Primary Production and Ecosystem Science Flashcards

Fundamental Definitions and the Role of Primary Production

• Primary Production Definition: Primary production is the storage of energy through the formation of organic matter from inorganic carbon compounds. It represents the first step in the capture, storage, and transfer of energy within most ecosystems.

• Autotrophic Organisms: These are the organisms responsible for primary production. The term "autotrophic" comes from the Greek words autos, meaning "self," and trophikos, meaning "pertaining to food." Autotrophs are "self-feeders" that produce the necessary food for their metabolism, growth, and reproduction.

• Photosynthesis: This is the most familiar form of primary production, conducted by plants and algae. It involves the incorporation of carbon dioxide ($CO_2$) into organic matter using energy derived from sunlight.

• Factors of Importance for Ecologists:

  • The Carbon Cycle: The ecosystem carbon cycle begins with carbon fixation ($CO_2$ incorporation into organic matter).

  • Food Web Foundation: Herbivores consume organic carbon produced by autotrophs to support growth and metabolism. Predatory and detritivore components of the food web depend directly or indirectly on this energy supply.

  • Biogeochemical Cycling: Primary producers require nutrients like nitrogen ($N$) and phosphorus ($P$) to build biomolecules (proteins and nucleic acids). The ratio of elements in primary producers influences various ecological processes.

  • Global Carbon Dynamics: Fixation and the fate of carbon influence atmospheric $CO_2$ concentrations.

Structural and Sensory Aspects of Primary Production

• Visible Structure: In terrestrial ecosystems, biomass accumulation provides structure. In forests, tree growth leads to wood accumulation and branch/root formation. In aquatic systems, growth of kelp, cattails, or submerged plants creates habitats supporting diverse organisms.

• Cryptic Production: Primary production is sometimes difficult to observe. For instance, phytoplankton biomass in lakes or oceans might remain stable day-to-day because loss processes (grazing by herbivores) occur as rapidly as production. Growth might be high even if biomass does not change.

• Conspicuous Production (Blooms): When growth rates consistently exceed loss rates, phytoplankton "blooms" occur, leading to visible, sometimes noxious scums that present environmental problems.

Components and Formulas of Primary Production

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

• Biomass vs. Production: Biomass is expressed as mass per area or volume independent of time. While often correlated within specific ecosystems, a system can have low biomass but high production rates (e.g., the ocean) or high biomass with low production (e.g., slow-growing old-growth trees).

• Gross Primary Production (GPP): The first step in productivity, representing the total amount of carbon dioxide fixed into organic matter regardless of respiratory losses.

• Net Primary Production (NPP): The rate at which organic matter is available for uses beyond the autotroph's own energy costs. It is the difference between GPP and autotrophic respiration ($R_a$):

  • $NPP = GPP - R_a$

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

  • $R_e = R_a + R_h$

• Net Ecosystem Production (NEP): The portion of GPP not respired by autotrophs or heterotrophs. It represents the balance of carbon flows:

  • $NEP = GPP - R_e$

  • $NEP = NPP - R_h$

• Organic Carbon Accumulation ($dC_{org}$): This is a function of NEP, imports ($I$), net consumer movement ($CM$), exports ($Ex$), and nonbiological oxidation ($Ox_{nb}$, such as fire):

  • $dC_{org} = NEP + I + CM - Ex - Ox_{nb}$

Autotrophic and Heterotrophic Ecosystems

• Autotrophic Ecosystems: These have a positive NEP, meaning they produce more carbon than they respire. Examples include most forests, grasslands, and vegetated wetlands.

• Heterotrophic Ecosystems: These have a negative NEP, respiring more carbon than they produce. The excess respiration depletes stored carbon or is subsidized by external imports. Examples include cities, many lakes, streams, and rivers.

• Carbon Sequestration: NEP is not strictly equivalent to carbon sequestration ($dC_{org}$). Sequestration requires considering the timescale and factors like fire (nonbiological oxidation) and export. For example, a forest may sequester carbon for 50 years, but a fire in year 51 could release much of the stored woody biomass carbon back to the atmosphere.

Chemosynthesis: The Alternative Foundation

• Definition: The use of chemical energy (oxidizing inorganic molecules) to produce organic matter from inorganic carbon, typically carried out by bacteria and archaea.

• Environments: Occurs primarily at the interface of aerobic and anaerobic environments (sediments, soils, stratified water columns) or in unlit ecosystems like caves and deep-sea thermal vents.

• Methane Oxidation: A common chemosynthetic reaction:

  • $CH_4 + O_2 \rightarrow CO_2 + 4H^+ + 2e^-$

• Anaerobic Methane Oxidation: Observed in environments like polluted canals where microbes use nitrate or sulfate instead of oxygen:

  • $5CH_4 + 8NO_3^- + 8H^+ \rightarrow 5CO_2 + 4H_2 + 14H_2O$

• Ecosystem Contribution: In most lit ecosystems, chemosynthesis is a small fraction ($0.3\%$ to $7\%$). In deep-sea vents, it supports entire ecosystems with high local biomass.

