Littoral living Ostracoda from the Bracciano caldera-lake (Sabatini volcanic complex, central Italy)

Bracciano Lake occupies a volcano-tectonic depression in the Regional Natural Park of Bracciano-Martignano (Latium, central Italy). It is a freshwater lake with a surface area of about 57 km2, a perimeter of about 31.5 km, and a maximum depth of 188 m below the hydrometric zero, located at 163 m a.s.l. Its water input is provided by precipitation, runoff waters, small ditches connected to meteoric events, and groundwater inflows. The natural outlet of Bracciano Lake is the Arrone River, whose discharge has gradually decreased over the last decades and has stopped since 2016. The lake was considered oligo-mesotrophic and warm monomictic, with a mixing phase from November to February (FERRARA et al., 2002). With the aim to improve the knowledge on the ecology of the lake, in the frame of the National Project CARG Bracciano in collaboration with ISPRA (National Institute for Environmental Protection and Research) and the Bracciano- Martignano Regional Natural Park, during 2022-2023, four seasonal campaigns were carried out to collect ostracods from the littoral area of the lake (0.5- 15 m of depth). Thirty-eight samples from 14 sampling sites located around the lake were collected each season using different methodologies depending on the depths. At each station, the main chemical and physical parameters of the bottom water were measured, and the characteristics of the substratum, including grain size and the composition of aquatic macrophytes, were evaluated. In this paper we present some preliminary data related only to the summer and autumn surveys. In those seasons, eleven taxa were collected: Darwinula stevensoni Brady & Robertson, 1885, Neglecandona angulata (G. W. Müller, 1900), Cyclocypris sp., Cypria sp., Ilyocypris sp., Herpetocypris sp., Heterocypris sp., Cypridopsis vidua (O.F. Müller, 1776), Limnocythere inopinata (Baird, 1843), Paralimnocythere sp., and Cyprideis torosa (Jones, 1850). In comparison with previous studies carried out on the lake (ZSCHOKKE 1911; MASTRANTUONO 1995; MASTRANTUONO & MANCINELLI, 2005) in our samples we failed to find living specimens of Cytherissa lacustris (Sars, 1863) and Strandesia sp. Conversely, we increased the ostracod list, adding Cyclocypris sp., Cypria sp., Herpetocypris sp., Heterocypris sp., Paralimnocythere sp., and Cyprideis torosa (Jones, 1850). Darwinula stevensoni and Cyprideis torosa were the most commonly found species in most sampling sites (12 and 11 respectively) at all the sampled depths. Neglecandona angulata mostly occurred in samples at 15 m of depth, associated with Cyprideis torosa and often also with Darwinula stevensoni and Ilyocypris sp. Extensive beds of Characeae were present at most of the sampling sites, especially at -7 m and to a lesser extent at -15 m. Few ostracod taxa were associated with dense macrophyte cover, namely Darwinula stevensoni, Cyprideis torosa, and Ilyocypris sp. The maximum number of ostracod taxa per sampling site was 9, collected at 0.5-1 m in the summer; two taxa (Cyclocypris sp. and Herpetocypris sp.) were exclusively found within the stands of Ludwigia hexapetala (Hook. & Arn.) Zardini, H.Y.Gu & P.H. Raven, an invasive alien macrophyte that was particularly widespread at that site.