# Lecture 27: Ecosystems and Biogeochemical Cycles
## General Biology II — Organismal, Evolution & Ecology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Distinguish between the biotic and abiotic components of an ecosystem
2. Explain the flow of energy through ecosystems including primary production, trophic levels, and ecological efficiency
3. Construct and interpret food chains, food webs, and ecological pyramids
4. Describe the major biogeochemical cycles (water, carbon, nitrogen, phosphorus) and the role of organisms in each
5. Explain how human activities are disrupting biogeochemical cycles
6. Describe the processes that govern net primary productivity in terrestrial and aquatic ecosystems
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## Lecture Content
### I. Ecosystem Concepts
**Ecosystem** — a biological community plus its abiotic (physical and chemical) environment, linked by energy flow and nutrient cycling. Two fundamental processes in ecosystems: **Energy flow** — unidirectional; energy enters as sunlight (or chemical energy for chemosynthetic ecosystems), passes through trophic levels, and is lost as heat at each step. **Nutrient (chemical) cycling** — matter is recycled; elements cycle between biotic and abiotic reservoirs (biogeochemical cycles) Ecosystems are open to energy (solar input, heat loss) but largely closed to matter (elements are recycled within the biosphere).
### II. Energy Flow and Trophic Structure
**Trophic levels** — feeding positions in an ecosystem: **Primary producers (autotrophs)** — convert inorganic energy sources to organic compounds. Photoautotrophs: plants, algae, cyanobacteria (photosynthesis) Chemoautotrophs: some bacteria and archaea (chemosynthesis — deep-sea vents, caves) **Primary consumers (herbivores)** — eat producers (e.g., insects, zooplankton, rabbits, deer) **Secondary consumers** — eat primary consumers (e.g., frogs, small fish, spiders) **Tertiary consumers** — eat secondary consumers (e.g., snakes, hawks, tuna) **Quaternary consumers (apex predators)** — top of the food chain (e.g., orcas, eagles) **Decomposers and detritivores** — break down dead organic matter and recycle nutrients. Decomposers: bacteria and fungi (chemical breakdown) Detritivores: earthworms, millipedes, dung beetles (physical breakdown and ingestion of detritus) **Food chain** — a linear sequence of trophic transfers (producer → primary consumer → secondary consumer → ...) **Food web** — an interconnected network of food chains in a community; more realistic representation; shows that most consumers feed at multiple trophic levels.
### III. Primary Production
**Primary production** — the amount of energy (or biomass) produced by autotrophs through photosynthesis or chemosynthesis. **Gross primary production (GPP)** — total energy fixed by autotrophs per unit time. **Net primary production (NPP)** — GPP minus energy used by autotrophs for cellular respiration (R_a): NPP = GPP - R_a. NPP represents the energy available to consumers and decomposers. Typically measured as g C/m^2/year or kcal/m^2/year. **Global patterns of NPP**: Terrestrial: tropical rainforests are the most productive biome; deserts and tundra are the least productive. Limiting factors on land: temperature, precipitation, sunlight, soil nutrients. Aquatic: open ocean has low NPP per unit area but contributes a large total due to its vast area; estuaries, coral reefs, and upwelling zones are highly productive per unit area. Limiting factors in aquatic systems: light (depth), nutrients (nitrogen, phosphorus, iron) **Net ecosystem production (NEP)** — NPP minus energy consumed by heterotrophs (respiration of all organisms): NEP = GPP - R_total. Positive NEP = ecosystem is a carbon sink; negative NEP = carbon source.
