Ecosystems are open systems in which energy and matter are exchanged. They are sustained by supplies of energy and matter.

Laws of Thermodynamics And Environmental Systems

Energy exists in a variety of forms (light, heat, chemical, electrical, and kinetic). The behaviour of energy in systems is defined by the laws of thermodynamics.

First Law of Thermodynamics

The first law of thermodynamics states that energy can neither be created nor destroyed - it can only change form. Two examples of the first law in our ecosystem are:

1. Photosynthesis: It is the conversion of light energy to chemical energy in the form of glucose. Glucose can be converted into other carbon compounds contained within biomass. Producers form the first trophic level in a food chain and produce their food using photosynthesis
2. Cellular respiration: Respiration releases energy from glucose by converting it into a chemical form that can easily be used in carrying out active processes within living cells. Some chemical energy released during cellular respiration is transformed into heat.

Second Law of Thermodynamics

The second law of thermodynamics states that energy transformations in ecosystems are inefficient. It relates to the quality of energy and that when energy is transformed, some must be degraded into a less useful form, such as heat.

Entropy

Entropy is linked with the second law of thermodynamics. It is a measure of the amount of disorder in a system. An increase in entropy arising from energy transformations reduces the energy available to do work.

Productivity in Ecosystems

Productivity is a measure of the rate at which energy is converted into biomass over time. There are several important concepts related to productivity:

Gross Primary Productivity (GPP): The total rate of energy capture through photosynthesis.

(HL) Net Primary Productivity (NPP): The energy remaining after the producer's respiratory needs are met.

𝑁𝑃𝑃 = 𝐺𝑃𝑃 − Respiratory losses

(HL) Gross Secondary Productivity (GSP): The total rate of energy assimilation by consumers.

𝐺𝑆𝑃 = Food eaten − Fecal loss

(HL) Net Secondary Productivity (NSP): The amount of energy available for growth and reproduction in consumers after accounting for respiratory losses.

𝑁𝑆𝑃 = 𝐺𝑆𝑃 − Respiratory losses

Maximum Sustainable Yield

The maximum sustainable yield in an ecosystem is equal to the net primary or secondary productivity. This represents the maximum amount of biomass that can be harvested without depleting the resource.

Human Impacts on Energy and Matter Flows

Human activities significantly affect energy flows and biogeochemical cycles:

1. Fossil fuel use: Releases stored carbon into the atmosphere, altering the carbon cycle and global energy balance.
2. Deforestation: Reduces the Earth's capacity to absorb CO₂ through photosynthesis and alters local energy flows.
3. Urbanization: Changes land surface albedo, affecting local energy absorption and reflection.
4. Agriculture: Alters nutrient cycles through fertilizer use and impacts energy flows through land-use changes.

Carbon Cycle

The carbon cycle describes how carbon moves through the atmosphere, biosphere (living organisms), hydrosphere (oceans), and lithosphere (rocks and soil).

Carbon Flows:

Carbon flows between reservoirs (stores) through natural processes:

• Photosynthesis: Plants absorb atmospheric CO₂ and convert it into glucose.
• Respiration: Animals and plants release CO₂ back into the atmosphere by breaking down glucose for energy.
• Decomposition: Dead organisms and waste products are broken down by decomposers, releasing carbon into soil or the atmosphere.
• Combustion: Burning fossil fuels or biomass releases stored carbon as CO₂.
• Ocean Uptake: Oceans absorb CO₂ from the atmosphere, storing it as dissolved carbon or in marine organisms.

Carbon Dioxide in Oceans:

• Storage: Oceans act as major carbon sinks, absorbing nearly a quarter of atmospheric CO₂.
  • Forms of Carbon in Oceans:
    • Dissolved CO₂.
    • Bicarbonates and carbonates.
    • Carbon in marine organisms (shells and skeletons made of calcium carbonate).
• Ocean Acidification:
  • Excessive CO₂ absorption reduces ocean pH, making it more acidic.
  • Consequences include:
    • Weakening of coral reefs due to reduced calcium carbonate availability.
    • Disruption of marine food webs and biodiversity loss.

Human Activities and the Carbon Cycle:

Human actions are altering the carbon cycle, leading to climate change:

• Burning Fossil Fuels: Adds CO₂ to the atmosphere faster than natural processes can remove it.
• Deforestation: Reduces carbon sequestration by cutting down forests, a major carbon sink.
• Industrial Agriculture: Increases greenhouse gases (e.g., methane from cattle).

Mitigation Strategies:

• Transitioning to renewable energy sources (e.g., solar, wind).
• Reforestation and afforestation projects to increase carbon sequestration.
• Sustainable agriculture to minimize carbon emissions.

Nitrogen Cycle

The nitrogen cycle describes the movement of nitrogen between the atmosphere, biosphere, hydrosphere, and lithosphere, essential for building proteins and nucleic acids in organisms.

Nitrogen Flows

• Nitrogen Fixation:
  • Conversion of atmospheric nitrogen (N₂) into a usable form like ammonia (NH₃) or nitrate (NO₃⁻).
  • Processes:
    • Biological fixation by bacteria (e.g., Rhizobium in legume root nodules).
    • Abiotic fixation via lightning or industrial processes (e.g., Haber process).
• Nitrification:
  • Ammonium (NH₄⁺) is converted to nitrites (NO₂⁻) and then to nitrates (NO₃⁻) by nitrifying bacteria (Nitrosomonas and Nitrobacter).
• Assimilation:
  • Plants absorb nitrates or ammonium to synthesize proteins and nucleic acids.
  • Consumers ingest plants to obtain nitrogen-containing compounds.
• Ammonification (Decomposition):
  • Decomposers (bacteria and fungi) break down organic matter, releasing ammonium into the soil.
• Denitrification:
  • Denitrifying bacteria (e.g., Pseudomonas) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), returning it to the atmosphere.