What is a likely effect on phytoplankton when large amounts of macrophytes are killed in a water body?

The correct choice indicating an increase in phytoplankton is based on the ecological dynamics that occur when large amounts of macrophytes (aquatic plants) are removed. Macrophytes typically play a significant role in aquatic ecosystems by providing habitat, stabilizing sediments, and consuming nutrients.

When these plants are killed off, one of the immediate effects is a disturbance in the nutrient balance within the water body. Macrophytes help absorb excess nutrients, particularly nitrogen and phosphorus, thus limiting phytoplankton growth. Without the macrophytes to take up these nutrients, there is often an influx of nutrients available in the water column.

This nutrient availability can lead to an increase in phytoplankton populations as they flourish in the nutrient-rich environment. The sudden increase in phytoplankton can also lead to changes in water clarity and oxygen levels, potentially causing further shifts in the ecosystem. Thus, the death of macrophytes directly contributes to an increase in phytoplankton due to reduced competition for nutrients and absence of plant cover.