Chemical stressors

Chemical stressors

Chemistry stressors are caused by alterations to water body or catchment chemistry, often prompted directly by human pressures.

Examples of pressures causing chemical stressors include:

  • nutrient enrichment as the result of point and/or diffuse pollution of fertilisers (particularly nitrogen and phosphorous) and slurry from agriculture,
  • nutrient enrichment from the waste products from aquaculture,
  • nutrient enrichment from wastewater from urban developments
  • pollution of toxins, microplastics, detergents, medicines, metals, pesticides, hydrocarbons and other chemical compounds from industry and urban developments through diffuse and point sources,
  • the atmospheric deposition of nitrogen compounds, pesticides, metals and other pollutants emitted into the air from industrial processes transferred to water bodies through rainfall and runoff.
  • the diffusion of metals, sulphide minerals, dissolved solids and salts from mines (both active and abandoned) into surface and ground waters.

Effects on ecological status of aquatic ecosystems

Chemical stressors can cause a number of impacts on the health and status of aquatic ecosystems and the ecosystem services they provide.

Eutrophication and algal blooms – may be the result of nutrient enrichment, resulting in lowered water quality, the growth of toxic cyanobacteria, increased turbidity, decreased dissolved oxygen and radiation inputs in the water column. Stressor effects may be detected through receptors such as decrease in bioindicator diversity or growth (e.g. of macroinvertebrates or macrophytes) or in decreases in ecosystem services provision (e.g. drinking water quality; recreation value for swimming).

Effects of organic and toxic pollutants – organic chemicals, trace and heavy metals (particularly pesticides, polycyclic aromatic hydrocarbons, and brominated flame retardants) can have acute lethal and chronic long-term effects on sensitive fish, invertebrate and algae species. There are a vast number of organic chemicals in circulation, with many new and emerging compounds created each year. Organic chemicals may thus occur in complex multiple stress combinations which place acute stress on the life functions – metabolism, reproduction, mobility and feeding – of aquatic organisms (Posthuma & de Zwart 2006, Malaj et al. 2014, De Castro-Català et al. 2015, Schäfer et al. 2016).

Acidification – acid rain was a key environmental issue in Europe in the 1970s and 1980s, where nitrogen oxides and sulphur dioxides from industrial emissions reduced the pH of rainwater across Europe, causing water bodies to become acidified, particularly in weakly buffered water bodies in Scandinavia and Wales. Many freshwater organisms cannot live in low pH conditions which place stress on their life functions, and as such acidification generally causes a reduction in biodiversity. It is predicted that increased CO2 concentrations in the atmosphere under future climate change pose an emerging acidification threat to water bodies.

Salinisation – freshwater organisms can only tolerate certain ranges of water salinity. Salinisation of water bodies – due to dissolved inorganic ions from irrigation, mining activity or urban run-off, salt intrusion due to sea-level rise,  – can thus reduce aquatic biodiversity and compromise the provision of ecosystem services.

Common combinations as multiple stressors

Nutrient stressors are the most common chemical stressors, and are involved in the three most frequent multiple stressor pairings: with hydrology, morphology and toxins (itself another chemical stressor). Hydrological and morphological stresses (alterations to water flows and water body structure, respectively) can alter  both nutrient concentrations (i.e. through dilution) and the amount of time nutrients are present in an ecosystem, the so-called ‘residence time’ (i.e. through water flows). Hydrology-nutrient pairings are particularly common in rivers and transitional waters. Nutrient and toxin pollutants are likely to often be co-emitted diffusely through urban and industrial wastewater. A fourth frequent stressor pairing is nutrient and thermal stress; largely because the growth of algal blooms and eutrophication is catalysed by warmer water temperatures (Nõges et al 2015).


Some chemicals in freshwater ecosystems have the potential to alter the hormonal balance and function of aquatic organisms. Such ‘endocrine disrupting chemicals’ (or EDCs) which include ibuprofen (a common painkiller), progesterone (used in contraceptive pills), and numerous other steroids, pharmaceuticals and organic compounds – can significantly affect how aquatic organisms live, feed and reproduce, and potentially cause stress on their populations (Freshwaterblog 2017a).

Pesticides from farming are responsible for the majority of acute chemical risks to freshwater life. The impact of chemical pollution on freshwaters is significantly increased close to agricultural land, sewage treatment works and urban areas where there is run-off of pollutants into rivers. An assessment of 4,000 monitroing sites across Europe indicated that pesticides from agricultural run-off posed the most acute chemical risk to freshwater life.  However, other chemicals were found to occur at potentially damaging concentrations, including the banned biocide tributyltin (an antifouling agent that is leached from ship’s hulls), brominated diphenyl ethers (which is used as a flame retardant in consumer goods) and polycyclic aromatic hydrocarbons (which are released from fossil fuels) (Freshwaterblog 2014).

