Origin and vertical dynamics of particulate organic matter in a salt-wedge estuary, the Ebro Delta, inferred from lipid molecular markers and compound-specific isotope carbon analysis

The abundance, molecular composition and δ13C values of free sterols, n-alkanols (C14–C30) and phytol have been investigated in the particulate matter of the Ebro river estuary (NE Spain) in order to assess the origin and fate of organic matter in a salt-wedge estuary. Water samples were collected in June and October 2003 at different depths in two water regimes, with and without the presence of salt-wedge. An important contribution of autochthonous productivity to the particulate organic matter was found in June as indicated by a higher concentration of phytol and phytoplankton sterols and an enrichment in δ13C values. In contrast, lower productivity and higher decomposition processes were noticeable in the October campaign, as indicated by more depleted δ13C values and higher stanol/stenol ratios. Molecular markers also indicated a change in the freshwater phytoplankton composition between the two sampling campaigns and showed some inputs derived from aquatic/terrestrial plants, particularly in surface samples. In the absence of the salt-wedge, particulate organic matter derived from similar origins occurred throughout the water column but a higher presence of organic matter decay was observed in bottom waters. On the other hand, a general depletion in the concentrations of lipid molecular markers was observed in the salt-wedge. This trend was attributed to the flocculation and mixing of different kinds of suspended particles (mineral and organic) in the salt-wedge originated in an abrupt salinity change that occurs at the stratification interface. An accumulation of freshwater phytoplankton was observed at the interface sampled in June, where this material is more susceptible to degradation and decomposition. Compound-specific carbon isotope data showed the influence of marine organic matter in the salt-wedge, mixed with terrestrial material which sinks from the surface freshwater.