The article discusses the diversity of coastal communities and the characteristics of their distribution at the river estuaries of the White Sea and the Chyoshskaya Bay, which differ in tidal ranges. The salinity of tidal estuaries of small rivers is non-uniform. It declines from the estuary mouth towards its head, influencing the helophytic vegetation of the marshes and altering their plant communities. Tides, in turn, promote the development of these marshes. The richest phytocenotic diversity is found in polyhaline marshes of the macrotidal estuaries of the Chizha and Chyosha rivers, which harbor communities dominated by obligate halophytes. However, the diversity diminishes in mesohaline marshes in the estuarine areas of Onega Bay, featuring communities dominated by facultative halophytes, and in oligohaline tidal wetlands at estuary heads, which are characterized by communities dominated by salt-tolerant glycophyte plants. In mesotidal and microtidal estuaries of the White Sea, euryhaline plant communities are widely represented, which primarily occur in marshes of different salinity types. Euryhaline coenoses are more diverse in the microtidal estuaries of Dvina Bay and the mesotidal estuaries of Onega Bay.