### IV. Ecological Efficiency and Trophic Pyramids
**Ecological efficiency** — the percentage of energy transferred from one trophic level to the next. Typically **~10%** (range: 5-20%) Energy losses at each trophic level: Not all organisms at a lower level are consumed. Not all consumed energy is assimilated (some passes through as feces) Assimilated energy is used for cellular respiration (~60-90% lost as heat) Only a small fraction is converted to new biomass (secondary production) **Production efficiency** = (net secondary production / assimilation energy) x 100. Endotherms: low production efficiency (~1-3%) because they use most assimilated energy for metabolism (maintaining body temperature) Ectotherms: higher production efficiency (~10-40%) **Trophic (ecological) pyramids** — graphical representations of trophic structure: **Pyramid of energy** — always upright; energy decreases at each successive trophic level (consequence of the second law of thermodynamics) **Pyramid of biomass** — usually upright on land; can be inverted in some aquatic ecosystems (e.g., open ocean: phytoplankton biomass < zooplankton biomass at any given time because phytoplankton reproduce rapidly and are consumed quickly — high turnover rate) **Pyramid of numbers** — can be inverted (e.g., one large tree supports many insects) **Biological magnification (biomagnification)** — toxins that are not metabolized (e.g., DDT, mercury, PCBs) become increasingly concentrated at higher trophic levels. Apex predators accumulate the highest concentrations → ecological and health consequences (e.g., eggshell thinning in raptors from DDT).
A multi-panel diagram of energy flow and trophic pyramids. Panel A (Energy flow diagram): A flow chart showing energy entering at the producer level as sunlight. A large green box represents producers (GPP), with a portion lost as heat via respiration and the remainder (NPP) available to the next level. Arrows flow to primary consumers, secondary consumers, and tertiary consumers, each represented as progressively smaller boxes. At each level, large arrows pointing to the side indicate energy lost as heat through respiration, and small arrows indicate energy lost as feces and detritus. A decomposer box at the bottom receives detritus from all levels and releases heat. Numerical values illustrate the approximately 10 percent efficiency: 1,000,000 J at producers, 100,000 J at primary consumers, 10,000 J at secondary consumers, 1,000 J at tertiary consumers. Panel B (Three ecological pyramids side by side): A pyramid of energy (always upright, with units in kcal/m2/year), a pyramid of biomass on land (upright, with units in g/m2), and an inverted pyramid of biomass in the open ocean (phytoplankton bar smaller than zooplankton bar, with an explanation of rapid turnover). Panel C (Biomagnification): A food chain from phytoplankton to zooplankton to small fish to large fish to a bald eagle, with DDT concentration (in ppm) labeled at each level, increasing from 0.04 ppm in water to 25 ppm in the eagle's tissues.
### V. The Water Cycle (Hydrological Cycle)
Water moves between the atmosphere, land, oceans, and organisms. Key processes: **Evaporation** — liquid water to water vapor; primarily from ocean surfaces (~86% of total evaporation) **Transpiration** — water loss from plant leaves through stomata; a major pathway from terrestrial ecosystems to the atmosphere. **Evapotranspiration** — combined evaporation and transpiration from land surfaces. **Condensation** — water vapor cools and forms clouds. **Precipitation** — rain, snow, sleet, hail. **Runoff** — water flows over land surface into streams, rivers, lakes, and oceans. **Infiltration** — water seeps into soil and groundwater (aquifers) Human impacts: withdrawal from aquifers faster than recharge; dam construction; deforestation (increases runoff, decreases transpiration); pollution of freshwater sources; climate change alters precipitation patterns.
### VI. The Carbon Cycle
Carbon is the backbone of all organic molecules; moves between the atmosphere, oceans, land organisms, soil, and rocks. Major reservoirs: atmosphere (CO2), oceans (dissolved CO2 and bicarbonate), fossil fuels, soil organic matter, terrestrial biomass, sedimentary rocks (limestone) Key processes: **Photosynthesis** — removes CO2 from the atmosphere → fixes carbon into organic molecules (plants, algae, cyanobacteria) **Cellular respiration** — releases CO2 back to the atmosphere (all organisms) **Decomposition** — microbial breakdown of dead organic matter releases CO2. **Ocean exchange** — CO2 dissolves in surface waters (physical and biological pumps); marine organisms incorporate carbon into shells (CaCO3) → sediments → limestone. **Fossil fuel formation** — over millions of years, dead organisms buried under sediment → compressed → coal, oil, natural gas (long-term carbon storage) **Volcanism** — releases CO2 from Earth's interior. **Human disruption**: Burning fossil fuels — releases ~9-10 Gt C/year back to the atmosphere. Deforestation — reduces photosynthetic carbon uptake and releases stored carbon. Cement production — releases CO2 from limestone. Atmospheric CO2 has risen from ~280 ppm (pre-industrial) to over 420 ppm. Consequences: enhanced greenhouse effect → global warming → climate change; ocean acidification (dissolved CO2 → carbonic acid → lowers ocean pH).
