Primary Production: The Foundation of Ecosystems

Introduction to Primary Production

• Primary Production Defined: This is the process of storing energy by forming organic matter from inorganic carbon compounds. It is performed by autotrophic organisms.

• Etymology: The term "autotrophic" comes from the Greek words "autos" (self) and "trophikos" (pertaining to food). Autotrophs are "self-feeders" producing food for metabolism, growth, and reproduction.

• Role of Producers: Plants and microbes, such as algae, are the primary autotrophs. Photosynthesis is the most familiar form, incorporating carbon dioxide ($CO_2$) into organic matter using sunlight energy.

• Ecosystem Importance:

  • The carbon cycle begins with carbon fixation (incorporating $CO_2$ into organic matter).

  • Autotrophs support herbivores, detritivores, and predators directly or indirectly.

  • The uptake of nutrients like nitrogen and phosphorus to build proteins and nucleic acids accompanies primary production.

  • The ratio of elements within primary producers influences global ecological processes (Sterner and Elser 2002).

• Global Impact: Primary production influences the global carbon cycle and atmospheric $CO_2$ concentrations on short- and long-term scales.

• Structural Role: In terrestrial systems, primary production creates physical structure (e.g., branches and wood in forests), which affects chemical and biological processes. Analogous structures exist in marine kelp forests, marsh cattails, and submerged plants.

• Cryptic Production: Primary production can be difficult to observe (cryptic). For example, in oceans or lakes, phytoplankton biomass might appear constant if loss processes, like grazing by herbivores, match growth rates.

  • Blooms: When growth rates significantly exceed loss rates, phytoplankton "blooms" occur, potentially leading to noxious scums and environmental problems.

Components and Energetics of Primary Production

• Units of Measurement: Primary production is a rate. Units are mass per area (or volume) per time, such as grams carbon per square meter per day ($g\,C\,m^{-2}\,d^{-1}$).

  • Yield vs. Production: While "yield" often refers to total mass (e.g., corn in a field), in ecosystem science, production always refers explicitly to rates.

  • Biomass vs. Production: Biomass is mass per area or volume independent of time. While often correlated, high production can occur with low biomass (e.g., in the ocean), and high biomass can occur with low production (e.g., slow-growing plants).

• Key Productivity Components:

  • Gross Primary Production (GPP): The total carbon dioxide fixed into organic matter, ignoring any respiratory losses.

  • Autotrophic Respiration ($R_a$): Energy spent by primary producers to support their own metabolism.

  • Net Primary Production (NPP): The rate at which organic matter is available for uses beyond autotroph respiration. Formula: $NPP = GPP - R_a$.

  • Heterotrophic Respiration ($R_h$): Respiration by consumers and decomposers.

  • Ecosystem Respiration ($R_e$): The sum of autotrophic and heterotrophic respiration. Formula: $R_e = R_a + R_h$.

  • Net Ecosystem Production (NEP): The difference between GPP and ecosystem respiration. Formula: $NEP = GPP - R_e$. It can also be expressed as $NEP = NPP - R_h$.

  • Net Ecosystem Exchange (NEE): An instantaneous measurement of NEP, often used in terrestrial gas-exchange studies. By convention, NEE is negative when the net flux of $CO_2$ is into the canopy (autotrophic state) and positive when it is out (heterotrophic state).

• Ecosystem States:

  • Autotrophic Ecosystems: Systems where GPP > R_e (Positive NEP), such as most forests, grasslands, and wetlands.

  • Heterotrophic Ecosystems: Systems where R_e > GPP (Negative NEP). These rely on stored carbon or imports from outside (e.g., cities, many lakes, streams, and estuaries).

• Organic Carbon Accumulation ($dC_{org}$): This represents the mass balance of the ecosystem. Formula: $dC_{org} = NEP + I + CM - Ex - Ox_{nb}$, where:

  • $I$ = Net imported organic carbon.

  • $CM$ = Net consumer movement.

  • $Ex$ = Exported organic carbon.

  • $Ox_{nb}$ = Nonbiological oxidation (e.g., fire or photo-oxidation).

Carbon Sequestration and Chemosynthesis

• Carbon Sequestration (Box 2.1): Storage of primary production in long-term reservoirs can influence atmospheric $CO_2$. NEP is not always equivalent to sequestration because exported carbon or future events (like fire) must be considered. In a regenerating forest, 90% of stored carbon in wood could be lost to fire, while only the 10% in soil might remain sequestered.

• Chemosynthesis (Box 2.2): Conversion of inorganic carbon to organic matter using chemical energy instead of light. Occurs mainly in prokaryotic microorganisms (bacteria and archaea).

  • Reaction Examples:

    • Methane oxidation: $CH_4 + O_2 \rightarrow CO_2 + 4H^+ + 2e^-$. Reducing power from ions fuels $CO_2$ fixation into formaldehyde ($HCHO$).

