Introduction

The formation of cyanobacterial blooms, which are usually linked to the development of cyanotoxins, is favored by excessive nutrient enrichment, water stillness, and favorable light and temperature conditions (Merel et al., 2013; Paerl, 2014). Cyanotoxins are commonly alkaloids of low molecular weight or cyclic peptides that cause harm to organisms, including humans (Carmichael et al., 2001; Metcalf & Codd, 2012). They are secondary metabolites, the organism does not utilize them for its primary metabolism and either retains them in producer cells or releases them into the water (Metcalf & Codd, 2012). Cyanotoxins comprise over a hundred different chemicals, each with a unique chemical structure and toxicity (Manning & Nobles, 2017).

Many authors have linked the poisoning and death of animals to cyanotoxins (Carmichael et al., 2001; Chellappa et al., 2008; Harding & Paxton, 2001; Masango et al., 2010; Scott et al., 1981; Shaw et al., 2002; Soll & Williams, 1985; Steyn, 1943; Van Halderen et al., 1995). Microcystins (MCs) are the most commonly reported cyanotoxins (Dawson, 1998; Gupta et al., 2003), and are known to cause cell necrosis, massive hemorrhage, and death (Hooser, 2000; Kuiper-Goodman et al., 1999). Carmichael et al. (2001), Funari and Testai (2008), and Buratti et al. (2017) documented several cases of human intoxication associated with cyanotoxins. Exposure to cyanotoxin-contaminated waters is known to cause skin irritations, conjunctivitis, earaches, gastroenteritis, respiratory diseases, allergic responses, and liver damage (Dietrich & Hoeger, 2005; Vidal et al., 2017).

Nearly every nation in the world has reported the occurrence of cyanobacteria and cyanotoxins (Fristachi et al., 2008), Microcystis and MCs being predominant in many countries (Harke et al., 2016; Ndlela et al., 2016). MCs are the most often reported cyanotoxins worldwide, followed by cylindrospermopsin (CYN), anatoxin-a (ATX-a), saxitoxins (STX), and nodularins (NOD) (Pelaez et al., 2010; Svirčev et al., 2019). In several cases, the levels of MCs, CYN, and STX have exceeded the provisional standards for drinking and recreational water around the world (Loftin et al., 2016; Roegner et al., 2020; Szlag et al., 2015). The most commonly reported cyanobacterial genera found worldwide with cyanotoxins in environmental samples are Microcystis spp., Dolichospermum (Formerly: Anabaena) spp., Aphanizomenon spp., Planktothrix spp., and Oscillatoria spp. (Ibelings et al., 2021; Svirčev et al., 2019).

There are numerous reports of cyanotoxin production in the waterbodies of Africa and its adverse effects. Animal illness and mass deaths have been documented for decades in the aquatic ecosystems of South Africa, Morocco, and Kenya (Krienitz et al., 2005; Oberholster et al., 2009; Oudra et al., 2001, 2002; Scott, 1991; Steyn, 1945). Many Africans have been experiencing broad toxicities resulting in itching, sore eyes, gastroenteritis, and skin rashes (Zilberg, 1966). The problem of eutrophication and algal blooming in Africa is exacerbated due to rapid population growth, urbanization, intensive agriculture, and industrialization (Giri, 2021; Juma et al., 2014).

There is paucity of compiled information on the presence and quantification of cyanotoxins and cyanobacteria in African waterbodies. This could probably be due to lack of resource, skilled technicians and analytical instruments to detect and quantify cyanotoxins. Therefore, this review aims to thoroughly examine the research articles about the prevalence of cyanotoxins and their adverse effects in African waters. This review on several cyanotoxins research aspects will serve as a foundation for ongoing research, monitoring, and management of cyanobacterial blooms in various water resources of Africa.

Research approach: inclusion and exclusion of literature

Articles, books, book chapters, and brief communications that addressed the occurrence, toxic effects, and health risk of cyanotoxins to humans and animals in African aquatic ecosystems was gathered from the Scopus, Web of Science, PubMed, and Science Direct databases. Articles that were published from 1990 to 2022 was searched using a combination of relevant key words/phrases that include “cyanotoxins,” “microcystin,” “cyanobacteria,” “mortality,” “adverse effects” “African waterbodies,” “Eutrophication,” “Southern African waterbodies,” “Eastern African aquatic ecosystems,” “North African waters,” “West African water resources,” and “Central Africa water systems.” Key words/phrases were combined using conjunctions like AND, AND NOT, NOT, and OR.

