Benthic foraminifera assemblages as bottom environmental indicators, Posiet Bay, Sea of Japan

Abstract

Benthic foraminiferal fauna patterns have been investigated as possible indicators of the various morphological features interpreted as the expression of slightly different environmental conditions within a ria bay. Eighty grab-top samples from the Posiet Bay and twenty littoral stations were examined concerning the distribution of benthic foraminifera in different bottom environments in one of the largest ria bays of the Western Japan Sea. Altogether 51 species of benthic foraminifera belonging to 32 genera were determined from the bay bottom sediments. The zones of high productivity and diversity (Shannon diversity) are established. The distribution of agglutinated and carbonate forms reflects mainly river input. Agglutinated foraminifera dominate within brackish shallow closed and semi-open inlets. Calcareous species dominate within the inshore part of open inlets and seaward part of the bay. The combination of the species composition, an information function (Shannon diversity), and population density are used to characterize modern bottom environments based on benthic foraminifera. Four faunal assemblages were identified by Q-mode cluster analysis. The first assemblage, dominated by Eggerella advena, is present from 7 to 40 m and is associated with temperatures of 9.7–16.9°C, that are typical for semi-open inlet environments with silt substrate. A second assemblage, dominated by Trochammina inflata, occurs in closed inlets at depths up to 21 m, and is associated with temperatures of 12.9–17.5°C, a brackish environment, and muddy substrate. The third Cribroelphidium asterenium assemblage represents a semi-open marine environment. The fourth assemblage, dominated by C. etigoense, is present to a depth of 18 m within an open shallow inlet. This assemblage with the highest species diversity is associated with bottom temperatures of 10–18°C and a sandy substrate. The difference in the benthic foraminiferal assemblages at similar depths in Posiet Bay is the result of different hydrologic regimes in closed, semi-open, and open environments. Littoral benthic foraminiferal assemblages are different from bottom ones. Regional environmental conditions can be recognized as deviations from the depth-bottom temperature range, river influx, and coastal configuration features.