Physical-chemical and ecological features of the Paterno sinkhole (Rieti, Lazio, Central Italy): a possible seasonal meromictic lake

Lake Paterno, located at 430 m a.s.l., not far from the city of Rieti (Lazio, Italy) and between the towns of Cittaducale and Antrodoco, originates from the formation of a sinkhole, a funnel-shaped cavity due to karst phenomena. The lake has an ellipsoidal shape (0.19 × 0.15 km), a surface of about 22.300 m 2 and a maximum depth of 54 m. It is likely fed by underground springs, as testified by the presence of an effluent having a flow rate of 16 L/sec. In hydrogeological terms, the lake belongs to the Velino aquifer hosted within carbonate formations of the northernmost part of the Latium–Abruzzi platform sequences. Plunging from the southern shore, the depth rapidly increases and the bottom has a stepped pattern. Previous studies carried out in summer 2010 reported an euphotic layer from the surface to the depth of 14-16 m (depending on weather condition) and two clear thermo-clines in the first 8 m, with a sharp temperature drop and no life forms visible by naked eye underneath. The first thermocline (4.5 m deep) is the lower limit of the water layer used by fish and macro-invertebrate populations, while the second one (at 8 m) represents the lower bound of the algal community. Few fish species were detected in summer, whereas in wintertime no fishes were observed. This phenomenon is likely due to the strong reduction of fish activity in the cold season. Fish fauna covers different trophic levels in aquatic systems, so it provides an interesting and complex ecological object to be associated with physical-chemical descriptors. In this work we present the results obtained in February 2011, during which the chemical and physical processes controlling the water composition were investigated in order to verify: 1) the occurrence of chemical and/or thermal lake stratification and 2) the presence of non-atmospheric dissolved gases. The results show that the chemical-physical parameters of the lake water remain basically homogenous, although at the lake bottom a slight decrease of dissolved O 2 , accompanied by an increase of the dissolved CO 2 and CH 4 concentrations, was measured. Further water and dissolved gas sampling campaigns are planned in summer 2011 to investigate the evolution of the chemical-physical features along the lake vertical column and their relationships with the external temperature and to assess the origin of dissolved CO 2 and CH 4 . Finally, the geochemical features and processes acting in the Paterno sinkhole may provide helpful insights on the horizontal and vertical fish space use and population dynamics among seasons.