, hydrodynamic transport, beach morphology, and sediment retention collectively determine MP distribution.

Vetrimurugan et al. [114] surveyed nine tourist beaches along the KwaZulu-Natal coast. The highest concentration occurred in Durban-area sediments, reaching 506 MPs per 30 g sediment. Microfibers predominated, reinforcing their importance in South African coastal sediments and identifying the highly urbanized Durban region as a potential MP hotspot.

Boshoff et al. [115] examined plastic contamination at two locations within the Knysna Estuary. The Ashmead Channel is influenced by an industrial area, the Bongani River, stormwater draining much of the central business district, and effluent from the Knysna wastewater-treatment plant. Brenton-on-Lake is less developed and subject to lower anthropogenic pressure. The authors found MP concentrations of 1.7 ± 1.6 MP m⁻³ to 2.5 ± 1.4 MP m⁻³ in the areas sampled. In these areas, Boshoff et al. [116] subsequently compared MP accumulation in sediments associated with Zosteracapensismeadows and adjacent unvegetated sediments over approximately 5 km. Total MP abundance did not differ significantly between vegetated and unvegetated sediments, although fiber abundance differed significantly between locations. These findings demonstrate that MP abundance and composition can vary considerably over very short spatial scales according to catchment characteristics and local pollution pressure.

De Villiers [117] investigated microfiber pollution at 175 sites spanning more than 2700 km of coastline during 2016-2017. Mean concentrations were 87 ± 84 fibers dm⁻³ in 2016 and 80 ± 102 fibers dm⁻³ in 2017, with individual sites ranging from 4 - 772 and 0 - 797 fibers dm⁻³, respectively. The highest concentrations occurred along the eastern coastline, particularly between Port Edward and Durban, where mean concentrations were approximately 160 fibers dm⁻³ compared with 28 - 43 fibers dm⁻³ in northern KwaZulu-Natal. The spatial gradient coincided with population density, riverine inputs, and wastewater infrastructure. The highest concentration in 2017 occurred at Merebank, near a major coastal wastewater-treatment discharge point. These associations support an important contribution from land-based sources.

The Port of Durban represents a particularly important MP hotspot because of the combined influence of urbanization, port activity, wastewater inputs, and local hydrodynamics. Naidoo et al. [118] reported a mean water-column concentration of 7.03 ± 11.93 particles m⁻³, increasing to 15.82 ± 18.54 particles m⁻³ in the Bayhead Canal. Sediment concentrations averaged 159.9 ± 271.2 particles per 500 mL and reached 745.4 ± 129.7 particles per 500 mL in the Bayhead Canal. Concentrations declined away from central Durban, suggesting a strong influence of localized urban sources.

Preston-Whyte et al. [119] documented substantial spatial heterogeneity throughout the harbour. Water-column concentrations increased with decreasing particle size, reaching 82 particles m⁻³ for 200 - 300 μm particles, compared with 45 particles m⁻³ for 300 - 500 μm particles and 22.5 particles m⁻³ for 500 μm - 5 mm particles at the most contaminated sites. Sediment concentrations ranged from 2400 ± 529 to 111,933 ± 29,189 particles kg⁻¹ dry sediment, with particularly high abundances at localized depositional sites. These patterns emphasize the importance of sediment characteristics and harbour hydrodynamics in controlling MP retention.

Methodological differences also strongly affect comparisons among studies. Nel et al. [120], for example, reported 1200 ± 133.2 particles m⁻³ after filtering 100 L through a 63 μm mesh, whereas Preston-Whyte et al. [119] reported 99.14 ± 36.29 particles m⁻³. Differences in mesh size and particle-size range scan substantially alter measured abundance because smaller meshes capture a greater proportion of fine particles. Comparisons among studies should therefore account for sampling volume, mesh size, particle-size range, extraction procedures, and reporting units.

Naidoo and Glassom [121] examined coastal plastic debris along the KwaZulu-Natal coastline using 43 manta-trawl samples collected over one year. Mean surface plastic abundance was 4.01 ± 3.28 particles 100 m⁻². Concentrations were significantly higher during winter (5.54 ± 3.26 particles 100 m⁻²) than summer (2.96 ± 2.94 particles 100 m⁻²), with the highest concentrations occurring south of Durban.

