untitled

Ecosystem Energy Transfer

I. Primary productivity

A. Definitions

The sun is the source of all energy used on the planet. The key to life is trapping solar energy in chemical bonds that can be stored until needed. Most energy is stored in molecules containing C, but some organisms use reduced forms of N, S or Feand later oxidize them for energy, e.g. bacteria involved in the nitrogen cycle or bacteria that live near underwater thermal vents where S concentrations are high. Organisms that make their own energy are either photoautotrophs if they use sunlight or chemoautotrophs if they use N or S.

B. Photosynthesis

1. Plants transform C from an oxidized (low-energy) state to a reduced (high-energy) state during photosynthesis:

2. 6CO2 + 6H20 -> C6H12O6 + 6O2 - photosynthesis, reverse is respiration

3. The plant gains 39 KJoules per gram of C assimilated. This is the gross production. The net production is the accumulation of energy in plant tissues - the energy expended in respiration used for tissue maintenance and biosynthesis. Note that plants only save 1-2% of the solar radiation they receive when they are not nutrient limited. The remainder is reflected or lost as heat.

C. Measurement

1. Increase in dry plant biomass: Weigh change in dry mass of plants over a fixed period. Hard to measure on terrestrial plants because of roots

2. 14C uptake: add known amount of 14C to an enclosure with a plant, then after a fixed period, measure the amount of 14C in the plant and divide by the proportion of 14C in the chamber at the beginning. If plant assimilates C at 10 mg/h and the proportion of C which was 14C at the beginning of the experiment was 0.05, then the plant assimilates C at 10/0.05 = 200 mg/h

3. CO2 flux: Appropriate for terrestrial measurements because CO2 is relatively rare in the air. Plants take up CO2 during photosynthesis, but release CO2 during respiration. Therefore, compare CO2 loss from air during the day (net production) with CO2 gain during the night to estimate gross production.

4. O2 flux: appropriate for aquatic measurements because dissolved O2 is rare in water. Compare O2 levels in a clear jar with phytoplankton to those in a dark jar with phytoplankton. In dark bottles respiration consumes O2 but in light bottles O2 is both consumed and released through photosynthesis. Basis for this week's lab.

II. Global variation

A. Patterns

1. Highest levels of productivity occur in tropical forests, marshes, coral reefs and estuaries, such as the Chesapeake bay. Estuaries export up to 10% of their gross primary productivity to the sea and provide refuges for immature forms. Consequently, they play major roles in larger scale ecosystems.

2. Lowest levels occur in open ocean, deserts and tundra.

B. Causes

1. the rate of photosynthesis increases with light intensity, temperature, and moisture content of air.

2. Humidity affects productivity because plants lose water through their stomata while taking in CO2 and eventually run out of water to use in photosynthesis.

3. Respiration increases exponentially with temperature while photosynthesis increases linearly with temperature.

4. As I have mentioned before, N is often limiting in terrestrial and marine communities while P is usually limiting in freshwater communities.

III. Energy transfer

A. Definitions

1. A biological community is a group of organisms that feed upon one another.

2. The combination of a biological community and the inorganic nutrient pools it relies on is called an ecosystem.

3. The links in the food chain are referred to as trophic levels with the organisms fixing energy referred to as primary producers, then herbivores, and primary, secondary or tertiary carnivores. Systems ecologists measure how energy is transferred between trophic levels.

B. Energy transfer

1. The amount of energy passing between trophic levels depends on the net primary productivity and the efficiency with which organisms convert food energy into biomass at each level. Ecological efficiency is the ratio of the energy stored in a trophic level divided by the energy stored in the next lower trophic level. On average, only 10% of the energy at each level is passed to the next higher level, although ecological efficiencies are usually higher in aquatic systems than terrestrial systems perhaps because aquatic plants use less hard to digest material as cellulose and lignins. Consequently, some aquatic systems have 5 trophic levels while most terrestrial communities only have 3-4 trophic levels. Furthermore, ecological efficiencies of endotherms may be only 3% while those of some insects may be 50%. Nevertheless, there will be an ecological pyramid of energy in all communities.

2. Because much plant material is unusable to herbivores, in many terrestrial systems more energy is available to the detritivore part of the community. Consequently, more energy is bound up in detritovores than in herbivores. The opposite is true in planktonic communities.