ABSTRACT

The historical trend of the concentrations of carotenoids originating from phytoplankton in sediment cores collected at two sites in Lake Shinji, a eutrophic oligohaline lake was investigated to clarify the changes in phytoplankton community structures in the lake over the last 100 years. Eight kinds of carotenoids were detected in the sediments, i.e., astaxanthin and lutein from chlorophytes, alloxanthin from cryptophytes, canthaxanthin, echinenone and zeaxanthin from cyanobacteria, diatoxanthin from diatoms, and β-carotene from unspecified algal groups. On the basis of the stoichiometric relation between total organic carbon (TOC) and total sulfur (TS), the historical changes in the TOC concentration of sediment cores since circa 1970 have been attributable to the decomposition of organic matter by sulfate reduction, confirming the importance of diagenetic processes in Lake Shinji sediments. It was found that labile chlorophyll a is transformed diagenetically to more stable pheophytin a and pyropheophytin a within the upper 5 cm of the cores. Similarly, the concentrations (μg g^-1 TOC) of carotenoids, other than diatoxanthin, in the cores decreased rapidly within this zone, and in the cores lower than 5 cm below lake floor, their concentrations showed a decreasing or fixed trend with depth. This suggests that these carotenoids have been degraded in sediments depending on their stabilities. Among the diatoxanthin, lutein and zeaxanthin of which the stability is similar, the relative abundance of lutein (chlorophytes) has increased since circa 1970, and, conversely, the relative abundance of diatoxanthin (diatoms) has decreased. Diatoms are likely a staple food for the shellfish Corbicula japonica in Lake Shinji, because they are rich in lipids, which are needed for the growth and survival of C. japonica. Hence, the relative decrease in the number of diatoms may have some effect on the decrease in the population of C. japonica since the 1970s in the lake.