l8- Ecosystem Dynamics and Primary Productivity

Fundamental Laws of Life: Thermodynamics in Ecosystems

• The Intersection of Physical Laws and Ecology: The rules of physics, specifically thermodynamics, set the foundational constraints for all biological life and ecosystem functions.

• The First Law of Thermodynamics: Also known as the Law of Conservation of Energy.

  • Principle: Energy cannot be created or destroyed; it can only be transformed from one form to another.

  • Ecosystem Application: The ultimate source of energy is the sun. Energy enters the system as photons.

  • Energy Transformation Pathways:

    • Sunlight hitting land or water is transformed into heat energy.

    • Sunlight captured by primary producers (plants) is converted into chemical building blocks (organic compounds) that fuel the rest of the food web.

• The Second Law of Thermodynamics: This law governs the transfer of energy.

  • Principle: During energy transfer, some energy is always lost (usually as heat), leading to increased entropy.

  • Entropy and Disorder: Entropy refers to the state of disorder.

  • Organization of Living Matter: Organisms are highly organized systems of molecules and structures.

  • Life to Death Transition: When an organism dies, its component parts (e.g., tree branches falling, trunk decaying) lose their organization and break down into basic atoms and molecules, returning to a state of disorder.

• Maintaining Order Against Entropy:

  • Ecosystems avoid total entropy through a constant influx of energy from the sun.

  • Primary producers utilize this solar energy, harvesting carbon and nutrients to continuously build new organic material, thereby maintaining order and sustaining life.

Defining and Measuring Primary Productivity

• Primary Productivity Definition: The rate at which carbon dioxide ($CO_2$) is converted into organic compounds.

• Photoautotrophs: The primary drivers of productivity in most ecosystems.

  • Land: Higher plants.

  • Water: Cyanobacteria and algae.

• Chemoautotrophs and Extremophiles: Non-photosynthetic primary productivity exists but is relatively minor on a global scale.

  • Thermal Vents: Environments[] where life is maintained without light.

  • Iron-Oxidizing Bacteria: Found in areas of heavy iron leaching; they create a substrate that supports other life forms by utilizing available iron.

• Gross Primary Productivity ($GPP$): The total rate of photosynthesis in a system.

• Net Primary Productivity ($NPP$): The rate of energy storage after the metabolic costs of the plant are subtracted.

  • Equation: $NPP = GPP - \text{Respiration}$.

  • Respiration Cost: Plants capture $CO_2$ during the day but release it at night via respiration.

• Measurement Units for NPP:

  • Energy Units: Joules per square meter per year ($J\,m^{-2}\,yr^{-1}$).

  • Biomass Units: Organic dry matter, often measured as grams per square meter per year ($g\,m^{-2}\,yr^{-1}$) or metric tons per hectare per year ($\text{metric tons}\,ha^{-1}\,yr^{-1}$).

  • Practical Application: Rangeland ecologists use exclusion plots to measure dry matter production to calculate how much forage is consumed by herbivores.

Determinants of Net Primary Productivity (NPP) in Terrestrial Ecosystems

• The Role of Climate and Nutrients: These are the primary controls for terrestrial NPP. Higher productivity creates a larger biological foundation capable of supporting more herbivores and carnivores.

• Precipitation:

  • There is a strong positive relationship between annual precipitation and NPP.

  • Asymptotic Growth: As rainfall increases, the NPP curve eventually levels off (becomes asymptotic), indicating that other factors eventually limit growth.

• Temperature:

  • There is a clear positive correlation between temperature and NPP.

  • Metabolic Constraints: Cold environments lead to lower metabolic rates, while warm environments facilitate higher metabolic rates.

• Day Length and Photosynthetic Period:

  • Productivity is strongly tied to the length of the photosynthetic period (the number of days leaves are active).

  • Deciduous Forest Example: In North American forests, the photosynthetic period ranges from $100$ to approximately $225$ days. Longer leaf duration typically correlates with higher NPP, though there is significant scatter in the data due to other variables.

• Interacting Factors: Evapotranspiration:

  • Definition: The combined measure of evaporation and plant transpiration.

  • High Productivity: Tropical forests, which are both hot and wet, have the highest above-ground productivity and evapotranspiration rates.

  • Low Productivity: Systems like Creosote bush environments are hot but dry, resulting in low evapotranspiration and low productivity.

Global Distribution of Productivity

• Terrestrial Patterns:

  • Highest NPP: Found in the Amazon, African rainforests, and Southeast Asia.

  • Lowest NPP: Found in Arctic/Boreal forests (limited by cold) and Deserts (Sahara, Arabian, and parts of Australia and North America) due to water limitation.

• Global Carbon Fixation Totals:

  • Terrestrial NPP: Fixes approximately $56 \times 10^{9}$ tons of carbon per year.

  • Oceanic NPP: Fixes approximately $48 \times 10^{9}$ tons of carbon per year.

  • Total: Roughly $104 \times 10^{9}$ tons of carbon fixed annually.

• Area vs. Rate: Global NPP contribution is influenced by both the rate of fixation and the total land/water area. Boreal forests cover massive areas but have low rates; tropical rainforests cover smaller areas but have very high rates (the "lungs of the earth").

Nutrient Constraints and Plant Allocation Strategies

• Soil Nutrient Availability: NPP increases as nutrient availability (especially Nitrogen and Phosphorus) increases.