Increasing attention has also been given to chemical additives associated with environmental MPs. Vilakati et al. [122] examined MPs from seven sites along the Atlantic coastline of Cape Town. PE was detected at six sites (87.5%), while PET and PVC occurred at five sites (71.4%). Other polymers included PS, PA-12, PAA, and EVA. Py-GC-TOF-MS identified six additives, predominantly fatty acid esters, and nine plasticizers derived from alcohols, carboxylic esters, and carboxylic acids.

Along the Indian Ocean coast, MP contamination has also been documented in mangrove-associated waters and sediments in KwaZulu-Natal [123]. Concentrations varied substantially among sites. In the St Lucia Estuary, concentrations were 1.19 ± 1.12 MP m⁻³ in water and 18.5 ± 34.4 MP per 500 g sediment. At uMgeni, concentrations reached 4.09 MP m⁻³ and 143.3 MP per 500 g sediment, while Durban Harbour and Isipingo Estuary exhibited similarly strong spatial variability. Mangrove waters showed higher MP concentrations during the wet season (1.31 - 9.42 MP m⁻³) than during the dry season (0.33 - 2.23 MP m⁻³), whereas sediment concentrations were higher during the dry season. This inverse relationship may reflect enhanced deposition during periods of reduced river discharge.

Ryan [124] emphasized the importance of South Africa as a contributor to global marine plastic pollution. Thus, in 2010 it was assumed that South Africa contributed 0.09 - 0.25 million metrictons of plastic debris to the marine environment [14]. Although offshore activities and long-distance transport contribute to marine plastic inputs, the composition and distribution of coastal debris indicate that local land-based sources are also highly important. Burial in beach sediments and retention in backshore vegetation can temporarily remove plastics from the active coastal environment, while beach cleaning removes larger debris. Conversely, sea-level rise and increasing extreme storms may remobilize previously buried plastics.

Oceanographic processes can subsequently transport plastics released from South African coastlines into both the South Atlantic and Indian Oceans. Floating macroplastics are particularly concentrated near urban centres, whereas offshore plastics tend to consist of larger and more buoyant items. Biofouling can increase particle density and promote sinking, while biological interactions may also influence the abundance of small floating particles. The seabed therefore represents a potentially important long-term sink, although existing observations account for only a small proportion of the plastic predicted to enter the marine environment. This discrepancy highlights substantial uncertainty regarding the transport, accumulation, burial, and degradation of marine plastics and reinforces the need for source-reduction measures.

6.5.1. Mozambique

Evidence for MP and plastic-debris contamination along Mozambique’s coastline remains comparatively limited. Karlsson [125] investigated plastic debris accumulation at beaches in Maputo Bay, including two sites on Inhaca Island and one at Catembe. Debris abundance was in the range of 40 - 205 and 0 - 47 particles per 100 m transect at the Marine Station and Saco sites on Inhaca Island, respectively, compared with 980 - 37,575 particles per 100 m at Catembe. The substantially greater abundance at Catembe likely reflects its closer association with the densely populated Maputo metropolitan area, although oceanographic transport and shoreline characteristics may also contribute.

Browne et al. [126] previously reported 21 - 30 MPs per 250 mL of beach sediment at Pemba, Mozambique. Barnes also investigated macro- and mesoplastic contamination (>1 cm²) on Quirimbas and Inhaca Islands [127].

6.5.2. Tanzania

Nchimbi et al. [128] documented widespread MP contamination in Tanzanian coastal sediments, with significantly higher concentrations in beach sediments (864.15 ± 275.10 MPs kg⁻¹ dry sediment) than in seabed sediments (79.9 ± 21.5 MPs kg⁻¹). Concentrations were also higher around Dar es Salaam than Zanzibar, with the Tegeta estuary representing the most contaminated Dar es Salaam site. Elevated abundances were associated within adequate waste management, riverine transport, urbanization, and shoreline activities. Fibers and fragments predominated, while polythene and polyester were the principal polymers.

Mayoma et al. [129] similarlyfound substantial spatial variability across 18 coastal sites. The highest abundance occurred at Mtoni Kijichi Creek, an industrial port in Dar es Salaam, where 2972 ± 238 particles kg⁻¹ dry sediment were recorded. Other sites ranged from 15 to 214 particles kg⁻¹. Fragments and fibers were detected at all sites, while PP and PE were the predominant polymers.

