BIOLOGICAL STUDIES OF SPANISH MEROMICTIC AND STRATIFIED KARSTIC LAKES M. R. Miraclel, E. Vicente 1 and C. Pedrós-Alió 2 I Departments of Ecology and Microbiology, University of Valencia, Burjasot, Valencia, Spain. 2 Institut de Cikncies del Mar. CSIC, Barcelona. Spain. Keywords: Karstic lakes, Meromictic, Stratification. Redoxcline, Oxycline, Anaerobic waters, Phytoplankton. Bacterioplankton, Ciliates, Zooplankton. ABSTRACT Surveys have been carried out in over 40 lakes in the Pre-Pyrenees, Cuenca mountains and Banyoles karstic areas in Spain. About 18 of these lakes are deep enough to become stratified and have been studied in more detail. Here we discuss the verti- cal distribution and species composition of their planktonic communities, and their relationship to physico-chemical parameters. Meromictic and stratified karstic lakes are characterized by the large accumulations of organisms at the redoxcline. Sharp gradients of oxygen and reduced compounds are found in this region as a consequence of the activities of the organisms. Diffe- rent organisms form abundance maxima at their preferred depths further contributing to the stratification. Special attention is given to these communities at the oxic-anoxic interface. Usually, rotifers accumulate in the microaerophilic zone. Algae may be found at the zero oxygen depth. Phototrophic anaerobic bacteria are found underneath when enough light reaches the inter- face. Different communities of ciliates appear in the aerobic waters, in the oxycline and in the anaerobic waters respectively. INTRODUCTION Karstic lakes originate by limestone and/or gypsum disso- lution. They have a peculiar morphometry, characterized by circular sinks with steep walls and a high relative depth (the ratio of maximum depth to mean diameter, see table 1). These features, together with a high mineralization of the bottom waters due to their dissolution origins, favor the stra- tification of the water layers. Permanent meromixis, howe- ver, is a peculiar condition found only in a few lakes. Many karstic lakes with similar morphometry and substratum are not meromictic, such as those next to the locality of Cañada del Hoyo (Cuenca), where only Laguna de la Cruz is perma- nently meromictic, out of a total of seven dissolution lakes. Considering the whole karstic area in the mountains of Cuenca, where we have studied more than 35 dissolution lakes and s~nall water bodies, only two (Laguna de la Cruz and Lake El Tobar) were found to be meromictic. Severa1 of these lakes can be seen in Plate l. Laguna de la Cruz presents a biogenic meromixis, due to enrichment of the monimolimnetic water in calcium, magnesium and iron bicarbonates. Lake El Tobar shows a crenogenic meromixis where the monimolimnion is constituted mostly by a sodium chloride brine (table 1). Another karstic area exists around Lake Banyoles (Girona). The main lake itself is a polje constituted by six main basins, but only one of them is permanently meromictic. There are about 15 small lakes surrounding the main lake and only one of them is mero- mictic (Lake Vilar). Thus, we find a continuos spectrum of lakes with different degrees and schedules of stratification. the most extreme and rase case being permanent meromixis. One of the most striking features of al1 these lakes is the sharp stratification of organisms, which is determined by their own activity. This is common to both meromictic lakes and to those stratified during an important part of the year. Actually, these stratified lakes were first studied for their accumulation of sulfur photosynthetic bacteria at the oxic- anoxic boundary (ABELLA et al., 1980, 1981, 1985: GASOL et al.. 1990; GUERRERO & ABELLA, 1978: GUERRERO et al., 1978, 1980, 1985, 1987; MAS et al., 1990; PEDRÓS-ALIÓ et al., 1986; VICENTE & MIRA- CLE, 1984, 1988, 1991). Additional information can be found in GUERRERO & PEDRÓS-ALIÓ (1992). More general limnological studies were only carried out later on. Limnetica, 8. 59-77 (1 992) O Asociación Española de Lirnnolopía, Madrid. Spain