Temporal variability of stable carbon and nitrogen isotopic composition of size-fractionated particulate organic matter in the hypertrophic Sumida River Estuary of Tokyo Bay, Japan

To clarify the major factors that control stable carbon and nitrogen isotopic compositions (δ13C and δ15N) of suspended particulate organic matter in a hypertrophic estuary, seasonal variation in concentrations and stable isotopic compositions of particulate organic carbon and nitrogen (POC, PN), chlorophyll a content (Chl) and dry weight of suspended solid (SS) was studied in the Sumida River estuary of Tokyo Bay. Particulate material was fractioned into four size-fractions (3–20, 20–53, 53–250, 250–1000 μm). At the lower-estuarine (LE) site, the finer size fraction (<20 μm) was predominant (>80% Chl) during the autumnal bloom (Aug–Sep 2003). The middle size fractions (20–53 and 53–250 μm) however increased during the spring bloom (50–80% Chl; Feb–Apr 2004), reflecting difference in taxonomic composition of phytoplankton. At the upper-estuarine and the riverine sites, the <20 μm fraction was most abundant throughout the survey period. δ13CPOC and δ15NPN varied between −32 and −16‰ and between −8 and +12‰, respectively. Relationships of δ13CPOC with POC/Chl and POC/PN ratios could be explained by assuming two end members: allochthonous (terrestrial) POC with a δ13C close to −26.5‰, and autochthonous (planktonic) POC whose δ13C was variable and apparently correlated with δ13C of ambient DIC (−10 to 0‰) and phytoplankton abundance (Chl). δ15NPN was apparently controlled by water temperature and NH4+ concentration. The variability of δ13C and δ15N was different between size fractions: the apparent dependence of δ13C on Chl as well as that of δ15N on water temperature was significantly stronger for the middle size fractions (mainly diatoms) than the finer fraction (mainly microflagellates), suggesting different physiological response between taxa. The second objective of this study was to evaluate contribution of terrestrial organic carbon (TPOC) to the POC size fractions by using δ13C of isolated chlorophyll a at LE site as surrogate for the autochthonous δ13C end member. During the autumnal bloom, TPOC was 23–36% on average for the middle size fractions (20–53 and 53–250 μm), and ≥59% for the finest (3–20 μm) and coarsest (250–1000 μm) fractions. During the spring bloom, the average contribution of TPOC was generally low (≤21%) except for the finest fraction (36%). Negative correlation between salinity and calculated TPOC suggested that transport of terrestrial organic carbon to this estuary was controlled principally by the river discharge.