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Berlin Environmental Atlas

02.01 Quality of Surface Waters (Edition 1993)

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Oxygen Saturation Index

Levels of dissolved oxygen in waters are primarily influenced by water temperature. Increasing water temperature reduces the capacity of water to take up and hold oxygen.

Figure 2
Fig. 2: Water Temperature and Oxygen Levels in the Spree for the Outflow Year 1991 (floating medien over 10 days)

Besides high temperatures in summer, the warming of waters by cooling water discharges leads to a further impact on the oxygen economy. All chemical and biological processes are accelerated; oxygen demand increases while the capability for absorption of oxygen drops. Increasingly critical oxygen levels are then shown precisely in slowly-flowing waters that form large surface areas, lake-like broadenings of running waters.

The oxygen saturation index indicates what percent of physically possible oxygen saturation has been reached at the time of sampling. Unimpacted waters usually have no large swings in the oxygen saturation index and oxygen amounts correspond roughly to those theoretically possible (oxygen saturation index about 100%). Since most decomposition processes in waters use up oxygen, but strong algae growth produces oxygen by photosynthesis, considerable swings can appear in nutrient-rich waters. Indices for impacted waters are not only low oxygen saturation indices, but also strong biogenic oxygen inputs and the oxygen oversaturation connected with it.

Figure 2 shows the course of water temperatures and measured oxygen amounts as exemplified at the measuring station Sophienwerder on the Spree for the outflow year 1991. Temperature-related possible oxygen amounts at 100% saturation are shown in order to make oversaturation and saturation deficit visible. While measured oxygen amounts in winter and spring basically match those to be expected in view of the temperatures, water in summer is not saturated. This is due to the predominance of oxygen-consuming decomposition processes in summer.

Oxygen Minimum

Oxygen needed for the respiration of all organisms is given into water through the air or through photosynthesis by water plants. Oxygen amounts of impacted, slowly-flowing waters are subject not only to climatic changes (wind speed, temperature, light penetration, etc.) but also to annual and daily variations due to excess algae growth. Additional oxygen by the assimilation capabilities of algae can only be produced in the upper layers of water. The penetration depth of sunlight into a body of water is a critical factor.

Individual types of fish each require certain environmental conditions for their survival. One such condition is a minimum amount of dissolved oxygen in waters.

Highly critical oxygen conditions can always occur in waters with large rain water or mixed rainwater and sewage system discharges after heavy rainfalls. Organic substances transported by inflows are decomposed by bacteria with considerable oxygen demand. More oxygen in waters may be used than can be reaerated from the air or from biogenic production. Oxygen amounts below a certain limit (about 4 mg/l for carp) are critical for fish. Any further drop in oxygen amounts will cause fish to die.

The complex and quickly occurring changes in the oxygen balance in waters with high levels of nutrients and intensive phytoplankton development can be only incompletely registered by measurements every month or every 14 days. The tense situation of oxygen conditions in Berlin waters is reflected in the fluctuations of oxygen amounts, which vary considerably depending on the time of day, at continually measured monitoring points.

Escherichia coli

Tests for Escherichia coli (E. coli) are made to determine bacteriological qualities of waters - particularly their suitability for swimming and water recreation. E. coli itself does not usually cause disease. Its presence, however, gives an indication of the pollution of a body of water from animal and human feces. If large amounts of E. coli are present, the water is heavily impacted with fecal effluents. That means the probability that disease bacteria are present increases with increasing amounts of E. coli. The measurement scale is Coli-Titer, the minimum amount of water in which E. coli can be detected.

EU bathing water guidelines for surface waters suitable for swimming give an E. coli Titer of 10-1 ml as barely tolerable.

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