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

Abstract

The main limnological features of Lake Arcas-2 were followed through two consecutive stratification periods. Its morphometrical characteristics, such as the high relative depth (31%) and steep basin walls, enhance the sharp water 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 SO ^2-₄ >Alkal.>Cl^- and Ca²⁺>Mg²⁺>Na⁺>K⁺ and the mineralization was high, with water conductivity higher than 2500 μS cm^-1. It is mesotrophic with epilimnetic chlorophyll a concentrations of 2–5 μg l^-1 and metalimnetic of 8 μg l^-1. The depth of the euphotic zone was established at around 8 m. Phosphorus concentration in the oxic waters was low but largely accumulated in the anoxic hypolimnion, together with other compounds such as ammonium, silicate, sulphide, etc. Nitrate was abundant in the oxic waters and is related to the use of fertilizers in the surrounding fields. 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 August. 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. ornata and Cryptomonas erosa. Those organisms, and mainly the cyanobacterium, accounted for the high chlorophyll a concentrations (>100 μg l^-1) recorded in the anoxic waters of the hypolimnion. Green bacteria were scarcely developed due to the shadowing effect caused mainly by the purple bacterial bloom.