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 freshwater and seawater, and related mixing of freshwater and marine particulate organic matter (POM) in the permanently stratified estuary of the River Krka, Croatia, are reflected in the stable isotope fingerprints of soft tissues and tubes of the serpulid Ficopomatus enigmaticus. The carbon stable isotope composition (δ13C values) of the river-borne POM is retained over long distances, causing a depletion in 13C of POM in brackish waters. A trophic depletion in 13C was recorded in serpulid soft tissues. The serpulid carbonate tubes were depleted in 13C even at locations with salinity close to that of the sea and were subject to large isotope fractionation between dissolved inorganic C (DIC) and carbonate caused by vital effects, making carbonate depleted in 13C by several per mil compared with DIC. These effects, though large in the freshwater zone, fade towards the sea. The carbonate δ18O values of tubes reflect the δ18O values of the water. The temperature-related differences in δ18O values of tubes from different sites are masked by source-related differences in the δ18O values of water arising from mixing of freshwater and seawater in the estuary. Therefore, in serpulide tubes, the terrestrial component can easily be overestimated because of vital effects during biomineralisation and trophic depletion in 13C in freshwater and brackish environments.