A decadal resolution record of coastal phyto- and zooplankton productivityin the Seto Inland Sea, Japan, over the last 7,000 years

Despite the fact that coastal ecosystems play an important role in biogeochemical cycles because they have high rates of primary production due to large inputs of nutrients from land and oceans, however, it is poorly understood how the primary production are driven by climate and environmental changes for several thousand years up to the present. In this study, we present a decadal (< 10 years) record of biomarker-based coastal lower trophic level productivity over the last 7,000 years from the sediment cores in Beppu Bay, Seto Inland Sea, Japan. The productivity was estimated based on the abundances of fossil pigments derived from chlorophyll a and its derivatives (phytoplankton), and steryl chlorin esters, SCEs (zooplankton). The reconstructed productivities of phyto- and zooplankton show (i) a wide variation in productivity through the period, and (ii) their maximum values at around 4.0 kyr, 1.9 kyr and 3.9 kyr cal BP, and an increase during the last several hundred years, and (iii) an enhancement in the amplitudes of their variations after around 4.2 kyr cal BP. Furthermore, spectral analysis indicated that the variations of phyto- and zooplankton productivity have an almost common periodicity at ca. 186 and 178 years, respectively. The Seto Inland Sea is thought to be greatly influenced by the variations in oceanic nutrient inputs related to the variability in Kuroshio path location and velocity. Therefore, not only the variation in the lower primary productivity but its periodicity might be controlled by large-scale environmental change in the Pacific. Here, we report the results and discuss about the controlling factors of their periodicities in the time series of the productivity.