Abiotic factors: How can climate change affect water itself?

Climate change can indirectly affect the ecological quality of urban surface water by causing physical or chemical changes in the water itself. For example, higher water temperatures will reduce oxygen concentrations in the water. Climate change can also affect water's abiotic factors: the concentrations of micropollutants, salt or nutrients it contains. All these abiotic factors affect urban water quality.

Would you like to know more about the different properties of the water system and how they can be affected by climate change? You will find more details on the page How vulnerable is a water system to climate change? and in the section on Indicators and Control Variables in the interactive knowledge document Urban Water Quality, Climate and Adaptation.

Hypoxia

As a result of climate change, hypoxia will occur more frequently in surface water. Hypoxia can be caused by heat and by severe precipitation:

• Heat: the oxygen concentration in water largely depends on the water temperature. If the water temperature is too high, the oxygen concentration will decrease. One of the reasons is that the water organisms use more oxygen in warmer water. In a heatwave, the water can even turn anoxic. The latter occurs particularly in shallow bodies of water whose bed is covered with a layer of organic matter, e.g., originating from fallen leaves. As this water will heat up more rapidly, fish and other wildlife species will consume more oxygen. In addition, the decomposition of the organic matter (mineralisation) requires a great deal of oxygen.
• Severe precipitation: torrential rain may affect the oxygen concentration in surface water, particularly during hot summers. And if severe downpours are causing more frequent sewer overflows, the demand for oxygen will increase, thus potentially resulting in anoxia. Fish and other small water creatures cannot survive in anoxic water. Furthermore, it will affect the composition of species.

More micropollutants and toxins

Climate change may result in more micropollutants and toxins ending up in urban surface water. In the event of severe precipitation, more micropollutants will flush into surface water. The combination of warm weather and persistent drought may also have a harmful impact: in former industrial locations, the waterbed may release toxins. Read on the questions below for a more detailed explanation.

How does severe precipitation impact micropollutants?

Particularly after a prolonged period of drought, intense downpours may cause micropollutants to end up in surface water. Such micropollutants could be tyre dust, copper from car brakes, microplastics, or oil residue. Micropollutants are harmful to life in and around the surface water. Hardly any data is available on the concentration of micropollutants in surface water. In most cases, the tests conducted by water management bodies only cover a few micropollutants, as such tests are quite costly.

How does the waterbed release toxins?

Under certain conditions, the soil underneath urban bodies of water in former industrial locations can release toxins. This risk is highest in hot weather and in water that has been stagnant for a prolonged period of time due to persistent drought. Such toxins, when ending up in the water, will seriously harm life in and around the surface water. This risk is going to increase in the future, as we will see increasingly frequent warm and dry periods.

More salt (salinisation)

Climate change may raise the salt concentration (chloride) in surface water. This is referred to as salinisation. Salinisation is mainly increasing in coastal areas. The main reason is that more and more saltwater seepage is flowing into freshwater bodies as a result of the rising sea level and the subsiding soil. Drought and heat can also cause salinisation of surface water. For instance, in dry periods, we occasionally let in “foreign” water from the major rivers, even though such water may have quite a high salt content during summer. In addition, more water will evaporate in warm periods, thus raising the water salt content even further. A persistently high salt content in surface water may have a biological impact on the water.

Too many nutrients due to eutrophication

As a result of climate change, urban surface water contains more phosphorus and nitrogen. This leads to increasing eutrophication: too many nutrients end up in the water. This may foster the growth of harmful blue-green algae, thus compromising the water quality. Climate change entails two processes that raise the phosphorus and nitrogen content. The first process takes place in the surface water itself and is referred to as internal eutrophication. The second process is caused by sources outside the surface water and is known as external eutrophication. Read below for a detailed explanation of internal and external eutrophication.

Internal eutrophication

Internal eutrophication means that the increasing concentration of phosphorus and nitrogen in the water can be attributed to the aquatic system itself. This happens as follows: higher temperatures accelerate the decomposition of organic matter, both in the water and in the sediment at the bottom. The decomposition of organic matter releases phosphorus and nitrogen. Especially in shallow water, the temperature will rise rapidly in warm weather. In such cases, the sediment may release double the volume of phosphorus. The thicker the layer of sediment, the more phosphorus it may release. Furthermore, the top layer of the sediment could even turn anoxic, which would release additional phosphorus into the water.

Stagnant water is conducive to the process

If the water remains stagnant for a prolonged period of time due to drought, the concentration of nitrogen and phosphorus will remain high. Flushing with flowing water may reduce the high nitrogen and phosphorus concentrations. In dry periods, inlet water may produce the same effect.

External eutrophication

External eutrophication means that the phosphorus and nitrogen concentrations in the water are increasing due to external sources. The main cause is changes in precipitation levels: on the one hand, increasing winter precipitation and torrential summer rain, and on the other, increasing summer drought, resulting in stagnant water. In addition, rain itself also contains nitrogen, so rain volumes play a part too.

Increase in nitrogen

The increase in nitrogen is mainly caused by increasing rainwater run-off and by sewer overflows onto surface water. This can be combated by creating a separate wastewater system and by greening: the soil will then absorb more water, which will reduce run-off. In addition, plants will sequester a large proportion of the nitrogen.

Increase in phosphorus

The increase in phosphorus is mainly caused by sewer overflows: sewage water contains a lot of phosphate, a common form of phosphorus. During severe precipitation, green rooftops and trees can also constitute a source of phosphorus. Another cause of the external increase in nutrients is the inlet of foreign water. If such water originates from rural farmland, it usually contains large concentrations of nutrients.