APES 1.3-1.4: Primary Productivity and Carbon Cycle

Primary production

The synthesis of organic compounds from inorganic carbon dioxide by living organisms. It occurs mainly by photosynthesis, but some organisms use chemosynthesis.

What organisms are responsible for primary production, and what role do they play in ecosystems?

Primary producers, or autotrophs, make organic matter from inorganic substances and form the base of food webs. Terrestrial producers are mainly plants, while aquatic producers are primarily algae and phytoplankton.

How does photosynthetic primary production differ from chemosynthetic primary production?

Photosynthesis uses light energy to reduce carbon dioxide into organic compounds. Chemosynthesis uses energy released by the oxidation or reduction of inorganic substances such as hydrogen sulfide.

Simplified equation for oxygenic photosynthesis

$\mathrm{CO_2 + H_2O + light \rightarrow CH_2O + O_2}$, where $\mathrm{CH_2O}$ represents a reduced carbohydrate unit. The carbohydrate can be used to build sugars, lipids, proteins, and nucleic acids.

Example equation for chemosynthetic carbon fixation using hydrogen sulfide

$\mathrm{CO_2 + O_2 + 4H_2S \rightarrow CH_2O + 4S + 3H_2O}$. Energy comes from the oxidation of inorganic hydrogen sulfide rather than from light.

Why is primary production essential to nearly all life on Earth?

It stores energy in organic molecules and introduces that chemical energy into food webs. Heterotrophs obtain both matter and energy by consuming producers or organisms that consumed producers.

Gross primary production (GPP)

The total rate at which primary producers fix carbon and store chemical energy as organic matter, before subtracting energy used in producer respiration. It is commonly reported as $\mathrm{g\ C\ m^{-2}\ yr^{-1}}$.

Net primary production (NPP)

The rate of organic matter or chemical energy remaining after primary producers use some of their fixed energy for cellular respiration and maintenance. NPP is available for producer growth, reproduction, and consumption by herbivores.

Quantitative relationship between GPP and NPP

$\mathrm{NPP = GPP - R_a}$, where $\mathrm{R_a}$ is respiration by the autotrophs. Therefore, GPP is always greater than or equal to NPP.

How do productivity and production differ quantitatively?

Production is the amount of material made, such as $\mathrm{g\ C\ m^{-2}}$. Productivity is the rate of production, such as $\mathrm{g\ C\ m^{-2}\ yr^{-1}}$.

Why can a system have high GPP but relatively low NPP?

A large fraction of the energy fixed by producers may be consumed by cellular respiration and maintenance. Since $\mathrm{NPP = GPP - respiration}$, high producer respiration lowers the energy available for growth and consumers.

What environmental factors primarily limit terrestrial primary production?

Temperature, water availability, and light—especially photosynthetically active radiation—are major controls. Extreme temperatures or shortages of water or light limit carbon fixation.

How does transpiration support terrestrial primary production?

Evaporation of water from leaves creates a driving force for transporting water and mineral nutrients from the soil. Transpiration also cools plants, but excessive water loss can cause stomata to close and reduce carbon dioxide uptake.

Stomata

Adjustable pores in plant leaves that regulate gas exchange. Opening stomata generally increases diffusion of carbon dioxide into the leaf but also increases water loss through transpiration.

Why does partial stomatal closure reduce photosynthesis?

Closing stomata decreases water loss, but it also restricts diffusion of atmospheric $\mathrm{CO_2}$ into the leaf. Lower internal carbon dioxide availability can reduce the rate of carbon fixation.

How do C4 and CAM photosynthetic pathways help plants in stressful environments?

They include adaptations that improve water-use efficiency and reduce limitations on carbon fixation under hot or dry conditions. These pathways can support greater production than typical C3 photosynthesis under such conditions.

What generally limits primary production in deserts and polar regions?

Deserts are primarily limited by water, while polar regions may be limited by low temperature, low light, frozen water, or short growing seasons. In both cases, at least one essential resource is insufficient for rapid photosynthesis.

How does primary production differ between terrestrial and oceanic ecosystems?

Most terrestrial production is performed by vascular plants. Most oceanic production is performed by free-floating microscopic phytoplankton, with seagrasses and macroalgae contributing mainly in shallow coastal regions.

Photic zone

The sunlit upper layer of an ocean where enough light is available for photosynthesis. It is often defined operationally as the depth at which light intensity has fallen to about $1\%$ of its surface value.

Why does light intensity decrease with depth in the ocean?

Water, dissolved substances, particles, and organisms absorb or scatter light. As light is attenuated, less energy is available to phytoplankton for photosynthesis.

