L2 - Nutrients and Algae

Question 1

What is the main ecological role of algae?

Answer 1

Primary Production:
- Photosynthesis - use solar energy to fix inorganic carbon (CO2) & turn it into organic carbon (sugar)
Form the base of aquatic food webs

Question 2

How much oxygen does Marine algae produce globally?

Answer 2

About 70%

Question 3

What is the range of Photosynthetically active radiation (PAR) a plant can absorb?

Answer 3

400-700 nm (nanometers)

Question 4

What are the major nutrients biological roles in marine ecosystems?

Answer 4

Carbon: energy and creation of all structures
Nitrogen: proteins (amino acids)
Phosphorus: nucleic acids (sugar-phosphate backbone)

Question 5

What is the issue with organism growth?

Answer 5

It is usually limited by one nutrient.
- organisms need nutrients in specific ratios

Question 6

what ratios of nutrients do marine algae need for maximum organism growth

Answer 6

C:N:P = 106:16:1 (Redfield Ratio)

Question 7

What is Stoichiometry?

Answer 7

Element ratios

Question 8

How does Stoichiometry vary in algae

Answer 8

• Algae can store nutrients they aren’t using in vacuoles
• Structural N:P varies by species
• A given species can be somewhat flexible depending on nutrient supply

Question 9

How are algae either N-limited (nitrogen) or P-limited (phosphorus)?

Answer 9

Without light for photosynthesis, they are light-limited
*algae are never C-limited (Carbon)

Question 10

What ecosystems are p-limited, and which are n-limited?

Answer 10

General tendencies:
→ Marine = N-limited
→ Freshwater = P-limited
BUT Co-limitation can occur

Question 11

What depth is the best place for algae to live?

Answer 11

Light is always highest at the surface
Nutrient levels are (often) highest at the bottom, because N & P:
- get stored in and then leach out of the sediments
- get trapped in the hypolimnion by stratification
- are not immediately absorbed by algae because of light limitation

Question 12

What is a trade-off?

Answer 12

an increase in one thing causes a decrease in another

Question 13

Why is it better for algae to be in the epilimnion than the hypolimnion?

Answer 13

they need to stay in euphotic zone (from the surface to 1% light level)

Question 14

How can algae slow down their sinking velocity?

Answer 14

Strategy #1: Be small
- Large spheres sink faster than small spheres - selection to stay small
Strategy #2: Be less dense
- Denser spheres sink faster - selection to add air vacuoles or other lightweight structures like oils that lower density
Strategy #3: Be flat
- Flatter objects with protuberances have more frictional drag and sink more slowly - selection to be needle or disc shaped and/or have spines
Strategy #4: Swim up
- Some algae have flagella and are motile

Question 15

How can conflict between strategies to slow algae sinking limit their usefulness?

Answer 15

Adding too many low-density structures or spines, or getting too oblong will increase cell size and cancel out the gains from density or form resistance

Question 16

What are the main 3 selective pressures on algae?

Answer 16

Sinking
Grazing
Competition (growth)

Question 17

What is the ecological role of herbivorous zooplankton?

Answer 17

Many are keystone species = the main food web link between producers (algae) and predators
Prevent algae overgrowth
Maintain algal diversity and nutrient availability

Question 18

What are defensive strategies to avoid grazing?

Answer 18

• Avoiding ingestion: be hard or big (e.g. form a colony/filament)
• Avoiding digestion: be covered in a gooey sheath and come out alive (‘viable gut passage’)
• Killing the grazer: be toxic

Question 19

What are constitutive and plastic defences of grazing?

Answer 19

Constitutive: They are always present and happen where induced/plastic defences get turned on and off based on signals from the environment, lead to changes in species composition - undefended species get replaced if grazing pressure increases

Plastic: induced defences mean that one species can be flexible and avoid getting replaced

Question 20

What are the problems with defences against grazing?

Answer 20

defensive strategies are costly! £££
- Resources (energy and nutrients) are finite! Anything invested in defence cannot be used to grow or reproduce
- When grazing pressure is low, defended algae will be outcompeted by undefended algae

Question 21

Compare Low Quality, Inedible algae and
High Quality, Edible algae.

Answer 21

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 22

What are the main 3 selective pressures on algae?

Answer 22

Sinking
Grazing
Competition (growth)

Question 23

How are different algal traits linked?

Answer 23

By resource allocation or biophysical processes:
- If you add defences to avoid grazing, algae grow more slowly
- If you get big to avoid grazing, algae sink faster