MEROMIXIS ORIGIN AND RECENT TROPHIC EVOLUTION IN THE SPANISH MOUNTAINLAKE LA CRUZ

A sediment core from a Mediterranean karstic lake was studied through its pollen, diatom, chydorid, ostracod, charcoal and authigenic minera l composition. Information about environmental history recorded in the sediment sequence since the Middle Ages is presented. The main fluctu ations of water volume and trophic status of the lake occurred during periods of great change in land management and during climatic cold ph ases. The synergetic effect of these two factors led to a high water l evel phase and triggered a rising of the trophic level which produced meromixis. The onset of meromictic conditions at about 1700 AD coincid es with the Maunder minimum in the Little Ice Age as well as with a pe riod of increasing human population, woodland clearance and agricultur al expansion to the detriment of the nomadic livestock-breeding or tra nshumance (''Mesta'').