Question

module 6 - primary productivity

1. explain the difference between gross primary productivity (gpp) and net primary productivity (npp). why is npp considered more useful for measuring the energy available to consumers in an ecosystem?
2. compare primary productivity across biomes. which biomes generally have the highest npp and which have the lowest? what abiotic factors explain these differences?
3. interpretation question: a researcher measures the gpp of a forest as 10,000 kcal/m²/year and respiration (r) as 4,000 kcal/m²/year. calculate the npp and explain what this value indicates about energy available in this ecosystem.
4. human impact question: how might deforestation or nutrient pollution (eutrophication) affect primary productivity in an ecosystem? provide an example for each.
5. connection question: why is primary productivity fundamental to understanding energy flow in ecosystems, and how does it relate to concepts like trophic levels, ecological efficiency, and the 10% rule?
6. photosynthesis vs. cellular respiration: write the balanced chemical equations for photosynthesis and for cellular respiration. then explain how the rates of these two processes within producers determine gpp and npp over a 24 - hour period (e.g., day vs. night), and predict how npp would change on an overcast week.

Explanation:

1. Gross Primary Productivity (GPP) is the total amount of energy that producers in an ecosystem capture through photosynthesis. Net Primary Productivity (NPP) is GPP minus the energy that producers use for cellular respiration. NPP is more useful for consumers as it represents the actual amount of energy available for them to consume after producers' energy - use for maintenance.
2. Tropical rainforests generally have the highest NPP due to high temperatures, abundant rainfall, and long growing seasons. Deserts have the lowest NPP because of limited water availability. Abiotic factors like sunlight, water, temperature, and soil nutrients explain these differences. More sunlight and water allow for more photosynthesis, while temperature affects the rate of metabolic processes and soil nutrients are needed for plant growth.
3. NPP = GPP - Respiration. Given GPP = 10,000 kcal/m²/year and Respiration = 4,000 kcal/m²/year, NPP = 10000 - 4000=6000 kcal/m²/year. This value indicates the amount of energy available for consumers and for the growth and reproduction of plants in the forest ecosystem.
4. Deforestation reduces primary productivity as there are fewer plants to perform photosynthesis. For example, in the Amazon rainforest, large - scale deforestation has led to a decrease in the number of trees, thus reducing the overall GPP and NPP. Nutrient pollution (eutrophication) can initially increase primary productivity in aquatic ecosystems as an excess of nutrients like nitrogen and phosphorus stimulates the growth of algae and other primary producers. However, it can also lead to oxygen depletion and die - offs if algae blooms occur, ultimately reducing long - term productivity.
5. Primary productivity is fundamental as it is the starting point of energy flow in ecosystems. The 10% rule states that only about 10% of the energy at one trophic level is passed on to the next. Primary productivity determines the amount of energy available at the base of the food web, which affects all higher trophic levels.
6. The balanced chemical equation for photosynthesis is \(6CO_{2}+6H_{2}O + light

ightarrow C_{6}H_{12}O_{6}+6O_{2}\). The balanced chemical equation for cellular respiration is \(C_{6}H_{12}O_{6}+6O_{2}
ightarrow6CO_{2}+6H_{2}O+energy\). During the day, if photosynthesis is occurring at a higher rate than cellular respiration in producers, GPP and NPP increase. At night, when only cellular respiration occurs, NPP decreases as energy is being used without photosynthetic energy capture.

Answer:

1. GPP is total energy capture by producers via photosynthesis; NPP = GPP - producer respiration. NPP is more useful for consumers as it's available energy after producer maintenance.
2. Tropical rainforests have highest NPP, deserts have lowest. Abiotic factors: sunlight, water, temperature, soil nutrients.
3. NPP = 6000 kcal/m²/year. It's energy available for consumers and plant growth/reproduction.
4. Deforestation reduces productivity (e.g., Amazon rainforest). Eutrophication can initially increase then decrease (e.g., aquatic ecosystems).
5. It's the start of energy flow; affects higher trophic levels via 10% rule.
6. Photosynthesis: \(6CO_{2}+6H_{2}O + light

ightarrow C_{6}H_{12}O_{6}+6O_{2}\), Cellular respiration: \(C_{6}H_{12}O_{6}+6O_{2}
ightarrow6CO_{2}+6H_{2}O+energy\). Day - time photosynthesis affects GPP and NPP; night - time only respiration affects NPP.