Spatial-temporal feeding dynamics of benthic communities in an estuary-marine gradient

We investigated the fluctuations of carbon and nitrogen stable isotope ratios in benthic consumers and their potential food sources to determine the spatial and temporal variations in the utilization of available organic matter, indicating the origin and pathways of energy from Yura Estuary to Tango Sea, Japan. Field samplings were conducted from the upper estuary to offshore with sampling frequency of twice per season from April (spring) 2006 to February (winter) 2007. The δ13C signatures of the upper and lower estuary benthos showed depleted and in wide range (−28.9‰ to −13.5‰) compared to the enriched and within narrow range signatures of marine benthos (−20.6‰ to −14.0‰) in all seasons. On the contrary, the δ15N signatures of benthic communities showed decreasing trend seaward and summer values were different from the other seasons. Using the dual isotope and multisource mixing models, we estimated the relative contributions of potential food sources to the benthos diet. River POM played an important source of energy for the estuarine benthos, especially in winter when river discharge was high. Marine POM served as an important alternative food for the estuarine benthos from spring to autumn when seawater intruded the bottom estuary. Benthic microalgae were the major food source at the shallow coast throughout the year, while marine POM fueled the deep coast and offshore benthic food webs. Spatial and temporal feeding variations in estuarine benthic communities were driven by the hydrology of the estuary, whereas primary production and transport of food source dictated diet variations of marine benthic communities. The elucidation of the dynamic energy subsidy among aquatic systems highlights the importance of the land–sea transition zones that is crucial for benthic secondary productions.