Ecosystem Energetics (Chapters 3 and 4) A little physics Primary production Transfer of energy across trophic levels

2 Laws of Thermodynamics Energy can neither be created nor destroyed, only transformed from one type to another conservation of energy In any transfer of energy, some energy is lost entropy

Where does the Energy Needed for Living Organisms Originate? 58% of electromagnetic radiation is reflected or absorbed as heat by our atmosphere! 1% for Photosynthesis This 1% is transformed on the earth’s surface as heat!

1st law: all energy in an ecosystem ultimately comes from… the sun chemical compounds (hydrogen sulfide) 2nd law: energy is constantly being lost to the environment as heat

10% Rule Consider living organisms as temporary storage units for useful energy Energy Cannot Recycle = Continuous Requirement for new energy to enter the system!

Primary production – capturing light energy and storing it in chemical bonds of carbon compounds primary productivity – the rate of primary production primary producers – photosynthetic autotrophs ATP NADPH

Each gram of C assimilated = 39 kilojoules (kJ) of energy stored Energy + 6CO2 + 6H2O → C6H12O6 + 6O2 Photosynthesis C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy! Respiration Each gram of C assimilated = 39 kilojoules (kJ) of energy stored Joules = unit of energy Biomass is dominated by carbon biomass and energy are equivalent

Plants use the products of photosynthesis in 2 ways new proteins, tissues, cells, structures – growth & reproduction fuel for the above processes

Assimilated Carbon structures fuel for biological processes

Gross primary production (GPP) – total amount of energy assimilated by photosynthesis Net primary production (NPP) – energy actually stored as biomass GPP – NPP = respiration

GPP NPP

Measuring NPP in nature Measure amount of CO2 absorbed by leaves extrapolate from a very small area Use radioactive isotopes to measure total uptake of C In aquatic systems, measure changes in O2 concentration

Light Dark

Limits on Primary Production Light in shade, forest understory photosynthetic efficiency – percentage of the energy in sunlight converted to NPP averages just 1 to 2% Temperature most plants have an optimum temperature respiration increases with temperature

Water transpiration (or water use) efficiency – amount of plant tissue produced per kilogram of water transpired 2-4 g tissue/kg water increasing precipitation -> increased NPP some water is lost to runoff

Nutrients

Some Ecosystems Produce Plant Matter Faster Than Others Do (2) Net primary productivity – rate at which producers use photosynthesis to produce and store energy minus the rate at which they use this stored energy for aerobic respiration. Ecosystems and aquatic life zones differ in their NPP (Fig. 3-16). Decreases from equator to pole. Estuaries are high Upwellings (water moving up from depths to surface) Open ocean, low NPP, but high absolute amount. Why?