Limnological and morphometrical data of eight karstic systems 'cenotes' of the Yucatan Peninsula, Mexico, during the dry season

The karstic nature of the Yucatan Peninsula allows the formation of natural sink-holes from the dissolution of calcareous rock. These systems are almost the only epigean source of fresh water available in this region. In spite of their biological importance, little is known about the morphometric and limnologic characteristics of these karstic systems. We measured limnological variables in eight cenotes in central Quintana Roo during February-May, 2001. Zooplankton biomass and chlorophyll a were also measured in order to determine if the behavior of primary and secondary production was related to environmental parameters. Important short-term changes were observed in nutrients (NO-3 , NO- 2 , PO3- 4 ), biomass, and chlorophyll a. The morphometrically conditioned productivity (MCP), which evaluates the cumulative effect of several morphometric variables on production (area, maximum length, shoreline development, perimeter), showed a negative correlation with respect to zooplankton biomass, as did also both pH and temperature. Conversely, NO- 3 and NO- 2 had a positive correlation with zooplankton biomass. No correlation was found for chlorophyll a. Significant differences in NO-3 (F = 61.52, p < 0.001), NO- 2 (F = 7.36, p < 0.001), zooplankton biomass (F = 17.57, p < 0.001), chlorophyll a (F = 62.19, p < 0.001), and conductivity (F = 497.49, p < 0.001)were found among the systems. These results indicate the existence of sharp differences between these karstic systems (oligotrophic, with smaller area, deep and less productive)and non-karstic ones, (eutrophic, larger area, shallow and more productive)but are similar to previous data from other karstic systems of Mexico and other parts of the world. However, understanding of these fragile tropical systems is in the initial phase. It is necessary to increase the intensity of these studies in order to allow a full explanation of their limnological behavior.