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Transcript
Cycles in Nature • Hydrologic Cycle • Carbon Cycle • Nitrogen Cycle • Phosphorus Cycle Let’s Focus on the Hydrologic Cycle First! Life on Earth would not exist without the hydrologic cycle. As you can see, from space our planet looks like a big blue marble, and is therefore nicknamed the water planet •Earth is covered by one of our most precious resources. •Nearly 97% of the Earth’s water is tied up in its oceans, however! How do we obtain fresh water resources then? Where does drinkable water come from? To understand, we need to turn to the hydrologic cycle. This is the process, powered by the sun’s energy, which moves water between the oceans, the sky, and the land. Let’s first look at the oceans... The sun causes evaporation of water on the surface of the oceans. Approximately 80% of all evaporation on Earth occurs over oceans Evaporation is the process whereby liquid water changes to a gas Once the gaseous water (water vapor) is in our atmosphere, it condenses into tiny droplets which cling to dust particles. The tiny dust and dirt particles known as “aerosols” are called “condensation nuclei” Condensation is the process whereby water is changed from a gaseous state into a liquid state. These tiny water droplets form clouds. Once heavy enough, these droplets fall to Earth as precipitation. • rain • sleet • snow • hail • freezing rain Precipitation is the primary mechanism for transporting water from the atmosphere to the surface of the Earth Some of the Earth’s precipitation is absorbed in the process of infiltration. Water that has penetrated Earth’s surface becomes groundwater When the ground is particularly impermeable, the water may runoff causing flooding Some of the Earth’s water is used and recycled by plants in a process known as transpiration. Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants through openings called “stomata”. Evapotranspiration accounts for approximately 10% of all evaporating water. • More of Earth’s water flows into streams, and rivers, and eventually back to the oceans, where the entire process starts over again. • The amount of precipitation and evaporation is balanced throughout the world. • Sometimes, one part of the world is experiencing a drought, while another is experiencing flooding rains. • On a global scale, over the years, however, everything balances out. It can take as little as a few days, to thousands of years for a single water molecule to complete the cycle. It can be trapped in glacial ice sheets for a very long time. To recap, the five main processes included in the hydrologic cycle are: • Condensation • Precipitation • infiltration • runoff • Evapotranspiration Combines the processes of transpiration and evaporation Considering so little of the water on Earth is drinkable to people, it is amazing the supply has survived as long as it has. The newest problem faced by the planet is not simply the consumption of freshwater, but the pollution of freshwater. Estimates show that 100 million billion gallons a year are cycled through the hydrologic cycle. Perhaps we should treat it as the precious commodity that it is, and stop taking it for granted. Carbon and Nitrogen Cycles •The nitrogen cycle represents one of the most important nutrient cycles found in both terrestrial, and aquatic ecosystems • nitrogen is critically important in forming amino acids which in turn form the proteins of your body. Proteins make up skin and muscle, among other important structural portions of your body, and all enzymes are proteins. Since enzymes carry out almost all of the chemical reactions in your body, it's easy to see how important nitrogen is. Just like the hydrologic cycle had processes at each stage, so does the nitrogen cycle. This cycle is carried out through abiotic and biotic means. • Fixation • Nitrification • Assimilation • Ammonification • Denitrification The chief reservoir of nitrogen is the atmosphere, which is about 78% nitrogen = Atmospehric nitrogen N2 •Nitrogen gas can be taken from the atmosphere (fixed…and made useable to organisms) in two basic ways: 1. lightning provides enough energy to "burn" the nitrogen and fix it in the form of nitrate, which is a nitrogen with three oxygens attached NO3 2. And nitrogen fixing bacteria called Rhizobium, found in soil and often associated with the root systems of legumes. • All nitrogen fixing bacteria fix nitrogen either in the form of nitrate (NO3) Like the lightning did •or in the form of ammonia (nitrogen with 3 hydrogens attached) (NH3) • Ammonia is rather toxic, but, fortunately there are nitrite bacteria in the soil and in the water which take up ammonia and convert it to nitrite, which is nitrogen bound to two oxygens. (NO2) • Nitrite is also somewhat toxic, but another type of bacteria, nitrate bacteria, take nitrite and convert it to nitrate, which can be taken up by plants to continue the cycle. (NO3) This is the process whereby plants absorb ammonium (NH3) , ammonia ions (NH4), and nitrate ions (NO3-) through their roots. • Most autotrophs can take up nitrate and convert it to amino acids, which are, of course, the building blocks of proteins.. •Heterotrophs acquire all of their amino acids when they eat plants (or other animals that consume plants) •When plants or animals die (or release waste) the nitrogen is returned to the soil, usually in the form of ammonia. This ammonia (NH3) or ammonium (NH4) is a product of decomposing bacteria, and in this state, may be reused by plants. •If this is indeed a cycle…how is N2 returned to the atmosphere? It turns out that there are denitrifying bacteria (usually anaerobic) which take the nitrites and nitrates and combine the nitrogen back into nitrogen gas, and nitrous oxide. (N2), (N2O) The carbon cycle models the movement and storage of carbon in the biosphere, lithosphere, hydrosphere and atmosphere. •Carbon is stored in the biosphere as living organisms. •in the atmosphere as carbon dioxide gas •in the lithosphere as soil organic matter as fossil fuel deposits, and as sedimentary rock deposits •and in the oceans as dissolved carbon dioxide gas and as calcium carbonate shells in marine organisms Some of the processes that move carbon in the cycle include: > photosynthesis > respiration > biomass combustion > fossil fuel burning > fossil fuel creation > sedimentary rock formation Humans have altered the carbon cycle through fossil fuel burning, deforestation, and land-use change. The net result of these processes is an increasing concentration of carbon dioxide in the atmosphere. Sink Amount in Billions of Metric Tons Atmosphere Soil Organic Matter Ocean Marine Sediments and 578 (as of 1700) - 766 (as of 1999) 1500 to 1600 38,000 to 40,000 66,000,000 to 100,000,000 Sedimentary Rocks Terrestrial Plants Fossil Fuel Deposits 540 to 610 4000 Judging from the data above, where on Earth is the largest Carbon Sink found? Oceans! • Ecosystems gain most of their carbon through atmospheric CO2 (through what processes?) Photosynthesis and Respiration This is the simplest of all the nutrient cycles, as Phosphorous doesn’t exist in the atmosphere for the most part…but found in soil, rock, and sediment. Also, of all the cycles, this one is more “local” than the others. It is released from the rocks and sediment through chemical weathering. It is usually released from sediments in the form of phosphate (PO4-3), which is very soluble and can be absorbed from the soil by plants. Phosphorus is a limiting factor in the growth of plants, so plants that are exposed to minimal amounts of phosphorus are usually stunted. Phosphorus is an important nutrient because it is an essential building block of nucleic acids. This can result in fish-kills. Humans have affected the natural cycling of this nutrient by mining phosphorous rich rocks for fertilizers. When we add those fertilizers to soil it is leached out, and washed to rivers and eventually into oceans where it can cause artificial eutrophication. The phosphorus is simply returned to the sediment when organisms that have incorporated it into their systems decompose. • Through geological processes, these sediments then either become rocks again…or are washed away and redeposited elsewhere.