Nutrient Cycles & Stoich (3)

What elements are major structural materials for organisms?

Carbon, nitrogen, and phosphorus.

Why is carbon important?

It is the main structural material for many organisms, especially plants.

What provides most of the energy for food webs?

Glucose and other fixed carbon molecules.

Why is nitrogen important biologically?

It is required for amino acids and proteins.

Why is phosphorus important biologically?

It forms the backbone of DNA and is essential for ATP and membranes.

What does CHNOPS stand for?

Carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur.

What percentage of living biomass is CHNOPS?

More than 99%.

What is stoichiometry in ecology?

The balance of energy and elemental ratios (like C:N:P) within organisms and ecosystems.

What is the N:P ratio in tadpoles?

20:1.

What is the N:P ratio in bony fish?

Between 5:1 and 15:1.

What is the human C:N:P ratio?

18:2:1.

Why does stoichiometry matter?

It affects mismatches between organisms and their food.

What is food quantity vs food quality?

Quantity is how much biomass is available; quality depends on nutrient composition.

Why is consuming low-quality food inefficient?

It forces organisms to process more biomass to obtain required nutrients.

How does stoichiometry relate to resource limitation?

Organisms may become limited by whichever nutrient is least available relative to their needs.

How does stoichiometry relate to competitive exclusion?

Species with lower R* for a limiting nutrient can competitively exclude others.

How does stoichiometry influence ecosystem functioning?

It strongly affects decomposition rates.

What is nutrient cycling?

The movement of nutrients between living organisms and the abiotic environment.

Why do nutrient cycles vary across ecosystems?

Sources, pool sizes, and turnover rates differ among ecosystems and elements.

What measures the speed of nutrient cycling?

Mean residence time of a nutrient in a pool.

What was the Hubbard Brook Ecosystem Study?

A catchment-level experiment measuring nutrient inputs and outputs to quantify whole-ecosystem nutrient processing.

What was measured in the Hubbard Brook study?

Input fluxes, output fluxes, and internal nutrient transformations.

Why was the Hubbard Brook approach useful?

It aggregated many species into a system-level understanding of nutrient flow.

What was a limitation of the Hubbard Brook approach?

It simplified species-level roles and reduced biological complexity.

What is the largest nitrogen stock on Earth?

Atmospheric N2.

How much of the atmosphere is N2?

About 79%.

Why is most nitrogen not biologically available?

N2 gas is inert and requires fixation to become usable.

Is N2O a greenhouse gas?

Yes, nitrous oxide is a powerful greenhouse gas.

Do nitrogen oxides (NOx) act as greenhouse gases?

No, but they form ozone, which is harmful.

Where are major terrestrial nitrogen pools?

Soils and plant biomass.

Where are major ocean nitrogen pools?

Surface waters and deep ocean reservoirs.

What produces most biologically available nitrogen?

Biological nitrogen fixation.

What chemical forms of nitrogen exist in ecosystems?

Organic nitrogen in organisms and necromass; inorganic nitrogen like N2, ammonium, and nitrate.

What controls nitrogen fixation rates?

Plants, bacteria, and their interactions.

What human activities affect nitrogen fixation?

Fertilizer production and industrial fixation.

How much nitrogen is fixed by biological processes globally?

Approximately 228 teragrams per year.

How much nitrogen is fixed by human processes?

About 100 teragrams per year.

Why is phosphorus limited in the atmosphere?

Phosphorus has no significant gaseous phase.

Where are the largest phosphorus pools?

Surface ocean waters, soil, bedrock, and benthic sediments.

What drives major phosphorus fluxes?

Physical weathering, geologic processes, and biological uptake.

Why is phosphorus recycling extremely tight?

P-limited ecosystems reuse phosphorus efficiently.

How does phosphorus cycling differ from nitrogen cycling?

Nitrogen has a large atmospheric reservoir, while phosphorus does not.

How are nitrogen and phosphorus cycles similar?

Both rely on biological processing to convert nutrients into usable forms.

What other major nutrients cycle in ecosystems?

Sulfur and potassium.

Why does nutrient availability influence food quality?

Consumers require certain elemental ratios for growth and maintenance.

What happens when food stoichiometry mismatches consumer needs?

Consumers must eat more or excrete excess elements inefficiently.

Why is stoichiometry important for ecosystem functioning?

Decomposition, nutrient cycling, and productivity all depend on elemental ratios.

What is the ecological significance of N:P ratios across organisms?

They determine nutrient demands, competitive outcomes, and trophic interactions.