Bivalve fauna and distribution in the Amur River estuary—a warm-water ecosystem in the cold-water Pacific region

Abstract

The understanding of the processes regulating the species richness and diversity in estuarine ecosystems is a fundamental problem of ecology. The Amur River estuary is charac- terised by high summer water temperatures (up to 20�21°C) anomalous for the cold-water north- western Pacific region. We hypothesized that the warm-water Amur estuary greatly influences the species richness of this cold-water region, contributing to the far northward spread of warm-water macrobenthic species. We investigated the fauna and distribution of bivalves in relation to environmental variables in the Amur estuary (Sakhalin Gulf, Amurskiy Liman, and northern Tatar Strait) on the basis of samples collected during 6 expeditions between 2003 and 2007. Most of the bivalve species found in the estuary (30 of 41) are marine species, which were recorded only in Sakhalin Gulf (Sea of Okhotsk) and the Tatar Strait (Sea of Japan). In the greater part of Amurskiy Liman, we found only brackish-water species Corbicula japonica, Corbula amurensis, and Macoma balthica, which were most abundant (mean ± SE: 74.6 ± 3.4% of the total biomass per sta- tion) among the macrobenthos. Qualitative (presence/absence of species) and quantitative (fourth-root transformed biomass) cluster and multidimensional scaling analyses revealed 6 dis- tinct areas in the estuary. Depth, temperature, and salinity of the bottom water and sediment char- acteristics (clay content) exert the greatest effect on the distribution of bivalves in the estuary. The environmental conditions in the warm-water Amur estuary are favourable for the reproduction and existence of the warm-water species C. japonica, C. amurensis, Venerupis philippinarum, Crassostrea gigas, Raeta pulchella, Protothaca euglypta, and Callithaca adamsi, of which C. japonica and C. amu rensis determine the bottom community structure and abundance in the oligohaline and meso haline zones of the estuary. The Amur estuary is the northern distribution limit of natural populations of these species and serves the role of a refugium for many warm- water species of the northwestern Pacific.