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Misleading measurements of water transparency

Image: Marlene Johansson

Published: 2020-06-11

FEATURE Measuring water transparency is easy and inexpensive, and the results are used to assess the status of eutrophication in the Baltic Sea. However, the method can lead to serious errors if disparities in the optical properties are not taken into account.

Text: Kristina Viklund

Eutrophication is one of the greatest challenges when trying to achieve good water quality in the Baltic Sea. To assess the status of eutrophication a variety of parameters are measured, including nutrient concentration, oxygen conditions and phytoplankton community composition. Other important parameters are chlorophyll-A concentration and water transparency. In both the EU Water and Marine Environment Directives, water transparency and chlorophyll concentrations are used for assessing eutrophication status along with the other parameters.

Easy and inexpensive

The reason that water transparency is used to assess eutrophication status is its coupling to the amount of phytoplankton in the water.

In the case of eutrophication, the amount of phytoplankton in the water increases and the water becomes turbid, which reduces the depth of view. The amount of phytoplankton can be measured indirectly by analyzing the water content of the pigment chlorophyll-a, which all phytoplankton contains.

Water transparency is measured by immersing a standardized white disc into the water until it is no longer visible to the eye. Measuring in this way is simple and inexpensive, and is used as a standard method in environmental monitoring.

Serious sources of error

However, new studies show that there may be serious sources of error in using this method to assess eutrophication, due to optical components not linked to eutrophication.

Water transparency depends in part on the amount of chlorophyll in the water, but there are also other factors that affect the optical properties of the water. This applies to the entire Baltic Sea, but especially to the Baltic Sea coastal areas.

The Baltic Sea is an inland sea, strongly affected by a large influx of river water, and has a slow turnover. In the coastal areas in particular, the optical conditions are affected by the river water and the suspended particulate matter that accompanies the river water out into the sea. The river water contains a large amount of organic material, which colors the water brown and adds particulate matter. This particulate matter can either absorb or scatter the light in the water, depending on whether it is completely dissolved in the water and whether the particles are organic or inorganic.

Especially misleading in coastal areas

There are substantial differences in optical properties between different areas of the sea, and also between different seasons. In coastal areas, the water content of dissolved organic matter and particles can even have a greater impact on water transparency than chlorophyll concentration has. This can lead to errors in reference values for water transparency with major consequences for, for example, what measures to take. For marine management authorities, it is therefore of the utmost importance to know the natural variation in water transparency in different areas and, above all, what causes it, and to adjust reference values and limit values with regard to the natural optical properties of the water.

The article was first published in the annual report EcoChange 2019. The text is based on the following scientific article:

Harvey et al 2019: The Effect of Optical Properties on Secchi Depth and Implications for Eutrophication Management.