River VL 8

Question 1

Why is eutrophication a major problem in many rivers?

Answer 1

Because of the increased potential of HABs due to higher nutrient supply.

Question 2

What were the main causes of low discharge in the Elbe River in 2018 and 2019?

Answer 2

Climate change, particularly long-term trends of heat and drought, caused extreme low discharge in the Elbe River.

Question 3

What is the MOSES project?

Answer 3

The Modular Observation Solutions for Earth Systems (MOSES) is a monitoring initiative that studies hydrological extremes like floods and droughts to analyze their impact on water quality and ecological processes.

Question 4

How does low discharge affect eutrophication in the Elbe River?

Answer 4

Low discharge in the Elbe River exacerbates eutrophication by reducing the flow of water, which has several consequences:

• Increased Residence Time: When river discharge is low, the water moves more slowly, allowing nutrients (such as nitrogen and phosphorus) to accumulate in the system. This prolonged residence time gives phytoplankton more time to absorb these nutrients and grow, leading to algal blooms.
• Enhanced Nutrient Concentration: Low discharge means less dilution of pollutants and nutrients entering the river, resulting in higher concentrations of these nutrients, which promote eutrophication.
• Oxygen Depletion: The excess growth of algae (due to nutrient accumulation) leads to high levels of organic matter in the water. As this organic matter decomposes, it consumes large amounts of oxygen, creating hypoxic or even anoxic conditions, particularly in deeper or slower-moving parts of the river.
• Sedimentation and Decomposition: Low flow rates allow more organic material, including algae, to settle and decompose at the bottom of the river. This decomposition further depletes oxygen, leading to dead zones where aquatic life cannot survive.
• Phytoplankton Blooms: The combination of high nutrient levels and slow water movement promotes phytoplankton blooms, which exacerbate eutrophication by creating more organic material that eventually dies and decomposes, further depleting oxygen levels.

In summary, low discharge intensifies eutrophication in the Elbe River by concentrating nutrients, increasing algal growth, and causing oxygen depletion due to the accumulation and decomposition of organic material.

Question 5

What e􀆯ects has very high photosynthesis on oxygen saturation and pH-value in a river?

Answer 5

Due to high photosynthesis, the oxygen saturation increases and so does the pH value, meaning that the water gets less acidic (ALKALINE) due to increased respiration

There is a positive relationship between pH-value and oxygen saturation.
- The higher the oxygen saturation (due to higher photosynthesis) the higher the pH-value (less acidic).

Question 6

Why is an unusual intense drought more severe for some fish species than for others (beside the obvious temperature stress)?

Answer 6

Droughts can lead to a connection loss of back waters (Nebengewässer).

• Back waters are spawning areas for some fish species.

Question 7

Name causes for algal sedimentation in rivers:

Answer 7

Causes of Algal Sedimentation in Rivers:
1. Nutrient Overload: Excess nutrients from agricultural runoff and wastewater.
2. Hydrological Changes: Altered water flow from dams and withdrawals.
3. Algal Bloom Die-Off: mostly natural causes.
4. Reduced Water Turbulence: Slow-moving or stagnant water promotes sedimentation.

Question 8

Name effects of algal sedimentation in rivers:

Answer 8

Effects of Algal Sedimentation in Rivers:
1. Oxygen Depletion: Decomposition of algae consumes oxygen, leading to hypoxia or anoxia.
2. Benthic Smothering: Accumulated algae cover and su􀆯ocate bottom-dwelling organisms.
3. Nutrient Release: Decomposing algae release nutrients, potentially causing more blooms.
4. Habitat Alteration: Changes in riverbed habitats, affecting aquatic life.
5. Water Quality Degradation: Increased turbidity and unpleasant odors.