Which of the following statements best explains why an increase in the number of phytoplankton in an aquatic ecosystem is not always beneficial?

• A. Phytoplankton always improve water quality regardless of their abundance.
• B. An increase in phytoplankton has no effect on nutrient cycles in aquatic ecosystems.
• C. Excessive phytoplankton growth can lead to oxygen depletion and harm aquatic life.
• D. More phytoplankton always results in greater biodiversity in aquatic environments.

Answer: (C)

Verified step by step guidance

1. Understand the role of phytoplankton in aquatic ecosystems: Phytoplankton are microscopic organisms that perform photosynthesis, producing oxygen and serving as the base of the aquatic food web. They are essential for nutrient cycling and energy flow in these ecosystems.
2. Recognize the concept of eutrophication: When phytoplankton populations grow excessively due to an abundance of nutrients (e.g., from agricultural runoff containing nitrogen and phosphorus), it can lead to a phenomenon called eutrophication. This disrupts the balance of the ecosystem.
3. Explain the consequences of excessive phytoplankton growth: When phytoplankton die and decompose, bacteria consume oxygen in the water during the decomposition process. This can lead to hypoxia (low oxygen levels) or anoxia (complete oxygen depletion), which can harm or kill aquatic organisms like fish and invertebrates.
4. Clarify why excessive growth is not always beneficial: While phytoplankton are crucial for ecosystem health, their overabundance can create harmful algal blooms (HABs). These blooms can release toxins, block sunlight from reaching other aquatic plants, and disrupt the food web.
5. Conclude with the correct explanation: The best explanation is that excessive phytoplankton growth can lead to oxygen depletion and harm aquatic life, highlighting the importance of maintaining a balanced ecosystem.