Inclusion and exclusion of literature was conducted based on the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines checklist (Salameh et al., 2020) that include journal ranking, impact factor, the relevance of the article to the research theme, reproducibility, and novelty. Abstracts and full-text contents were thoroughly assessed to check the quality and relevance of literature to study objectives. Consequently, 32 publications out of a total of 172 assessed literature found were rejected for not meeting the minimum set requirements (Fig. 1). Numerical data of cyanotoxin concentrations were extracted from only 64 articles studied in various African aquatic ecosystems.

Fig. 1
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Identification, screening, inclusion, and exclusion of literature used for numerical analysis

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Lake Victoria was classified as a three-ecosystem since it was regarded as a water resource for Kenya, Tanzania, and Uganda. The cyanotoxin concentrations are presented as maximum content in two different units (µg L–1 and µg g–1). Toxin concentrations that were reported in µg L−1 only were compared with the WHO recommended values of drinking and recreational water. The analysis did not include studies conducted on tap water and experimental research involving cyanotoxins.

The role of eutrophication in the occurrence of cyanotoxins in African water bodies

In many African countries, industrialization and urbanization coupled with intensive agricultural practices driven by rapid population growth led to land degradation with knock-on effect on excess nutrient enrichment into aquatic systems (Fetahi, 2019; Giri, 2021; Zinabu et al., 2018). Domestic and municipal wastewater, increased applications of chemical fertilizers, and discharging industrial effluents into nearby aquatic systems without adequate treatment have led to increased nutrient loads into surface water resulting in eutrophication (Akale et al., 2018; Lanckriet et al., 2017). About 28% of African waterbodies are seriously impaired by eutrophication due to increased developmental activities with some sort of production work (Nyenje et al., 2010). The concentration of total phosphorus greater than 0.01 mg L−1 and total nitrogen above 0.3 mg L−1 are enough to cause eutrophication (Tibebe et al., 2022).

In many African countries, recent agricultural intensification with crop–livestock production has led to excess applications of chemical fertilizers along the catchment (Fetahi, 2019; Nyenje et al., 2010; Van Ginkel, 2011). Consequently, runoff from agricultural land washing fertilizers and organic-rich surface soil accelerates the eutrophication of aquatic systems (Lijklema, 1995; Van Ginkel, 2011). Fertilizers applied on farmlands are not completely utilized, a rather large quantity of fertilizer either remain in the soil or gradually washed into aquatic systems due to poor farming practice and reduced water and soil conservation measures (Buratti et al., 2017; Lijklema, 1995). It has been acknowledged that eutrophication is the main factor favoring cyanotoxin production (Chorus et al., 2021; Dolman et al., 2012; Sasner et al., 1981; Tanvir et al., 2021). In various African waterbodies, there are numerous examples of cyanotoxins presence in high concentrations in eutrophic and hypertrophic aquatic systems (Eguzozie et al., 2016; Harding et al., 1995; Masango et al., 2010; Ndlela et al., 2016; Scott et al., 1981, 2018).

Occurrence of cyanotoxins and cyanobacteria in African waterbodies

We documented cyanotoxins occurrence reports on 95 waterbodies (29 lakes, 41 reservoirs, 10 ponds, 9 rivers, 5 coastal waters, and 1 irrigation canal) in 15 African countries (Fig. 2). Svirčev et al. (2019) reviewed publications until 2018 and indicated the presence of cyanotoxins occurrence in fourteen African countries in 76 freshwater ecosystems. In the present review, almost all aquatic ecosystems (98.9%) contained MCs, while ATX-a (5.3%), CYN (2.1%), NOD (2.1), homoanatoxin-a (HTX) (1.1%), and β-N-methylamino-l-alanine (BMAA) were reported in few waters. MCs are the most frequently reported toxin (60) in the literature surveyed, while other cyanotoxins such as ATX-a (5), NOD (2), CYN (2), HTX (1), and BMAA (1) have minor contributions. ATX-a was quantified in Kenyan ecosystems only. All concentrated extracts of the Ethiopian Rift Valley Lakes had trace amounts of CYN, but none of the concentrations were high enough to be detected by ELISA (Willén et al., 2011). HTX and BMAA, each were reported in one African aquatic system only. The highest amounts of NOD and MCs were 347,000 µg g–1 and 49,410 µg L–1 in lakes Zeekoevlei and Nhlanganzwani, South Africa, respectively (Table 3).

