Spatiotemporal dynamics of stable carbon isotope ratios in two sympatric oligohaline copepods in relation to the estuarine turbidity maximum (Chikugo River, Japan): implications for food sources

Abstract

To improve our understanding of high copepod productivity in the estuarine turbidity maximum (ETM) of the macrotidal Chikugo River estuary in southwestern Japan, we determined stable carbon isotope ratios (δ[13]C) in the sympatric oligohaline copepods Sinocalanus sinensis andPseudodiaptomus inopinus from 2005 to 2006. Terrestrial-plant and phytoplankton detritus always accumulated in the ETM (salinity 0.1–3), whereas outside the ETM phytoplankton dominated especially in the warm season (>20°C). In contrast to the year-round concentration of S. sinensisin the ETM, P. inopinus occurred widely along the upper estuary under phytoplankton-dominated conditions. Terrestrial-plant detritus was characterized by relatively constant δ[13]C (∼−24‰), suggesting that significant spatiotemporal variability in copepod δ[13]C was attributable to the feeding of copepods on phytoplankton and/or its detritus. Both copepods had relatively depleted δ[13]C values in the ETM, reflecting δ[13]C in freshwater/oligohaline phytoplankton ( < −24‰). However, relatively enriched δ[13]C values (>−24‰) associated with meso/polyhaline phytoplankton downstream from the ETM were found only in P. inopinus. Although the contribution of terrestrial-plant detritus to copepod production remains to be determined, our results indicate that both copepods selectively utilize freshwater/oligohaline phytoplankton and/or its detritus in the ETM, whereas only P. inopinus utilizes meso/polyhaline phytoplankton downstream from the ETM.