Climate change impacts on lakes. (EG)

Question 1

What is a mesocosm

Answer 1

“A mesocosm (meso- or ‘medium’ and -cosm ‘world’) is any outdoor experimental system that examines the natural environment under controlled conditions.”
Cna be within lakes or on land.
Link between field and lab studies.

Question 2

Why do we use mesocosms for climate change
research?

Answer 2

Close to the real world without losing the advantage of reliable reference conditions and replication.
Allows rigorous testing of climate impacts on populations & communities, improving our theoretical understanding of ecological responses to likely climate shifts.

Question 3

Advantages of mesocosm

Answer 3

• Treatments are easily replicated
• Multiple variables can be manipulated
• As close to natural environment as possible

Question 4

Disadvantages of mesocosm

Answer 4

• Expensive
• Time intensive
• Many not adequately imitate environment
• Spatially restricted
• Ethical considerations

Question 5

3 ecological impacts of climate change on lakes

Answer 5

Physical, Geochemical & Biological

Question 6

What physical ecological impacts does climate change have on lakes

Answer 6

Thermal regime and mixing
Underwater solar radiation
P/E, water volume and residence time

Question 7

What geochemical ecological impacts does climate change have on lakes

Answer 7

Dissolved oxygen
Nitrogen & Phosphorus
Dissolved organic matter

Question 8

What biological ecological impacts does climate change have on lakes

Answer 8

Biodiversity
Biomass and production
Food web relationships

Question 9

What ecosystem services link, physical, geochemical and biological ecological impacts together

Answer 9

Water supplies
Flood control
Fishing & hunting
Waste treatment
Hydroelectricity
Water sports
Aesthetics

Question 10

Case study 1 looks at:

Answer 10

Cyanobacteria responses to warming, extreme rainfall events and nutrient enrichment

Question 11

Background of CS1

Answer 11

• Cyanobacteria blooms are a major water quality hazard
• Appear to increase with warmer temperatures and nutrient enrichment
• Extreme rainfall events also expected to increase with climate change

Question 12

Hypothesis of CS1

Answer 12

(a) warming would favour the growth of cyanobacteria over other phytoplankton
(b) that the effect would be synergistic with nutrient addition that is greater than the sum of their individual effects
(c) that cyanobacteria may be more sensitive to flushing (slower to recover) because of slower growth rates compared to other competing phytoplankton taxa

Question 13

Key results of each hypothesis in CS1

Answer 13

a) Warming increased the abundance of bloom-forming cyanobacteria
b) Warming in combination with high nutrient enrichment reduced the abundance of cyanobacteria
c) Flushing had no effect on cyanobacteria

Question 14

Case study 2 looks at:

Answer 14

Fish-mediated plankton responses to increased temperature in subtropical aquatic mesocosm ecosystems

Question 15

Background of CS2

Answer 15

• Warming can reinforce eutrophication in shallow lakes
• Some evidence that adverse impacts of rising temperatures are diminished in fishless systems
• Unclear if removal of zooplanktivorous fish may be useful in attempts to mitigate eutrophication in warmer climates

Question 16

Hypothesis of CS2

Answer 16

(a) fish and phytoplankton in fish-present mesocosms would benefit from warming, while zooplankton (especially Daphnia spp.) would suffer
(b) warming would favour zooplankton growth and enhance herbivory in mesocosms without fish, leading to reduced phytoplankton biomass.

Question 17

Methods of CS2

Answer 17

*12 mesocosms.
- heated (+3C) and fish present x 3
- Unheated and fish present x3
- Heated and fish absent x3
- Unheated and fish absent x3
* Nitrogen and phosphorus added daily

Question 18

Key results of CS2

Answer 18

a) In fish-present mesocosms, phytoplankton biomass and fish weight were both higher in heated mesocosms than in unheated controls
- Biomasses of both large-sized Daphnia and total zooplankton were reduced by warming
b) No difference between unheated and heated treatments for phyto or zoo plankton biomass.
- Population development for large Daphnia in mesocosms may have been constrained by low food availability.

Question 19

Case study 3 looks at:

Answer 19

Effects of brownification and warming on algal blooms and higher trophic levels

Question 20

Backround for CS3

Answer 20

• Increases in DOC ‘brownification’ reported across Northern Europe and North America
• Water temperatures are rising
• Extreme rainfall events also expected to increase with climate change

Question 21

Hypothesis for CS3

Answer 21

(a) shading, resulting from increasing DOM, would reduce phytoplankton biomass
(b) higher trophic level biomass (macroinvertebrates and fish) would consequently decline
(c) higher temperatures, particularly in more eutrophic conditions, would increase primary production
(d) higher trophic level biomass (macroinvertebrates and fish) would increase because of higher food availability

Question 22

Key results of CS3

Answer 22

A) phytoplankton growth benefited from brownification in these shallow systems
B) - total macroinvertebrate abundance increased with added OM
- fish biomass showed no statistically significant effect overall because of increased OM
C) phytoplankton biomass showed very similar abundances and phenology in ambient and warmed mesocosm
D) food availability in the heated mesocosms was not different to that in the mesocosms maintained at ambient temperature, higher trophic level biomass was not expected to differ between treatments

Question 23

Case study 4 looks at:

Answer 23

Impacts of water level, nutrients and temperature on macrophytes

Question 24

Background of CS4

Answer 24

• Increases in DOC ‘brownification’ reported across Northern Europe and North America
• Water temperatures are rising
• Extreme rainfall events also expected to increase with climate change

Question 25

Hypothesis for CS4

Answer 25

(a) mesotrophic conditions in combination with low water levels lead to favourable light conditions that
enable high macrophyte growth (temperature benefits growth)
(b) high water levels with mesotrophic conditions or low water levels with eutrophic conditions reduce
macrophyte growth (temperature has dampened positive impact on growth)
(c) high water levels with eutrophic conditions reduce macrophyte growth (higher temperatures do not
directly benefit growth)
(d) higher temperatures indirectly stimulate macrophyte growth due to evaporation-driven reduction of water levels

Question 26

Methods of CS4

Answer 26

Experiment carried out in 6 countries (Sweden, Estonia, Germany, Czech Republic, Turkey and Greece)
- 16 mesocosms in each country
- Shallow and mesotrophic x4
- Shallow and eutrophic x4
- Deep and mesotrophic x4
- Deep and eutrophic x4

Question 27

Additional methodology for CS$

Answer 27

Macrophyte coverage and water
depth were measured monthly
from July to November.
Coverage was estimated by visual
inspection

Question 28

Results for a & b hypothesis in CS4

Answer 28

a) Highest growth of macrophytes observed in shallow, low nutrient mesocosms and was
positively influenced by temperature
b) * Shallow + high nutrient growth was similar to deep + low nutrient growth
* But deep + low nutrient growth was higher than deep + high nutrient growth
* Increased temperature had a positive impact on growth

Question 29

Results for c & d hypothesis in CS4

Answer 29

c) * Deep mesocosms with high nutrients had the lowest growth
* Higher temperatures increased growth but the effect was lower than shallow + low nutrient conditions
d) * Only Greece and Turkey experienced marked water level declines
* This increased the available light available to the macrophytes (compared to theoretical values)