Balasubramaniam and Phillott [130] detected 3 - 7 MPs per 25 g sediment at Tebeke Beach, with a mean of 4.6 MPs per 25 g. Although geographically limited, this observation further confirms MP occurrence along the Tanzanian coast and highlights spatial variability associated with anthropogenic inputs, sediment characteristics, and hydrodynamics.

6.5.3. Kenya

Kerubo et al. [131] compared MP contamination in surface waters of urbanized Tudor and Port-Reitz creeks in Mombasa with Mida Creek near Watamu, a comparatively less urbanized system within a Marine National Reserve. MPs were detected at all sites, including Mida Creek. Mean concentrations were 3365.1 ± 431.2 and 2539.3 ± 223.5 particles m⁻³ during the two sampling periods. Fibers predominated. The occurrence of MPs within Mida Creek demonstrates that protected and comparatively remote coastal environments remain exposed to plastic pollution through local settlements, tourism, fishing, and regional transport.

Kerubo et al. [132] subsequently examined sediments from the same systems. Mean MP concentration was 1.61 ± 0.14 particles g⁻¹ dry weight, with significant spatial variability. Tudor Creek exhibited significantly greater contamination than Mida and Port-Reitz, although high concentrations were also recorded at Mikindani, Dabaso, and Mayonda. Fibers predominated, and LDPE, HDPE, and PP were identified. The results demonstrate the combined influence of urban sources and regional transport.

Kosore et al. [133] further demonstrated the importance of riverine and catchment-scale inputs by investigating six coastal centres: Lamu, Kipini, Malindi, Mombasa, Gazi, and Vanga. Concentrations ranged from 0.02 ± 0.00 to 3.22 ± 2.04 MP m⁻³, with an overall mean of 0.58 ± 1.29 MP m⁻³. The highest concentration occurred at Malindi, potentially reflecting its connection to the Sabaki River and its heavily impacted catchment. Sampling shortly after the October-November short-rain period may also have contributed to elevated concentrations through enhanced runoff, flooding, and river discharge.

As already mentioned above, mangrove forests represent another important environment for MP retention. Okuku et al. [134] examined debris accumulation across landward, middle, and seaward zones of seven Kenyan mangrove forests. The landward zone primarily retained land-derived debris, whereas the seaward zone preferentially trapped marine-derived litter. The middle zone exhibited the highest debris density (0.162 ± 0.150 items m⁻²), compared with 0.066 ± 0.049 items m⁻² in the landward zone and 0.036 ± 0.028 items m⁻² in the seaward zone. These findings demonstrate that mangroves function as spatially structured retention systems in which vegetation, tidal exchange, hydrodynamics, and litter-transport direction determine where plastic accumulates.

Taken together, Kenyan studies demonstrate widespread MP contamination across surface waters, sediments, estuaries, and mangrove environments, extending from highly urbanized systems to comparatively protected areas. The prominence of fibers suggests important contributions from wastewater, domestic activities, fishing, and other land-based sources, while rainfall, river discharge, sediment characteristics, and mangrove zonation modify their distribution. The occurrence of MPs in protected coastal environments indicates that marine protected areas cannot be isolated from regional plastic pollution and that mitigation must address sources throughout connected catchments.

KeChi Okafor et al. [135] examined microfibers in surface waters along the Kenyan and Tanzanian coasts. Of 2403 microfibers recovered from 37 sites, 55% were natural, 8% regenerated cellulosic, and 37% synthetic. Natural microfibers were more abundant than synthetic microfibers at 33 of the 37 sites, demonstrating the importance of distinguishing synthetic microplastics from other fiber types during environmental assessments.

Weideman et al. [136] investigated meso-, microplastics, and microfibers off Mozambique and Tanzania during October-November 2017 when aboard the research vessel S.A. Agulhas II. Net and bulk-water samples were collected at 60 stations, including 44 off Mozambique and 16 off Tanzania. Synthetic microfibers were several orders of magnitude more abundant numerically than net-collected litter (45 ± 50 fibers m⁻² versus 0.009 ± 0.020 items m⁻²), although net-collected litter contributed more by mass (0.044 ± 0.195 mg m⁻² versus 0.01 ± 0.01 mg m⁻²).

