Level muddy surface bordering an estuary , alternately submerged and exposed to the air by changing tidal levels.
In addition to the alternating submergence and exposure, the varying influences of fresh river water and salty marine waters cause physical conditions to vary more widely than in any other marine environment. The mud of a tidal flat is usually rich in dissolved nutrients, plankton , and organic debris, and it supports large numbers of small animals such as crabs and worms. Vegetation is generally sparse, but mats of blue or blue-green algae (see cyanobacteria ) may be present.