Final answer:
NPP is often low in rivers and streams due to their fast-moving water, which inhibits the growth of phytoplankton. Instead, photosynthesis is mostly attributed to algae that grow on rocks.
Step-by-step explanation:
The main reason why NPP (Net Primary Productivity) is often low in rivers and streams is due to their fast-moving water.
This fast flow results in minimal silt accumulation at the bottom of the river or stream, making the water clear. However, this clear water inhibits the growth of phytoplankton, which are the primary producers responsible for photosynthesis.
Instead, photosynthesis in rivers and streams is mostly attributed to algae that grow on rocks. Additionally, the fast-moving water and shading from tree cover keep temperatures lower, further reducing the rate of photosynthesis.