3 Environmental systems: Connections, energy, and ecosystems
This lecture will help you understand: • The nature of systems • Ecosystem-level ecology • Earth’s biomes • Nutrient cycles: Nitrogen, Carbon, Phosphorus • The rock cycle • The hydrologic cycle
Central Case: The Gulf of Mexico’s “Dead Zone” • Major fisheries off Louisiana were devastated by die-offs. • Scientists found large regions of low oxygen in the Gulf. • The recurring “dead zone” resulted from nitrogen pollution traveling down the Mississippi River.
Earth’s environmental systems Our planet consists of many complex, large-scale, interacting systems. System = a network of relationships among a group of parts, elements, or components that interact with and influence one another through the exchange of energy, matter, and/or information • Feedback loop = a circular process whereby a system’s output serves as input to that same system.
Feedback loops: Negative feedback • In a negative feedback loop, output acts as input that moves the system in the opposite direction. • This compensation stabilizes the system Figure 6.1a
Feedback loops: Positive feedback • In a positive feedback loop, output acts as input that moves the system further in the same direction. • This magnification of effects destabilizes the system. Figure 6.1b
An environmental system • Mississippi River as a system: • Input of water, fish, pollution, etc. • Output to Gulf of Mexico Figure 6.3
Two systems or one? • The Mississippi River system and the system of the Gulf of Mexico interact. • Understanding the dead zone requires viewing the Mississippi River and the Gulf of Mexico as a single system. • This holistic kind of view is necessary for comprehending many environmental issues and processes.
Eutrophication • Key to the dead zone = • Eutrophication: excess nutrient enrichment in water, which increases production of organic matter... • … which when decomposed by oxygen-using microbes can deplete water of oxygen
Creation of the hypoxic dead zone • Nitrogen input boosts phytoplankton… • …which die and are decomposed by microbes that suck oxygen from water, killing fish and shrimp. Figure 6.5
Chemistry and the environment Chemistry is central to environmental science: • • Carbon dioxide and climate change • • Sulfur dioxide and acid rain • • Pesticides and public health • • Nitrogen and wastewater treatment • • Ozone and its atmospheric depletion
Atoms and elements • An element is a fundamental type of chemical substance. • Elements are composed of atoms. • Each atom has a certain number of: • protons (+ charge) • electrons (– charge) • neutrons (no charge) Figure 4.1
Atoms and elements • 92 elements occur in nature, each with its characteristic number of protons, neutrons, and electrons. Figure 4.1
Chemical symbols • Each element is abbreviated with a chemical symbol: • H = hydrogen • C = carbon • N = nitrogen • O = oxygen • P = phosphorus • Cl = chlorine • Fe = iron
Isotopes • Isotopes are alternate versions of elements, which differ in mass by having a different number of neutrons. • Carbon-14 has two extra neutrons beyond normal carbon’s 6. Figure 4.2
Ions Atoms electrically charged, due to gain or loss of electrons Figure 4.3
Molecules, compounds, and bonds • Molecules = combinations of two or more atoms • Compounds = molecules consisting of multiple elements • Atoms are held together by bonds: • covalent bond = uncharged atoms sharing electrons (CO2) • ionic bond = charged atoms held together by electrical attraction (NaCl)
Water is a unique compound • Hydrogen bonds give water properties that make it a vital molecule for life: • • Is cohesive • • Resists temperature change • • Ice insulates • • Dissolves many chemicals Figure 4.4
Acidity In an aqueous solution, If H+ concentration is greater than OH– concentration, then solution is acidic. If OH– is greater than H +, then solution is basic.
pH scale • pH scale measures acidity and basicity. • Pure water = 7 • Acids < 7 • Bases > 7 Figure 4.6
Organic compounds • Consist of carbon atoms and, generally, hydrogen atoms • Joined by covalent bonds • May include other elements • Highly diverse; C can form many elaborate molecules • Vitally important to life ethane
Hydrocarbons • C and H only; major type of organic compoundMixtures of hydrocarbons make up fossil fuels. Figure 4.7
Macromolecules • Large molecules essential for life: • • Proteins • • Nucleic acids • • Carbohydrates • • Lipids • The first three are polymers, long chains of repeated molecules.
