A 200-year record of natural and anthropogenic changes in water quality from coastal lagoon sediments of Lake Shinji, Japan

Abstract

We analyzed organic carbon, total nitrogen, and total phosphorus concentrations together with the stable isotope ratios of organic carbon and total nitrogen from three cores of sediments collected at Lake Shinji, an oligohaline lagoon situated in southwest Japan, to trace both natural and anthropogenic effects on the water quality of the coastal lagoon for the past 200 years. The increase of carbon and nitrogen concentrations in the core sediments collected near the mouth of the freshwater river suggested that the anthropogenic eutrophication first started in the 1940s due to the increase of loads caused by the increased population in the drainage area. δ 15N of the sediments also implied that the eutrophication continues to date even after the end of population growth, due presumably to the use of chemical fertilizer. Organic carbon and total nitrogen concentrations showed higher values during the early 19th century than the present in the core sediments collected at the east end of the lake where polyhaline water occasionally flows back. These higher concentrations were attributed to more frequent backflow of polyhaline water.