Global Ecology and Biogeochemical Cycles: Comprehensive Study Guide

Global Ecology and Biogeochemical Cycles

• Concept 25.1 Summary: Elements move among geologic, atmospheric, oceanic, and biological pools at a global scale.
• Biological Importance: Global cycling of carbon ($C$), nitrogen ($N$), phosphorus ($P$), and sulfur ($S$) are emphasized due to their necessity for life and their roles in human-induced alterations of the global environment.
• Key Definitions:     * Pool (or Reservoir): The amount of an element contained within a specific component of the biosphere.     * Flux: The rate of movement of an element between different pools.     * Economic Analogy:         * Pool = The total amount of money currently in a bank account.         * Flux = The rate at which money is earned (income) and spent (expenses).

The Global Carbon Cycle

• Measurement Units: Carbon amounts are measured in Petagrams ($Pg$).     * $1\,Pg = 1 \times 10^{15}\,g$.
• Global Carbon Pools (Approximate values):     * Atmosphere: $760\,Pg$.     * Vegetation: $650\,Pg$.     * Soils: $1,500\,Pg$.     * Surface Ocean Waters: $920\,Pg$.     * Marine Biota: $3\,Pg$.     * Dissolved Organic Carbon (Detritus): $700\,Pg$.     * Deep Ocean Waters: $38,000\,Pg$.     * Benthic Sediments: $150\,Pg$.     * Rocks: $60 \times 10^{6}\,Pg$.
• Primary Carbon Fluxes:     * Gross Primary Production (GPP): $123\,Pg/yr$ from atmosphere to vegetation.     * Respiration: $120\,Pg/yr$ from terrestrial biota/soil to atmosphere.     * Atmosphere-Ocean Flux: $92\,Pg$ moving into the ocean and $91\,Pg$ moving into the atmosphere.     * Ocean Mixing: $92\,Pg$ moving between surface and deep waters.     * Rivers: Transport $0.8\,Pg$ from land to surface ocean.
• Atmospheric Carbon Dynamics:     * Anthropogenic Release: Resulting from land use change (primarily deforestation, contributing ~$0.9\,Pg$) and burning fossil fuels (contributing ~$9.5\,Pg$).     * Forest Impact: Removing canopies warms the soil, which accelerates decomposition and respiration rates. Burning trees releases $CO_2$ and small amounts of $CO$ and $CH_4$.     * Trends: Anthropogenic emissions of $CO_2$ more than doubled between 1970 and 2011.     * Uptake: Roughly half of these emissions are absorbed by oceans and terrestrial biota, though this proportion is predicted to decrease as uptake cannot keep pace with the rate of atmospheric increase.

Ocean Acidification and Marine Impacts

• Chemical Process: Atmospheric $CO_2$ diffuses into ocean water and reacts to form carbonic acid.     * Carbonic Acid: $H_2CO_3$     * Bicarbonate: $HCO_3^-$     * Carbonate: $CO_3^{2-}$
• Impact on Shell-Forming Organisms:     * Many marine organisms use calcium carbonate ($CaCO_3$) for shells.     * Increased acidity dissolves existing shells and lowers carbonate concentrations, hindering the synthesis of new shells.
• Observed Data:     * Foraminiferans: Near natural $CO_2$ seeps, population density drops significantly as pH decreases (from ~250 individuals/g at $pH\,8.0$ to near zero at $pH\,6.8$). Species diversity (Shannon index) also drops from ~25 to <5 across that same pH range.     * Great Barrier Reef Corals: Calcification rates ($g/cm^2/yr$) remained relatively stable (~$1.6$ to $1.7$) from 1900 to 1980 but showed a sharp decrease after 1980, correlating with observed seawater pH declines.     * Fish Larvae (Inland Silverside): Survival rates drop from ~$50\% - 60\%$ at $400\,ppm\,CO_2$ to <$10\%$ at $1,000\,ppm$ ($r^2 = 0.80$). Standard length also decreases from ~$4.0\,mm$ to ~$3.2\,mm$ across the same concentration range ($r^2 = 0.84$).

Atmospheric Methane ($CH_4$)

• Concentration vs. Potency: While atmospheric $CH_4$ levels are lower than $CO_2$, methane is a significantly more effective greenhouse gas.
• Sources: Emitted by anaerobic methanogenic bacteria found in:     * Wetlands.     * Shallow sediments.     * The rumens of ruminant animals (e.g., cows).

Global Climate Change and Greenhouse Effect

• Concept 25.2: Earth is warming due to anthropogenic greenhouse gas emissions.
• Historical Context: Atmospheric $C$ concentrations have fluctuated over geologic time.     * 60 million years ago: >$3,000\,ppm$.     * 140,000 years ago: <$200\,ppm$.
• Energy Balance (Units of 100 incoming solar radiation):     * Reflected: $32$ units (23 by clouds/aerosols, 9 by surface).     * Absorbed by Atmosphere: $19$ units.     * Absorbed by Surface: $49$ units.     * Outgoing Longwave Radiation: $68$ units total (emitted by atmosphere, clouds, and surface).     * Back Radiation: $95$ units of energy are returned to the surface by greenhouse gases.
• Measurement and Proxies:     * Direct Measurement: Air bubbles in ice cores provide records up to ~800,000 years ago.     * Proxies for CO2: Stomata density in fossil leaves and Boron isotopes (pH proxy).     * Proxies for Temperature: The ratio of $O^{18}$ to $O^{16}$.
• Definitions:     * Weather: The current state of the atmosphere at a given time.     * Climate: Long-term description of weather, including averages and variation ranges over at least three decades.     * Observed Change: Global average surface temperature increased by $0.8^\circ C$ between 1880 and 2012.

