INTRODUCTION TO ENVIRONMENTAL SCIENCES

LECTURE NOTES

Ecosystems I: Systems, Matter, and Energy

I. Systems and Models

System: A set of components that operate in a connected and predictable way. It is a defined, physical part of the universe (e.g. the human body, a lake, the atmosphere).

Large systems are often made of many smaller systems.

Use system models to simulate in a controlled way the system being studied. The model can be a scaled down version of the real system (scale-model of a river), a mathematical model (equations in a computer climate model), a graphical model (drawing of a process), or a mental model (an idea or conception of how something works).

Models improved by comparing predictions with real world results and then adjusting model to force it to predict what really happened. You keep doing this until the model prediction matches what actually happened. This is why weather forecasting is getting better. However, all models are, by definition, simplifications of the real world. Conclusions reached only as good as the validity of the model and the accuracy of the measured data. That is, garbage in, garbage out!

A. System Components

All environmental systems have inputs (matter, energy, and information), throughputs (flows), and outputs (wastes). The latter may become inputs for other systems. For example, we put food in our mouth (input), metabolize it (throughput) and expel wastes (output). This waste then becomes the input to a waste treatment system or bacteria.

Environmental systems often have feedback loops where a system output is used as an input to the same system (aluminum cans are recycled back to the aluminum smelter to make new aluminum products). Feedback loops can be either positive or negative in nature (no moral judgment implied).

A positive feedback loop reinforces the behavior of the system. For example, a dog barks, scares birds, they fly away, excites dog, it barks even more, scares more birds, more fly away, dog gets even more excited, barks even more, and on and on.

If left unchecked causes an uncontrolled run-away situation (dog suffers a stroke!).

A negative feedback loop works against the behavior of the system. For example, on a hot day you sweat, this causes evaporation, this cools you down, and this causes you to stop sweating.

Can act as a check on a positive feedback loop.

Most environmental systems have both types of feedback loops. Leads to homeostasis (equilibrium) where a constant condition is maintained. Does not mean nothing is happening, just that everything is in balance. Can be a very dynamic, complex, and fragile situation.

II. Matter

Matter is anything that has mass and takes up space. Includes all solids, liquids, and gases. Made up of atoms, either as single elements (C) or as compounds with multiple elements (H2O). Can be combined into mixtures.

A. Organic and Inorganic Compounds

Organic compounds: Carbon based. Include hydrocarbons (C and H; methane, gasoline), chlorinated hydrocarbons (contain C, H, Cl; DDT and PCBs), Chlorofluorocarbons (C, Cl, F; Freon), and simple carbohydrates (C, H, O; glucose).

Can link simple organic molecules (monomers) to form complex ring and chain organic compounds (polymers). These include complex carbohydrates (starch, cellulose), proteins, DNA, and RNA.

Inorganic compounds: Not based on carbon (although they may contain it). Include minerals, water, and gases. Some may polymerize (silicate minerals).

B. Matter Quality

Subjective classification. High-quality matter is organized, concentrated, easily exploited and useful to us (oil deposits). Low-quality matter is dilute, dispersed, harder to get at, and not particularly useful (methane in the atmosphere).

III. Energy

Energy is the capacity to do work or transfer heat. There are a variety of different forms.

Kinetic Energy: Energy due to motion. Because E=1/2MV², the greater the mass or velocity, the greater the energy. Includes electromagnetic energy (radiation) such as visible light, x-rays, UV, radio waves, etc. Shorter wavelengths (higher energy) than visible light are ionizing (can knock electrons out of atoms) and are a health risk. Longer wavelengths (lower energy) than visible light are non-ionizing.

Also includes heat (thermal) energy, which is due to all the internal motion of molecules and atoms in a substance. It is measured by temperature.

Potential Energy: Stored energy due to position. Includes chemical energy stored in bonds.

A. Energy Quality

This is a subjective evaluation. High-quality energy can do useful (at least to humans) work. Usually concentrated in a limited volume. Low-quality energy does less useful work because it is very dispersed (atmospheric heat).

Try to match the quality of the energy with the intended task. Don't use a nuclear reactor to heat your hot water tank.

IV. Conservation of Energy and Matter

Matter and energy can be neither created nor destroyed. What you start with is what you wind up with. There is no away when you throw something away.

However, matter and energy can be changed from one type to another (kinetic to thermal or potential to kinetic) and even converted, one to the other (E=MC²). You just can't create matter or energy out of nothing or return it to nothing. Matter and energy are two sides of the same coin.

Matter can be changed physically (shape, size, or state) or chemically (composition). However, none of the atoms are created or destroyed, just rearranged. Energy also is required to drive the change.

A. Energy Quality Change

Converting energy from one form to another always decreases the total energy quality. Useful energy converted to less useful energy, often in the form of waste heat to the surrounding environment. The conversion is usually an inefficient process. For example, only 10% of the potential energy in gasoline is converted to kinetic energy of a car in motion. Rest lost as heat (engine, transmission, tires). Only 5% of electricity in a bulb converted to light. Rest lost as heat (really a heat bulb).

