Differences in stable isotope compositions of freshwater snails from surface sediments of two Polish shallow lakes

The study reports and discusses the differences in delta C-13 and delta O-18 values of shells between several species of freshwater snails. Shells were derived from sediment samples collected from depths of 0.5, 1, 2 and 3 m along transects in two shallow eutrophic lakes located in mid-western Poland. Mean delta C-13 values of the shells ranged between -7.5 and -3.8 parts per thousand in Lake Jarostawieckie and between -8.1 and -5.2 parts per thousand in Lake Rosnowskie Duke, whereas mean delta O-18 values ranged between -2.2 and -0.2 parts per thousand and between -2.2 and 0.4 parts per thousand respectively in the studied lakes. A similar order of species in terms of shell isotope values, from least to most C-13 and (18) O-depleted was observed in both lakes and seems to indicate constancy of the factors controlling the stable isotope compositions of snail shells. We postulate that the nearly 4 parts per thousand difference in the mean carbon stable isotope values between the species was primarily controlled by the amount of metabolic carbon incorporated into the shells and the delta C-13 values of the snail food. Different growth cessation temperatures and microhabitats of the species studied result in temporally and spatially varied DIC delta C-13 values, water delta O-18 values and water temperature of shell precipitation, and may thus differentiate the delta C-13 and delta O-18 values of shells. The range of delta C-13 and delta O-18 values of individual shells from a sediment sample (mean 2.35 and 2.15 parts per thousand, respectively) is interpreted as reflecting an intraspecific variability of isotope compositions in shells from a population and changes of the ambient conditions during the accumulation of the sediment layer. The species-specificity and intraspecific variability in C and O isotopic compositions of shells allow concluding that in palaeolimnological studies, stable isotope analyses should be performed on a set of mono-specific shells representing mean isotope compositions of the species for the interval studied rather than single shells or multispecific bulk shell material. (C) 2015 Elsevier GmbH. All rights reserved.