Water pollution, the accidental or intentional dumping of pesticides; herbicides; oil products; fertilizers; sewage; industrial waste; detergents; and other foreign chemicals and objects into aquatic environments is arguably the biggest current pollution concern.
The dumping of products containing heavy metals into aquatic environments is particularly concerning because heavy metals are toxic even in small concentrations, and likely to biomagnify. When aquatic organisms process contaminated water, they absorb or ingest the heavy metals along with other essential nutrients. With each additional step along the food chain, organisms ingest and accumulate increasingly higher concentrations of these toxic elements.
In this way, even small amounts of heavy metals can become lethal across several levels of the food web over time. Biomagnification is especially a concern with long-lived predatory marine fishes that people consumed as food, such as swordfish (Xiphias gladius, LC), marlins, sharks, and some tunas and sea basses.
For example, mercury (emitted mainly during fossil fuel use), lead, and arsenic have bioaccumulated so much in sharks off South Africa that many species are now considered unsafe for human consumption.
Oil pollution involves the release of petroleum products into the environment, which can originate from damaged ships, failed drilling rigs, leaking offshore platforms, or other unexpected events. The released oil causes mammals and birds to lose the insulating abilities of their fur and feathers, leaving those animals vulnerable to hypothermia and drowning.
Other aquatic animals, including fish and shellfish, may ingest oil products, causing them to sicken and die. Because of the way oil is extracted and transported, marine ecosystems are particularly at risk. Furthermore, because of
the massive amount of oil that is involved in oil extraction and transport, an oil pollution event often represents a serious ecological disaster.
Africa has been hit hard by oil spills in recent years, particularly around oil-producing countries like Angola and Nigeria, and along shipping lanes passing along the coasts of Namibia, South Africa, and Mozambique. Nigeria is perhaps the biggest victim of oil spills; between 1976 and 2001, there were an estimated 6,817 oil spills around Africa’s largest wetland, the Niger Delta!
These oil spills have destroyed thousands of hectares of mangrove swamps, estuarine wetlands, and other coastal ecosystems, causing severe hardship to marginalized local communities who depended on those areas for subsistence fishing and farming.
Because of biomagnification, many long-lived predatory marine fishes are now considered unsafe for human
consumption. Plastic pollution is fast becoming a ubiquitous threat to Africa’s environment, its wildlife, and its people. To visualize the magnitude of the problem, consider that there are more than 1.6 trillion pieces of plastic, collectively weighing over 70,000 tonnes, currently floating in the Atlantic and Indian Oceans surrounding Africa.
While many of these plastic items were dumped directly in the ocean, many also have a terrestrial origin. For
example, if someone throws a plastic wrapper on a sidewalk, there is a good chance that the wrapper will find its way into a nearby stream at some point, carried by wind or rain runoff. From here, the wrapper will float along various streams and rivers until it reaches the ocean.
A recent review in Africa found that 88–95% of plastics floating into the world’s oceans originated from just 10 rivers, which include West Africa’s Niger River and East Africa’s Nile River. In the process, thousands of seabirds, dolphins, whales, turtles, seals, and fish die each year from suffocation or starvation after ingesting plastics and other pieces of trash that they confused with food.
This plastic pollution also impacts humans: researchers recently found microfibers (many of which are plastic) in
over 80% of tap water samples from Uganda, as well as food-grade commercial sea salt originating from South Africa.
Some of the biggest impacts from plastic pollution are caused not by visible scraps of plastic, but by microplastics, the collective name for plastic particles smaller than 1 mm (some are microscopic). Microplastics may originate from the breakdown of larger pieces of plastic and polystyrene products, or they may be manufactured intentionally small, such as beads added to cosmetics and other personal care products that are flushed down drains after use.
Because microplastics are so small, they easily pass through the standard filters used at sewage treatment
plants. Consequently, microplastics generally end up in the aquatic environment, where they are unintentionally consumed by crustaceans (crabs, lobsters, and krill), molluscs (mussels, oysters, and clams), echinoderms (sea stars, sea urchins, sea cucumbers), and small fish.
