L7 - Spatial and Temporal variation in lakes Flashcards
Explain the seasonal stratification in temperate lakes, in winter
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
Explain the seasonal stratification in temperate lakes, in spring
In spring, the top layer warms and the wind is able to fully mix the water, leading to spring circulation
Explain the seasonal stratification in temperate lakes, in summer
In summer, eventually the surface becomes so warm (and less dense) that the wind cannot fully mix the water, leading to summer stratification.
Explain the seasonal stratification in temperate lakes, in autumn
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.
Explain the seasonal stratification in temperate dimictic lakes
SEASONAL STRATIFICATION IN TEMPERATE DIMICTIC LAKES
Alternating periods of stratification (winter and summer) and circulation (spring and autumn)
What is ecological succession?
the process by which a community of organisms shifts overtime
How does ecological succession cause changes in algae and zooplankton?
Quantity - predator-prey cycles
Quality - succession
In winter, why are algae and zooplankton at low densites
Low light and cold temperature limit growth
Most zooplankton are overwintering – in diapause or as resting eggs
Explain overwintering strategies within zooplankton species such as cladocerans, rotifers, copepods
Cladocerans:
Produce resting eggs (“ephippia”) that sink down to the sediment
Rotifers:
Produce resting eggs (“cysts”)
Copepods:
Enter diapause (basically go to sleep)
How do zooplankton exit an overwintering strategy?
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
How does zooplankton over grazing in summer, lead to an algal crash?
Zooplankton eat algae faster than it can reproduce – they run out of food and their population also crashes;
How do algae and zooplankton rebound in autumn?
Lower grazing levels from zooplankton allow algae to begin increasing again, with zooplankton following again soon after (a classic predator-prey cycle)
Explain how algae and zooplankton decline with the onset of winter
The zooplankton either enter diapause (copepods) or produce resting eggs (Daphnia: “ephippia”; rotifers: “cysts”) before dying
What does the zooplankton’s seasonal cycle of famines and feasts coincide with?
Increase and decrease of algal biomass
Explain how growth-defence trade-off leads to spectrum of food quality for grazers
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
Give an overview of algal succession
1) Mostly a mix of edible eukaryotes, except during autumn feast sequence
2) edible diatoms
3) inedible cyanobacteria
4) inedible N-fixing cyanobacteria
Explain efficient vs powerful grazers (zooplankton)
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
What are the 2 strategies copepod’s use for low algal food
- Lipid storage (visible as orange globules)
- Adults can become carnivorous and eat smaller cladocerans
What does PEG stand for and what is it?
Plankton Ecology Group
A model for succession
*developed for temperate lakes, especially in germany
What are the limitations of the PEG model?
> 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
When do bacteria peak in PEG lakes and why?
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
Why does the PEG model fail to describe tropical lakes?
- 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!
Where do fish hunt in the day, and what does this lead to?
Hunt in the euphotic zone, leading to diurnal vertical migration (DVM) to avoid predation.
Explain diurnal vertical migration (DVM)
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)
What does DVM behaviour depend on?
May depend on predator presence
e.g. Zooplankton migrate vertically in lakes with fish, but not in lakes without fish
What are the chemical cues of predator presence called that drive DVM?
kairomones
