USA Biology Olympiad (USABO) Practice Exam

In which zone of a lake would the most phytoplankton be found?

1. Profundal Zone

2. Benthic Zone

3. Limnetic Zone

4. Littoral Zone

The correct answer is: Limnetic Zone

The Limnetic Zone is the region of a lake that is well-lit, allowing for photosynthesis to occur. This zone extends from the surface of the water down to the depth where light penetration is insufficient for photosynthetic activity. Phytoplankton, which are microscopic aquatic plants, thrive in this zone because they require sunlight to produce energy through photosynthesis. The abundance of light, coupled with nutrient availability, creates an ideal environment for phytoplankton growth and reproduction. In contrast, the other zones have characteristics that limit phytoplankton presence. The Profundal Zone, being deeper, experiences lower light levels, making it less suitable for phytoplankton. The Benthic Zone, located at the bottom of the lake, consists of sediments and is generally too dark and lacks the necessary light for phytoplankton. The Littoral Zone, while it does support some phytoplankton, is primarily characterized by rooted aquatic plants and may not contain as high a concentration of phytoplankton as the Limnetic Zone due to its shallow nature and increased competition with these larger plants for resources.