Heat constantly lost as energy is transferred up the food chain.

It is hard to recycle energy because it is always being converted to lower quality. Therefore, systems constantly need a new source of high-quality energy. True for individuals (need to eat), societies (need more fuels to burn), and ecosystems (need a constant supply of solar and/or chemical energy to maintain them).

V. Large-Scale Earth Systems

Atmosphere: Envelope of gases surrounding planet. Mostly composed of nitrogen (78%) and oxygen (21%). The rest is argon, neon, CO2, and other gases.

Hydrosphere: All water at or near earth's surface. Most is in the oceans. Rest is in ice, streams, lakes, groundwater, and atmosphere.

Lithosphere: Earth's brittle outer shell (up to 100+ km). Includes continental and oceanic crusts and upper mantle. It is the source of most important resources.

Biosphere or Ecosphere: All living organisms. Found in all of the hydrosphere, lower atmosphere, and upper lithosphere.

The presence of the latter requires the prior presence of the first three. Also need renewable, high-quality energy sources, such as photosynthesis and chemosynthesis, and gravity to keep atmosphere from escaping.

B. Solar Energy

It is the ultimate source of most high-quality energy in the biosphere. Chemosynthetic bacteria and earthπs internal geothermal heat supply the rest.

Earth receives one billionth of total solar output. 34% reflected back into space (earth's albedo). Rest is used to warm atmosphere, earth's surface, evaporate water, create wind, and in photosynthesis (0.023%). All of it ultimately winds up as low-quality heat energy that is reradiated back into space and, therefore, must be continually replaced.

C. Nutrient or Biogeochemical Cycles

Nutrient: Any matter required by an organism. Some (C, H, N, P, S, Ca, Mg, Fe) needed in large quantities. Others (Zn, Cu, Cl, Mn, I) are needed in trace quantities.

These are all cycled through the four spheres in closed loops. Constantly recycled. Cycles are powered by solar energy and earth's internal heat.

In summary, in all four of the Earth's large-scale systems, matter is recycled, but energy is lost.

VI. Ecosystems

A. What is an Ecosystem?

Ecosystem: A community of different species interacting with one another and their surrounding physical environment. Size may vary (stream, lake, forest, prairie) and they may be natural (glacial lake) or artificial (Centennial ≥Lake≤).

Ecology is the study of ecosystems and their component parts.

Ecosystems made up of individual communities of species, which are made up of populations of individual species, which are made up of individual organisms.

Individual ecosystems overlap with adjoining ones at ecotones (for example, a marsh between a grassland and a lake). These can be considered as transitional ecosystems.

Multiple ecosystems combine to form terrestrial environments, known as biomes (forest, mountain, savanna), or aquatic life zones (stream, estuary, deep ocean).

Large amounts of biodiversity (genetic, species, and ecological) found in all the various ecosystems. Assures future evolutionary adaptations. As ecosystems are destroyed, diversity is lessened, as is the possibility for future adaptations. Extinctions become more likely.

B. Ecosystem Components

Two major divisions, biotic (living) and abiotic (nonliving)

Living organisms are either producers (autotrophs) or consumers (heterotrophs).

Producers (plants, algae, bacteria) make their own food via photosynthesis (CO2 + H2O + solar energy > glucose + O2) or chemosynthesis (H2S + CO2 + geothermal energy > organics).

Consumers feed on other organisms. They may be primary consumers (herbivores or plant eaters), which feed directly on producers, or secondary consumers (carnivores), which feed on other consumers. Tertiary consumers feed on other carnivores.

Omnivores are both herbivores and carnivores. Scavengers feed off dead carcasses.

Detritivores live off dead fragments and waste. They are either detritus feeders (small pieces) or decomposers who break down detritus to organic nutrients that can be reused by producers. Dust to dust, ashes to ashes.

Energy stored in organic nutrients released to an organism either by aerobic (glucose + O2 > CO2 + H2O +energy) or anaerobic (no water or CO2 produced) respiration.

Abiotic components are the physical and chemical variables of the surrounding environment. Include climate (temperature, precipitation, humidity, sunlight, altitude, latitude, wind), currents, soil nutrients, and salinity.

Every species can tolerate a range of physical and chemical variables around an optimum set of conditions. Exceed that range (tolerance limits) and the species cannot survive. Some species have a very wide range of tolerances, others a very narrow one.

Some species can acclimate slowly to a condition normally outside its tolerance limits. Others are fine until a finite limit is reached (threshold effect).

Often one environmental factor is more important than others. This is the limiting factor where a particular species has a very limited tolerance. For example, a plant tolerates a wide range of temperature, soil moisture, and sunshine conditions, but requires a very specific level of a particular nutrient (high potassium for dandelions).

VII. Energy Flow in Ecosystems

Energy flows through an ecosystem via the food chain (each organism is a source of food for the next). Each link in the food chain is a feeding or trophic level. Actually, it is usually much more complex than a simple chain and is more like an interconnected web. Because no organism is 100% efficient in utilizing the energy stored in its food, energy always is lost in going from one trophic level to another.

