The UK’s freshwater bodies - lakes, rivers and streams - are an essential part of our lives, from drinking water and crop irrigation to sailing. But there is more to freshwater than H2O - its chemical nature varies with both geography and human activities. The diversity and abundance of organisms found in lakes and rivers greatly depends on the water conditions. Chemical water testing is an essential part of assessing water quality for humans and wildlife alike.
The geology of an area can have an effect on the chemical content of freshwaters. Igneous rocks are hard and relatively impervious and so contribute little to the ionic content of water. In contrast sedimentary rocks are softer and permeable, contributing ions such as calcium. Rain is naturally acidic, so as it falls on igneous uplands and forms streams they tend to remain acidic. As the stream runs to lower sedimentary land and becomes a river, the pH generally increases with the increase in dissolved minerals. The pH of a waterbody influences the wildlife found within it - invertebrates are particularly sensitive. For example the diversity of crustaceans and molluscs tends to increase with higher pH, as they require certain amounts of calcium for their shells and exoskeletons. There are a variety of ways to measure the pH of water. A simple cost-effective method is to use test strips which give a fast indication of acidity but not an accurate measure. Portable digital meters are more costly but will also give a fast measure, to a much higher accuracy than a paper strip.
Oxygen is as essential to aquatic life as terrestrial organisms but water holds only about a third of what air contains. Some aquatic life can return to the surface to utilise aerial oxygen whilst others rely on dissolved oxygen in the water itself. The oxygen content of water varies with temperature - it can hold more the cooler the temperature. The organisms in the water can themselves influence oxygen levels. Aquatic plants produce oxygen during photosynthesis whilst animals, decomposing bacteria and so on use up oxygen whilst respiring. Oxygen can be measured as an absolute concentration or % saturation. % saturation is a measure of the proportion of oxygen the water should contain at that temperature. This measure is often favoured by ecologists as it can indicate biological factors influencing water conditions (eg. high rates of decomposition leading to low oxygen saturation). Oxygen can be measured using digital meters; these can be specific meters or in combination with other water condition parameters such as temperature and pH.
Measuring conductivity gives a crude indication of the ionic content of water. The greater the ionic content the greater the conductivity. Again this is typically measured using an electronic meter. Measurement of specific ions such as calcium or potential toxins like copper can also be done using meters. Whilst they are more expensive, they give a highly accurate measures of almost any substance of interest to freshwater ecologists.
Although testing the chemical conditions of water is an essential tool for aquatic biologists and ecologists, it will not always give you the complete picture of a water body’s health. Testing a water sample only gives a snapshot of conditions at that time. For example temperature and oxygen levels will undoubtedly vary throughout the year. A pollution event such as fertiliser spillage in a river can occur, and nutrient levels can appear to return to normal within hours. Yet it will still have had major impacts on the aquatic organisms - all of which can be missed by taking a water reading at the wrong time. While it is still a powerful tool, chemical water testing should be used in conjunction with other methods. Organisation involved in assessing the UK’s waterways quality such as the Environment Agency typically use a combination of water testing and monitoring the abundance and diversity of aquatic organisms (particularly invertebrates) to give an indication of a water body’s health.
