River VL 3

Question 1

What are some consequences of low discharge in a river?

Answer 1

• high residence time (Aufentshaltszeit des Wassers)
• Low water depth.
• hig underwater light availability!

Question 2

What is the tidal part of a river?

Answer 2

• refers to the section of the river where the water level and flow are influenced by the tides from the sea
• this area experiences regular rises and falls in water levels due to tidal movements
• its basically an esturay (that ends in the sea or ocean, but not a lake)

Question 3

Why is there a phytoplankton decrease in the tidal part of a river?

Answer 3

• High water depth -> low light intensity
• Mixing with salt water -> osmotic stress
• Oxygen decrease -> due to microbial degradation (also called “Sauerstoffloch im Tidebereich”)

Question 4

As an example: What caused a severe light limitation in river Elbe until the year 1990?

Answer 4

A lignin pollution from cellulose production and activated sludge from waste water treatment plants

Question 5

Why was the concentration of “chlorophyll a” not decreasing in the river Elbe AFTER 1990? Which factors limited phytoplankton growth before and after 1990?

Answer 5

Light Limitation (Pre-1990):

• Before 1990, the primary factor limiting phytoplankton growth was light availability. This was due to high levels of lignin pollution from cellulose production and the discharge of activated sludge from wastewater treatment plants, which reduced water clarity.

Eutrophication and Nutrient Levels (Post-1990):

• After 1990, despite improvements in water clarity due to reduced lignin pollution and better wastewater treatment, the concentration of chlorophyll A did not decrease. This is attributed to the river’s high eutrophication levels, primarily due to elevated nutrient loads (nitrogen and phosphorus) which promote phytoplankton growth.
• The river remained highly eutrophic, a state where nutrient over-enrichment supports excessive phytoplankton growth. This was further compounded by low grazer biomass, meaning there were insufficient zooplankton to control the phytoplankton population through grazing.

Summary:

• The concentration of chlorophyll A in the River Elbe did not decrease post-1990 despite improved water clarity due to persistent high nutrient levels and low grazer biomass.

Question 6

How does oxygen concentration and pH value fluctuate in the River Elbe on a daily basis, and what are the reasons?

Answer 6

Oxygen Concentration: Photosynthesis and Respiration:

• During daylight hours, photosynthesis by phytoplankton increases oxygen levels in the water as they produce oxygen while converting carbon dioxide and sunlight into organic matter.
• At night, the absence of photosynthesis coupled with continued respiration by aquatic organisms and microbial degradation of organic matter leads to a decrease in oxygen concentration.

pH Value: Photosynthetic Activity:

• The pH value in the river fluctuates with the cycle of photosynthesis and respiration. During the day,
  photosynthetic activity consumes carbon dioxide, reducing the concentration of carbonic acid in the water, which raises the pH (making the water more alkaline).
• At night, respiration processes dominate, releasing carbon dioxide back into the water, increasing carbonic acid concentration, and thus lowering the pH (making the water more acidic).

Summary

Daily fluctuations in oxygen concentration and pH value are primarily driven by the cyclical processes of photosynthesis during the day, which increases oxygen levels and pH, and respiration at night, which decreases oxygen levels and pH.

Question 7

What is the pelagic zone? How is it subdivided?

Answer 7

The pelagic zone refers to the open water column of oceans, seas, and large lakes, excluding the shallow coastal areas and the ocean floor (benthic zone).

1. Neritic Zone: The shallower part of the pelagic zone, extending from the coastline to the edge of the continental shelf. It includes the epipelagic and mesopelagic zones and typically ranges from the surface down to about 200 meters in depth.
2. Oceanic Zone: The deeper part of the pelagic zone, beyond the continental shelf, where the water column is deeper and the ocean floor drops off sharply. It includes the bathypelagic, abyssopelagic, and hadalpelagic zones, extending from depths of about 200 meters down to the ocean floor, which can reach depths of several thousand meters or more.

Question 8

Right or Wrong (&why): “The Elbe is a highly eutrophic river, due to low grazer biomass”

Answer 8

Right

• Grazer biomass, also called zooplankton, is not abundant enough to control the high amount of phytoplankton in the river and therefore the Elbe is very nutrient rich.
• The zooplankton isn’t building up despite the vast amounts of phytoplankton. This could have several reasons:
  1) Too much predation by fish or higher trophic level organisms
  2) Too turbulent water high flow velocity

Question 9

Is zooplankton growth in groines larger or smaller than mid-river? (Why)?

Answer 9

Zooplankton growth rate in groine fields is larger.

• Groine fields are retention areas favored by small organisms with high reproduction rates due to the calmer and more stable conditions compared to the high flow velocity river.
• Zooplankton growth rate is reduced in high flow velocity and gets disturbed by turbulence.

Question 10

Which one likes turbulent waters and which one doesn’t? – Rotifers, Crustaceans;
Therefore, which ones growth rate gets boosted by groine fields?

Answer 10

Do like turbulent waters: Rotifers
* Groines (calmer and more stable water flow conditions) do not significantly increase the abundance of phytoplankton and rotifers.

Do not like turbulent waters: Crustacean (zooplankton)
* Groines (calmer and more stable water flow conditions) do significantly increase the abundance of crustacean zooplankton.

Question 11

Groine fields: Why are they build and what consequences are there?

Answer 11

Groines are structures that extend into the river to direct the flow of water and increase water levels during low discharge periods for shipping purposes.

Consequences:

1. Increases the flow velocity in the main channel, which enhances the erosion of the river bed.
2. Enhanced erosion results in a lowering of the river bed. This can lead to a reduction in the
   base level of the river relative to the surrounding floodplain.
3. As the river bed erodes and lowers, the hydraulic connection between the river and the adjacent floodplain is reduced. This means that during normal and low flow conditions, the river is less likely to recharge the groundwater in the floodplain through seepage (Versickerung).
4. Groundwater levels in floodplains are often maintained by periodic flooding and the slow
   seepage of river water into the groundwater system. With a lower river bed and higher flow velocities, the frequency and duration of these recharging events decrease.
5. With less frequent flooding and reduced seepage from the river, the water table in the floodplain begins to drop, leading to a decrease in groundwater levels.

The construction of groines, while beneficial for maintaining navigation and increasing water levels during low discharge, inadvertently causes increased flow velocities and erosion of the river bed, which lowers groundwater levels in the adjacent floodplains by reducing hydraulic connectivity and groundwater recharge opportunities.

-> Groines may reduce beach erosion (seitliche Erosion), but increase overall erosion and vertical erosion long term.