A. Energy and Biomass Pyramids

Typically only 5%-20% of the energy at one trophic level gets transferred to the next one. This means each level has much less energy to work with. Graphically shown as a pyramid of energy flow.

Since there is less energy at each trophic level, there also tends to be less biomass (dry weight of all organic matter) at each one. However, in some ecosystems, producers are eaten almost immediately by primary consumers. Therefore, there is little of the former, but much of the latter.

Pyramid of numbers (of organisms) usually looks similar, although in some instances a few big producers (trees) can support a large population of primary consumers.

In general, as you go from one trophic level to the next, the number of species and individuals declines. Reason why there usually are only a few carnivores at the top of the food chain, but a lot of prey and even more producers on which the prey feeds.

B. Productivity Rates.

Rate at which energy is converted by producers from solar to energy stored in the biomass is the gross primary productivity (GPP). Typically GPP is highest in shallow marine environments.

However, the producers use some of this energy to support their metabolic activity. Have to subtract it from the GPP. Get net primary productivity (NPP). This is what is available to consumers at higher trophic levels. Measured in kcal/m²/yr (energy rate) or g/m²/yr (mass rate).

Various ecosystems have widely different NPP values.

The earth's total NPP limits the number of consumers (including us) that can be supported. The earth has a finite carrying capacity of organisms. We now consume about 27% of the earth's total potential NPP. What happens if our population doubles? Quadruples?

VIII. Matter Flow in Ecosystems

All matter cycles through the various ecosystems. They are not one-way flows like energy. Five major cycles (C, N, P, S, H₂O). Each has different paths and sinks (storage sites or reservoirs)

A. Carbon Cycle

Major sinks are the atmosphere (CO₂, actually only a very small amount of carbon is stored here), the lithosphere (carbonate rocks and fossil fuels, the largest sink), the hydrosphere (CO₂, carbonate, and bicarbonate molecules in water), and as biomass (carbon based organic molecules) in the ecosphere.

Humans are upsetting this cycle by burning fossil fuels and cutting down forests. Increasing CO₂ in our atmosphere very rapidly (30+% in last 150 years). The flow to other sinks (oceans) is not fast enough to offset this (requires thousands of years). CO₂ reduces heat lost to space and, therefore, results in warming of the atmosphere.

B. Nitrogen Cycle

Most nitrogen in the atmosphere is in the form of N₂. This gaseous molecule is not reactive and, therefore, needs to be converted to a usable, reactive form. Done by nitrogen fixation (by bacteria and lightning). Forms ammonia (NH₃) and ammonium (NH₄). Then converted to nitrite (NO₂) and nitrate (NO₃). Nitrate is easily used by plants and then by consumers

Returned to the atmosphere in the form of N₂ by ammonification (by decomposers) and then denitrification (by bacteria) processes.

Humans are upsetting this cycle by emitting large amounts of nitric oxides into the atmosphere. Combines with water to make nitric acid, which forms acid rain. We also add excess nitrogen into rivers, lakes, and oceans because of the agricultural runoff of fertilizers, soil erosion, and animal wastes (including our own). This stimulates algal growth, which when they die and decay, depletes water of oxygen (anoxic), making it incapable of supporting higher trophic levels (eutrophic condition).

C. Phosphorus Cycle

P mostly cycles through the environment in the form of the phosphate (PO₄) molecule with very little in the atmosphere. Most phosphate is cycled between the ocean and land, to the biomass, and then back to ocean and land. Moves slowly through ocean and land, but quickly through organism (constantly being excreted). Can be stored for long periods of time (millions of years) in phosphate rocks. Phosphate level often is a limiting factor for plant and algal growth.

Humans are affecting this cycle by destruction of tropical rain forests, causing the rapid loss of phosphorus (stored in detritus and leached from soil). Also adding excess phosphorus to aquatic systems for same reason as nitrogen. Same impacts.

D. Sulfur Cycle

Much of it is stored in the lithosphere as sulfide and sulfate minerals. Returned to the atmosphere as hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) during volcanic eruptions. In the atmosphere, SO₂ reacts with water to form sulfuric acid (H₂SO₄) and acid rain. Utilized by producers in the form of sulfate (SO₄)

Humans are adding huge volumes of SO₄ to the atmosphere by the burning of sulfur-bearing fossil fuels (coal and oil) and by the refining of sulfide minerals for their metals.

E. Hydrological Cycle

Water molecules are cycled through the atmosphere, lithosphere, and hydrosphere by a sequence of evaporation, transpiration, condensation, precipitation, infiltration, percolation, and runoff. May take thousands of years for a single water molecule to go through entire cycle.

97.5 % of the Earthπs water is in the oceans. Only a small fraction is in surface waters (lakes and rivers). Most fresh, liquid water is in the groundwater.

Humans affecting the cycle by polluting surface water and groundwater, over consuming these same sources, and by increasing surface runoff and decreasing infiltration. Reduces available groundwater and increases flooding, landslides, and soil erosion.