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Limnology of Gallocanta Lake, Aragon, northeastern Spain

Abstract:

A limnological study was sustained from September 1980 to October 1981 to show the evolution of Gallocanta Lake (N.E. Spain) under very dry climatic conditions. It is the physical terminus within an endorheic basin of 500 km2 situated 1 000 m over the sea level. In 1977 its maximum depth was 2.5 m but it decreased to 60 cm in 1981 as a consequence of the last very dry years. It is actually 6 km long and 2.5 km wide, and its area is approximately 12 km2. Salt concentration was quite constant (32-43 g l-1) since September 1980 until June 1981. The equivalent ratio Cl-:SOinf4sup2-varied between 2.2 and 2.8. From June through October 1981 salinity increased up to 105 g l-1 and Cl-:SOinf4sup2-equivalent ratio varied between 2.5 and 3.4. The relative concentrations of ions were retained all year ordinated as follows: Cl- > SOinf4sup2-> HCOinf3sup->. COinf3sup2-; Na+ > Mg2+ ≫ Ca2+ > K+. Total reactive phosphorus was less than 1.5 μg-at l-1 from September 1980 to January 1981. During the rest of the studied period it varied between 2 and 8 μg-at l-1 (the maximum, in February 1981). Nitrogen oxidized forms were relatively high in winter (4-8 μg-at N-NOinf3sup-l-1; 0.5-2.5 μg-at N-NOinf2sup-l-1), and early May 1981 (25 μg-at N-NOinf3sup-l-1; 1 μg-at N-NOinf2sup-l-1). Neither reached 1 μg-at l-1 from September through December 1980, or from June through October 1981. Planktonic algae increased in density during the period of the study from 103 to 5 105 cells ml-1. Chromulina sp. was the main species during autumn and winter (>50% in number of cells ml-1) while the copepod Arctodiaptomus salinus decreased its population. Lobocystis dichotoma increased its density from December 1980 to May 1981 and was dominant in spring (>90%), when a typical reproductive activity was observed in the Arctodiaptomus population. This species disappeared in late spring. In June 1981 Brachionus plicatilis reached up to near 2 000 individuals ml-1. This species was not observed after August 1981. Then the ciliate Fabrea salina was the only zooplankter staying in the free waters of the lake and Nannochloris sp. the dominant alga (>90%). Dense macrophytic beds constituted of Lamprothamnium papulosum and Chara galioides covered the bottom of the lake in September 1980 and before that date since the observations began in 1977. They decreased in covered area since October 1980 and disappeared during summer 1981. Gallocanta Lake is in a very stressed situation produced by prolonged drought. The increased salt concentration together with the extremely cold and hot temperatures, and the lack of nitrogen relative to phosphorus are the main environmental factors that controlled the biological populations during the period of the present study. © 1983 Dr W. Junk Publishers.