### VII. The Nitrogen Cycle
Nitrogen is essential for amino acids, nucleotides, ATP, and chlorophyll. Atmosphere is 78% N2, but most organisms cannot use N2 directly (triple bond is very stable) Key processes: **Nitrogen fixation** — conversion of N2 to ammonia (NH3/NH4+): Biological: nitrogen-fixing bacteria (Rhizobium in legume root nodules; free-living Azotobacter; cyanobacteria) using nitrogenase enzyme. Abiotic: lightning; industrial Haber-Bosch process (converts N2 + H2 → NH3 for fertilizer) **Nitrification** — conversion of ammonia to nitrate (by nitrifying bacteria): NH4+ → NO2- (nitrite) by Nitrosomonas → NO3- (nitrate) by Nitrobacter. Nitrate is the primary form absorbed by plant roots. **Assimilation** — plants absorb NH4+ or NO3- and incorporate nitrogen into organic molecules (amino acids, nucleotides) **Ammonification (decomposition)** — decomposers break down organic nitrogen in dead organisms and waste → release NH4+ (ammonium) back to soil. **Denitrification** — anaerobic bacteria convert NO3- → N2O → N2 (returned to atmosphere); occurs in waterlogged, oxygen-depleted soils and sediments. **Human disruption**: Haber-Bosch process has doubled the rate of nitrogen fixation globally. Excess nitrogen fertilizer → runoff into waterways → eutrophication (algal blooms → decomposition → oxygen depletion → dead zones) Burning fossil fuels → NOx emissions → acid rain, smog. Nitrous oxide (N2O) — potent greenhouse gas.
### VIII. The Phosphorus Cycle
Phosphorus is essential for nucleic acids, ATP, phospholipids, bones, and teeth. **No gaseous phase** — phosphorus cycles through rock, soil, water, and organisms (unlike C and N) Key processes: **Weathering** of phosphate-containing rocks → releases phosphate ions (PO4^3-) into soil and water. Plants absorb phosphate from soil; animals obtain phosphorus from food. Decomposition returns phosphorus to the soil. Phosphorus enters aquatic systems via runoff → incorporated into marine sediments → over geological time, uplifted into new rock formations (very slow) Phosphorus is often the limiting nutrient in freshwater ecosystems. **Human disruption**: Mining phosphate rock for fertilizer. Agricultural runoff → excess phosphorus in waterways → eutrophication (same consequences as nitrogen enrichment) Detergents containing phosphates (now largely banned in many countries).
A comprehensive diagram of the nitrogen cycle. The atmosphere occupies the top of the figure, filled with N2 molecules. Arrows show the following processes, each labeled and color-coded. Nitrogen fixation: arrows from the atmospheric N2 pool lead down to NH4+ (ammonium) in the soil, passing through root nodules of a legume plant (with Rhizobium bacteria shown inside the nodules) and through free-living soil bacteria. An additional lightning bolt arrow indicates abiotic fixation of N2 to NO3-. Nitrification: arrows in the soil show NH4+ converted to NO2- (by Nitrosomonas) and then to NO3- (by Nitrobacter). Assimilation: arrows from NO3- and NH4+ lead into plant roots, then up through the plant, with nitrogen incorporated into proteins and nucleic acids; an animal eating the plant is shown to illustrate nitrogen transfer through the food web. Ammonification: arrows from dead plant and animal matter and from animal waste (urea, uric acid) lead back to NH4+ in the soil via decomposer bacteria and fungi. Denitrification: arrows from NO3- in waterlogged soil lead back up to the atmosphere as N2 and N2O, passing through anaerobic denitrifying bacteria. A sidebar shows human impacts: a factory representing the Haber-Bosch process converting N2 to NH3 for fertilizer, runoff arrows from agricultural fields carrying excess NO3- into a lake showing eutrophication (green algal bloom on the surface, a dead zone with depleted O2 at the bottom), and smoke stacks emitting NOx contributing to acid rain.
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