Organic carbon in estuarine sediments can have many different sources. Terrestrial, riverine, estuarine and marine C pools may all contribute to and influence the organic C (C
org) inventory of the estuarine sediments and the differing stable isotope signatures of the sources are reflected in the sediment's overall
13C content. Ecological interpretations of sedimentary isotope data may, however, be limited by the fact the total C
org inventory of a sediment may not be an accurate representation of the fraction that is labile and being actively turned over by the sedimentary community. To gain a better understanding of sedimentary C
org dynamics in estuaries and the relationship between the sedimentary C pool and the C
org undergoing mineralisation, we studied three components of an estuarine system: (1) the sedimentary C
org inventory on a transect from the mouth to the upper end of the estuary, (2) temporal changes of sedimentary C
org at one station throughout a year, and (3) the δ
13C of respired CO
2 compared to the δ
13C of available source material and sedimentary C
org in a novel application of methods developed for soil science. Our experiments demonstrated that material of marine origin dominated the studied estuary. At the time-series station, material of marine origin dominated the sedimentary C
org throughout the 1-yr study period. δ
13C values of CO
2 released from the sediment differed significantly from the sedimentary C
org inventory at all study sites, but also clearly reflected differences between the main sections of the estuary. These results suggest that δ
13C measurements of respired CO
2 are promising as a tool to advance our understanding of C cycling in estuaries, and highlight that the sedimentary C
org pool alone may not be a satisfactory indicator of OM utilisation in estuarine sediments.
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