Pools and Fluxes

Final Lecture - Pools and Fluxes

  • Overview of Lecture

    • Focus on the concepts of pools and fluxes, important for understanding nutrient movement in both aquatic and terrestrial ecosystems.

    • Reminder of the reading by Janet Compton about mass balance analysis of nitrogen in the Willamette Valley.

  • Learning Outcomes

    • Understand the concepts of pools and fluxes alongside residence time.

    • Fundamental understanding of mass balance analysis, particularly in streams.

Definitions

  • Pool

    • Definition: A standing stock or amount of a given element or resource within an area of interest.

    • In this context, examples include water and dissolved elements or molecules.

    • Pools can be expressed in terms of mass (e.g., kilograms) or volume (e.g., cubic meters).

    • May be expressed as standardized values per unit area.

  • Flux

    • Definition: The movement or transformation of a given element or resource from one pool to another.

    • Includes:

    • Physical movement of material (e.g., water flowing from one area to another).

    • Changing forms of elements (e.g., nitrogen in its gas form converting to bioavailable forms).

    • For simplicity, initial discussions focus mainly on the physical movement of water since elements are often dissolved in water.

Importance of Studying Pools and Fluxes

  • Quote from Terry Chapin's "Terrestrial Ecosystem Ecology":

    • The flow of energy and materials through organisms and the physical environment provides a framework for understanding the diversity and form and function of earth's physical and biological processes.

    • Understanding how elements, resources, and energy (often in the form of fixed carbon) move through an environment aids in comprehending the ecosystem and its organisms.

Bathtub Analogy for Pools and Fluxes

  • Basic Model

    • Inputs: Water entering the bathtub at a rate of 10 m³/s.

    • Pool Volume: Bathtub holds 1 x 10⁹ m³ of water.

    • Outputs: Water exiting the bathtub at a rate of 10 m³/s.

    • This setup presents a mass balance analysis where the input must equal the output for the system to be in "steady state".

  • Steady State Assumption

    • Definition: If inputs equal outputs, the system is considered to be in balance despite ongoing changes.

    • If inputs decrease or increase, outputs must follow suit to maintain balance.

  • Example of Budget Closing

    • If input decreases to 7 m³/s, then investigation begins to identify where water is being lost (e.g., crack or evapotranspiration).

    • Closing a budget means attempting to balance inputs with outputs under steady state conditions.

Residence Time

  • Definition: The average time a quantity of something remains in a specific pool.

    • Note: This is the average and doesn't mean all materials stay for the same duration.

    • Formula:

    • ext{Residence Time} = rac{ ext{Average Pool Mass}}{ ext{Flux Rate Out of the Pool}}

    • Alternatively, under steady state, can use flux rate into the pool.

  • Example Calculation of Residence Time

    • For instance, water in the bathtub:

    • Average pool mass = 1 x 10⁹ m³.

    • Flux rate out = 10 m³/s.

    • Calculation: ext{Residence Time} = rac{1 imes 10^9 ext{ m}^3}{10 ext{ m}^3/ ext{s}} = 3.17 ext{ years}

    • Average residence time of water in the bathtub is approximately 3.17 years.

Conceptual Framework: Boxes and Arrows

  • In ecosystem ecology, systems are often simplified to be represented as boxes (pools) and arrows (fluxes).

  • Example Structure:

    • Input Mass → Box (Pool) → Outputs (Fluxes).

Real-World Example of Mass Balance Analysis

  • Example: Analyzing nitrogen loading into a water body.

    • Total Maximum Daily Load (TMDL): E.g., 5 kg/day allowed into a reservoir.

    • Inputs:

    • Discharge: 6 L/s with a concentration of 10 mg/L computes to an annual total.

      • 6 ext{ L/s} imes 10 ext{ mg/L} o 518,400 ext{ mg/day} = 5.18 ext{ kg/day}.

      • Given TMDL is exceeded, indicating potential pollution issues.

  • Mass Balance Calculation for Streams

    • Addresses situation where nitrogen might be polluted by development or other activities.

    • Each stream's nitrate concentration yields the total incoming load calculated against the outlet to see if the system is balanced.

Discussion of Variable Inputs and Outputs

  • Evaluating if nitrogen pollution and development around a lake correlate requires assessing inputs over time.

  • Possible Methods:

    • Examine historical data trends comparing nitrogen loads and development rates.

    • Use groundwater sampling to assess contamination tied to nearby land use.

Streams and Nutrient Uptake

  • Fundamental Mass Balance in Streams

    • Mass flowing downstream must balance with flow upstream unless influenced by tributaries or groundwater inputs.

  • Differences Between Conservative and Non-Conservative Constituents

    • Conservatively Transported Constituents (e.g., chloride):

    • Mass upstream + contributions (from groundwater or tributaries) - mass downstream = 0.

    • Non-Conservatively Transported Constituents (e.g., nitrogen, phosphorus):

    • Change in mass will reflect biological or abiotic processes occurring along the stream reach after inputs and outputs are considered.

Conclusion

  • Upcoming lecture will focus on nutrient uptake and further examining nutrient pathways and fates within aquatic systems.