The impact of alien invasive plant species on water quality

Article cover photo by Wouter Haagan (Public Domain).

Aquatic alien invasive species is a worldwide problem. They have a negative effect on native aquatic plants and can also cause big problems for animal life and of course affect the quality of water. Within South Africa alone there are hundreds of invasive plant species in and near water sources. We will concentrate on only one of them and its effect on water quality and the efficient functioning of the nutrient and hydrological (water) cycles.

These cycles are defined as follows:

Nutrient cycle

The nutrient cycle describes how nutrients move from the physical environment into living organisms, and subsequently are recycled back to the physical environment. This movement of nutrients, essential for life, from the environment into plants and animals and back again, is a vital function of the ecology of any region. In any particular environment, the nutrient cycle must be balanced and stable if the organisms that live in that environment are to flourish and be maintained in a constant population (Martin 2010). Currently, large parts of mankind influence the nutrient cycle in such a way that we remove nutrients from the land and discharge them into aquatic environments. On the one hand, this leads to soil depletion on the land, and on the other hand, an overabundance of nutrients and pollution of water sources.

Nutrients are chemical elements that all plants and animals require for growth. On Earth, there is a constant and natural cycle how these elements are incorporated when an organism grows, and degraded if an organism dies. The nutrients used in the largest amounts are the non-mineral elements, i.e. carbon (C), hydrogen (H) and oxygen (O). These elements are mainly taken up by plants as carbon dioxide (CO2) from the air, and water (H2O) by the roots (Joensson et al. 2004). They make up 95-98% of the mass of all living beings (Mahendrappa 2007). But they are, however, not sufficient for life to exist. Further elements are necessary to fuel life on earth: Nitrogen (N) and Phosphorus (P), Potassium (K) as well as Calcium (Ca) and Magnesium (Mg) are highly important, in particular for plant growth and agriculture. These elements are often referred to as macro nutrients. Their uptake is about 100 times that of micro nutrients. Micro nutrients, that plants take up in a much smaller amount and that are essentially consumed by humans, include Boron (Bh), Copper (Cu), Iron (Fe), Chloride (Cl), Manganese (Mn), Molybdenum (Mo), Zinc (Zn) and others.

These nutrients, essentially chemical elements, are continuously in a circular movement. The nutrient cycle is hence a general term that describes how nutrients move from the physical environment into living organisms, and are subsequently recycled back to the physical environment (Martin 2010). Nutrients in the soil are taken up by plants, which are consumed by humans or animals, and excreted again by them, or they are released back into the environment when organisms die (e.g. plants lose their leaves). Microorganisms in the soil break this matter down, and again make nutrients available in their mineral form, which makes it possible for plants to take them up again. Essentially, all nutrients that plants and humans require to survive are cycled in this way. In relation to water management and sanitation, it is mainly N, P and K that are of high priority. They are the most important nutrients to sustain plant growth and agriculture, and thus humanity.

Human-induced alterations in the nutrient cycles lead to an imbalance in the availability of nutrients, whose consequences, in particular with regard to water, are grave. Fertiliser runoff and wastewater discharge contribute to uncontrolled blooms of algae in rivers, lakes and oceans, feeding on nitrogen and phosphorus from fertilisers. When they die, their decomposition depletes the water of oxygen and slowly chokes aquatic life, producing “dead zones.” The largest dead zone in American waters, topping 20,000 square kilometres in July 2008, is off the Mississippi delta. More than 400 dead zones now exist worldwide, covering a combined area of more than 245,000 square kilometres (Castelvecchi 2009).

Hydrological cycle

The amount of water on earth has been constant over centuries. It is in permanent circulation and regeneration and is therefore a renewable resource. However, most of the water on earth is salty (97%) and thus not suitable for the majority of uses. Only roughly 2.5% is freshwater in rivers, lakes, groundwater, fixed in soil or frozen in icecaps and a mere 0.5% of the total is easily accessible for human use (Inforesources Focus 2006). This liquid water travels the earth constantly in the water cycle. Humans largely influence the water cycle today, be it quantitatively, by using large parts of the water available, or be it qualitatively, by changing the quality of water (e.g. pollution). On the Earth, water never occurs in pure form, it always contains dissolved substances, be it minerals, nutrients, and also pollutants. The water cycle is thus inherently linked to the nutrient cycle. The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate. The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet. For both the nutrient and hydrological cycles to occur neither one of them may be interrupted.

Let us take Eichhornia crassipes, better known as common water hyacinth, for example. It is an aquatic plant native to the Amazon basin, and is often considered a highly problematic invasive species outside its native range. Outside of the Amazon, water hyacinth is a aquatic weed that can be found in lakes, rivers and ponds or areas filled with freshwater. It is a perennial aquatic plant and lives for more than 2 years.

These plant species are known as a free-floating weed but may be anchored in more shallow water. The invasive status which indicates to what extend the level of invasion is “transformer”. Transformer means that it can, as a monospecies, dominate or replace a natural or semi-natural ecosystem, altering its structure, integrity and functioning.

One of the fastest growing plants known, water hyacinth reproduces primarily by way of runners or stolons, which eventually form daughter plants. Each plant can produce thousands of seeds each year, and these seeds can remain viable for more than 28 years (Sullivan & Wood 2012). Some water hyacinths were found to grow up to 2 to 5 metres a day in some sites in Southeast Asia (Gopal 1987). The common water hyacinth are vigorous growers known to double their population in two weeks.

When not controlled, water hyacinth will cover lakes and ponds entirely; this dramatically impacts water flow, blocks sunlight from reaching native aquatic plants, and starves the water of oxygen, often killing fish and other aquatic life.

There's a number of factors that ensures that the quality of water is sufficient. Water flow can also be influenced due to the formation of dense mats by water hyacinths. These dense mats increases silt build-up in water bodies and the diffusion of oxygen is decreased resulting in lower concentrations of dissolved oxygen.

In conclusion the quality of water decreases because of lower oxygen concentrations combined with the increased amounts of organic detritus that collect beneath these floating mats, increases sediment accumulation rates and accelerates hypertrophication* processes that is harmful and eventually lethal to fish and other aquatic life, causes algal blooms and renders water toxic and uninhabitable for indigenous animal and plant species.

*Hypertrophication is the ecosystem's response to the addition of artificial or natural substances, mainly phosphates, through detergents, fertilisers, or sewage, to an aquatic system. One example is the "bloom" or great increase of phytoplankton in a water body as a response to increased levels of nutrients. Negative environmental effects include hypoxia, the depletion of oxygen in the water, which may cause death to aquatic animals.