The word "benthic" is an adjective that describes something related to the bottom of a body of water, like a lake, river, or ocean. Think of it as everything that happens on or lives at the very bottom of these water bodies.
"Benthic" primarily has one meaning related to aquatic environments. However, in a broader sense, you could consider "benthic" to imply anything that is foundational or essential, like the underlying support of an ecosystem, but this usage is rare.
There are no common idioms or phrasal verbs directly related to "benthic," as it is a specific scientific term.
In summary, benthic is a scientific term that describes anything associated with the bottom of water bodies.