♻️AP Environmental Science
2. A freshwater lake ecosystem supports a diverse food web. The lake receives nutrient inputs from surrounding agricultural land and experiences seasonal algal blooms. Scientists are studying the ecosystem to understand energy transfer efficiency and nutrient cycling patterns. The lake contains phytoplankton (primary producers), zooplankton (primary consumers), small fish (secondary consumers), and large predatory fish (tertiary consumers).
Trophic Level | Energy Content (kcal/m²/year) |
|---|---|
Phytoplankton (primary producers) | 87,500 |
Zooplankton (primary consumers) | 14,000 |
Small fish (secondary consumers) | 1,400 |
Large predatory fish (tertiary consumers) | 140 |
Figure 1. Nitrogen Cycle in a Freshwater Lake Ecosystem (enhanced, exact labels and arrow directions)
Identify the bacterial process shown in Figure 1 that converts atmospheric nitrogen (N₂) into a form usable by phytoplankton.
Identify the specific form of nitrogen shown in Figure 1 that phytoplankton directly absorb from the water for growth.
Based on the information in Figure 1, identify the bacterial process that converts nitrate back into atmospheric nitrogen gas.
Agricultural runoff containing high levels of nitrate enters the lake, causing excessive algal growth. Explain how this increased nutrient availability leads to decreased dissolved oxygen levels in the lake, harming fish populations.
Describe one negative environmental effect of excessive algal blooms on aquatic organisms other than the oxygen depletion already discussed.
Propose one realistic solution that farmers could implement to reduce the amount of nitrogen-containing runoff entering the lake.
Figure 2. Energy Content at Different Trophic Levels in the Lake Ecosystem
Using the data in Figure 2, calculate the percent energy transfer efficiency from phytoplankton to zooplankton. Show all work for full credit. The lake ecosystem data shows phytoplankton contain 87,500 kcal/m²/year and zooplankton contain 14,000 kcal/m²/year. Energy transfer efficiency can be calculated using the formula: (Energy at consumer level / Energy at producer level) × 100%
Justify the solution you proposed in part F by describing one additional environmental benefit, other than reducing nitrogen runoff into the lake.
Freshwater lakes are a type of aquatic biome with distinct characteristics. Describe one difference between the distribution of nutrients in a lake ecosystem compared to an open ocean ecosystem.
The phosphorus cycle differs from the nitrogen cycle because phosphorus has no atmospheric reservoir. Describe how phosphorus typically enters freshwater ecosystems and explain why this makes phosphorus pollution particularly difficult to reverse.
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