♻️AP Environmental Science
1. Phytoplankton are microscopic photosynthetic organisms that form the base of aquatic food webs. In freshwater lakes, phytoplankton abundance is influenced by nutrient availability, particularly nitrogen and phosphorus. Scientists studied a temperate lake ecosystem to understand how nutrient concentrations affect primary productivity and energy flow through trophic levels.
Describe one way that phytoplankton acquire solar energy and convert it to chemical energy.
Explain how an increase in phytoplankton biomass could affect the zooplankton population in the lake ecosystem.
Figure 1. Relationship Between Dissolved Phosphorus Concentration and Phytoplankton Biomass (temperate freshwater lake).
Based on the data in Figure 1, identify the phytoplankton biomass at a dissolved phosphorus concentration of 50 μg/L.
Figure 2. Energy Available at Each Trophic Level in High-Nutrient vs. Low-Nutrient Lake Regions (kcal/m²/year).
Based on the data in Figure 2, describe the relationship between trophic level and energy availability in the high-nutrient region.
Describe one way that the data in Figure 2 support this hypothesis. Scientists hypothesized that energy transfer efficiency between trophic levels would be approximately 10% in both lake regions.
A group of students wanted to investigate how nitrogen availability affects phytoplankton growth rates. They collected water samples from a local pond and divided them into five experimental groups. Each group received a different concentration of nitrogen: 0 mg/L (control), 5 mg/L, 10 mg/L, 15 mg/L, and 20 mg/L. All samples were placed under identical light conditions at 22°C for 7 days. Students measured phytoplankton cell density using a microscope at the end of the experiment.
Identify the independent variable in the students' investigation.
Identify one variable that was held constant in the students' investigation.
Site | Species A | Species B | Species C | Species D | Species E | Species F |
|---|---|---|---|---|---|---|
Nutrient-enriched pond | X | X | ||||
Natural pond | X | X | X | X | X |
The students expanded their investigation to compare phytoplankton biodiversity in a nutrient-enriched pond (receiving fertilizer runoff) and a natural pond. They collected samples from each pond and identified the phytoplankton species present. The data are shown in the following table. An 'X' in the table indicates that the species was present at that site.
Explain why the natural pond would be more resilient to environmental disturbances, such as temperature fluctuations or disease outbreaks, than the nutrient-enriched pond would.
Explain why agricultural runoff containing excess phosphorus could lead to decreased dissolved oxygen levels in the lake.
Describe one step in the phosphorus cycle that returns phosphorus from organisms to the soil or sediment. Nitrogen and phosphorus cycle through ecosystems via biological and geological processes. Phosphorus differs from nitrogen in that it lacks a significant atmospheric reservoir.
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