A very natural process

A very natural process

There’s not much left of the surface of many local waters like the hablach at the moment. The water in some areas is almost completely covered with a layer of algae, which has spread rapidly within a few days. "This is a natural process that cannot be prevented", reassures matthias schrepfermann. According to the deputy manager of the water management office, the high temperatures of the past few days are the reason for the rapid growth of algae.

A natural process

The phenomenon can be explained by the reaction rate-temperature rule: "if the temperature rises by ten degrees, the biological turnover doubles". In winter the water temperature is four degrees, now it has been measured 24 degrees." The sunlight and the increasing concentration of nutrients in the water further stimulate the growth of algae.

"To grow, the algae need nitrate and phosphate", explains schrepfermann. "Because the discharge velocity is currently much lower due to the lower water mass, the concentration of nutrients increases."

Phosphate and nitrates

The amount of nutrients supplied to the hablach is constant throughout the year. The phosphate and the nitrate get into the water in two different ways. "Clarification plants discharge the purified wastewater into the downstream water", explains the chemist. Manure from agriculture is also being washed into the rivers by the rain.

Fish migrate

In technical terminology, clear plant inflows are called point sources, and agriculture is called a shallow source. A fraction of the nitrate and phosphate also enters the water through the excretions of animals living in the water and air pollution from traffic. "When it rains, farmers are not allowed to fertilize and are also encouraged to apply only as much fertilizer as the plants can absorb", said biologist julia krawina.

As the temperature rises, the oxygen content of the water decreases. In order for the fish to breathe, the oxygen in the water must dissolve: "at a water temperature of zero degrees, the oxygen content is 14.6 milligrams per liter. At 20 degrees it is only 9.1 milligrams per liter – under laboratory conditions", female shoemaker. When the concentration of oxygen drops below three milligrams per liter, the situation for fish becomes critical.

"The fish are under stress and actually needed even more oxygen.", stresses krawina. The biologist suspects that they migrate to deeper and shadier parts of the hablach. "There is also the possibility that coarser fish can be reeled to the next weir and wait until the water level rises again." In her opinion, smaller fish have it easier because they can be spooled through the weirs.

Which depends on the species

How much oxygen the individual fish need depends on the species. "While eels and crucian carp survive at low oxygen levels, trout, barbel, ash and bullhead need more oxygen", explains krawina. A fish kill has not yet broken out in kronach, he says.

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