Mixed layer

The upper ocean layer vertically homogenized by turbulence, especially wind-driven mixing. Mixing can supply nutrients from deeper water but also move phytoplankton below the well-lit region.

Critical depth

The greatest mixed-layer depth at which phytoplankton can achieve net growth. If the mixed layer is deeper than this, phytoplankton spend too much time in darkness and respiration can exceed photosynthesis.

How do ocean mixing and the photic zone jointly regulate net primary production?

A shallow mixed layer keeps phytoplankton in adequate light but may become nutrient-depleted. A deep mixed layer can replenish nutrients but may keep phytoplankton in darkness long enough that respiration exceeds photosynthesis.

Why is oceanic primary production often seasonal in temperate regions?

Seasonal changes in surface light and wind-driven mixing alter both photosynthetic energy supply and nutrient delivery. Winter mixing can replenish nutrients, while summer stratification can increase light exposure but reduce nutrient resupply.

How can strong summer stratification reduce oceanic primary production even when sunlight is abundant?

Stratification inhibits vertical mixing, so nutrients from deeper water are not replenished in the mixed layer. Phytoplankton may consume available nitrate, phosphate, and other nutrients until nutrient limitation offsets the benefit of increased light.

Which inorganic nutrients are especially important for phytoplankton growth?

Nitrate, phosphate, and silicic acid are important nutrients used to synthesize cellular structures and machinery. Their removal by sinking particles and their resupply by mixing or upwelling strongly affect production.

Why can sinking plankton and organic particles reduce primary production in surface waters?

Sinking exports nutrients from the photic zone. Unless mixing or upwelling returns those nutrients, phytoplankton eventually experience nutrient limitation.

Why is iron an important oceanic micronutrient?

Iron acts as a cofactor in enzymes involved in processes such as nitrate reduction and nitrogen fixation. In some high-nutrient, low-chlorophyll regions, insufficient iron limits phytoplankton growth despite the presence of other nutrients.

High-nutrient, low-chlorophyll (HNLC) region

An ocean region where macronutrients remain abundant but phytoplankton biomass is low because a micronutrient—often iron—is insufficient. Adding iron can stimulate phytoplankton growth, although ecological consequences must be considered.

Why is desert dust an important source of oceanic iron?

Wind transports iron-containing mineral dust from continents to the ocean. Remote regions that receive little dust may experience iron limitation.

Why is measuring GPP more difficult than measuring NPP?

Respiration occurs continuously and consumes some newly produced organic compounds before they can be measured. NPP is the net change after respiration, whereas GPP requires estimating or accounting for respiratory losses.

Why do biomass-based terrestrial NPP estimates often underestimate actual NPP?

Field measurements may omit belowground production, root turnover, herbivory, litterfall, volatile compounds, root exudates, and carbon allocated to symbiotic microorganisms. The unmeasured components cause the calculated NPP to be too low.

How can ecosystem respiration be estimated in terrestrial systems?

Gas-flux measurements can quantify carbon dioxide released by the ecosystem. This includes respiration from plants, soil organisms, and other ecosystem components.

What is the purpose of using aboveground-to-belowground NPP ratios?

Directly measuring root production and turnover is difficult, so belowground NPP is often estimated from the relationship between aboveground NPP and belowground NPP. This provides an indirect estimate of total terrestrial production.

How does the light-and-dark-bottle method estimate aquatic primary production?

The dark bottle measures oxygen consumption from respiration because photosynthesis cannot occur. The light bottle measures net oxygen change from photosynthesis minus respiration; adding dark-bottle respiration to light-bottle net production estimates GPP.

In a light-and-dark-bottle experiment, what does a decrease in oxygen concentration in the dark bottle indicate?

It indicates ecosystem respiration, in which organisms consume oxygen while oxidizing organic molecules. The magnitude of oxygen loss estimates the respiration rate during incubation.

In a light-and-dark-bottle experiment, why is the light bottle's oxygen change a measure of net photosynthesis?

The light bottle contains both photosynthesis, which produces oxygen, and respiration, which consumes oxygen. Thus its net oxygen increase represents oxygen production minus oxygen consumption.

How does radioactive carbon-14 labeling measure aquatic primary production?

Phytoplankton incorporate added $^{14}\mathrm{C}$-labeled inorganic carbon into organic matter. Measuring the incorporated radioactivity estimates carbon fixation; short incubations more closely approximate GPP, while longer incubations include more losses and tend toward NPP.

Why can carbon-14 incubation estimates depend strongly on incubation time?

Respiration, excretion, and consumption can remove labeled carbon from the measured organic pool. These losses become more significant as incubation time increases, causing the estimate to reflect net rather than gross production.

