AP Environmental Science Unit 1: Primary Productivity

🔆 Source of Energy

  • Ultimate source of energy = Sun.

  • Algae + plants capture sunlight via photosynthesis → convert into chemical energy (sugars).

  • This energy supports food webs (producers → consumers → decomposers).

⚡ Key Terms

  • Gross Primary Productivity (GPP):

    • Total energy captured through photosynthesis in a given area/time.

    • Think: all the energy plants make before costs are deducted.

  • Respiration (R):

    • Energy burned by producers to maintain life functions (growth, repair, metabolism).

    • Always subtracts from the total available to the ecosystem.

  • Net Primary Productivity (NPP):

    • Energy available for consumers (herbivores + higher trophic levels) after respiration.

    • Formula:

      NPP=GPP−RNPP = GPP - RNPP=GPP−R

    • Units: energy per area per time (e.g., kcal/m²/year).

🧪 Example: Light–Dark Bottle Experiment

  • Used by limnologists to study algae productivity in aquatic systems.

  • Method: measure dissolved O₂ (mg/L) in light and dark bottles over time.

Data given:

  • Initial bottle = 8 mg O₂/L

  • Light bottle = 10 mg O₂/L

  • Dark bottle = 5 mg O₂/L

Calculations:

  • Respiration (R): Initial – Dark = 8 – 5 = 3 mg/L/hr

  • Net Primary Productivity (NPP): Light – Initial = 10 – 8 = 2 mg/L/hr

  • Gross Primary Productivity (GPP): Light – Dark = 10 – 5 = 5 mg/L/hr

Check: GPP = NPP + R → 2 + 3 = 5 ✅

🌍 Big Picture of Productivity

  • Measured as:

    • Energy per unit area per unit time (e.g., kcal/m²/year).

    • Snapshot alternative = standing crop (biomass at a single point in time).

  • High productivity ecosystems: tropical rainforests, estuaries, coral reefs.

  • Low productivity ecosystems: deserts, open ocean (nutrient-limited).

💡 Light Absorption in Water

  • Visible spectrum: 380–740 nm.

  • Red light absorbed quickly → only penetrates upper 1 m of water.

  • Blue light penetrates deepest (~100 m).

  • Photosynthesizers adapt by:

    • Using accessory pigments (e.g., carotenoids, phycobilins).

    • Living at depths that match their light absorption needs.