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Hydrologic and Carbon Cycles
Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic molecules are carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur. These elements take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth’s surface.
Geologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates, all play a role in this recycling of materials. Because geology and chemistry are involved in this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle.
Water contains hydrogen and oxygen, which is essential to all living processes.The hydrosphere is the area of the Earth where water movement and storage occurs: as liquid water on the surface and beneath the surface or frozen (rivers, lakes, oceans, groundwater, polar ice caps, and glaciers), and as water vapor in the atmosphere. Carbon is found in all organic macromolecules and is an important constituent of fossil fuels. Nitrogen is a major component of our nucleic acids and proteins and is critical to human agriculture.
Phosphorus, a major component of nucleic acid (along with nitrogen), is one of the main ingredients in artificial fertilizers used in agriculture and their associated environmental impacts on our surface water. Sulfur, critical to the 3-D folding of proteins (as in disulfide binding), is released into the atmosphere by the burning of fossil fuels, such as coal.
The cycling of these elements (hydrogen, oxygen, carbon, nitrogen, phosphorus and sulfur), is interconnected. For example, the movement of water is critical for the leaching of nitrogen and phosphate into rivers, lakes, and oceans. Furthermore, the ocean itself is a major reservoir for carbon. Thus, mineral nutrients are cycled, either rapidly or slowly, through the entire biosphere, from one living organism to another, and between the biotic and abiotic world.
Water is the basis of all living processes. The human body is more than 1/2 water and human cells are more than 70 percent water. Thus, most land animals need a supply of fresh water to survive. When examining the stores of water on Earth, 97.5 percent of it is non-potable salt water. Of the remaining water, 99 percent is locked underground or as ice. Thus, less than 1 percent of fresh water is easily accessible from lakes and rivers.
Many living things, such as plants, animals, and fungi, are dependent on the small amount of fresh surface water supply, a lack of which can have massive effects on ecosystem dynamics. Humans, of course, have developed technologies to increase water availability, such as digging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, the supply of fresh water is still a major issue in modern times.
Water cycling is extremely important to ecosystem dynamics. Water has a major influence on climate and, thus, on the environments of ecosystems, some located on distant parts of the Earth. Most of the water on Earth is stored for long periods in the oceans, underground, and as ice. Residence time is a measure of the average time an individual water molecule stays in a particular reservoir. A large amount of the Earth’s water is locked in place in these reservoirs as ice, beneath the ground, and in the ocean, and, thus, is unavailable for short-term cycling (only surface water can evaporate).
There are various processes that occur during the cycling of water. These processes include the following:
• Subsurface water flow
• Surface runoff/Snowmelt
The Carbon Cycle
Carbon is the second most abundant element in living organisms. Carbon is present in all organic molecules, and its role in the structure of macromolecules is of primary importance to living organisms. Carbon compounds contain especially high energy, particularly those derived from fossilized organisms, mainly plants, which humans use as fuel. The carbon cycle has two interconnected sub-cycles: one dealing with rapid carbon exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic processes.
Since the 1800s, the number of countries using massive amounts of fossil fuels has increased. Since the beginning of the Industrial Revolution, global demand for the Earth’s limited fossil fuel supplies has risen; therefore, the amount of carbon dioxide in our atmosphere has increased.
This increase in carbon dioxide has been associated with climate change and other disturbances of the Earth’s ecosystems and is a major environmental concern worldwide. Thus, the “carbon footprint” is based on how much carbon dioxide is produced and how much fossil fuel countries consume.
Living organisms are connected in many ways, even between ecosystems. A good example of this connection is the exchange of carbon between autotrophs and heterotrophs within and between ecosystems by way of atmospheric carbon dioxide. Carbon dioxide is the basic building block that most autotrophs use to build multi-carbon, high energy compounds, such as glucose.
The energy harnessed from the sun is used by these organisms to form the covalent bonds that link carbon atoms together. These chemical bonds thereby store this energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid, HCO3 ). No matter how carbon dioxide is acquired, a by-product of the process, is oxygen.
The photosynthetic organisms are responsible for depositing approximately 21 percent oxygen content of the atmosphere that we observe today.
