Grade 9 Ecology Practice Exam

Eutrophication happens when nutrients such as nitrogen and phosphorus build up in a freshwater system, fueling rapid growth of algae and aquatic plants. This nutrient enrichment often leads to algal blooms, which reduce water clarity and alter light availability for other organisms. When the blooms die, decomposers use a lot of oxygen to break down the organic matter, causing dissolved oxygen levels to drop. The resulting hypoxic conditions can create dead zones and lead to biodiversity loss, as many species can’t survive low-oxygen areas. So, instead of increasing clarity, eutrophication typically makes water murkier and less hospitable for aquatic life. The opposite ideas—nutrient depletion, clearer water, or fewer algal blooms—do not describe what happens with eutrophication.