Biological Impacts of Warming

• Habitat Shifts:     * Alps: Species richness is increasing at higher elevations as plants move up summits (e.g., from ~10 species at $2,800\,m$ to ~60+ at certain summits).     * Marine Taxa: Species like the Red hake are moving poleward and into deeper, cooler waters.     * Mosquitoes: Range expansion in East Africa; Mt. Kilimanjaro areas (Kenya/Tanzania) now see ~160 days/year between $25^\circ C$ and $29^\circ C$, facilitating mosquito breeding.
• Phenology (Timing of Events):     * Higher temperatures move plant and insect phenology earlier in the season.     * Mismatch Example: Migratory birds like the Red knot rely on horseshoe crab eggs in Delaware Bay. If warming causes crabs to lay eggs earlier than the birds arrive, it creates a survival threat.     * Caterpillars: Bird nestling feeding must sync with peak caterpillar abundance.
• Temperature-dependent Sex Determination (TSD):     * In some reptiles (e.g., Alligator mississippiensis, Trachemys scripta, Macroclemys temminckii), incubation temperature determines the sex ratio of offspring, making populations vulnerable to fixed warming trends.
• Net Primary Production (NPP): Complexity arises from opposing factors. Higher $CO_2$ increases photosynthesis, but increased droughts reduce it.

Climate Feedback Loops

• Positive Feedback (Magnifies change):     * Albedo: Warming $\rightarrow$ reduced ice cover $\rightarrow$ lower albedo (less reflection) $\rightarrow$ more absorption/warming.     * Permafrost: Warming $\rightarrow$ permafrost thawing $\rightarrow$ increased soil microbial activity $\rightarrow$ increased greenhouse gas ($GHG$) release $\rightarrow$ more warming.
• Negative Feedback (Mitigates change):     * Ocean Circulation: Warming $\rightarrow$ ice sheet melt $\rightarrow$ increased freshwater/reduced salinity $\rightarrow$ disrupted deep water formation $\rightarrow$ slowed ocean heat transfer (e.g., Gulf Stream/AMOC) $\rightarrow$ potential cooling in Europe.     * Scientific Context: There is growing evidence for declines in the Atlantic Meridional Overturning Circulation (AMOC).

The Global Nitrogen Cycle ($N$)

• Concept 25.3: Anthropogenic sulfur and nitrogen emissions cause acid deposition and alter soil chemistry.
• Pools and Fluxes:     * Atmospheric $N_2$: Largest pool ($3.9 \times 10^{9}\,Pg$), but unavailable to most organisms.     * Nitrogen Fixation:         * Biological: $128\,Pg/yr$ (oceans), $120\,Pg/yr$ (terrestrial).         * Human-driven: Industrial fixation ($100\,Pg/yr$) and crop fixation ($30\,Pg/yr$).     * Reactive N: Fixed N compounds that can participate in chemical reactions.
• Human Alteration: Human $N_2$ fixation rates now exceed natural biological rates.
• Nitrogen Saturation: Occurs when N deposition exceeds the capacity of plants and microbes to absorb it, leading to nutrient leaching and diversity loss.     * Great Britain Study: N deposition rate was the primary factor explaining variation in grassland species richness.
• Eutrophication and Hypoxia: Nutrient runoff (N and P) from agriculture, livestock, and urban sources leads to algal blooms, which consume oxygen upon decomposition, resulting in "Ocean Dead Zones."

The Global Sulfur Cycle and Acid Rain

• Sulfur ($S$) Fluxes:     * Atmosphere: Pool of $3\,Pg$.     * Sources: Volcanoes ($5\,Pg/yr$), Sea spray ($144\,Pg/yr$), Fossil fuels/Industry ($90\,Pg/yr$).     * Atmospheric Deposition: $180\,Pg/yr$ to the ocean, $75\,Pg/yr$ to land.
• Acid Deposition Mechanism:     * $S$ and $N$ compounds in the atmosphere convert to sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$).     * Soil Chemistry: As $H^+$ percolates through soil, it replaces essential cations ($Ca^{2+}$, $Mg^{2+}$, $K^+$) at exchange sites on clay particles.     * Consequences: Leaching of nutrients out of the root zone, increased soil acidity, and increased levels of toxic metals.
• Historical Context: Large-scale mortality in European forests (1970s-1980s) was linked to acid rain and cation deficiencies. Trends since 1990 show a significant decrease in acid precipitation acidity due to regulations.
• Sulfate Aerosols: Stratospheric sulfur from industrial activity and volcanoes may have caused the temporary cooling/lack of warming observed between the 1950s and 1970s.

Stratospheric Ozone ($O_3$)

• Concept 25.4: Ozone levels in the stratosphere (loss) and troposphere (increase) pose risks.
• Function: Stratospheric ozone protects the surface from ultraviolet-B (UVB) radiation.
• UVB Risks: Causes DNA damage, impairs immune responses, damages photosynthetic pigments, and causes skin tumors.
• The Ozone Hole: An area of unusually low springtime ozone concentration over Antarctica, first measured in 1980.
• Chlorofluorocarbons (CFCs):     * Created by Thomas Midgley, Jr. for safer refrigeration.     * In the stratosphere, CFCs release reactive chlorine atoms.     * Potency: A single free chlorine atom can destroy $10^{5}$ ($100,000$) ozone molecules.
• Montreal Protocol (1989): Signed by >150 countries to phase out CFCs. Concentrations like CFC-11 and carbon tetrachloride have since decreased or stabilized.
• Recovery: Ozone layer recovery will take decades due to the long atmospheric life of CFCs.

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