The physical, chemical and biological characteristics of the holomictic sulphated Lake Arcas-2 (Cuenca, Spain)

The main limnological features of Lake Arcas-2 were followed through two co nsecutive stratification periods. Its morphometrical characteristics, such as the high relative depth (31%) and steep basin walls, enhance the sharp w ater stratification with the formation of an oxic-anoxic boundary at 8.8-9 m and a sulphide-rich hypolimnion during the thermal stagnation. The ionic sequence was SO42-> Alkal.> Cl- and Ca2+> Mg2+> Na+> K+ and the mineralizat ion was high, with water conductivity higher than 2500 mu S cm(-1). It is m esotrophic with epilimnetic chlorophyll a concentrations of 2-5 mu g l(-1) and metalimnetic of 8 mu g l(-1). The depth of the euphotic zone was establ ished at around 8 m. Phosphorus concentration in the oxic waters was low bu t largely accumulated in the anoxic hypolimnion, together with other compou nds such as ammonium, silicate, sulphide, etc. Nitrate was abundant in the oxic waters and is related to the use of fertilizers in the surrounding fie lds. A fine-layer sampler was used to study the oxic-anoxic interface where a dense plate of Chromatiaceae developed. The dominant species, Chromatium weissei, reached a maximum integrated biomass of 121 gWW m(-2) during Augu st. Thiocapsa sp., representing less than 1% of total purple bacteria, had an integrated biomass of 0.8 gWW m(-2) and Amoebobacter sp. (1%) had 1 gWW m(-2). Other populations were sharply stratified i.e. Oscillatoria cf. orna ta and Cryptomonas erosa. Those organisms, and mainly the cyanobacterium, a ccounted for the high chlorophyll a concentrations (> 100 mu g l(-1)) recor ded in the anoxic waters of the hypolimnion. Green bacteria were scarcely d eveloped due to the shadowing effect caused mainly by the purple bacterial bloom.