Proteins • Consist of chains of amino acids; fold into complex shapesFor structure, energy, immune system, hormones, enzymes Figure 4.8
Carbohydrates • Complex carbohydrates consist of chains of sugars.For energy, also structural (cellulose, chitin) Figure 4.11
Lipids • Do not dissolve in water • • Fats and oils • • Phospholipids • • Waxes • • Steroids
Nucleic acids • DNA and RNA • Encode genetic information and pass it on from generation to generation • DNA = double-stranded chain (double helix) • RNA = single-stranded chain
Nucleic acids • Paired strands of nucleotides make up DNA’s double helix. Figure 4.9
Genes and heredity Genes, functional stretches of DNA, code for the synthesis of proteins. Figure 4.10
Cells Plant cell Basic unit of organismal organization; compartmentalize macromolecules and organelles Animal cell Prokaryotic cell Eukaryotic cell Figure 4.12
Energy Can change position, physical composition, or temperature of matter Potential energy = energy of position (water held behind a dam) Kinetic energy = energy of movement (rushing water released from a dam)
Potential and kinetic energy • Potential energy stored in food is converted to kinetic energy when we exercise. Figure 4.13
Laws of thermodynamics • First Law: Energy can change form, but cannot be created or lost. • Second Law: Energy will tend to progress from a more-ordered state to a less-ordered state (increase in entropy).
Increase in entropy • Burning firewood demonstrates the second law of thermodynamics. Figure 4.14
Energy from the sun Energy from the sun powers most living systems. Visible light is only part of the sun’s electromagnetic radiation. Figure 4.15
Autotrophs and photosynthesis • The sun’s energy is used by autotrophic organisms, or primary producers (e.g., plants), to manufacture food. • Photosynthesis turns light energy from the sun into chemical energy that organisms can use.
Photosynthesis • In the presence of chlorophyll and sunlight,Water and carbon dioxide • are converted to • sugars and oxygen. Figure 4.16
Photosynthesis 6 CO2 + 12 H2O + energy from sun ————> C6H12O6 (sugar) + 6 O2 + 6 H2O
Streamlined 6 CO2 + 6 H2O + energy from sun ————> C6H12O6 (sugar) + 6 O2
Respiration and heterotrophs • Organisms use stored energy via respiration, which splits sugar molecules to release chemical energy. • This occurs in autotrophs and in the heterotrophs (animals, fungi, most microbes) that eat them.
Respiration • The equation for respiration is the exact opposite of the equation for photosynthesis. Some organisms and communities live without sunlight and are powered by chemosynthesis. C6H12O6 (sugar) + 6 O2 ————> 6 CO2 + 6 H2O + chemical energy
Ecosystems • Ecosystem = all the interacting organisms and abiotic factors that occur in a particular place and time • Energy and nutrients flow among all parts of an ecosystem. • Conception of an ecosystem can vary in scale: • small pond • large forest • entire planet
Energy in ecosystems • Energy from sun • converted to • biomass (matter in organisms) • by producers • through photosynthesis • Rapid conversion = high primary productivity • (coral reefs) • Rapid plant biomass availability for consumers = high net primary productivity • (wetlands, tropical rainforests)
Nutrient (biogeochemical) cycles These describe how particular chemicals cycle through the biotic and abiotic portions of our environment. Nutrients = elements and compounds organisms consume and require for nutrition and survival A carbon atom in your body could have been part of a dinosaur 100 million years ago.
Nutrient (biogeochemical) cycles Nitrogen: 78% of atmosphere In proteins and DNA In limited supply to organisms; requires lightning or bacteria to become usable A potent fertilizer Phosphorus: In ADP and ATP for metabolism In DNA and RNA In limited supply to organisms A potent fertilizer Nitrogen, carbon, and phosphorus are key nutrients. Carbon: Key component of organic molecules Atmospheric CO2 regulates climate
The nitrogen cycle • How nitrogen (N) moves through our environment • • Atmospheric N2 is fixed by lightning or specialized bacteria and becomes available to plants and animals in the form of ammonium ions (NH4+). • • Nitrifying bacteria turn ammonium ions into nitrite (NO2–) and nitrate (NO3–) ions. Nitrate can be taken up by plants. • • Animals eat plants, and when plants and animals die, decomposers consume their tissues and return ammonium ions to the soil. • • Denitrifying bacteria convert nitrates to gaseous nitrogen that reenters the atmosphere.
The nitrogen cycle Figure 6.25
Human impacts on the nitrogen cycle • Haber and Bosch during WWI developed a way to fix nitrogen artificially. • Synthetic nitrogen fertilizers have boosted agricultural production since then. • Today we are fixing as much nitrogen artificially as the nitrogen cycle does naturally. • We have thrown the nitrogen cycle out of whack.
Human impacts on the nitrogen cycle Figure 6.26
Nitrogen and the dead zone • Excess nitrogen flowing down the Mississippi River into the Gulf causes hypoxia, worse in some regions than others. From The Science behind the Stories