• Nitrogen Mineralization Rate: The rate at which organic nitrogen is converted into inorganic forms (e.g., Ammonium or Nitrate) that plants can absorb. Measured in kilograms per hectare per year ($kg\,ha^{-1}\,yr^{-1}$).

• Indeterminate Growth Responses:

  • Plants can target growth toward specific limiting resources.

  • Low Nutrient Soil: Plants invest more energy into root development to find nutrients.

  • High Nutrient Soil: Plants invest more into leaf production to maximize photosynthesis.

• Case Studies:

  • Wisconsin Forest Sites: Positive relationship between above-ground NPP and nitrogen mineralization.

  • Minnesota Grassland Sites: A study of $20$ different sites showed a strong positive correlation between nitrogen availability and NPP.

Dynamics of Aquatic Ecosystems

• Primary Controls: Light and nutrient availability.

• Light and Depth:

  • Light intensity decreases exponentially with depth.

  • Spectral Limitation: Red and blue wavelengths (critical for photosynthesis) do not penetrate deep water well.

• The Euphotic Zone and Compensation Depth:

  • Photoinhibition: At the surface, light can be too intense, potentially damaging photosynthetic machinery.

  • NPP Peak: Productivity usually peaks just below the surface.

  • Compensation Depth: The depth (stylized at $100\,m$) where the rate of photosynthesis equals the rate of respiration ($NPP = 0$). Below this depth, plants lose more carbon than they fix and cannot survive.

• Nutrient Cycling in Oceans:

  • Nutrients (Nitrogen, Iron, Phosphorus) often sink to the bottom as organisms die.

  • Upwelling: Essential for transporting nutrients back to the surface where light is available.

  • Temperature Turnover: In lakes, seasonal cooling makes surface water denser, causing it to sink and circulate nutrients.

• Limiting Nutrients in Marine Systems:

  • Iron Limitation: Common in the Southern Ocean due to the low solubility of iron in cold, oxygen-rich waters.

  • Phosphorus Limitation: Initially controversial because anthropogenic inputs (farming runoff/human activity) in estuaries and harbors masked its importance. Deep coastal and oceanic studies confirm phosphorus is a major limiter in pristine environments.

• Global Marine Patterns:

  • Unlike land, ocean productivity does not follow a simple latitudinal gradient.

  • Coastal Productivity: Coastlines are highly productive due to shallow water (high light), nutrient runoff from land, and the Coriolis-driven upwelling.

Nutrients and Productivity in Freshwater Systems

• Lakes:

  • Solar radiation and temperature are key, but phosphorus concentration is the primary nutrient driver.

  • Proxy Measurement: Chlorophyll concentration is used as an estimate for phytoplankton activity and overall NPP.

• Streams and Rivers (Lotic Systems):

  • Generally have lower NPP than lakes due to moving water and foliage shade (light limitation).

  • Autochthonous Systems: Organic carbon is produced within the stream (e.g., by internal plants/algae). This becomes more important as stream size increases and flow slows.

  • Allochthonous Systems: Organic carbon comes from outside the system (e.g., leaf fall from terrestrial forests).

    • DOM: Dissolved Organic Matter.

    • POM: Particulate Organic Matter.

  • In small streams, up to $100\%$ of energy can come from external terrestrial sources.

Temporal Variation and Successional Changes in Productivity

• Short-Term Variation: NPP fluctuates seasonally. Deciduous forests show zero productivity in winter when leaves are absent and a massive spike during the growing season.

• Year-to-Year Climatic Variation: Productivity can change annually based on environmental shifts.

  • UK Grassland Study: High maximum temperatures resulted in reduced yield/NPP because high temperatures increased evaporation and reduced water availability.

• Woodland Stand Age: NPP varies with the age of the forest stand (time since the last disturbance, such as fire).

  • Biomass Allocation Shifts:

    • Young Stands: Allocate resources roughly equally between foliage and wood (stem).

    • Older Stands: Foliage production decreases as a proportion of biomass. Most energy goes into maintaining a massive woody trunk (measured as Diameter at Breast Height, or $DBH$).

  • Efficiency Drop: As trees age, the biomass of the trunk/wood drastically outnumbers the leaves, leading to a quick dip in net primary productivity.

Questions & Discussion

• Question on Asymptotic Graphs: A student asked if the leveling off of the temperature/productivity graph was due to light saturation points.

  • Response: The lecturer acknowledged that light saturation could contribute, as well as biological limits when it simply becomes too hot for metabolic efficiency.

• Question on Root Productivity: A student asked if below-ground NPP is significant in terrestrial systems like grasslands.

  • Response: It is significant, as roots take up nutrients and develop underground biomass. Most textbook examples focus on above-ground NPP for ease of measurement, but total productivity includes roots.

• Variation in Decadal Data: A student asked what causes the year-to-year variation in the decadal carbon fixation charts.

  • Response: It is driven by year-to-year climatic changes (e.g., fluctuations in temperature and rainfall).

• Student Logistics (Course Change): A student mistakenly attended the lecture, thinking it was for Political Science, but the lecturer clarified it was Biology $274$. The student had recently switched courses and was looking for catch-up material, to which the lecturer directed them to the online land site for recordings and slides.

• Upcoming Test: The lecturer mentioned that a test was being scheduled and details regarding types of questions would be shared on Monday.