    • Anaerobic methane oxidation (documented in a polluted canal): $5CH_4 + 8NO_3^- + 8H^+ \rightarrow 5CO_2 + 4H_2 + 14H_2O$.

  • Habitats: Interfaces of aerobic and anaerobic environments, soils, sediments, and deep-sea thermal vents where reduced compounds (sulfides, methane, ammonium) emerge.

  • Significance: In most systems, chemosynthesis is 0.3%–7% of production, but in unlit ecosystems like caves or thermal vents, it supports entire ecosystems with high biomass.

Measuring Primary Production

• Aquatic Methods:

  • Traditional $^{14}C$ Method: Adding radioactive bicarbonate ($H^{14}CO_3$) to water samples and measuring the carbon captured on a filter. Estimates a rate between GPP and NPP depending on incubation length.

  • Free Water Oxygen Method: Uses sensors to measure dissolved oxygen changes continuously.

    • $\Delta O_2 = GPP - R_e + D$ ($D$ is diffusion across air-water interface).

    • $GPP = \Delta O_2(\text{day}) + |R_{\text{night}}| + D$.

• Terrestrial Methods:

  • Harvest and Increment: Clipping grassland material or measuring tree diameter changes. Uses allometric equations to correlate diameter to woody biomass. Measures NPP.

  • Remote Sensing: Lidar (pulsed laser light) and radar provide detailed forest structure images to estimate biomass.

  • Foliar and Root Production: NPP includes leaf fall, seed production (e.g., oak masting), and root growth. Root production is measured via minirhizotrons (video cameras in tubes). Mycorrhizal fungal production can account for 10%–40% of NPP.

  • Eddy Covariance: Pairs a high-speed $CO_2$ sensor with a wind speed sensor on a tower to calculate carbon flux into/out of a canopy (NEE).

Regulation and Factors Controlling Production

• Light: Photosynthesis increases with light flux up to a maximum.

  • Model Parameters: Initial slope ($\alpha$) and maximum rate ($P^b_{max}$).

• Nutrients: Primary production is nutrient-limited in most systems.

  • Liebig’s Law of the Minimum: A single factor typically limits production.

  • Limiting Nutrients: Nitrogen ($N$) and Phosphorus ($P$) in aquatic systems; $N$, $P$, Potassium ($K$), and Water in terrestrial systems.

  • Co-limitation: Diatoms may be limited by silica ($Si$) while others are limited by $P$, $N$, or Iron ($Fe$). In Lake Erie, adding all three ($Fe, N, P$) produced the greatest biomass increase.

• Relative Stoichiometry: Macronutrients (e.g., $C, N, H, O, P, S, K, Mg, Na, Ca$) vs. Micronutrients (e.g., $Fe, Mn, Zn, Cu, B, Mo, Cl, V, Co$).

  • C:N:P Ratios:

    • Terrestrial leaves: Mean C:N = 36; Mean C:P = 970.

    • Marine seston: Mean C:N = 7.7; Mean C:P = 143.

    • Freshwater seston: Mean C:N = 30; Mean C:P = 307.

• Hawaii Forest Study: Vitousek (2004) showed limitation changed from $N$ on young substrate ($0.3\,ky$) to $P$ on old substrate ($1400\,ky$).

• Precipitation: Terrestrial ANPP is strongly correlated with precipitation in dry biomes (deserts, grasslands). Forests (>1000\,mm) are less sensitive to interannual precipitation variation.

• Herbivory:

  • Direct effects: Consumption of tissue, trampling.

  • Indirect effects: Nutrient regeneration (excretion), seed dispersal, changing plant composition through selective foraging.

  • Intensity: Consumes <10\% of NPP in forests/grasslands; often >50\% in aquatic algal systems.

Global Patterns and Fates

• Productive Ecosystems: Tropical forests, upwelling zones, coral reefs (>700\,g\,C\,m^{-2}\,y^{-1}).

• Low-Productivity Ecosystems: Deserts, tundra, boreal woodlands, oligotrophic lakes/oceans (<200\,g\,C\,m^{-2}\,y^{-1}).

• Global Totals:

  • Terrestrial NPP: $\approx 54\,Pg\,C\,y^{-1}$.

  • Marine NPP: $\approx 52\,Pg\,C\,y^{-1}$.

  • Global GPP: $\approx 120\text{--}130\,Pg\,C\,y^{-1}$ for both land and sea.

• Fates of NPP:

  • Phytoplankton: Largely grazed by herbivores.

  • Forests/Kelp: Up to 90% goes to detritus.

  • Export: DOC moves from land to water. Largest rivers export $0.4\text{--}8.3\,g\,C\,m^{-2}\,y^{-1}$.

  • Respiration: Most carbon is ultimately respired by heterotrophs (especially bacteria) back to $CO_2$.