Fig. 2
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Geographical distribution of the cyanotoxins reported in African waterbodies

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Most of the waterbodies surveyed and reported in μg L−1 (83.6%) contained less than 10 μg L−1 cyanotoxin from 73 waterbodies. The majority of the toxin content reported as µg g–1 (59.1%) was higher than 200 µg g–1. The MCs concentrations in 63% of the water systems were greater than the WHO guideline threshold of lifetime drinking water (1 μg L−1). Twelve waterbodies found in six countries contained MCs concentrations greater than WHO’s provisional guideline values for short-term drinking water (12 μg L−1) and recreational water (24 μg L−1) (Tables 1 and 2). The ATX-a, NOD, CYN, HTX, and BMAA concentrations reported did not exceed threshold values of short-term drinking-water and recreational water. However, fish ponds in Sohag province, Egypt, were reported to have CYN amounts (2.76 µg L−1) above the lifetime drinking-water guideline value set by WHO (0.7 μg L−1).

Table 1 Maximum values/standards of cyanotoxins concentrations in drinking and recreational water set by WHO (Chorus & Welker, 2021; WHO, 2020)
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Table 2 African waterbodies that contained MCs concentrations greater than the WHO’s guideline value for short-term drinking-water and recreational water
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Aquatic ecosystems studied at least twice such as Hartbeespoort Reservoir (South Africa), Nile River (Egypt), Lake Oubeira (Algeria), and Lake Koka (Ethiopia), showed generally declining trends of cyanotoxins concentrations between 1995 and 2015. Lake Victoria’s MCs concentration has been steadily increasing until 2017 before it decreased to one of the lowest values (0.02 μg L−1). However, Lake Mburo (Uganda), Lebna Reservoir (Tunisia), Ahmadu Bello University (ABU) Reservoir (Nigeria), Lake Bomo (Nigeria), and Lake Makwaye (Nigeria), have showed increments in MCs content between 2003 and 2013. These variations can be linked to management efforts and nutrient enrichment driven by anthropogenic activities.

The most frequently found cyanobacteria with cyanotoxins in water samples in African aquatic ecosystems were Microcystis spp. (73.7%), followed by Oscillatoria spp. (35.8%) and Dolichospermum spp. (33.7%). Other cyanobacterial genera that were present with cyanotoxins in environmental samples in at least 10% of the waterbodies include Merismopedia (24.2%), Pseudanabaena (22.1%), Chroococcus (17.9%), Planktothrix (16.8%), Spirulina (14.7%), Raphidiopsis (9.5%), and Synechococcus (9.5%).

Southern Africa

The occurrence and distribution of cyanotoxins and cyanobacteria have been extensively researched in South African waterbodies. In contrast to NOD, which was only identified in Lake Zeekoevlei, MCs were found in 94.1% of the 17 Southern African aquatic systems surveyed (Table 3). A total of 88.2% of the water systems surveyed had MCs levels over the WHO-recommended limit of 1 μg L−1 for drinking water, and of those, 46.2% had levels above the threshold for both short-term drinking water and recreational water use. In Lake Zeekoevlei, Nhlanganzwane Reservoir, and Kruger National Park Reservoirs (Nhlanganzwani, Mpanamana, Makhohlola, and Sunset), the concentration of cyanotoxins (MCs and NOD) was so high (up to 49,410 μg L−1) that it was linked to serious illness and mortality in domestic and wild animals (Harding et al., 1995; Masango et al., 2010; Oberholster et al., 2009). Between 1984 and 2011, Lake Hartbeespoort's particulate MCs remained stable, while its dissolved MCs decreased between 2005 and 2013. This is consistent with a decline in productivity between 2010 and 2018, which is probably caused by a fall in the water temperature (Ali et al., 2022).

Table 3 Occurrence of cyanotoxins and cyanobacteria in some Southern African waterbodies
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MCs measurements in Botswana, Mozambique, and Zimbabwe waterbodies were less than guideline value for short-term drinking water set by WHO (12 μg L−1), although many of them were higher than the limit set by WHO for drinking water (1 μg L−1) (Table 3). Oreochromis mossambicus and Labeo rosae had estimated daily intake values of MCs concentration in muscle that were 4.1 and 4.6 times, respectively, higher than the recommended tolerable daily intake (TDI) value on 0.04 μg kg−1 of body weight (Nchabeleng et al., 2014). In some Southern African water ecosystems that are sources of drinking water, the presence of MCs at concentrations higher than 1 μg L−1 is especially worrisome (Mhlanga et al., 2006; Pedro et al., 2012; Recknagel et al., 2017).

