Energy Flow
This term describes the total amount of energy captured by producers through photosynthesis before any is used for respiration.
Gross Primary Production (GPP)
This dominant plant species is a primary producer in salt marsh ecosystems and was the focus of Dr. Silliman's research.
Cordgrass
These bacteria in legume root nodules convert atmospheric N₂ into ammonia — making nitrogen available to plants.
Nitrogen-fixing bacteria
When crops are harvested and removed from a field, this is the main way that farming depletes soil nitrogen and phosphorus over time.
Harvesting removes nutrients from the ecosystem permanently, and they don't return to the soil through natural cycling.
In the ecosystem energy flow diagram, what is the key difference between the blue arrows (chemical cycling) and the orange arrows (energy flow)?
Chemical cycling loops continuously, matter is recycled. Energy flow is one-directional, energy is lost as heat and must be resupplied by the sun.
In Teal's Georgia salt marsh, GPP was 34,580 and NPP was 6,585 kcal/m²/yr. How much energy was lost to respiration?
27,995 kcal/m²/yr (34,580 − 6,585)
Before Silliman's work, scientists believed salt marsh ecosystems were regulated primarily by this type of control: Driven by nutrients, water, and sunlight.
Bottom-up control
This process, carried out by bacteria, converts organic nitrogen from dead organisms back into ammonium in the soil.
Ammonification
Excess nitrogen and phosphorus from agricultural runoff enters waterways and triggers this process, which can create aquatic dead zones.
Eutrophication: Algal blooms fueled by excess nutrients die and decompose, consuming oxygen.
Plants get carbon from the air and nitrogen from the soil. What specific form of carbon do they absorb, and what is the primary process used?
Plants absorb inorganic CO₂ from the atmosphere and convert it to organic carbon (C₆H₁₂O₆) via photosynthesis.
In the salt marsh study, 3,671 kcal/m²/yr left as detritus and NPP was 6,585 kcal/m²/yr. What percentage of NPP left the marsh as detritus?
56% (3,671/6,585 = 0.557)
What surprising observation caused Silliman to question the bottom-up model of cordgrass regulation?
He observed large numbers of snails that were grazing and consuming cordgrass
Identify the TWO-step nitrification pathway: what converts ammonium, and what is the final plant-usable product?
Ammonium (NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻) Nitrifying bacteria carry out both steps, so that plants can absorb nitrate.
Farmers use this input to compensate for nutrient depletion from harvesting crops. Name it and explain why it's necessary based on the nitrogen cycle.
Fertilizers (Synthetic nitrogen/phosphorus). Necessary because harvesting breaks the natural cycling loop, so nutrients must be artificially replaced.
The Serengeti ecosystem is described as 'nitrogen limited.' Define this term and explain what would happen to plant biomass if nitrogen suddenly became abundant.
Nitrogen-limited means nitrogen is the nutrient in shortest supply relative to plant needs, it constrains growth most. If abundant, plant biomass would increase (bottom-up cascade), potentially supporting more herbivores.
A caterpillar eats 200 J, loses 100 J as feces, and loses 67 J to respiration. How much becomes new biomass, and what is this value called?
33 J
This is net secondary production (new biomass)
In Silliman's 2×2 experiment (snails present/absent × nitrogen added/not), which treatment produced the MOST cordgrass? What does this reveal?
No snails + nitrogen added. It reveals that both top-down and bottom-up factors interact to control plant biomass.
Which type of bacteria returns nitrogen to the atmosphere, and what is the chemical transformation involved?
Denitrifying bacteria. They convert nitrate (NO₃⁻) → N₂ gas, and return inorganic nitrogen to the atmosphere.
Replacing native plants with crop monocultures can change nitrogen cycling efficiency. Explain why, referencing the organisms that drive the nitrogen cycle.
Native plant communities support diverse microbial communities, including nitrogen-fixing bacteria. Monocultures may support fewer nitrogen-fixing organisms, reducing the rate at which atmospheric N₂ is converted to plant-usable forms.
The salt marsh exports 56% of its NPP as detritus, yet the Serengeti is nitrogen-limited. How do both facts reflect the same underlying principle about matter vs. energy?
Both reflect that matter cycles, while energy doesn't.
Explain why energy flows in ONE direction through an ecosystem while matter cycles repeatedly. What happens to energy at each trophic level?
Energy is lost as heat by cellular respiration at every trophic transfer and cannot be recycled, it must be continuously resupplied by the sun.
Matter is never destroyed and cycles between organisms and the abiotic environment.
Blue crab populations crash due to overfishing. Using the trophic cascade model (crabs → snails → cordgrass → nutrients), trace the full cascade and name the phenomenon.
Fewer crabs → snail populations increase due to less predation → cordgrass decreases due to more grazing → possible marsh decline. This is a trophic cascade, which has an indirect effect rippling across trophic levels.
Wildebeest eat grass, respire, defecate, and die. For each step, identify whether organic or inorganic carbon/nitrogen is involved and what happens to it.
Eating: Consume organic carbon (glucose/plant biomass).
Respiration: Release inorganic CO₂ to atmosphere.
Defecation: Release organic N/P → Decomposers convert it to inorganic forms.
Death: Decomposed → N returned via ammonification; C returns as CO₂.
A student argues: "Farming can't really affect the nitrogen cycle — nitrogen is 78% of the atmosphere, so there's always plenty." Refute this argument using what you know about nitrogen availability and limiting nutrients.
Atmospheric N₂ is unusable by most organisms directly. It must be converted to ammonium or nitrate by nitrogen-fixing bacteria first. The rate of biological nitrogen fixation limits plant-available nitrogen, not the total atmospheric pool. The Serengeti is nitrogen-limited despite being surrounded by air. Availability, not abundance in the atmosphere, is what matters.
Define 'ecosystem services' and give three specific examples from salt marshes discussed in lecture.
Ecosystem services: Practical benefits humans receive from the natural world.
Salt marsh examples: coastal protection, habitat for commercially important species, carbon sequestration (storing organic carbon in sediments), and water filtration.