What is the 'Bottom-up:Top-down Theory' in regards to trophic interactions of aquatic ecosystems suggest?

Answer 1

It simply explains the way some trophic levels affect others.

The bottom-up part suggest that the lower trophic levels limit higher trophic levels. For example, off the coast of Peru, plankton (lower trophic level) usually provide food for anchovies (higher trophic level). But during El Nino years, the nutrient supply for the plankton shrinks, which causes the anchovy population to shrink, and the anchovy population is replace by sardines.

A top-down scenario suggests exactly the opposite. For example, in the Arctic Ocean, killer whales traditionally feed on seals, but when the seal populations started to dwindle, the killer whales started eating sea otters. Since the otters are the sole predator of barnacles, and barnacles feed on other stuff, the implications of the shift on the ecosystem are huge. In this situation the top trophic layer is causing the lower trophic levels to shift.