Microcystis is the most frequently reported (82.3%) cyanobacteria together with the cyanotoxin in Southern African water resources. Some South African waterbodies, including the Vaal Reservoir, Theewaterskloof Reservoir, Hartbeespoort Reservoir, Lake Zeekoevlei, and Orange River, have been found to contain potentially toxin-producing cyanobacteria such as Microcystis aeruginosa, Microcystis flos-aquae, Dolichospermum spp., Oscillatoria spp., Nodularin spumigena and Raphidiopsis raciborskii, Spirulina spp., and Planktothrix (Harding & Paxton, 2001; Scott et al., 1981; Steyn, 1945; Van Halderen et al., 1995).

East Africa

The concentrations of MCs in East African waterbodies ranged from trace to hazardous levels, reaching up to 453.89 μg L−1 (Legedadi Reservoir, Ethiopia) and 19,800 µg g–1 (Lake Baringo, Kenya) (Table 4). From 33 water resources surveyed, the majority of them (57.6%) had MCs concentrations over the WHO standard for drinking water (1 μg L−1). Lake Koka (Ethiopia), Legedadi Reservoir (Ethiopia), and Lake Victoria (Uganda) contained MCs levels that exceed WHO guideline limits of drinking water for short duration and recreational water. Legedadi Reservoir, a major supply of drinking water for Addis Ababa, Ethiopia, had an extremely high MCs concentration (453.89 μg L−1), which could be hazardous to end users' health (Major et al., 2018). Five lakes in Kenya (Lakes Baringo, Simbi, Sonachi, Bogoria, and Nakuru) contain the neurotoxic ATX-a, with concentrations up to 1260 µg g–1 (Lake Baringo).

Table 4 Occurrence of cyanotoxins and cyanobacteria in some Eastern African waterbodies
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Lesser Flamingo deaths were associated with the hot spring shore of Lake Bogoria, which had three times more MCs content than the western shore (Ballot et al., 2004; Krienitz et al., 2003). Different MC variants, specifically MC-LR, MC-YR, MC-RR, MC-LA, MC-LF, MC-dmLR, MC-dmRR, LA[NMeSer7]-MC-YR, [Asp3]-MC-RY, [MeAsp3]-MC-RY, and [NMeSer7]-MC-YR), were found in both the water samples and algal seston (Habtemariam et al., 2021; Major et al., 2018; Okello et al., 2010; Olokotum et al., 2022). CYN was found to be the most prevalent cyanotoxin found around the lakeshores in Lake Victoria, Tanzania, where NOD and MCs also co-occurred (Mchau et al., 2021). HTX was also reported in Lake Victoria, Uganda.

Microcystis dominated the freshwater bodies of East Africa, which occurred in 81.8% of the surveyed waterbodies. The cyanobacterial genera Merismopedia, Dolichospermum, Arthrospira, Pseudanabaena, and Anabaenopsis are present in at least five water ecosystems of East Africa in the habitats that exhibited detectable cyanotoxins. Arthrospira fusiformis dominated three of the five water resources that contained ATX-a (Lakes Simbi, Sonachi, and Bogoria). ATX-a and MC-YR were produced by A. fusiformis monocyanobacterial strain that was isolated from Lake Sonachi (Ballot et al., 2005). The mcyE gene, a member of the microcystin synthesis gene cluster, in field samples taken from Lake Naivasha in Kenya, and the mcyB genotype of Microcystis and Planktothrix species in freshwater lakes in Uganda were identified to produce cyanotoxins (Krienitz et al., 2013; Okello et al., 2010).

North Africa

MCs occurred in all of the North African waterbodies surveyed, with many congeners detected from various ecosystems (up to 21 in Lake des Oiseaux) (Bouhaddada et al., 2016). Of the 19 aquatic ecosystems documented, 63.2% had MCs concentrations higher than the WHO's provisional recommendation limit of 1 μg L−1 for drinking water (Table 5). Fish ponds in the Sohag province (Egypt), Lalla Takerkoust Reservoir (Morocco), and Lake Oubeira (Algeria) have cyanotoxins levels beyond the WHO guidelines for short-term drinking water use and recreational water. In the province of Sohag, a fish pond with a high concentration of CYN (2.76 µg L−1) may cause the accumulation of the toxin in fish, putting consumers at risk. The main sources of drinking water, Lake Oubeira and Lalla Takerkoust Reservoir contained extremely high levels of MCs up to 29,163 μg L−1. The presence of high concentrations of MCs was linked to turtle deaths that occurred during a Microcystis spp. bloom in Lake Oubeira (Nasri et al., 2008). Tomato plants exposed to a crude extract of a harmful cyanobacterial bloom from Lalla Takerkoust Reservoir experienced leaf tissue necrosis (El Khalloufi et al., 2012).