Meso-litter, micro-litter, and microfibers were widespread along northern Mozambique and southern Tanzania. Litter loads were generally higher along the Tanzanian coast, potentially reflecting higher population density. Greater proportions of films and polystyrene off Tanzania were consistent with predominantly local sources. Litter size decreased with increasing distance from shore, while higher densities near river mouths and greater litter mass close to shore suggested an important contribution from land-based sources.

However, the absence of increased offshore litter concentrations near major coastal cities, including Dar es Salaam and Beira, complicates a simple land-source interpretation. One possible explanation is rapid retention of land-derived debris on nearby beaches through tidal processes and long shore drift. Alternatively, small particles may accumulate along salinity fronts where rivers discharge into the sea. Further synoptic investigations of river systems are required to distinguish among these mechanisms.

6.5.4. Sudan

Ibrahim et al. [137] investigated the composition, distribution, and abundance of beach litter on selected beaches within the Dungonab Bay and Mukkawar Island National Park (DMNP), a World Heritage Site located along the Sudanese Red Sea coast. The southern boundary of the DMNP lies approximately 125 km north of Port Sudan Harbour. The protected area covers approximately 2800 km², including around 800 km² of coastal land and 2000 km² of marine waters. Dungonab Bay is a shallow-water basin that is partially enclosed by the long, narrow Dungonab Peninsula. The bay extends approximately 31 km from north to south and is about 13 km wide at its southern end, covering an area of approximately 284.5 km². Mukkawar Island is an uninhabited rocky island located approximately 30 km offshore from the Dungonab Peninsula.

The DMNP is situated in a remote and sparsely populated rural area, with approximately 2000 inhabitants distributed among several small villages. Artisanal fishing represents the principal economic activity, supplemented by other small-scale activities such as shellfish collection and animal husbandry. Despite the relatively low population density and limited development in the region, Ibrahim et al. [137] recorded considerable quantities of beach litter. A total of 24 beach-litter surveys were conducted along approximately 600 m of coastline, corresponding to an area of around 7700 m². In total, 3037 litter items were collected, corresponding to an overall mean abundance of 126.72 ± 76.86 items per 100 m of shoreline, or approximately 0.4 items m⁻².

Plastic was the dominant litter category, accounting for 1738 items, or 57.23% of all collected debris. The mean density of plastic litter was 72.52 items per 100 m, equivalent to approximately 0.23 items m⁻². However, substantial spatial variation was observed among the sampled beaches. Plastic densities ranged from 11.5 ± 13.9 items per 100 m on a beach used for oyster cleaning to as high as 140 ± 89.11 items per 100 m at Mohammed Gol Jetty, which includes a semi-rural beach. These differences suggest that local human activities and patterns of beach use may influence the accumulation of plastic debris even within this relatively remote coastal environment.

Temporal variation was also evident in the accumulation of beach litter. The litter flux ranged from 16.43 to 21.9 items per 100 m per day during the study period, with an overall mean of 18.82 ± 2.8 items per 100 m per day. Overall, the findings demonstrate that plastic constitutes the dominant component of beach litter in the DMNP and that significant plastic accumulation can occur even in sparsely populated and relatively undeveloped coastal areas. The observed spatial and temporal variability further highlights the influence of local activities, coastal dynamics, and potentially marine sources on the distribution and accumulation of beach litter along the Sudanese Red Sea coast.

During the dry season 2024, Salah et al. [138] investigated microplastic (MP) contamination at four coastal sites near Port Sudan, comprising two semi-enclosed bays (Flamingo Bay and Al Khaleej Bay) and two open-coast sites (Flamingo Coast and Al Kilo Coast). This sampling design enabled a direct comparison between semi-enclosed and exposed coastal environments. Water and sediment samples were collected from 19 stations in total: four in Flamingo Bay, five at Flamingo Coast, six in Al Khaleej Bay, and four at Al Kilo Coast, yielding 57 water and 57 sediment samples.