Investigations on the River Mulde from the Elbe catchment provide evidence on possible drivers of mutagenicity  - where chemicals interact with our genes, resulting in harmful mutations, potentially causing cancer and damaging our offspring – and of its effects on wild populations of freshwater shrimps (Gammarus pulex). In short, chemical pollution on the river with compounds that probably stem from dye production,  is causing mutations in aquatic organisms, causing significant stress to the ecosystem health and status (Freshwaterblog 2017b).

The breakdown of organic pollutants such as sewage and farm run-off uses oxygen, meaning that polluted waterways often suffer severe drops in dissolved oxygen levels. Polluted rivers with low oxygen levels are more susceptible to the harmful effects of climate change. Studies show that two common mayfly species are less able to tolerate rising water temperatures in polluted rivers with low oxygen levels (Freshwaterblog 2016a).

Combined future effects of climate warming and nutrient enrichment are likely to significantly alter the diversity and composition of bacterioplankton communities in shallow lakes. Bacterioplankton play a number of important roles in aquatic ecosystems, particularly the decomposition of organic matter and nitrogen fixation (Freshwaterblog 2016b).

Changes in water levels - whether caused by low rainfall, human water abstraction, or a mixture of both - and salinity have significant effects on the lake ecosystems, nutrient dynamics, nutrient concentrations and water quality. Increased lake salinity often “markedly alter the community composition of phytoplankton, zooplankton, macrophytes and fish and often lead to a decrease in the biomass and diversity of each of these organism groups” (Freshwaterblog 2015).

The populations of both land-locked and migratory Atlantic salmon in the Norwegian river Otra were severely impacted by acidification due to acid rain and industrial and municipal pollution in the latter half of the 20th century (Freshwaterblog 2016c).

(Freshwaterblog 2016c)

Further reading

MARS publications:

Nõges P., Argillier C., Borja Á., Garmendia J.M., Hanganu J., Kodeš V., Pletterbauer F., Sagouis A. & S. Birk (2016). Quantified biotic and abiotic responses to multiple stress in freshwater, marine and ground waters. Science of the Total Environment, 540: 43-52. DOI: 10.1016/j.scitotenv.2015.06.045 (Read abstract)

Reports and publications:

Cañedo-Argüelles M., Kefford B.J., Piscart C., Prat N., Schäfer R.B. & C.J. Schulz (2013). Salinisation of rivers: An urgent ecological issue, Environmental Pollution, 173: pp 157-167. (Read abstract)

De Castro-Català N., Muñoz I., Armendáriz L., Campos B., Barceló D., López-Doval J., Pérez S., Petrovic M., Picó Y. & J.L.Riera (2015). Invertebrate community responses to emerging water pollutants in Iberian river basins, Sci. Total Environ., 503-504 (2015), pp. 142-150 (Read abstract)

Malaj E., von der Ohe P.C., Grote M., Kühne R., Mondy C.P., Usseglio-Polatera P., Brack W. & R.B. Schäfer (2014). Organic chemicals jeopardize the health of freshwater ecosystems on the continental scale. Proc. Natl. Acad. Sci., 111: 9549–9554. (Read abstract)

Posthuma, L. & D. de Zwart (2006). Predicted effects of toxicant mixtures are confirmed by changes in fish species assemblages in Ohio, USA, rivers. Environmental Toxicology and Chemistry, 25: 1094–1105. doi:10.1897/05-305R.1 (Read abstract)

Schäfer R.B., Kühn B., Malaj E., König A.& R. Gergs (2016). Contribution of organic toxicants to multiple stress in river ecosystems. Freshw Biol, 61: 2116–2128. doi:10.1111/fwb.12811 (Read abstract)

Selected Freshwaterblogs:

Freshwaterblog (2017a). Broader scale research needed for emerging threat of endocrine disrupting chemicals (External website)

Freshwaterblog (2017b). Mutagenicity in surface waters: new insights into an old problem (External website)

Freshwaterblog (2016a). Water pollution makes river biodiversity more vulnerable to climate warming (External website)

Freshwaterblog (2016b). Climate warming and nutrient pollution may interact to alter future shallow lake ecosystems (External website)

Freshwaterblog (2016c). Multiple stressors in Science of the Total Environment (External website)

Freshwaterblog (2015). Low water and high salinity: the effects of climate change and water abstraction on lake ecosystems (External website)

Freshwaterblog (2014). Chemical pollution threatens Europe’s freshwaters (External website)