Diatom communities of the Casteldoria thermo-mineral spring (Northern Sardinia, Italy): first results

The thermo-mineral springs of Sardinia have been historically the subject of several geochemical studies. By contrast, their communities and ecological aspects are totally unknown, until now. The Casteldoria spring, located in the region of Anglona in northern Sardinia, is an interesting thermo-mineral spring of the Island, characterised by high temperature (72 °C at the point of emergence) and salinity (5 g L-1). The water emerges from granite sands and has a Na-Cl-Ca composition, with N2 gas emissions, according to literature. The aim of this study was to document the diatom biodiversity of the spring by optical and scanning electron microscopy and to acquire first information about relationships between species and environmental variables. Samplings were conducted monthly for environmental variables (chemical, physical and nutrients) and seasonally for epilithic and epipelic diatoms, from May 2016 to April 2017. pH range was between 6.3 and 7.6, sodium between 1200 and 1439 mg L-1, sulfates between 88.7 and 100.8 mg L-1, chlorides between 2530 and 3701 mg L-1, nitrates between 0.023 and 0.179 mg L-1, total phosphorus between 0.018 and 0.177 mg L-1, and reactive silica between 25.2 and 29.9 mg L-1. After a preliminary analysis of part of the samples collected, 142 taxa belonging to 51 genera were inventoried. The most abundant taxa (relative abundance > 5%) were Aulacoseira ambigua, A. granulata, Amphora sp., Nitzschia frustulum, N. microcephala, N. sp., Pinnularia joculata and Stephanodiscus neoastraea. These species have a cosmopolitan distribution with the exception of P. joculata, present mainly in tropical environments at high temperatures of South America. The most abundant and frequent taxa (relative abundance of 1.5 to > 5%) mostly prefer brackish to saline waters (eg Nitzschia clausii), high nutrient content (eg Achnanthidium exiguum) and medium-high content of organic matter (eg Melosira varians).