Mean MP abundances were substantially higher in the semi-enclosed bays than along the open coast, reaching 154.84 ± 4.44 items m⁻³ in seawater and 171.79 ± 2.66 items kg⁻¹ in sediments in the bays, compared with 91.85 ± 1.51 items m⁻³ and 61.15 ± 1.64 items kg⁻¹, respectively, at open-coast sites. Fibers were the predominant MP morphology, accounting for 51.84% of particles in seawater and 45.80% in sediments, while PE and PVC were the dominant polymer types. A distinct spatial gradient in MP abundance was observed in surface water, following the order Flamingo Bay > Al Khaleej Bay > Al Kilo Coast > Flamingo Coast. In contrast, sediment samples showed a higher MP concentration in Al Khaleej Bay than in Flamingo Bay.

Diversity and correlation analyses indicated that the semi-enclosed bays receive more complex MP inputs and are strongly influenced by local anthropogenic pressures and site-specific hydrodynamic conditions. In contrast, MPs at the open-coast sites appeared to be influenced more strongly by long-range transport, salinity, and water depth. Ecological risk was also greater in the bays, where elevated risk levels were associated with the presence of potentially hazardous polymers and stronger direct anthropogenic inputs. Overall, the study demonstrates that coastal geomorphology and hydrodynamic setting strongly influence MP accumulation, with semi-enclosed bays acting as important hotspots for both MP contamination and associated ecological risk.

6.6.1. Mayotte

Mulochau et al. [139] investigated marine litter accumulation across 22 stations on fringing reefs within the Marine Natural Park of Mayotte. Litter densities ranged from 0 to 50 items per 500 m², with similar mean abundances in winter (3.9 ± 1.3 items per 500 m²) and summer (3.8 ± 1.1 items per 500 m²). Plastic represented 92% of litter, approximately 66% of which consisted of fishing gear. The predominance of fishing-related debris is particularly concerning because entanglement and attachment to coral colonies can cause physical damage and potentially impair coral growth and recovery. Approximately half of coral colonies containing litter exhibited visible impacts.

6.6.2. Seychelles and the Western Indian Ocean

Evidence from remote Seychelles islands demonstrates that geographical isolation does not prevent marine plastic accumulation. On Cousine Island [140], 9119 litter items were recorded across 40 surveys, corresponding to an overall accumulation rate of 0.0082 items m⁻¹ d⁻¹. This increased significantly over time, reaching 0.0255 items m⁻¹ d⁻¹ in 2019. More than 80% of collected litter was wholly or partly plastic, with polystyrene fragments and pieces representing the most frequent category. The increasing deposition rate indicates that remote islandscanact as sinks for plastics transported by regional ocean circulation.

Alphonse Island [71] similarly accumulated substantial marine debris. A 2013 survey of a 1-km windward beach recovered 4743 items weighing approximately 142 kg. Small plastic fragments represented 25.89% of the total. Prevailing winds, currents, and wave action likely facilitate long-range transport to the island. Marine debris on remote western Indian Ocean islands has also been linked to sources in Indonesia, India, and Sri Lanka [68], emphasizing the transboundary nature of plastic pollution.

A particularly striking example is Aldabra Atoll. During a large-scale clean-up in March 2019, approximately 25 tonnes of plastic were removed at a cost of US$224,537. Remaining plastic waste was estimated at approximately 513 tonnes (95% CI: 212 - 814 tonnes), representing one of the largest reported accumulations of plastic litter on a single island. Fishing-related debris accounted for 83% of the total mass, followed by flip-flops (7%), fragments (4%), packaging (3%), other litter (2%), and consumer items (1%) [141]. Complete removal was estimated to require approximately US$4.68 million and 18,000 person-hours. These findings demonstrate both the scale of plastic accumulation on remote islands and the substantial logistical and financial burden associated with remediation. Clean-up therefore needs to complement, rather than replace, measures aimed at preventing plastic leakage at source.

6.6.3. Mauritius

MP contamination has also been reported from beaches and mangrove systems in Mauritius. Beach sediments at Troud’Eau Douce and La Cambuse, together with measurements from beaches across the western, northern, southern, southeastern, and eastern coasts, demonstrate considerable spatial variability [142] [143]. Reported concentrations ranged from relatively low values at Baie du Cap and Bel Ombre to substantially higher concentrations at Flat Island, La Cambuse, and Pointe D’Esny. At La Cambuse, for example, concentrations reached 89.2 MP m⁻² in the upper layer, 67.8 MP m⁻² in the beach-surface layer, and 39.3 MP m⁻² in the lower sampled layer.

