Oxygen depletion zone, fish mortality and fish stock development: questions and facts

How much oxygen is dissolved in the water is the result of oxygen input and oxygen consumption. Oxygen is introduced physically from the air via the water surface and biologically via photosynthesis, i.e. the conversion of carbon dioxide and water into biomass and oxygen with the help of sunlight by algae, also known as phytoplankton. Oxygen is consumed through the respiration of animals and plants, as well as during the microbial decomposition of dead organic matter (e.g. plant remains). Oxygen deficiency always occurs when oxygen consumption outweighs oxygen input for a longer period of time.</p

In fact, oxygen valleys have been occurring in the tidal Elbe for many decades. In earlier years, until around 1990, oxygen valleys with critical oxygen levels and mass fish mortality were even more pronounced than today, which was partly due to the discharge of industrial oxygen-depleting wastewater in the GDR and Czechoslovakia. After 1990, the number of days with low oxygen levels fluctuates greatly from year to year, but overall the situation has improved significantly compared to the past.

The formation of the summer oxygen valley has several causes:

1. algal bloom in the Middle Elbe

Too many nutrients from agricultural land enter the water in the Middle Elbe. The result of this over-fertilisation is an unnatural mass development of algae, known as an algal bloom. As the Middle Elbe is very shallow, light can penetrate a larger part of the water. The intensive photosynthesis of the algae and the oxygen input from the air then initially lead to high oxygen levels in the water.</p

<p>2. influence of the tidal Elbe:

However, the conditions change fundamentally when the algae- and oxygen-rich water of the Middle Elbe flows over the Geesthacht weir into the tidally influenced and deeper area of the Elbe with its high turbidity. This is because there is a lack of light in the deeper layers of water (from about one metre), which leads to mass death of the algae. The microbial decomposition processes of the dead algae consume a lot of oxygen. In addition, the dead algae are only transported slowly down the Elbe because the tidal current keeps pushing them back towards Hamburg.

<p>In addition, as the water depth increases, the ratio of water surface to water volume, the so-called specific water surface, decreases and with it the oxygen input from the air.

3. temperature and headwater discharge:

Oxygen is much less soluble in warmer water than in cold water. In addition, the microbiological decomposition processes of dead algae are strongly temperature-dependent. The higher the temperature, the more intensive the biological degradation and thus the oxygen depletion.</p

<p>When the outflow from the Middle Elbe towards Hamburg is low, less oxygen-rich water flows into the tidal Elbe. In addition, the residence time of the water in the harbour area is longer than with high outflows. At the same time, the natural turbidity zone, which normally lies between Brunsbüttel and Stade, shifts upstream towards Hamburg. As a result, the turbidity in Hamburg increases, the light-flooded upper water layer decreases and with it the oxygen production of the algae.</p

As the greater water depths of the tidal Elbe (created by man over the centuries) play an important role in the formation of the oxygen valley, it is reasonable to assume that the recently completed fairway upgrade has also led to an additional deterioration in the oxygen situation. However, the environmental impact assessment has shown that the additional effect of the deepening of the fairway is too small to measurably reduce the oxygen content. Even after the previous fairway upgrade, there was no discernible impact on the number of days with oxygen deficiency.

<p>Adverse effects from the continuous water depth maintenance, i.e. from the maintenance dredging, could be caused by the fact that the dredging activities and the relocation of the dredged material lead to an increase in turbidity and thus a reduction in oxygen production. For this reason, the HPA has taken precautionary measures to rule out such impairments as far as possible. For example, trailing suction hopper dredgers are technically optimised so that their use only leads to a very small amount of sediment being stirred up locally. Bed levellers and water injection devices are only used if the oxygen content is above 4 mg/l. The relocation of dredged material in the Hamburg Tidal Elbe area is also limited to the oxygen-rich winter months (November to March).

A currently demanded "reduction" of the fairway upgrade to shallower water depths would in any case not change the problem of the summer oxygen valley described above. In order to "get to the root of the problem", on the other hand, the unnatural mass proliferation of algae in the Upper and Middle Elbe would have to be significantly reduced by a large-scale change in the use of agricultural fertilisers.</p

<p>In the tidal Elbe, however, it makes sense to preserve and create shallow water areas. Such shallow water areas, which still have a water depth of around 2 metres even at low tide, contribute to increasing the biogenic and physical oxygen input due to their shallow water depth. On the other hand, they are valuable refuges for many fish species, especially in times of low and therefore fish-critical oxygen levels. With the shallow water area Kreetsand recently completed by the HPA, we have now successfully implemented the first such measure. Together with BUKEA, the HPA is currently looking for other suitable locations - including areas where more shallow water could be created again by removing silted-up areas.</p

Fish stocks are generally subject to strong fluctuations. After reunification, the water quality in the Elbe improved rapidly and fish stocks initially grew strongly. However, a sharp decline was then observed between 2010 and 2022. In particular, smelt, the main species in the Elbe, declined sharply. It is certain that the extremely low Elbe discharges in the years 2014-2023 contributed significantly to a deterioration in conditions for fish (high turbidity due to a shift in the turbidity zone towards HH, salinity, oxygen). In addition, the strong fluctuations in fish stocks are also dependent on many other factors. These include "in addition to a pronounced seasonal component, physical and chemical parameters [oxygen, temperature], (...) also water morphological, meteorological and even astronomical factors such as the tide", according to a study commissioned by BUKEA. The evidence studies for WSV and HPA (Bioconsult 2025) also cite a variety of factors ("...nutrient development, predators, competition with other species..."). In addition, the extensive cooling water withdrawals and fishing itself have an impact on the stocks. Reducing the causes to maintenance dredging or fairway upgrades is therefore far too one-sided. As a precautionary measure, closed seasons are observed during maintenance work in fish-sensitive periods.</p

<p>In the years since 2022, the stocks have developed positively again ("The Elbe fishermen report a good smelt season this year...", Fischmagazin 17.5.2025; "Elbfischer Lothar Buckow is pleased. ... the catches will be significantly better again in 2025", Blinker 31 January 2025), which is also confirmed by the study commissioned by BUKEA. To ensure that this remains the case, we are continuing to work together with the environmental departments to sustainably improve the living conditions in the tidal Elbe (see above).</p