Why is it important?
Next to water temperature, salinity and transparency (see Water transparency), water colour observations belong to the oldest time series of climate data. Water colour is an Essential Climate Variable (ECV) designated by the World Meteorological Organization (WMO) for which sustained and climate quality measurements are needed to track and analyze climate change. This is nowadays considered an important aspect of the science of natural-water-optics, since the colour of the ocean is determined, partly, by phytoplankton. A variation in phytoplankton abundance implies a change in the uptake of CO2, the primary greenhouse warming gas, suggesting a possible role of these organisms in the regulation of climate.
Coastal waters, rivers and lakes can vary in colour due to natural events, such as algal growth, during spring that can make the water greener. However, the colour can also be affected by human activities, for example, due to an addition of nutrients, such as phosphates and nitrates (through sewage or fertilizers), that cause phytoplankton to grow. This phenomenon is known as eutrophication. A change in water colour not only affects the aesthetics and the recreational value of a water body (generally people prefer to swim in blue-green clear waters than dark and murky waters), but could also have a harmful effect on the environment as the one caused by harmful algal blooms.
So, how do we know if coloured water is natural? A good way to start is by collecting long-term colour data along with other water quality indicators, such as transparency and fluorescence. This information can be used to determine what is happening in a water body. For example, if the water in a coastal area has been blue-green for a long period of time, a change towards a more brownish colour can indicate something is altering this environment.
Long-term monitoring of a simple attribute of water such as the apparent colour, using low-cost devices and the aid of citizens, could help detect changes taking place in aquatic environments in a rapid way, without the need of costly and time-consuming water quality analyses.