Mangrove forests provide additional evidence of plastic retention. At Mahebourg and Ferney, plastic represented 42.9% and 43.7% of anthropogenic litter, respectively, with plastic abundances of 0.20 ± 0.11 and 0.10 ± 0.09 particles m⁻² [144]. These observations reinforce the role of mangrove environments as important retention zones for anthropogenic plastic debris.

Collectively, the available evidence demonstrates that plastic and MP contamination is widespread across African coastal environments, spanning the Mediterranean and Red Sea coasts of North Africa, the Atlantic and Gulf of Guinea coasts of West and Central Africa, the Indian Ocean coast of East and Southern Africa, and remote islands of the western Indian Ocean. MPs and larger plastic debris have been documented in surface waters, beach and seabed sediments, lagoons, estuaries, rivers, harbours, mangroves, coral reefs, and remote island shorelines.

Despite substantial differences in reported concentrations, several consistent patterns emerge. Fibers and fragments are among the most frequently dominant particle morphologies, while PE and PP commonly represent the principal polymers. However, polymer composition can vary considerably among regions and environmental compartments, emphasizing the diversity of plastic sources and the importance of local material use.

Anthropogenic activity is a recurring driver of contamination. Urbanization, industrial activity, wastewater discharge, fishing, tourism, port operations, and inadequate solid-waste management are repeatedly associated with elevated plastic concentrations. Rivers, streams, stormwater, and wastewater provide important pathways through which land-derived plastics reach coastal environments. Conversely, fine-grained sediments, beaches, lagoons, estuaries, mangroves, harbours, and semi-enclosed bays frequently function as accumulation zones.

At the same time, MP abundance cannot be interpreted solely as a measure of local emissions. Hydrodynamic connectivity is a central control on the distribution and fate of plastic debris. Tidal circulation, surface currents, waves, wind, river discharge, seasonal rainfall, and sediment dynamics can transport particles away from their sources or concentrate them in particular depositional environments. This is evident in the seasonal redistribution of MPs in Algeria, rainfall-driven transport in Cameroon, riverine transport in Kenya, and the accumulation of marine-derived debris on remote Seychelles islands.

The evidence from mangrove systems is particularly important because these ecosystems can act as both sinks and temporary reservoirs. Their complex root structures, tidal channels, and spatial zonation can retain debris originating from both land and marine environments. However, retention is not necessarily permanent, and changes in hydrodynamics, storms, river discharge, and sediment dynamics may remobilize previously deposited material.

The South African literature additionally highlights the importance of methodological standardization. Reported concentration scan differ by orders of magnitude depending on mesh size, sampled volume, particle-size range, extraction procedure, polymer-identification method, and reporting units. The contrasting estimates from the Port of Durban illustrate why numerical comparisons among studies should be made cautiously unless methodological characteristics are comparable.

Finally, recent evidence from the Port of Durban suggests that stormwater drains may represent an important pathway for MP delivery to the harbour and may contribute more MPs than riverine sources [119]. This observation reinforces the broader conclusion that coastal plastic pollution is governed by interconnected drainage, wastewater, riverine, and coastal transport pathways rather than by individual point sources alone.

Plastic pollution in the African marine environment affects not only marine organisms but also industries that depend on them, particularly the fishing industry. In this context, a report by the FAO’s EAF-Nansen Programme examined the prevalence of plastic debris in the catches of seine fishers across four West African countries—Côte d’Ivoire, Ghana, Togo, and Benin. The study found that plastic debris accounted for between 1.1% and 15.7% of the total catch by weight (Table 5) [145].

Table 5. Plastic debris found in the catch of artisanal fishers in four countries along the Gulf of Guinea.

Overall, the African coastal literature indicates that MP contamination is widespread, spatially heterogeneous, and strongly controlled by the interaction between anthropogenic sources and environmental transport processes. The occurrence of MPs even in protected, sparsely populated, and remote environments demonstrates the regional and transboundary nature of the problem. Effective mitigation therefore requires a combination of source reduction, improved solid-waste and wastewater management, control of riverine and stormwater inputs, responsible fisheries and port practices, and targeted monitoring of major accumulation environments. At the same time, greater harmonization of sampling and analytical methods is essential to enable robust comparisons of MP abundance and trends across African coastal ecosystems.