What is a major advantage of stable-isotope and oxygen/argon methods for measuring aquatic production?

Some of these methods can estimate respiration in the light and avoid the need for dark incubations. Oxygen/argon measurements can also be made continuously at sea under suitable conditions.

Why might carbon-based methods be preferred when studying the carbon cycle?

They directly track carbon assimilation rather than inferring carbon fixation from oxygen production. This can be more relevant when the goal is to quantify carbon transfer through ecosystems.

How is global primary production estimated using remote sensing?

Terrestrial estimates can use satellite measurements such as the Normalized Difference Vegetation Index, while oceanic estimates commonly use sea-surface chlorophyll. These observations are combined with models relating vegetation or chlorophyll to carbon fixation.

What approximate global rates of photoautotrophic primary production are reported in the source?

The global total is about $105\ \mathrm{Pg\ C\ yr^{-1}}$, divided into roughly $56\ \mathrm{Pg\ C\ yr^{-1}}$ on land and $49\ \mathrm{Pg\ C\ yr^{-1}}$ in the oceans. These are estimates and depend on the measurement and modeling approach.

How do average areal production rates compare between land and oceans?

The source estimates approximately $426\ \mathrm{g\ C\ m^{-2}\ yr^{-1}}$ on land excluding permanent ice and $140\ \mathrm{g\ C\ m^{-2}\ yr^{-1}}$ in the oceans. Ocean production is spread across a much larger area and is dominated by microscopic organisms.

Why can oceanic primary producers account for a large fraction of global production but only a small fraction of standing biomass?

Phytoplankton reproduce and are consumed rapidly, giving them high turnover. Their small standing biomass can therefore support substantial production over time.

What evidence can be used to reconstruct primary production in Earth's past?

Biogeochemical models and geochemical proxies in sediments can be used. Examples include sedimentary barite associated with carbon export and isotope patterns that record changes in oxygen or sulfur cycling.

How can human land use affect net primary production?

Agriculture, grazing, urbanization, and other land changes can alter vegetation and reduce actual NPP relative to potential natural vegetation. Irrigation can increase production locally, but global land-use change has produced an overall reduction in many regions.

How might climate change affect oceanic NPP?

Changes in ocean temperature, stratification, circulation, nutrient supply, and light conditions may reduce oceanic NPP. Model estimates in the source project reductions of roughly $3\%$ to $10\%$, depending on the emissions scenario.

What is the carbon cycle?

The movement and exchange of carbon among the atmosphere, oceans, living organisms, soils, sediments, and geological reservoirs. It includes processes such as photosynthesis, cellular respiration, decomposition, and combustion.

How does photosynthesis affect atmospheric carbon dioxide?

Photosynthesis removes $\mathrm{CO_2}$ from the atmosphere or water and converts the carbon into organic molecules. This transfers carbon into producer biomass and is a major carbon sink.

How does cellular respiration affect the carbon cycle?

Cellular respiration breaks down organic molecules for energy and releases carbon dioxide back to the atmosphere or water. It occurs in producers, consumers, and decomposers.

How does decomposition affect the carbon cycle?

Decomposers break down dead organisms and waste, releasing carbon dioxide through respiration and returning carbon-containing compounds and nutrients to soils and water. Under anaerobic conditions, decomposition can also produce methane.

How does combustion affect the carbon cycle?

Burning biomass or fossil fuels rapidly oxidizes organic carbon and releases it primarily as atmospheric carbon dioxide. Combustion transfers carbon from biological or geological reservoirs to the atmosphere.

What is a carbon sink?

A carbon sink is a reservoir or process that absorbs and stores more carbon than it releases over a given period. Forests, soils, oceans, and long-term sedimentary or geological storage can function as carbon sinks.

What is a carbon source?

A carbon source is a reservoir or process that releases more carbon than it absorbs over a given period. Fossil-fuel combustion, deforestation, respiration, decomposition, and some ocean regions can act as carbon sources.

How can ecosystems alternate between carbon sinks and carbon sources?

The balance between photosynthetic uptake and carbon release changes with temperature, water availability, disturbance, season, and human activity. An ecosystem is a sink when uptake exceeds release and a source when release exceeds uptake.

How do oceans exchange carbon dioxide with the atmosphere?

Carbon dioxide dissolves into seawater and can be taken up by marine primary producers, making the ocean a carbon sink. Warming, circulation, and biological or chemical changes can reduce uptake or cause some regions to release carbon dioxide.

How does carbon move through a food web?

Producers fix inorganic carbon into organic matter. Consumers obtain that carbon by feeding, and carbon is transferred through successive trophic levels before being returned by respiration, waste, death, and decomposition.