Head over to the refrigerator and pour a glass of apple juice for yourself.
Go to the kitchen sink. Place an empty bowl under the tap and turn on the tap.
Grab a balloon and blow air into it.
All three activities are examples “pools” and “fluxes” in action.
Ecosystem analysis—the study of ecosystems, their structure and functions—seeks to understand how energy and matter behave in ecosystems. To describe this behavior, ecologists conceptualized pools and fluxes.
What are ecosystem pools?
Pools (or stocks or reservoirs) describe the storage spaces for energy and matter*.
We try to measure the quantity of energy and matter stored. Pools are usually common for different forms of energy and matter. For example, oceans are carbon pools as well as water pools. Other examples of pools include: soil, lakes, and the atmosphere.
What are ecosystem fluxes?
Fluxes describe the movement of energy and matter between different pools.
When you pour a glass of juice for yourself, the juice bottle and the glass are both “pools” for the apple juice. The act of pouring it from the bottle to the glass is the flux.
Similarly, the tap and the bowl inside the kitchen sink are pools for water. When you run the tap to fill the bowl, you are watching a water “flux” in action.
When you blow air into a balloon, you are helping the air flux from one pool—your lungs—to another: the balloon.
A single ecosystem process usually facilities fluxes for different forms of energy and matter. For example, photosynthesis facilities fluxes for energy, oxygen and carbon. Other examples of fluxes include: respiration, evapotranspiration and the food web.
During fluxes, energy and matter may transform from one form to another. Consider Nitrogen fluxes in ecosystems. Nitrogen is present in the form of N2 in the atmospheric pool. Nitrogen-fixing bacteria converts N2 into NH3 through various processes, but the base matter—nitrogen—remains the same.
How do we measure ecosystem pools?
Pools are easy to measure and study. Every part of the ecosystem is a pool for some form of energy or matter. Even microscopic bacteria contain mitochondria, which is a storehouse for energy in the form of ATP. To study pools, we need to identify them, and quantify the energy/matter they store.
Pools are quantified as mass (for solid matter like carbon), temperature (for heat) or concentration (for trace gases) over a defined boundary.
A simple example is you! When we say that 70% of the human body’s weight is made of water, we are staying that your body is a pool, storing 70% of your weight in the form of water.
How do we measure ecosystem fluxes?
Measuring fluxes is more complicated, because of the various forms in which energy and matter can move through an ecosystem. We also need to define the temporal and spatial scale over which fluxes will be measured.
To help you appreciate this complexity, consider this example.
Suppose you want to study the carbon dioxide flux (CO2) over a tree canopy. This requires us to observe how leaves of the tree interact with the atmosphere, and understand the internal leaf processes to come up with an accurate estimation of the flux.
But, each leaf behaves differently based on where it is located in the canopy. CO2 flux from a leaf varies based on 1) the structure of the leaf, 2) the position of the leaf in the canopy, 3) the orientation of the leaf towards the sun (which controls its rate of photosynthesis) and 4) the relative amount of shade it experiences (source).
Temporally, CO2 flux varies throughout the day. CO2 absorption is greater during mornings and afternoons, when leaves photosynthesize. During the night, respiration is dominant and leaves release more CO2 than they absorb.
It is impossible to measure each leaf in the canopy. So, this measurement requires some amount of experimental gymnastics to arrive at representative figures.
Fluxes are measured as changes in mass/concentration/temperature per unit area/volume per unit time.
Different methods are used to measure fluxes, depending on our spatial and temporal boundaries.
Biomass and carbon flux in forests/trees are often measured by their body size, and by calculating the Net Primary Productivity (NPP). These techniques are useful when we are describing fluxes annually.
For smaller spatial scales of biomass (like a leaf or a patch of soil) or gas exchanges from soil, static and dynamic chambers are used. These chambers help us observe fluxes over small temporal scales, of the order of a few hours and days.
Modelling is used for studying water fluxes, air fluxes within the atmosphere and large-scale energy fluxes. Modelling is versatile and can be employed over a range of spatial and temporal scales. For example, water fluxes are studies through water balance studies, and describe water fluxes over many timescales. The spatial unit is often a watershed. For this technique to work, comprehensive data are required as inputs into the model.
For flux measurements—especially gaseous fluxes—over an entire ecosystem or a landscape in small temporal scales, the eddy covariance method is employed. This method describes the movement of air currents in a defined system and measures concentrations of specific gases. Eddy covariance is measured through flux towers, and a global network of flux towers feeds into the FLUXNET system to create integrated datasets.
Why should we study of pools and fluxes?
Some of the major applications of this concept is in:
Climate science: Pools and fluxes are overwhelmingly investigated in climate science to understand the movement of GHG gases, carbon and water in the world’s ecosystems. For example, take the carbon pool and flux studies. Pools can describe how carbon is stored in the atmosphere and oceans. If the concentration of CO2 in the atmosphere is described as 400 ppm, it means that the atmosphere, as a carbon pool, holds 400 molecules of CO2 gas per million molecules of air. Then, fluxes can give us an insight into how these pools interact—how carbon moves from soil to the atmosphere to oceans. This insight helps us predict how carbon pools will respond to changing fluxes and if this response would have a positive or a negative impact on the climate.
Environment management: By understanding the threshold of pools, we can determine how much energy and matter a pool can store and if at all we can enhance them. The associated fluxes can then be regulated in order to optimize pools. For example, UNFCCC’s REDD+ program was developed to improve and maintain carbon pools in forest ecosystems. Similar applications are found in water supply management for cities.
Agricultural sciences: Pools and fluxes can help us understand nutrient flows in ecosystems. Nutrient flows are vital in agricultural sciences. Farmers often employ too much or too little nutrients to the soil and cannot reap the benefits of modern agriculture practices. To scientifically rationalize the use of nutrients, we need to understand how the soil stores nutrients and how nutrients move within the soil and into plant systems.
This concept is critical in ecosystem analysis today. Humans continue to alter ecosystems. Only by thoroughly understanding how these changes impact natural ecosystem processes can we take remedial measures.
*Energy usually manifests itself in ecosystems in the form of heat, and matter is present in the form of nutrients and gases.