This consumption can block or damage the victim’s digestive and respiratory systems, cause reduced food uptake by creating a false sense of satiation, or even poison animals through leaching of synthetic chemicals. Each of these threats increases death rates and lowers reproductive rates.
Just as with the biomagnification we discussed earlier, the consumption of microplastics also affects other consumers (including humans), because the small organisms that ingest the microplastics are often food for other animals, allowing plastic pollution to move through an entire food chain.
For example, a recent study from Lake Victoria found microplastics imbedded in the digestive tracts of perch
and tilapia bought at a local market and meant for human consumption.
Because microplastics are so hard to remove once in an ecosystem, the best method for their containment may be to reduce plastic use, to ban products containing microplastics, or develop microplastics that are biodegradable within a reasonable timeframe. But for this to happen, there is a need to educate the public and lawmakers about the dangers posed by this threat to the environment and local economies.
Nutrient pollution represents another growing threat to Africa’s aquatic environments. Many lakes, streams, and other freshwater and marine environments naturally contain low concentrations of essential nutrients, such as nitrates and phosphates. In order to survive, the species living in these nutrient-poor waters must then be adapted to this natural nutrient scarcity.
However, raw sewage, agricultural fertilizers, concentrated animal feeding operations, and industrial processes release large amounts of additional nitrates and phosphates into the environment, which are washed into the aquatic environment. Minor additions of essential nutrients stimulate plant growth, providing more food for organisms at higher trophic levels.
However, at high concentrations, the system becomes subject to nutrient pollution. One of the worst outcomes of nutrient pollution is eutrophication. During eutrophication, surface algae grow so rapidly (known as an algae bloom) that it starts blocking sunlight from reaching aquatic organisms below the surface.
Because each individual alga is short-lived, their rapid growth also adds large amounts of decaying matter to the environment. In response, decomposers that feed on the dead algae can become so abundant that they consume most of the water’s dissolved oxygen.
Without oxygen and sunlight, aquatic plant and animal life may die off in large numbers. The resultant dead zones are sometimes visibly in the form of fish kills, with large numbers of dead fish floating on the surface of the affected water body. The organisms that die during this process are generally toxic to humans because of bacteria build-up and other imbalances.
Eutrophication is an increasingly common problem in Africa; for example, a recent review found that 41–76% of South Africa’s lakes may be eutrophic. Eutrophication has already negatively impacted Africa’s tourism and fisheries sectors and even led to a temporary shutdown of water supplies on the Kenyan side of Lake Victoria.
Preventing further eutrophication should thus be a high priority—not only will it prevent harmful algae blooms but may even play an important role in controlling invasive aquatic plants such as the water hyacinth (Eichhornia crassipes).
Groundwater pollution—the release of pollutants into aquifers and other sources of groundwater—is also becoming a serious issue. This type of pollution generally originates from landfills, on-site sanitation systems, leaking sewage systems, mining leachate, agriculture runoff (fertilizer, pesticides, animal waste, etc.), and other types of waste dumping.
The pollutants may sometimes be released directly into aquifers; however, more often the contaminants and pathogens leak into the soil, from where it seeps into groundwater.
One of the most important emerging threats to groundwater is hydrological fracturing or fracking, in short. During this process, pressurized liquids that contain suspended particles and thickening agents are blasted into rock formations deep underground to break them open. When the pressure and liquids are removed, the suspended
particles keep the fractures open, which enables the extraction of natural gas and petroleum.
While fracking was initially hailed as a method to access previously inaccessible fossil fuels, scientists subsequently found that it poses a wide variety of very serious environmental and health risks. Most importantly, the liquids used in fracking contain toxic chemicals which pose a high risk for groundwater pollution, which in turn lead to miscarriages and birth defects, cancer, as well as skin and respiratory diseases.
In addition, fracking increases greenhouse gas emissions and induces infrastructure-damaging earthquakes. Because of these myriad serious risks, several national governments in Europe, and several local governments in the USA, UK, Canada, and Australia have banned the practice from their lands.