Final answer:
In rivers, the top predator trophic level is most lacking due to energy and nutrient decrease.
Step-by-step explanation:
In rivers, the trophic level that is most lacking is usually the top predator. This is because energy and nutrients decrease as you move up the food chain, so there is less available for organisms at higher trophic levels. In the case of rivers, the top predators, such as large fish species, may be limited due to this energy transfer.