Use of optical sensors
Natural waters, such as the sea, lakes, or small garden ponds are of blue, green, brown, or even red colour. Sometimes you see the bottom, in other cases even shallow waters impede the view on the ground. These colours and underwater visibility can change over the seasons, years or even days. Did you ever wonder, what causes these changes?
Marine scientists do, as these changes tell us a lot about processes that can be related to water quality, or the extent to which human activities have altered the quality of the water. Therefore, even these simple observations, such as water colour and visibility can be related to a bigger picture. Any single observation at different times and locations is one piece of the puzzle that aids in understanding the whole. Everybody can help! In Citclops, we introduce three different bio-optical measures that can be simply applied with Smartphone Apps: Water colour, water transparency and fluorescence. You find more information on the technical background and on how to conduct measurements here.
Basics for bio-optical observations
To dive into bio-optical measurements of water, it is necessary to first have a look at the sky, as all light which leaves water, and thereby makes up its colour, originates from the sun. This light, even though it appears yellowish white to the human eye, consists of different colours, the so-called visible spectrum. This reaches from violet-blue, over green and yellow to red. These colours are visible in rainbows or when reflected from water drops in waterfalls.
Picture: Part of spectrum of visible light. White sunlight consists of different colours, which are visible e.g., in rainbows or waterfalls.
In natural waters the following happens with the different colours of light: 1. it is absorbed and thereby “disappears” or 2. it is bounced back from all matter- the so-called backscattering.
The three main components in the water (besides water itself) that absorb or scatter light are:
- Microscopic algal cells (phytoplankton)
As an example, algal cells absorb the red and blue fractions of light, while green light is backscattered. Therefore, water appears green, if there are many algal cells in the water.
- Non-algal matter, such as coarse sediment or fine soils (chalk).
- Dissolved coloured matter, mainly organic compounds including humic acids and tannins that originate from many types of terrestrial and aquatic plants.
The type and mutual proportion of the concentrations of these main components determine the specific water colour, and different types of natural waters (rivers, lakes, seas and oceans) can be recognised and classified in relation to these proportions. Learn more about water colour.
The amount of these three main components is determining the underwater visibility, or transparency of water. Learn more about water transparency.
From certain substances, a minor part of absorbed portions of light is “released” back into the water in a different colour as fluorescence. Learn more about water fluorescence.
Why is it important to measure bio-optical parameters?
The three measurements, water colour, transparency and fluorescence, are indications of microscopic algal cells (phytoplankton), sediments, and coloured dissolved organic matter (CDOM) in the water. Changes in the concentrations of these materials can be related to important issues of societal concern. Two examples are climate change and eutrophication:
Climate change: Next to water temperature and salinity, transparency and water colour observations belong to the oldest time series of climate data. Ocean colour and phytoplankton also belong to the oceanic Essential Climate Variables (ECV) designated by the World Meteorological Organization (WMO) for which sustained and climate quality measurements are needed to track and analyse climate change. A variation in phytoplankton concentration 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. All variables measured within Citclops, transparency, water colour and fluorescence can be related to the phytoplankton concentration of respective areas.
Eutrophication: Especially in coastal areas or lakes, human activities may cause an unnatural growth of phytoplankton. These may be due to the inflow of nutrients, such as phosphates and nitrates through sewage or agricultural fertilizers that cause phytoplankton to grow. This phenomenon is known as eutrophication. Algae can form dense mats that lead to oxygen depletion and shading of underlying vegetation which has profound negative effects on the ecosystem.
So, how do we know if the measured concentrations of phytoplankton, sediment or CDOM are 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 these simple attribute of water, such as the apparent colour, transparency and fluorescence, by using low-cost devices and the aid of citizens, helps to detect changes taking place in aquatic environments in a rapid way, without the need of costly and time-consuming water quality analyses.