Макрозообентос эстуария реки Сусуя (остров Сахалин): I. Гидрологическая характеристика эстуария, видовой состав и распределение макрозообентоса

Abstract

The paper provides description of species composition of macrozoobenthos, its structure, quantitative indicators, and features of distribution along the riverbed of the Susuya River estuary (Sakhalin Island). By hydrological characteristics and macrobenthos parameters, the main biotopic zones of the estuary are distinguished: the mouth (polyhaline zone), the lower estuary polyhaline–mesohaline zone, the middle estuary oligohaline zone, the upper estuary δ-chorohaline zone, and the freshwater zone. In total, 58 species of bottom hydrobionts were found in the river estuary. Malacostraca, 22 species, forms the basis of the species composition. Amphibiotic insects account for 15 species; Polychaeta, 9 species; and Mollusca, 7 species. Other groups are represented by 1–2 species. Each identified zone features its own, specific composition of bottom hydrobionts. Species richness decreases from the mouth with the sea salinity of water (30 species) to the α-chorohaline boundary (12 species) which corresponds to provisions of the theory of critical salinity. The value increases to 20 species when moving to the freshwater zone of the riverbed. The density of macrobenthos rises from (476 ± 59) ind.·m⁻² at the river mouth to (6,653 ± 915) ind.·m⁻² in the center of the lower estuary. The minimum density, (653 ± 72) ind.·m⁻², characterizes the α-chorohalinicum, while the maximum one, (3,529 ± 336) ind.·m⁻², is confined to the upper estuary. The basis of macrozoobenthos density is formed by polychaetes, gastropods, and amphipods in the lower estuary; by polychaetes, amphipods, and oligochaetes in the middle estuary; and by oligochaetes and dipterans in the upper estuary. Four areas with high biomass were registered: the lower estuary, (51.2 ± 5.7) g·m⁻²; the middle estuary, (190.5 ± 41.2) g·m⁻²; the upper estuary, (397.5 ± 82.0) g·m⁻²; and the riffle separating the estuary from the freshwater zone of the riverbed, (23.4 ± 2.78) g·m⁻². At the river mouth, decapods form the basis of macrozoobenthos biomass. Upstream, the main contributors are bivalves, chiefly Macoma balthica, and gastropods, mostly Fluviocingula nipponica. In the middle estuary oligohaline zone, bivalves form the basis of macrozoobenthos biomass; those are represented almost by Corbicula japonica alone. At the boundary of the oligohaline zone and upper estuary, Diptera species (Chironomidae) predominate. On the freshwater riffle, decapods become the main group again; those are represented by a crab Eriocheir japonica alone. The boundaries between the mouth and lower estuary zone, α-chorohalinicum and δ-chorohalinicum, are clearly distinguished by macrobenthos abundance and structural indices. The boundary between the upper estuary oligohaline zone and freshwater zone extends for several hundred meters along the estuary.

Keywords: estuary, salinity, macrozoobenthos, density, biomass, Sakhalin Island