why estuaries and tidal marshes are high level of biodiversity?

Answer 1

edges between any zone always has greater diversity, more specialist niches, more opportunity, generaly more productive too.

in estuaries therre is also a greater flow of nutrients, and the changing environment means there is no optimum steady-state solution to living there, so lots of species can adapt in different ways with no one being THE fittest

Answer 2

Because the circulation of water in them tends to be slow & regular. This tends to provide a constant, dependable supply of food (& support a larger variety of creatures) right up the food chain.