Foraminiferal assemblages response to anthropogenic influence and parallel to decadal sea-level changes over the last 70 years in Lake Kugushi, Fukui Prefecture, southwest Japan

Abstract

Foraminiferal assemblages in Lake Kugushi in the Sea of Japan coast of Southwest Japan responded to both human activity and sea-level changes over the last 70 years. Human modifications in and around this lake were mainly related to the dredging and widening of the Urami River in 1935 and to shore protection projects that removed aquatic plants around the lake in the middle 1970s. About 60% of the natural shoreline was replaced by artificial ones. Marine foraminifera migrated into the southern part of the lake after the modifications of 1935. Brackish foraminifera preferring phytodetritus became dominant from the middle 1970s, while the original assemblage, preferring low-salinity and probably land-derived organic matter, decreased. The sea-level rise over the last 70 years is clearly indicated by two kinds of foraminiferal assemblages: the appearance of marine species in the 1950s and 1970s, and the development of a brackish assemblage preferring eutrophic water from the 1980s onward. The latter is an assemblage dominated by Trochammina hadai, which is common in coastal lagoons with a well developed halocline and highly eutrophied argillaceous substrates. Based on the different response of these foraminiferal assemblages to sea-level rise, the most recent sea-level rise is driven by a mechanism unlike those in the 1970s and 1950s. The thermal expansion of seawater will lead to coastal lagoons that will be more stagnant and eutrophic, with little exchange of coastal marine water.