Eutrophication in South Africa

 

Eutrophication is one of the most prevalent threats to freshwater biodiversity. 28% of all African Lakes are impaired by eutrophication (Nyenje et al 2010). This might be considered quite low compared to other continents, e.g. Europe is 53% and Asia has the top spot at 54% (Nyenje et al 2010). However, with the increasing use of fertilizers (that I talked about in my previous blog post) this percentage will be on the rise.

Over the past 50 years eutrophication has become a major threat to South Africa’s freshwater resource. Freshwater is already South Africa’s most limiting resource (Van Ginkel et al 2001). It is an arid country, receiving an average annual rainfall of 450mm/yr, which is half the global average (Harding 2015). In fact the net rainfall (precipitation – evapotranspiration) is negative (NWRS 2004). They have one of the lowest available freshwater per person rates, at only 1131m³ per annum (Revenga & Cassar 2002). The 2018 droughts have brought about global awareness of the real threat of water shortage in South Africa. The fear of a ‘day zero’ scenario, where water taps would be turned off and replaced by strict rationing, has been loaming over South Africa in recent years.

Reservoirs are at the heart of South Africa’s socio-economic wellbeing. Together with rivers they produce 10 000Mm³ of their water resource, a much greater importance currently than groundwater which only supplies 2000Mm³ (Harding 2015). There are 580 major reservoirs in South Africa, but up to an estimated 76% of their storage is at a Eutrophic or hypertrophic status (Harding 2015).

The issue of Eutrophication started being recognised in the 1970s, and a 1 mg/L phosphorus (P) standard for all water treatment was established by the Department of Water Affairs and Forestry (Van Ginkel 2011) . This restriction has been criticized by many for being to high (Van Ginkel 2011, Coetzee & Hill 2012, Harding 2015). Anthropogenic eutrophication has come from many point sources, including industrial discharge and wastewater treatment works, and also non-point sources, the largest of which is agricultural runoff which this post focuses on. A study by Mudaly and Van der Laan (2020) looked at the interaction of agriculture and surface water in the Middle Olifants catchment in North Eastern South Africa. They measured phosphorus and nitrogen levels from the Loskop and Flag Boshielo Dams as well as the irrigation canals that make up the second largest irrigation scheme in South Africa (figure 2) (Mudaly and Van der Laan 2020).They discovered that nutrient levels were above the eutrophication threshold for the majority of the sampling period. Rising nutrient levels in the summer rainfall months pointed towards an agricultural source for at least part of the nutrient loading.

Figure 2. Google maps view over irrigated intensive farming in the Middle Olifants catchment

Cyanobacterial blooms are one of the major consequences of the nutrient loading (Van Ginkel 2011). When there cells die they release cyanotoxins into the water, which, when ingested in high concentrations can pose as a risk to human and animal life (Van Ginkel 2011). This has proven to be especially dangerous in areas where water purification is minimal or not fully functional (Van Ginkel 2011).  As of yet the concentrations haven’t reached higher enough levels for human death, although in Kruger National Park several mammals died in 2005 and 2007 (Oberholster 2009).

The Rietvlei Dam, which has seen large blooms Microystis (a type of Cyanobacteria), have been attempting to use a piece of tech called the ‘Solarbee’ to stop these bacterial blooms (figure 3) (Van Ginkel 2011). The solarbee produces laminar flow that disturbs the epilimnion, this creates unfavourable conditions for cyanobacterial growth (Hart and Hart 2006). Coetzee, deputy director of Scientific Services for South Africa at the time, said “The water quality in the dam has improved tremendously and the occurrence of cyanobacterial blooms has almost disappeared.” The success of the management scheme is debatable. Certainly it has improved the water quality, but it hasn’t attempted to address the heart of the issue, namely the input of nutrients (Van Ginkel 2011).

Figure 3. diagram of how the Solarbee works. (Blue - green algae is a term for cyanobacteria)

Biomanipulation is another technique that has been used in South Africa to tackle eutrophication. On the hypertrophic Hartbeesport dam, just north of Johannesburg,  they implemented a food web manipulation programme to reduce cyanobacterial blooms (Venter et al 2010). Wherein they restricted certain undesirable fish species: catfish, carp and canary cooper. The theory being that these fish feed on the zooplankton that in turn graze the cyanobacteria, therefore removing fish will increase the zooplankton and the result will reduce the cyanobacteria population (Venter et al 2010). This is a top -down biomanipulation technique. A study of Hartbeesport dam biomanipulation using medium resolution imaging spectrometer satellite data, by Hart and Matthews (2018), compared cyanobacteria between 2002 and 2012, alongside 6 control lakes. They observed some reduction in blooms post 2008, however similar reductions were seen in control lakes, so no definite link could be claimed (Hart and Matthews 2018). Furthermore, stable isotope analysis in the reservoir showed that although phytoplankton (cyanobacteria) were significantly controlled by zooplankton, zooplankton were not significantly consumed by fish species (Harding and Hart 2013). They concluded that this method of top-down biomanipulation was unsuccessful and that what is needed is reduction in nutrients entering the system.

Eutrophication as a result of agricultural fertilizers is a major concern for the water quality of South Africa’s limited water supply. Although we have seen much effort to limit the effects of nutrient loading, more is going to be needed to reduce the supply. Water is precious and so are the ecosystems that are essential for all life, they both need to be conserved.