An Introduction to Environmental Biophysics

The discipline of environmental biophysics relates to the study of energy and mass exchange between living organisms and their environment. The study of environmental biophysics probably began earlier than that of any other science, since knowledge of organism-environment interaction provided a key to survival and progress. Systematic study of the science and recording of experimental results, however, goes back only a few hundred years. Recognition of environmental biophysics as a discipline has occurred just within the past few decades.

Rates of biochemical reactions within an organism are strongly dependent on its temperature. The rates of reactions may be doubled or tripled for each 10° C increase in temperature. Temperatures above or below critical values may result in denaturation of enzymes and death of the organism.

Terrestrial organisms live in a gaseous medium composed mostly of nitrogen and oxygen. Water vapor is present in varying amounts, and carbon dioxide and other gases, in trace amounts. Organisms exchange oxygen, carbon dioxide, and water vapor with their surroundings. Carbon dioxide is a substrate for photosynthesis and oxygen is a product; while oxygen is a substrate for respiration and carbon dioxide is a product. Exchange of these gases with the environment is therefore a requirement for life. Water vapor almost always moves from the organism to the environment. The humidity of the organism is near 100 percent, while the surroundings are nearly always much drier. The organism must remain in a highly hydrated state in order for biochemical reactions to occur, so the constant loss of water is a threat to survival, and frequent access to liquid water is a necessity for most terrestrial organisms. The intake of liquid water and the loss of water vapor to the environment are usually the most important components of the water budget of an organism.

Almost all of the water in living organisms is liquid, rather than vapor. In addition, water is taken up from the organism environment mainly in the liquid phase. Good physical descriptions of water in the liquid phase are necessary to understand liquid-phase water exchange and organism response. The energy state of liquid water can also affect the vapor pressure and concentration of water at evaporating surfaces. Vapor exchange is therefore also influenced by the state of the liquid water.

As living organisms, we are most acutely aware of three things about the wind. We know that it exerts a force on us and other objects against which it blows, it is effective in transporting heat from us, and it is highly variable in space and time. A fourth property of the wind, less obvious to the casual observer, but essential to terrestrial life as we know it, is its effective mixing of the atmospheric boundary layer of the earth. This can be illustrated by a simple example. On a hot summer day about 10 kilograms (550 moles) of water can be evaporated into the atmosphere from each square meter of vegetated ground surface. This amount of water would increase the vapor concentration in a 100 m thick air layer by 100 g m⁻³ (136 mmol/mol) if there were no transport out of this layer or condensation within it. This is much more water than the air could hold at normal temperature. The observed increase in vapor concentration in the first 100 meters of the atmosphere is typically less than 1 g m⁻³, so we can see how effective the atmosphere is for transporting and mixing. A similar calculation (Monteith, 1973) shows that photosynthesis in a normally growing crop would use all of the CO₂ in a 30 m air layer above a crop in a day, yet measured CO₂ concentrations have diurnal fluctuations of 15 percent or less. Without the vertical turbulent transport of heat, water vapor, CO₂, oxygen, and other atmospheric constituents, the microenvironment we live in would be very inhospitable.

Life depends on heat and mass transfer between organisms and their surroundings. Such processes as carbon dioxide exchange between leaves and the atmosphere, oxygen uptake by micro-organisms, oxygen and carbon dioxide exchange in the lungs of animals, or convective heat loss from the surfaces of animal coats are fundamental to the existence of life. A thorough understanding of these exchange processes is therefore a necessary part of the study of biophysical ecology.

This chapter continues the discussion of transport, and focuses on methods for computing the conductances and resistances needed for the calculations in Ch. 6. We first discuss conductances on the smallest spatial scale; molecular diffusion. It is by this process that heat and mass are transported in still air or water, such as in parts of the lungs of animals, in soils, in the substomatal cavities of leaves, and in animal coats. The equations for turbulent transport of heat and mass on larger scales in the atmosphere are similar to those for molecular diffusion, so those equations are discussed following the molecular diffusion equations. After diffusion processes are discussed, we consider an intermediate scale; namely, convective heat and mass transfer theory as it applies to fluids moving over plates, cylinders, and spheres (simulating leaves, stems, fruits, and animals).

