Utilisation of different carbon sources in a shallow estuary identified through stable isotope techniques

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 (13)C 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 delta(13)C of respired CO(2) compared to the delta(13)C 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. delta(13)C 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 delta(13)C 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. (C) 2011 Elsevier Ltd. All rights reserved.