L7 - Spatial and Temporal variation in lakes

Question 1

Explain the seasonal stratification in temperate lakes, in winter

Answer 1

In winter, the densest water (4 degrees) sinks to the bottom.
In cold locations, the surface gets even colder (and less dense) leading to winter stratification

Question 2

Explain the seasonal stratification in temperate lakes, in spring

Answer 2

In spring, the top layer warms and the wind is able to fully mix the water, leading to spring circulation

Question 3

Explain the seasonal stratification in temperate lakes, in summer

Answer 3

In summer, eventually the surface becomes so warm (and less dense) that the wind cannot fully mix the water, leading to summer stratification.

Question 4

Explain the seasonal stratification in temperate lakes, in autumn

Answer 4

In autumn, the surface layer (epilimnion) cools down, the difference in water density between layers decreases, and the wind can once again fully mix the lake, leading to autumn circulation.

Question 5

Explain the seasonal stratification in temperate dimictic lakes

Answer 5

SEASONAL STRATIFICATION IN TEMPERATE DIMICTIC LAKES
Alternating periods of stratification (winter and summer) and circulation (spring and autumn)

Question 6

What is ecological succession?

Answer 6

the process by which a community of organisms shifts overtime

Question 7

How does ecological succession cause changes in algae and zooplankton?

Answer 7

Quantity - predator-prey cycles
Quality - succession

Question 8

In winter, why are algae and zooplankton at low densites

Answer 8

Low light and cold temperature limit growth
Most zooplankton are overwintering – in diapause or as resting eggs

Question 9

Explain overwintering strategies within zooplankton species such as cladocerans, rotifers, copepods

Answer 9

Cladocerans:
Produce resting eggs (“ephippia”) that sink down to the sediment
Rotifers:
Produce resting eggs (“cysts”)
Copepods:
Enter diapause (basically go to sleep)

Question 10

How do zooplankton exit an overwintering strategy?

Answer 10

Major cues for exiting diapause or hatching from resting eggs:
Algal concentration
Temperature
Sunlight
*The sensitivity to cues (which ones, amount needed) varies by species → staggered hatching/awakening across the spring and summer

Question 11

How does zooplankton over grazing in summer, lead to an algal crash?

Answer 11

Zooplankton eat algae faster than it can reproduce – they run out of food and their population also crashes;

Question 12

How do algae and zooplankton rebound in autumn?

Answer 12

Lower grazing levels from zooplankton allow algae to begin increasing again, with zooplankton following again soon after (a classic predator-prey cycle)

Question 13

Explain how algae and zooplankton decline with the onset of winter

Answer 13

The zooplankton either enter diapause (copepods) or produce resting eggs (Daphnia: “ephippia”; rotifers: “cysts”) before dying

Question 14

What does the zooplankton’s seasonal cycle of famines and feasts coincide with?

Answer 14

Increase and decrease of algal biomass

Question 15

Explain how growth-defence trade-off leads to spectrum of food quality for grazers

Answer 15

Low quality, Inedible algae:
> Large or defended
> slow growing
> Poor competitors, but thrive when grazers are present.
High quality, Edible algae:
> Small, undefended, fast growing
> Good competitors, but suppressed by grazers

Question 16

Give an overview of algal succession

Answer 16

1) Mostly a mix of edible eukaryotes, except during autumn feast sequence
2) edible diatoms
3) inedible cyanobacteria
4) inedible N-fixing cyanobacteria

Question 17

Explain efficient vs powerful grazers (zooplankton)

Answer 17

Efficient zooplankton:
> can do okay when feel level is low
Powerful zooplankton:
> grow really quickly when food level is high
*most zooplankton are specialised for a specific amount (or quality) of food

Question 18

What are the 2 strategies copepod’s use for low algal food

Answer 18

1. Lipid storage (visible as orange globules)
2. Adults can become carnivorous and eat smaller cladocerans

Question 19

What does PEG stand for and what is it?

Answer 19

Plankton Ecology Group
A model for succession
*developed for temperate lakes, especially in germany

Question 20

What are the limitations of the PEG model?

Answer 20

> doesn’t include the microbial loop (only looks at the main bit of the food chain)
fail in humic lakes (High DOC = brown water) because of the heterotrophic food web driven by microbial loop
fail to describe tropical lakes

Question 21

When do bacteria peak in PEG lakes and why?

Answer 21

During the clean-water phase.
When algae drop, a lot more nutrients which bacteria can use to grow
Good food for rotifers, some copepods, smaller cladocerans, and protozoa (also food) → which eat the bacteria

Question 22

Why does the PEG model fail to describe tropical lakes?

Answer 22

• High sunlight and warm temperatures year round → no winter bottleneck in algae and zooplankton populations
• Seasonality is driven by rainfall patterns
• Many tropical lakes are polymictic – some even mix daily!

Question 23

Where do fish hunt in the day, and what does this lead to?

Answer 23

Hunt in the euphotic zone, leading to diurnal vertical migration (DVM) to avoid predation.

Question 24

Explain diurnal vertical migration (DVM)

Answer 24

Its the movement between deeper hypolimnion (daytime) and surface epilimnion (at night)
Practiced by cladocerans, copepods, rotifers, and macroinvertebrates – even some algae (to acquire nutrients at night)

Question 25

What does DVM behaviour depend on?

Answer 25

May depend on predator presence
e.g. Zooplankton migrate vertically in lakes with fish, but not in lakes without fish

Question 26

What are the chemical cues of predator presence called that drive DVM?

Answer 26

kairomones