When the sun shines on the soil surface, some of the energy is absorbed, heating the soil surface. This heat is lost from the surface through conduction to lower layers of the soil, through heating the atmosphere, and through evaporation of water. Heat transport from the surface to the atmosphere was discussed in Ch. 7. This chapter considers heat transport into the soil. Some of the results from an analysis of heat transport in soil are presented in Ch. 2 to show typical temporal and spatial patterns of soil temperature. Here we show how those equations are derived and how they depend on soil properties.

The final transport equation that we need to consider is Darcy’s law (Eq. (6.4)). This law describes the transport of water in porous materials such as soils. Darcy’s law describes most of the water flow that takes place in soils. Since water plays such an important role in the energy balance of soils, plants, and animals, an understanding of at least some simple applications of Darcy’s law is important to environmental biophysicists. The processes that are important in determining the water budget of a soil are infiltration of applied water, redistribution of water in the soil profile, evaporation of water from the soil surface, and transpiration of water by plants.

The modes of energy transport discussed so far (conduction, convection, and latent heat) all are somewhat intuitive. Radiative energy transport, on the other hand, is not intuitive at all. Radiant energy is transferred by photons, discrete bundles of electromagnetic energy that travel at the speed of light (c = 3 × 10¹⁰ m/s in vacuum) and behave both as particles and waves. These photons are emitted or absorbed by matter as a result of quantum jumps in electronic energy levels in atoms, or changes in vibrational and rotational energy levels in molecules. The wavelength of the radiation is uniquely related to the photon energy in an equation due to Planck: where h is Planck’s constant (6.63 × 10⁻³⁴ J s) and λ is the wavelength of the photon. Thus green photons, having a wavelength of 0.55μm would have an energy

Before beginning a detailed discussion of radiant energy budgets of plants, animals, canopies, and soils, we need to determine what information will be required and how to obtain that information. An environmental biophysicist may approach the study of radiant energy exchange in two different ways. For the first, detailed observations of radiant flux densities to and from an organism are needed to compute a detailed energy budget. These detailed observations must be obtained by direct measurement at the time the energy budget is being determined. The second type of study simulates the behavior of parts of an ecosystem. Knowing the exact value of a radiant flux density may not be as important as having the correct relationship among variables. Models of the fluxes, extended from the basics covered in Ch. 10, are used for studies of the second type. The models can be counted on to give reasonable estimates (± 10%) of average flux densities, but they are usually not adequate as substitutes for careful field measurements of radiant fluxes for detailed energy budget studies. This chapter presents models for estimating solar and thermal radiant fluxes in the natural environment.

The principles discussed thus far become more meaningful as they are applied to problems in nature. The first problem considered is that of describing the fitness of the physical environment for survival of an animal whose requirements we specify. Survival of the animal can depend on many factors; we consider only those related to maintaining body temperature within acceptable limits and those related to maintaining proper body water status. Even these aspects are only discussed to a limited extent. For example, maintenance of body temperature in endotherms (animals which maintain body temperature through internal metabolic heat production) involves production of metabolic heat. Stored chemical energy from the animal’s food is used to produce the heat, so availability of food in the environment could be construed as part of the animal’s physical environment. Food availability does not enter into our discussions in this way, but we do compute the amount of food an endotherm needs in order to maintain constant body temperature. Food requirements are of interest to those modeling ecosystems as well as those managing range lands for wild or domestic animals.

Human-environment interaction involves the same principles discussed in Ch. 12. However, we need to look at three additional factors. They are clothing, sweating, and comfort. These are examined by considering survival in cold environments, survival in hot environments, and the human thermoneutral energy budget. The variables that need to be considered are metabolic rate, surface area, latent heat exchange, body temperature, and body (clothing and tissue) conductance.

Our discussions in Chs. 12 and 13 focus on determining which environments were energetically acceptable to animals and on energetic costs of living in those environments. Similar questions apply to the study of plants and plant communities. In this chapter we are interested in the environmental factors that determine temperatures and transpiration rates, and in the factors that control carbon assimilation. The energy budget again plays the central role in these analyses.

In Ch. 14 plant canopies are treated as big leaves. We did not worry about their structure or the details of how the leaves make up the canopy, we just assumed that we could find a canopy conductance for vapor and boundary layer conductances for heat and vapor. Combining these with the absorbed radiation and soil heat flux densities allowed us to compute canopy temperatures and transpiration rates. We even estimated carbon assimilation rates by knowing transpiration rate or light interception.