Question

which organisms form the base of most aquatic food webs? aquatic food web squid seal seagull penguin krill blue whale plankton phytoplankton small fish decomposers

Explanation:

In an aquatic food web, the base is formed by producers. Phytoplankton are primary producers as they perform photosynthesis (like plants on land but in water). Small fish are consumers (either primary/secondary etc., depending on what they eat). Decomposers break down dead organisms and are not the base. Looking at the food web diagram, plankton (phytoplankton are a type of plankton) is at the bottom - all other organisms (krill, which are then eaten by seal, squid, seagull, blue whale etc.) ultimately rely on it for energy (as krill feed on plankton and then other organisms feed on krill or directly/indirectly on plankton - though in this web krill is shown as feeding on plankton and others on krill).

Answer:

Phytoplankton