Table 5 Occurrence of cyanotoxins and cyanobacteria in some Northern African waterbodies
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There are a few instances in Egypt and elsewhere where water treatment plants not only failed to completely remove cyanobacterial cells and degrade toxins, but also lysed the cyanobacteria's cells and released significant amounts of MCs (Eynard et al., 2000; Ling, 2000; Mohamed et al., 2015; Mohamed, 2016a). The final treated water's MCs concentration was 3.8 µg L−1, much higher than the quantity found in the untreated water source from Egypt's Nile River (Mohamed, 2016a). Fishponds in Egypt contained considerably high concentrations of MCs and CYN that exceed WHO guideline values of 1 μg L−1. The muscle of common carp from Lake Oubeira exceeded the WHO lifetime limit for the tolerated daily consumption of MCs (Amrani et al., 2014).

Oscillatoria was the most frequently reported (84.2%) cyanobacteria found with the cyanotoxins in water samples from North African waterbodies surveyed. Other cyanobacteria with a smaller frequency of occurrence include Microcystis (73.7%), Pseudanabaena (57.9%), Merismopedia (57.9%), Chroococcus (47.4%), Dolichospermum (36.8%), and Planktothrix (31.6%). The cyanobacterial cell density of some North African water systems positively correlated with MCs contents (Mohamed & Bakr, 2018; Mohamed et al., 2015; Nasri et al., 2007). Anabaena affinis, Planktothrix agardhii, Raphidiopsis catemaco, and Capillaria philippinensis were assigned as CYN producers for the first time in Egyptian fishponds (Mohamed & Bakr, 2018). The presence of the MCs synthetase genes (mcyA, -B, -C, -D, -E, and -G) in the Microcystis spp. isolates indicate that these species are responsible for the MCs production in a few North African water ecosystems (El Herry et al., 2008; Fathalli et al., 2011).

West Africa

All of the 26 waterbodies of three surveyed West African nations (Ghana, Nigeria, and Zimbabwe) contained MCs (Table 6). Major congeners of MCs such as MC-LR, MC-LF, MC-RR, and MC-YR were reported (Addico et al., 2017; Chia & Kwaghe, 2015). The WHO's guideline limit of drinking water 1 μg L−1 for MCs concentration was exceeded in 76.9% of the aquatic ecosystems. None of the water bodies’ MCs levels, however, rose above the WHO threshold limits for recreation usage and drinking water for short periods. The Owabi Reservoir in Ghana, which provides drinking water to the Kumasi metropolitan area, had the highest MCs content (8.73 µg L–1) of all the West African waters examined (Addico et al., 2017). The presence of MCs levels exceeding 1 μg L−1 in about ten coastal ecosystems in Nigeria shows that cyanotoxins are a concern that is not just present in inland waters. In Nigeria, Kwaru stream irrigated vegetables and the fish from some fish ponds in Zaria contained MCs values that exceed WHO recommended total daily intake value (0.04 μg kg−1 body weight) (Abdullahi et al., 2022; Chia et al., 2021).

Table 6 Occurrence of cyanotoxins and cyanobacteria in some Western African waterbodies
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Dolichospermum is the most frequently found (61.5%) cyanobacteria in the surveyed West African waterbodies where cyantoxins were measured. Other cyanobacterial genera found in at least 5 ecosystems examined include Microcystis (57.7%), Oscillatoria (57.7%), Spirulina (26.9%), Chroococcus (26.9%), Marssoniella (26.9%), Planktothrix (23%), Trichodesmium (23.1%), and Merismopedia(19.2%). All of the samples examined from 11 Nigerian coastal waters that contained the mcyE gene of the MC synthetase (mcy) cluster showed the presence of toxic strains of cyanobacteria (Kadiri et al., 2020). Many West African water systems showed a substantial positive association between MCs concentration and cyanobacterial density (Addico et al., 2017; Chia & Kwaghe, 2015; Chia et al., 2021).

