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RESEARCH PRODUCT

Lake La Cruz, an Iron-Rich Karstic Meromictic Lake in Central Spain

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description

Laguna de la Cruz (Lake La Cruz ) is a small karstic meromictic lake located in Central-Eastern Spain. Its biogenic meromixis (originated ca.1660) is favoured by the high relative depth and the steep doline walls. Calcium and magnesium bicarbonate are the main salts. The monimolimnion permanently spans from 18 m to the lake bottom (ca.21 m), whereas thermal stratification occurs above from April to October. Steep chemical gradients appear both at the bottom of the metalimnion (seasonally) and, permanently, at the permanent chemocline (16–18 m). Nutrients in surface waters decay as stratification advances, but deeper they are abundant. Linked to physical and chemical gradients, sharply stratified planktonic phototrophic populations develop, with the highest rates of inorganic carbon fixation occurring at the oxic–anoxic interface during the stratification period. In the oxic and microaerobic layers, the photosynthetic planktonic assemblage is dominated by picocyanobacteria , seasonally forming a deep chlorophyll maximum. Instead, photosynthetic sulphur bacteria thrive in anoxic waters. Chemolithoautotrophic bacteria, some linked to the iron cycle , show rates of dark inorganic carbon fixation that overpass the contribution of anoxygenic photosynthesis to total carbon fixation in the lake, though this is far dominated by oxygenic photosynthesis . Most of the deep primary production sinks to anoxic waters being unconsumed by the metazooplankton dominated by rotifers and the copepod Tropocyclops prasinus. Only a few species of anaerobic ciliates consume part of this sinking organic matter , which is anaerobically decomposed in the anoxic layers or reaches the lake sediments annually forming a dark layer that alternates with a white layer. This white layer results from massive precipitation of calcium carbonate (whiting ) yearly occurring in summer. Remains of organisms and minerals in Lake La Cruz sediments serve as palaeoecological archives, allowing to describe the past ecological conditions of the lake.