L2 - Nutrients and Algae Flashcards
What is the main ecological role of algae?
Primary Production:
- Photosynthesis - use solar energy to fix inorganic carbon (CO2) & turn it into organic carbon (sugar)
Form the base of aquatic food webs
How much oxygen does Marine algae produce globally?
About 70%
What is the range of Photosynthetically active radiation (PAR) a plant can absorb?
400-700 nm (nanometers)
What are the major nutrients biological roles in marine ecosystems?
Carbon: energy and creation of all structures
Nitrogen: proteins (amino acids)
Phosphorus: nucleic acids (sugar-phosphate backbone)
What is the issue with organism growth?
It is usually limited by one nutrient.
- organisms need nutrients in specific ratios
what ratios of nutrients do marine algae need for maximum organism growth
C:N:P = 106:16:1 (Redfield Ratio)
What is Stoichiometry?
Element ratios
How does Stoichiometry vary in algae
- 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
How are algae either N-limited (nitrogen) or P-limited (phosphorus)?
Without light for photosynthesis, they are light-limited
*algae are never C-limited (Carbon)
What ecosystems are p-limited, and which are n-limited?
General tendencies:
→ Marine = N-limited
→ Freshwater = P-limited
BUT Co-limitation can occur
What depth is the best place for algae to live?
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
What is a trade-off?
an increase in one thing causes a decrease in another
Why is it better for algae to be in the epilimnion than the hypolimnion?
they need to stay in euphotic zone (from the surface to 1% light level)
How can algae slow down their sinking velocity?
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
How can conflict between strategies to slow algae sinking limit their usefulness?
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
What are the main 3 selective pressures on algae?
Sinking
Grazing
Competition (growth)
What is the ecological role of herbivorous zooplankton?
Many are keystone species = the main food web link between producers (algae) and predators
Prevent algae overgrowth
Maintain algal diversity and nutrient availability
What are defensive strategies to avoid grazing?
- 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
What are constitutive and plastic defences of grazing?
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
What are the problems with defences against grazing?
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
Compare Low Quality, Inedible algae and
High Quality, Edible algae.
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
What are the main 3 selective pressures on algae?
Sinking
Grazing
Competition (growth)
How are different algal traits linked?
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