Adverse effects of cyanotoxins in Africa

Research on the Vaal Reservoir, South Africa is the earliest known study on animal toxicity caused by algal blooms in Africa (Steyn, 1945). According to Steyn (1945), the algal-infested water was poisonous, and the degree of poisoning depended upon the number of algae consumed. Since then, many scholars have linked the poisoning of humans and animals in Africa to cyanotoxins (Harding et al., 1995; Krienitz et al., 2005; Nasri et al., 2008; Scott et al., 1981; Soll & Williams, 1985; Van Halderen et al., 1995; Zilberg, 1966).

In South Africa, between 1973 and 1974, cattle kills around Hartbeespoort Reservoir was linked to M. aeruginosa toxicity (Scott et al., 1981). In 1979, three white rhinoceroses (Ceratotherium simum) died related to Microcystis toxicity from Klipvoor Reservoir, Barakologadi Game Reserve (Soll & Williams, 1985). Postmortem macroscopic and microscopic pathology revealed acute hepatotoxicity, hepatomegaly with hemorrhage, severe liver necrosis, numerous ecchymoses, and petechiae. In 1994, a dog died after drinking water from Lake Zeekoevlei. Histopathological examination of the liver revealed periacinar fibrosis with duplication of the central veins, degeneration of hepatocytes, and the distention of the bile canuliculi with bile pigment (Van Halderen et al., 1995). The water contained cyanobacterial bloom of Nodularia spumigena (95%) and Microcystis aeruginosa that produced 3479 µg g–1 nodularin.

Oberholster et al. (2009) associated the possible cause of the mortalities of the white rhinoceroses, zebra, wildebeest, and other wild animals in Nhlanganzwane Reservoir, South Africa with acute exposure to MCs toxins. Similarly, the wildlife mortality near four South African reservoirs (Nhlanganzwani, Mpanamana, Makhohlola, and Sunset) in Kruger National Park was also associated with Microcystis blooms, which were confirmed to be toxic using mouse and fish (Clarias gariepinus) bioassays (Masango et al., 2010). Animal (e.g., fish, giraffes, cows, sheep, etc.) poisoning and deaths around some South African waterbodies, including the Vaal Reservoir, Eastern Transvaal Reservoir, Bloemh Reservoir, Klipdrif Reservoir, Theewaterskloof Reservoir, Erfenis Reservoir, and Orange River, were linked to the toxicity of cyanotoxins (Harding, 1997; Harding & Paxton, 2001; Scott, 1991; Theron, 1990a, 1990b; Van Ginkel & Hohls, 1999).

Cyanotoxins (MCs and ATX-a) ingestion was associated with the mass mortality of lesser flamingos at Lake Bogoria, Kenya (Krienitz et al., 2003, 2005). Death of lesser flamingos related to cyanotoxins is also reported in Lakes Big and Manyara, Tanzania (Lugomela et al., 2006). The mortality of freshwater turtles (Emys orbicularis and Mauremys leprosa) was associated with MCs in Lake Oubeira, Algeria (Nasri et al., 2008). Similarly, deaths and illnesses in wild and domestic animals in the Ethiopian Rift Valley lake areas were also linked to cyanotoxins (Willén et al., 2011). Moreover, Zilberg (1966) related gastroenteritis, skin rashes, itching, and sore eyes of children to the production of algal toxins from Lake Mcllwaine, Salisbury (now Harare).

Awareness, efforts of control, and possible solutions

Several efforts have been made to lessen eutrophication in South Africa since the 1970s (Van Ginkel, 2010, 2011). As a result, many eutrophication control strategies were put into practice, including biomanipulation, lowering detergent P levels, predicting cyanobacterial blooms, and disrupting epilimnion by laminar flow (Van Ginkel, 2011). Lake McIlwaine (now Lake Chivero) in Zimbabwe is an example of a successful eutrophication recovery. The long-term research by the Hydrobiology Research Unit of the University of Rhodesia (now the University of Zimbabwe) since 1967 was combined with intensive monitoring efforts by the City of Salisbury. These efforts included diverting nutrients to pasture irrigation schemes, extensive publicity and developing water pollution control legislation and control of the water hyacinth. As a result, the lake was changed to mesotrophy by the end of the 1970s from hypereutrophic condition in 1971 (Thornton, 1982).

