What condition reduces excessive algae productivity in major rivers compared to ponds?

Higher turbidity due to erosion is a key condition that can reduce excessive algae productivity in major rivers compared to ponds. In rivers, increased turbidity often comes from sediment erosion and transportation. This turbidity can block sunlight from penetrating the water, which limits the photosynthesis process that algae rely on for growth. In contrast, ponds typically have clearer water, allowing for more sunlight to reach the bottom, which can encourage higher rates of algae growth.

Additionally, in rivers, the continuous flow of water can help to disperse nutrients and algae, further reducing the likelihood of algal blooms. The dynamic nature of rivers allows for greater dilution and flushing of nutrients, as opposed to the stagnant conditions often found in ponds. This combination of factors helps to limit the excessive productivity of algae in river environments.