Swimming in the Elbe – An assessment of the water quality

In this project for the first time the "Elbe swimming relay" accompanied with a research ship which was equipped with measuring water quality instruments. It made possible for researchers to characterize the quality of the water of the river Elbe for a 575 km long in detail which is shown below figure.

The main purpose of this monitoring was to shed light on the general water quality situation of the Elbe River (KIT WAQUAVID project).
This monitoring was mainly carried out with two probes, which allowed a high temporal and geographical resolution analysis of the parameters important for the water quality.
In this project, a multiparameter probe (BIOFISH) with eight relevant physicochemical probes and an in situ microalgae monitoring probe (FluoroProbe) were used.
The FluoroProbe measures chlorophyll concentration and algal classes including green algae, blue-green algae (cyanobacteria), diatoms, and cryptophytes at various water depths.
By combining the measurements of the two systems, it was possible to obtain a general overview of the dynamics of water quality and to evaluate the water quality of the Elbe in terms of its suitability as a bathing water for researchers and other scenarios.

The analysis and processing of the BIOFISH data shows that at no time did these parameters reach the threshold values of the water quality standards. However, water composition along the monitoring reach was subject to both temporal and regional influences and changes.

Regional changes could be mainly due to the larger Elbe tributaries such as Saale or Havel, which have different water composition.
An example of this is the change in electrical conductivity of the Elbe River, shown in the following images. This image shows electrical conductivity, or the amount of dissolved charged particles (ions), which is an indicator of the salinity of a body of water.
Due to the higher salinity of the Saale River, a significant increase in the electrical conductivity of the Elbe River was observed after the confluence of two rivers in the Elbe River.
In the area of the mouth of the Havel, the salt concentration of the Elbe decreases again, because at the time of measurement the Havel has a high flow with a low salt concentration, which could be an influence in particular of the heavy rainfall in the Berlin area on 07.07.2017.

The water quality of the river changed frequently during the day and depending on the weather conditions.
The sensitively high algae concentration during the measurement campaign is due to the chemical processes in the Elbe's water system during the warm seasons.
Photosynthesis by algae in water, an activity to produce organic compounds from carbon dioxide (CO2) and water, produces dissolved oxygen (O2) as a byproduct.
In a water body, this process leads to an increase in dissolved oxygen saturation, while at the same time the pH of the water increases due to the consumption of CO2 (the acidity of the water decreases).
These relationships can be seen in the measured data from the positive linear correlation between the amount of algae (chlorophyll concentration), pH, and oxygen saturation shown in the graphs below:

However, since the photosynthetic activity of algae is dependent on solar energy, these processes occur more intensively at higher solar irradiance.
As a result, an increase in pH and oxygen saturation was generally observed during the day in sunny weather, while the corresponding values were significantly lower on cooler days, cloudy or rainy days due to lower algal activity.
With regard to the suitability of the Elbe as a bathing water, it should be mentioned in this context that the occurrence of algae per se should not be viewed negatively.
After all, in a flowing body of water such as the Elbe, a constant supply of atmospheric oxygen is guaranteed even if the algae die off, so that there is no undersupply of oxygen.
Analysis of the algae classes present also showed that the green algae and diatoms, which are harmless to humans, predominate in the Elbe.
Blue-green algae (cyanobacteria), which can severely impair the quality of a body of water by releasing toxins. This can lead to a restriction of bathing. Fortunately, this did not happen during the monitoring period of the Elbe River. Below are some more pictures of this project.

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