Freshwater Ecology Practice Test 2026 – Comprehensive Exam Prep

Invasive species can alter nutrient cycling in freshwater systems by changing how nutrients move between the water, sediments, and living organisms. Zebra mussels are a classic example because they are strong filter feeders that remove large amounts of particulate matter from the water. This increases water clarity, which shifts which phytoplankton groups dominate and how nutrients are taken up in the system. Their feeding and waste products return dissolved inorganic nutrients to the water, effectively speeding up the recycling of nutrients. They also influence sediments by attaching to surfaces and disturbing the bottom, which can release stored nutrients back into the water column. Taken together, these changes demonstrate a documented way an invasive species can modify nutrient cycling.

The other scenarios describe different ecosystem effects that aren’t as clearly linked to nutrient cycling in a documented, mechanism-based way. For example, cattails changing sedimentation rates, crayfish affecting dissolved oxygen, or snails altering nutrient uptake by algae without a clear, system-wide shift in nutrient flows, do not illustrate the same well-supported nutrient-cycle modification seen with zebra mussels.