ReviewA review of the socioecological causes and consequences of cyanobacterial blooms in Lake Victoria
Introduction
Numerous studies have been conducted in the past 20 years with the goal of improving our knowledge of the causes and consequences of cyanobacterial blooms. Taranu et al. (2015) have shown that the increasing occurrence of cyanobacterial blooms is clearly associated with the increasing impact of human activities on freshwater ecosystems during the Anthropocene, and it is well established that the main cause of cyanobacterial blooms is the nutrient enrichment of phosphorus (P) and nitrogen (N) (O'Neil et al., 2012; Huisman et al., 2018). Recently, several papers also suggested that climate change might directly or indirectly promote cyanobacterial blooms (e.g., Moss et al., 2011; Paerl et al., 2016; Ho et al., 2019). Blooms are also well known to have multiple impacts on the ecological functioning of freshwater ecosystems (e.g., Paerl et al., 2016; Huisman et al., 2018; Escalas et al., 2019) and on the goods and services (G&S) they provide (e.g., Dodds et al., 2009). Finally, many papers deal with the production of harmful toxins by cyanobacteria and the sanitary risks associated with them (e.g., Briand et al., 2003; Merel et al., 2013; Meriluoto et al., 2017).
Among these previous studies, few papers have focused on developing countries, with the exception of a few countries such as Brazil and China (Merel et al., 2013). In particular, the issue of cyanobacterial blooms has been poorly investigated on the African continent, as illustrated recently by Svircev et al. (2019). When looking in this review at the distribution of cyanotoxins worldwide, it appears that data on cyanotoxins are available only for 14 of the 54 African countries and for 76 ecosystems on the continent. Among these ecosystems, Lake Victoria (LV) is the most studied, probably because it is the second largest lake in the world and provides goods and services (G&S) to millions of people (Downing et al., 2014; El-Noshokaty, 2017). Moreover, LV has experienced rapid water quality degradation that has led to eutrophication, which is considered a major threat to the ecological function of the lake (Hecky et al., 1994; Juma et al., 2014) as it results in the recurrent proliferations of aquatic weeds (e.g., water hyacinth) and cyanobacteria (Lung'ayia et al., 2000; Opande et al., 2004; Juma et al., 2014). The proliferation of water hyacinth has been more or less controlled since the end of the 1990s with biological, mechanical or physical strategies (Opande et al., 2004; Wanda et al., 2015), but cyanobacteria blooms persist in LV, particularly in the bays and gulfs (B&Gs) (Haande et al., 2011; Sitoki et al., 2012; Mbonde et al., 2015).
The LV basin has one of the highest population densities in Africa, with several large cities located along the shores of the large bays and gulfs (e.g., Kampala in Uganda, Kisumu in Kenya, and Mwanza in Tanzania) (Fig. 1). An understanding of the interplay of ecological and socioeconomic processes acting directly or indirectly on the lake is of primary interest. In this context, we address the state of knowledge on (i) the distribution of cyanobacterial blooms and cyanotoxins in LV; (ii) the social and environmental factors and processes potentially explaining the occurrence of cyanobacterial blooms, with a particular emphasis on changes that have occurred in its watershed during the last 50 years; (iii) the consequences of these blooms on the ecosystem G&S provided by the lake and (iv) the management practices implemented with the goal of reducing nutrient loads and consequently the cyanobacterial blooms.
Section snippets
Study site: the Lake Victoria basin
The LV catchment area has a surface area of 184,200 km2 and is shared between five countries (Burundi, Kenya, Rwanda, Tanzania and Uganda). As shown in Fig. 1, the catchment is dominated by cropland and grassland, with major build-up areas (for example Kampala, Kisumu, Musoma and Mwanza) located on the shorelines of the B&Gs. LV is located 1,100 meters above sea level in East Africa and is the source of the White Nile. The lake is shared between the three countries of Kenya (6%), Tanzania (51%)
Historical and current status of cyanobacterial blooms
Recent evolution of the LV phytoplankton community. As shown from paleolimnological data by Verschuren et al. (2002), the phytoplankton production in LV has increased since the 1930s. Though there is insufficient nutrient data for this time period, it appears that the phosphorus concentrations increased (from 1.1 to 2.9 µmoles L−1 between the 1960s and 1990s) and that eutrophication manifested towards the end of the 1980s (Hecky, 1993; Hecky et al., 2010). The increasing nutrient content led to
Linking demographic changes with the increasing occurrence of cyanobacterial blooms
From the analysis of the data available in the literature on the causes of cyanobacterial blooms in LV, we built a flow diagram (Fig. 3) describing the multiple ways in which the human population growth in the LV watershed is connected to the increasing occurrence of cyanobacterial blooms in the OL and B&Gs.
As summarized in Fig. 3, the increase in the populations density in the LV basin, such as everywhere in the world, has resulted in increasing food, housing and product demands (FAO, 2016).
Consequences on food resources and fishing
Fishing is an important food resource for approximately 1.5 million people living on the lake shores as well as for the 42 million inhabitants in the watershed of LV. In this context, cyanobacterial blooms have two main impacts on the fish communities: (i) the changes in the environmental conditions of the fish due to the blooms, with potential negative impacts on the abundance and diversity of the LV fish community, and (ii) the accumulation of cyanotoxins in the fish and the associated risks
Efforts to reduce eutrophication and the occurrence/intensity of cyanobacterial blooms
Knowing that it is well established that the eutrophication of freshwater ecosystems promotes cyanobacterial blooms, the reduction of external nutrient inputs in the lakes is the main sustainable strategy to reduce cyanobacterial blooms (e.g. Huisman et al., 2018). LV is a very complex socioecological ecosystem, and there is limited documented knowledge (and understanding) regarding the social and ecological dynamics and interactions involving LV. However, based on existing knowledge, we
Conclusion
Due to the demographic growth and the rising concentration of the population into urban areas in Africa (United Nation, 2018), we may fear that in the next decades, eutrophication of freshwater ecosystems and associated cyanobacterial blooms will continue to increase in LV and also in many other African lakes. Knowing that all these lakes provide numerous ecosystem G&S and are vital for the water and food supply, the high microcystin concentrations recorded in water and fishes of LV and the
Acknowledgment
This review paper has been prepared in the framework of the WaSAf program (Sustainable Monitoring and Management of Surface Water Sources in Africa), which is funded by the French Facility for Global Environment (Fonds Français pour l'Environnement Mondial, FFEM).
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