Stable carbon isotopic composition of dissolved inorganic carbon (DIC) as a driving factor of aquatic plants organic matter build-up related to salinity

Stable isotopes probing is among the most used methods applied in the studies of paleoenvironment. Isotope signatures of sediments are influenced by different environmental conditions such as climate, salinity, and plant coverage at the time those sediments were deposited, representing an inestimable value for paleoenvironmental reconstruction and interpretation. In this study the carbon stable isotopic composition of organic matter (delta C-13(ORG)) and lignin monophenols (delta C-13(VSC)) extracted from this organic matter of submerged vascular plant Stuckenia pectinata (L.) Boerner 1912 syn. Potamogeton pectinatus L 1753 was investigated. Samples were collected from five different environments along a gradient of salinity from brackish Baltic Sea and freshwater lake in Estonia. The salinity influenced delta C-13(DIC) of ambient water, in the freshwater C-13 depletion was observed compare to brackish sites. S. pectinata have an intraspecific variation of the carbon stable isotope signature of organic matter depending on the environmental conditions, samples were C-13 enriched in the more brackish sites. Strong positive correlation between delta C-13(ORG) and C/N ratio for both leaves and stems were found. Lignin monophenols were C-13 depleted if compared with bulk organic matter, reflecting a common biochemical pattern of C fixation during lignin synthesis as for terrestrial plants. To conclude, organic matter isotopic signature and lignin composition of aquatic plants responded to environmental salinity. The innovative combination of these analyses provides a more refined interpretation of delta C-13(ORG) recorded in sediment deposited in the zones colonized by those plants.