Many African countries have legislations that set standards for effluent discharges including the Republic of South Africa Water Act; Zimbabwe Water Act; Kenyan Water Act; Malawian Water Resources Act; Zambian Natural Resources Conservation Act and Water Pollution Control Act (Brown et al., 2020; Thornton & Boddington, 1989). For instance, South Africa limits ammonia release to water resources to less than 6 mg L−1, whereas Zimbabwe limits nitrate, ammonia, and phosphorus concentrations to under 10, 0.5, and 1 mg L−1, respectively (McKendrick, 1982; NWA, 1998; Wiechers & Heynike, 1986). The Ethiopian Environmental Protection Authority has set the concentrations of ammonia, nitrate, and dissolved phosphorus to be less than 5, 20, and 5 mg L–1, respectively, before discharge to inland waters (EEPA, 2011; FDRE, 2002). Additionally, numerous measures for soil and water conservation have been put into place, but it is important to regularly examine how they will affect the improvement of water quality (Reij, 1991).

Different approaches are used worldwide to control cyanotoxin concentration below acceptable drinking and recreational water limits. These approaches prevent bloom occurrence and eradicate cyanobacterial blooms (Merel et al., 2013). To prevent cyanobacterial blooms occurrence, monitoring the quantity of nutrients (C, N, and P), sediment dredging, sediment inactivation (usually by using aluminum sulfate), dosing with aluminum sulfate (to bind P and make it limiting for cyanobacterial growth), biomanipulation of the food web (using microorganisms such as viruses, fungi, bacteria, zooplankton, and algae), accurate prediction of cyanobacterial growth cycles, and mechanical disturbance of the epilimnion are essential (Harding & Paxton, 2001; Merel et al., 2013; Van Ginkel, 2011). On the other hand, eradication of occurring cyanobacterial blooms requires applying an algaecide, usually copper sulfate (Hrudey et al., 1999). However, this technique should be applied with caution since it releases intracellular toxins as a result of cell lysis (Jones & Orr, 1994; Peterson et al., 1995) and induce Cu2+ accumulation on the sediment causing copper toxicity that allows cyanobacterial blooms a few weeks later (Merel et al., 2013).

Regular monitoring of nutrient, cyanobacteria and cyanotoxin load is required in African waterbodies. Waterbody management, watershed management, and land-use planning are necessary to reduce nutrient enrichment of water ecosystems (Chorus & Welker, 2021; WHO, 2016). Controlling human exposure is the only remaining option for management if control measures on these levels are absent or fall short of their goals(Chorus & Welker, 2021). Options for addressing this range from educating users—that is, raising knowledge and guiding personal reactions to bloom situations—to temporarily limiting access to aquatic ecosystems during the bloom (Chorus, 2005; Stauffer et al., 2019).

Contrary to freshwaters, the public is generally well aware about algal toxins for the marine environment (Chorus & Welker, 2021). Even though scientific understanding has been expanding in Africa for decades (Harding & Paxton, 2001), the general public does not seem to be sufficiently informed about cyanotoxins (Harding et al., 2009; Mohamed, 2016a). Therefore, public gatherings, radio broadcasts, warning signs near impacted waterbodies and the dispersal of information booklets must be used to distribute pertinent information.

Conclusions

This meta-analysis review documented occurrences of cyanobacteria and cyanotoxins among African waterbodies. It showed how widely cyanotoxins have spread throughout African aquatic ecosystems and the resulting health dangers to people and animals. MCs were reported in at least 95 African water resources. ATX-a, CYN, NOD, and HTX were quantified in few aquatic ecosystems. Microcystis spp. dominated the East and Southern African water systems with cyanotoxins in water samples, while Oscillatoria spp. and Dolichospermum spp. were most frequently found in North and West African waters. Extremely high concentrations of MCs are reported from African aquatic resources and large concentrations of cyanotoxins have been linked to animal mass mortalities. Most of the water resources examined had MCs concentrations higher than the WHO recommendation of drinking water (1 μg L−1). This indicates a high potential risk to public health when exposed to the toxins during recreational, domestic, and daily activities. Therefore, it is crucial to regularly monitor cyanotoxins in fish, treatment centers, and drinking water sources. Watershed management activities and point-source pollution control are essential for reducing the excessive levels of nutrients loading and eutrophication into the African waterbodies. Furthermore, the standards set in relation to nutrients’ concentration for African waterbodies should be revised to adopt the international standards.