Specifics of the bottom biogeocenosis formation on tidal flats of the White Sea mesotidal estuaries

We studied the peculiarities of bottom biogeocenoses formation on tidal flats of mesotidal estuaries (case-study of the Tamitsa River estuary in the Onega Bay of the White Sea). Mesotidal estuaries that dominate in the White Sea are characterized by extensive tidal flats and the highest tide of 1.7–2.8 m. Such habitats are currently very poorly understood. In May 2022, expedition works were carried out in the Tamitsa River estuary. Quantitative zoobenthos development parameters and organic matter content in benthic sediments identified by chemical oxygen demand were determined at five sites on the tidal flat. It is shown that the sorting of suspended sediments by hydraulic coarseness in the estuary water entails the formation of relatively narrow bands enriched or depleted in organic content. Such bands stretch along the shoreline and are confined to water levels close to low and high water of the tidal cycle. The characteristic width of bottom sediment bands enriched with organic matter with the presence of a large number of benthic animals is several meters. It depends largely on the difference in tidal magnitude during different phases of the moon-solar cycle. The extent of the above mentioned bands will depend on the geomorphology of the river mouth. If we focus on mesotidal estuaries of minor rivers of the White Sea, their characteristic length should be from several hundred meters to 1–3 km. Bivalves and polychaetes dominate in the bands with maximum biomass of benthic invertebrates, while crustaceans and some species of bivalves dominate in the zones with minimum biomass. It is recommended to take into account the possibility of accumulation of highly toxic pollutants, such as organochlorine and organophosphorus pesticides, biphenyls and a number of heavy metals, in monitoring of bottom sediment pollution in tidal estuaries.