River VL 9

Question 1

How deep are shallow lakes typically?

Answer 1

Typically not deeper than 6 meters.

Question 2

Do deep or shallow lakes rather tend to stratify dimictic?

Answer 2

Deep lakes are more likely to stratify dimictic (in temperate regions).
* Because they have sufficient depth to develop distinct thermal layers (epilimnion, metalimnion, hypolimnion)
* that mix during the spring and fall turnovers, but remain stratified during the summer and winter.

Shallow lakes generally lack the depth needed to sustain these separate layers and tend to mix more frequently, often resulting in polymictic conditions.

Question 3

Lakes are climate…?

Answer 3

1. Sentinels:
   * Lakes respond quickly to changes in temperature, precipitation, and atmospheric inputs, providing early warning signals of climate change.
2. Integrators:
   * Lakes accumulate sediments and integrate inputs from surrounding ecosystems, capturing long-term environmental changes.
3. Regulators:
   * Lakes influence carbon cycling and local climates by storing and processing carbon and affecting evaporation and precipitation patterns.

Summary:
lakes quickly reflect changes (sentinels), store environmental data (integrators), and impact carbon and local climate (regulators).

Question 4

Which two types of plankton are involved in seasonal plankton dynamics for dimictic lakes? Describe the cycle!

Answer 4

It is about phytoplankton and zooplankton -> Predator prey relations:
* Bottom-up regulation by phytoplankton supply and
* Top-down regulation by grazing of (edible) phytoplankton by zooplankton.

1) There is typically a first phytoplankton bloom in early summer.
* These are edible phytoplankton and soon after, there is a spike in zooplankton species, grazing the first phytoplankton bloom and regulating it top-down.

2) Then in mid or late summer, there often is a second phytoplankton (algal) bloom in dimictic lakes.
* These are phytoplankton species that are inedible for zooplankton.
* Due to the lack of top-down pressure through grazing by zooplankton, the second algal bloom is often more dominant, leading to potentially harmful algal blooms.

Question 5

On which two factors does the mixing regime of a lake depend on?

Answer 5

• 1: climate zone
• 2: lake characteristics (e.g. depth, flow through regime).

Question 6

With global warming over the next decades, dimictic and monomictic lakes tend to…?

Answer 6

• Have a prolonged summer Stratification (verlängertes Nicht-Durchmischen im Sommer)
• In extreme cases, dimictic mix only once per year instead of twice  They get monomictic!
• Monomictic lakes tend to get meromictic and thereby oxygen and nutrient-poor due to insu􀆯icient/ only partial mixing.
• Lakes would tend to get oligotrophic (from eutrophic -> Oligotrophic) which influences biodiversity