engleski

Variability of stable isotope fingerprints of the serpulid Ficopomatus enigmaticus within a permanently stratified estuary : implications for (palaeo)environmental interpretations

This paper examines how the mixing of seawater and freshwater, and related salinity gradients and mixing of freshwater and marine particulate organic matter (POM) in a permanently stratified estuary of the river Krka, Croatia, are reflected in the stable isotope fingerprints of soft tissues and carbonate tubes of serpulids Ficopomatus enigmaticus, which are often used as (palaeo)environmental indicators of temperature and salinity. The conspicuous carbon stable isotope composition (δ13C values) of the river-borne (freshwater) POM is retained over long distances, causing a significant depletion of estuarine brackish POM and serpulids in 13C even at locations with salinity close to that of the sea. A trophic depletion in 13C was recorded in serpulids’ soft tissues. The δ13C values of carbonate tubes are subject to large isotope fractionation between dissolved inorganic C (DIC) and carbonate caused by vital effects, making the carbonate depleted in 13C by several ‰ compared to the DIC. These effects, however, though large in the freshwater zone, fade towards the sea, where biomineralisation seems to occur in conditions close to the equilibrium. The carbonate δ18O values of tubes reflect those of the water. The temperature-related differences in δ18O values of tubes from different sites are masked by source-related differences in δ18O values of water arising from mixing of freshwater and seawater in the estuary. Therefore, caution is required in interpretation of both organic and inorganic C, as well as oxygen isotope compositions of serpulids in transition environments where the terrestrial component can easily be overestimated because of vital effects during biomineralisation and trophic depletion in 13C in freshwater and brackish environment.

polychaete; particulate organic matter; organic carbon; carbonate; stable isotopes; estuary

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