Heterotrophs and autotrophs are partners in biological carbon exchange (especially the primary consumers, herbivores).Heterotrophs acquire the high-energy carbon compounds from the autotrophs by consuming them, and breaking them down by respiration to obtain cellular energy, such as ATP. The most efficient type of respiration, aerobic respiration, requires oxygen obtained from the atmosphere or dissolved in water. Thus, there is a constant exchange of oxygen and carbon dioxide between the autotrophs (which need the carbon) and the heterotrophs (which need the oxygen).
The movement of carbon through the land, water, and air is complex, and in many cases, it occurs much more slowly geologically than as seen between living organisms. Carbon is stored for long periods in what are known as carbon reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), ocean sediment, soil, land sediments (including fossil fuels), and the Earth’s interior. As stated, the atmosphere is a major reservoir of carbon in the form of carbon dioxide and is essential to the process of photosynthesis.
The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each location, and each one affects the other reciprocally. Carbon dioxide (CO2) from the atmosphere dissolves in water and combines with water molecules to form carbonic acid, and then it ionizes to carbonate and bicarbonate ions.
The equilibrium coefficients are such that more than 90 percent of the carbon in the ocean is found as bicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate (CaCO3), a major component of marine organism shells. These organisms eventually form sediments on the ocean floor, which over geologic time, forms limestone, which comprises the largest carbon reservoir on Earth.
On land, carbon is stored in soil as a result of the decomposition of living organisms or from weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically decomposed remains of plants that take millions of years to form. Fossil fuels are considered a non-renewable resource because their use far exceeds their rate of formation. A nonrenewable resource, such as fossil fuel, is either regenerated very slowly or not at all.
Another way for carbon to enter the atmosphere is from land (including land beneath the surface of the ocean) by the eruption of volcanoes and other geothermal systems. Carbon sediments from the ocean floor are taken deep within the Earth by the process of subduction: the movement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano erupts or from underwater volcanic hydrothermal vents.
Carbon dioxide is also added to the atmosphere by the animal husbandry practices of humans.
The large numbers of land animals raised to feed the Earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming practices and respiration and methane production. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way.
The movement of mineral nutrients through organisms and their environment is called a _____________ cycle.
a. biological, b. bioaccumulation, c. biogeochemical, d. biochemical
Answer C) The recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle.
The majority of water found on Earth is:
a. ice, b. water vapor, c. fresh water, d. salt water
Answer D) When examining the stores of water on Earth, 97.5 percent of it is non-potable salt water. Of the remaining water, 99 percent is locked underground or as ice. Thus, less than 1 percent of fresh water is easily accessible from lakes and rivers.
assimilation biomass consumed and assimilated from the previous trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste
biogeochemical cycle cycling of mineral nutrients through ecosystems and through the non-living world
biomagnification increasing concentrations of persistent, toxic substances in organisms at each trophic level, from the primary producers to the apex consumers
biomass total weight, at the time of measurement, of living or previously living organisms in a unit area within a trophic level
ecological pyramid (also, Eltonian pyramid) graphical representation of different trophic levels in an ecosystem based of organism numbers, biomass, or energy content
ecosystem dynamics study of the changes in ecosystem structure caused by changes in the environment or internal forces
ecosystem community of living organisms and their interactions with their abiotic environment
equilibrium steady state of an ecosystem where all organisms are in balance with their environment and each other
eutrophication process whereby nutrient runoff causes the excess growth of microorganisms, depleting dissolved oxygen levels and killing ecosystem fauna
fallout direct deposit of solid minerals on land or in the ocean from the atmosphere
food chain linear representation of a chain of primary producers, primary consumers, and higher- level consumers used to describe ecosystem structure and dynamics
food web graphic representation of a holistic, non-linear web of primary producers, primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics
grazing food web type of food web in which the primary producers are either plants on land or phytoplankton in the water; often associated with a detrital food web within the same ecosystem
hydrosphere area of the Earth where water movement and storage occurs
residence time measure of the average time an individual water molecule stays in a particular reservoir
subduction movement of one tectonic plate beneath another
trophic level transfer efficiency (TLTE) energy transfer efficiency between two successive trophic levels
trophic level position of a species or group of species in a food chain or a food web
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