Methodologies for Measuring Primary Production

• Aquatic Methods:

  • $^{14}C$ Method: Adding radioactive bicarbonate ($H^{14}CO_3$) to water samples. It is highly sensitive and typically estimates a rate between GPP and NPP.

  • Dissolved Oxygen Method: Measuring changes in $O_2$ levels. Modern sensors allow continuous measurement. The change in oxygen ($\Delta O_2$) is calculated as:

    • $\Delta O_2 = GPP - R_e + D$ (where $D$ is atmospheric diffusion).

• Terrestrial Methods:

  • Harvest and Increment: Measuring leaf fall and wood production. Wood production often uses allometric equations relating tree diameter to mass. Lidar and radar are now used to estimate biomass more accurately.

  • Root Production: Difficult to measure; often excluded, leading to data being reported as ANPP (above-ground net primary production). Mycorrhizal production can constitute $10\%$ to $40\%$ of NPP in temperate forests.

  • Eddy Covariance: Uses towers with $CO_2$ and wind sensors to measure net ecosystem exchange (NEE). NEE is conceptually an instantaneous measurement of NEP. If NEE is negative, the ecosystem is absorbing more $CO_2$ than it releases.

Regulation and Limitation of Primary Production

• Light: Photosynthesis increases with light flux up to a saturating point. This is modeled using the initial slope ($\alpha$) and the maximum rate ($P_{bmax}$).

• Nutrients (Liebig’s Law of the Minimum): Proposes that production is limited by the single scarcest factor. In terrestrial systems, limitations are often water, $K$, $Ca$, $N$, or $P$. In aquatic systems, $N$ and $P$ are frequently limiting.

• Colimitation: Multiple resources can interact. For example, adding iron ($Fe$) to Lake Erie facilitates the uptake of nitrate, helping overcome nitrogen limitation when phosphorus is also present.

• Nutrient Stoichiometry:

  • Terrestrial Plants: High carbon ratios due to structural investments (Mean Foliar $C:N = 36$; $C:P = 970$).

  • Marine Seston: Dominated by phytoplankton (Mean $C:N = 7.7$; $C:P = 143$).

  • Freshwater Seston: Intermediate ratios (Mean $C:N = 30$; $C:P = 307$).

• Precipitation: In terrestrial systems, ANPP is strongly correlated with annual precipitation, particularly in arid regions where the regression slope is positive. In regions with over $1000\,mm$ of rain, the sensitivity to interannual variation decreases (slope near zero).

• Herbivory: Consumers can reduce biomass directly or stimulate production indirectly through nutrient recycling (excretion/egestion). Consumption is typically low in terrestrial systems (<10\%) but can exceed $50\%$ in aquatic algal systems.

Global Patterns of Productivity

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

• Low Productivity Ecosystems (<200\,g\,C\,m^{-2}\,y^{-1}): Deserts, tundra, boreal woodlands, and open ocean mid-latitudes.

• Global Totals: Global NPP is split roughly equally between land and sea.

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

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

  • Global GPP: approximately $120$ to $130\,Pg\,C\,y^{-1}$ for each realm.

Fates of Primary Production

• Immediate Fates: Most NPP is either grazed by herbivores or converted to detritus. In forests, up to $90\%$ goes to detritus. In Yellowstone National Park, grazers like elk and bison consume about $45\%$ of annual NPP.

• Carbon Sequestration/Storage: Unrespired carbon may accumulate in soils or sediments for millennia or in woody biomass for decades.

• Downstream Export: Dissolved Organic Carbon (DOC) export from the 30 largest rivers ranges from $0.4$ to $8.3\,g\,C\,m^{-2}\,y^{-1}$, representing $0.1\%$ to $10\%$ of terrestrial NPP.

Introduction to Ecosystem Science Concepts

• Ecosystem Definition: An interacting system made up of all living and non-living objects in a specified volume. Both are given equal status.

• Scale: Can range from a biofilm on a rock or a tree cavity to a watershed or the entire Earth.

• Black-Box Concept: The ability to measure the function (inputs and outputs) of an ecosystem without knowing every internal process.

• Holism vs. Reductionism: Holism studies the intact system, while reductionism studies parts. Ecosystem science often combines both.

• Ecosystem Services:

  • Provisioning: Food, water, fiber.

  • Regulating: Climate, pollination, erosion control.

  • Cultural: Recreation, education, spiritual values.

  • Supporting: Soil formation, nutrient cycling.

• Mass Balance Tool: Defines connectivity. In a watershed, if input exceeds output, material is retained. The Hubbard Brook Forest established a calcium budget using the equation $P + W = S + B$ ($P$: Precipitation, $W$: Weathering, $S$: Stream loss, $B$: Biomass storage).

• Tracers: Radioactive ($^{14}C$, $^{32}P$) or stable ($H$, $N$, $C$, $O$, $S$) isotopes used to monitor material movement. A Catskill Mountains study used $^{15}N$ to find that forest floors are the largest nitrogen sink over 300 days.