Answer:
C. Both will gradually decrease.
Here's the reasoning:
1. **Increase in Herbivorous Fish**: With the migration of some carnivorous fish from the river, there is less predation pressure on the herbivorous fish, leading to their increase in population.
2. **Impact on Primary Producers**: As the number of herbivorous fish increases, they consume more primary producers (like aquatic plants and algae). This increased consumption leads to a decrease in the population of primary producers.
3. **Impact on Net Primary Productivity (NPP)**: NPP is the rate at which all the plants in an ecosystem produce net useful chemical energy; it equals the difference between the energy they produce in photosynthesis and the energy they use in cellular respiration. With fewer primary producers available due to increased herbivory, the overall NPP of the ecosystem will likely decrease because there will be less biomass conversion of sunlight into energy via photosynthesis.
Thus, the increase in herbivorous fish, due to decreased predation, leads to increased consumption of primary producers, ultimately resulting in a decrease in both the number of primary producers and the NPP of the ecosystem.
