Modeling calcification periods of Cytheridella ilosvayi from Florida based on isotopic signatures and hydrological data

Abstract. The isotopic signatures of ostracod shells are the result of the temperature and composition of their host water and the phenology and ecology of the target species. Investigations addressing the influence of site-specific environmental variations on the isotopic ranges of ostracod shells are still rare but can provide important information on habitat-dependent variations and may signify a seasonally restricted timing of calcification periods. Here we present isotopic signatures (δ¹⁸O_ostr, δ¹³C_ostr) of living Cytheridella ilosvayi (Ostracoda) and physical, chemical, and isotopic (δD, δ¹⁸O_water, δ¹³C_DIC) compositions of 14 freshwater habitats (rivers, lakes, canals, marshes, sinkholes) in South Florida from winter 2013 and summer 2014. We also present instrumental data of river temperatures and δ¹⁸O of precipitation (δ¹⁸O_prec) from this region. The physicochemical and isotopic compositions of the selected sites characterize the different habitats and show the influence of the source water, biological activity, and duration of exposure to the surface. Mean δ¹⁸O_ostr and δ¹³C_ostr signatures of C. ilosvayi shells correlate well with the isotopic composition of their host waters. Within-sample variabilities in repeated isotopic measurements of single ostracod shells reflect habitat-dependent ranges. The similarly high range of ostracod δ¹⁸O in rivers and one marsh sample indicates that both temperature and δ¹⁸O_prec are responsible for their variation in the whole study area. Rivers and canals, which are predominantly influenced by the input and mixing of inorganic carbon from the catchment, show smaller δ¹³C_ostr ranges than the marsh dominated by local fluctuations in biological activities.

Based on these observations, background data of water temperatures and δ¹⁸O_prec were used to calculate monthly δ¹⁸O variations in a theoretical calcite formed in rivers in Florida assuming a direct reaction on precipitation changes. The calculated values showed a high variation coupled with low mean values during the summer wet season, while during the winter dry season the variation remains small and mean values increased. Inferred configurations were used to approximate possible calcification periods of C. ilosvayi. For a plausible calcification period, mean values and ranges of δ¹⁸O_ostr had to be equal to the theoretical calcite with a slight positive offset (vital effect). The applied model suggests a seasonal calcification period of C. ilosvayi in early spring that is probably coupled to the hydrologic cycle of Florida.