Sudden drying of large karst lakes in the Lacandon Maya region of southern Mexico
Final Report Abstract
This project evaluated the ecological effects of the sudden loss of water in 2019 experienced by two large karst lakes in the Lacandon Forest, southern Mexico. Water levels in Lakes Metzabok and Tzibaná declined dramatically within a two-week period in July 2019. Lake Metzabok (0.83 km2; 25 m max. depth) dried completely, whereas lake level in Lake Tzibaná (1.24 km2; 70 m max. depth) fell by ~15 m. Meteorological data from nearby stations (Yaquintela and Ocosingo) do not show any clear correlation with the large loss in lake surface area. Field observation and geophysical data collected during a field campaign in October 2019 suggest that the sudden lake level decline was caused by rapid subterranean drainage. Irrespective of the specific drainage mechanism, the sudden loss of such a large volume of water had profound ecological and environmental effects and caused significant biodiversity loss. Many crocodiles, turtles and most fish did not survive such harsh conditions. Similarly, our results indicate a diversity decline of highly sensitive microorganisms such as ostracodes, testate amoeba, chironomids, and cladocerans. At the same time, we were able to observe and investigate the commencing transition from an aquatic to a terrestrial habitat in Lake Metzabok. First groups colonizing the exposed lake bed included spiders and plants belonging to the families Poaceae (grasses), Amaranthaceae (amaranths/chenopods) and Fabaceae (legumes). Following the sudden desiccation event, vegetation indices (NVDI) determined from satellite imagery significantly increased indicating an extensive, rapid increase in vegetation density. Results from neo- and paleolimnological analysis suggest that both lakes are highly dynamic, unstable, and the limnogeological context is extremely complex. This makes environmental and climate inferences from the sediment records challenging. Therefore, a sound knowledge of the study area and related processes is essential. With this in mind, we conducted additional geophysical studies to characterize the lakebed geometry of Metzabok and Tzibaná to identify potential sites for future recovery of longer sediment cores. The joint interpretation of seismic refraction and time-domain induced polarization data collected on the exposed lake bed on the one hand and older sub-bottom profiler and transient electromagnetic sounding data on the other hand enabled the derivation of a comprehensive geological picture of the lake bed and the geological setting of the two lakes. Our geophysical imaging results permitted to determine suitable locations for future paleoenvironmental coring campaigns, which are characterized by thick (5-6 m), undisturbed fine-grained lake sediments covering the flat parts of the two lakes. These fine-grained sediments, which have a low hydraulic permeability, form a continuous layer covering large areas of both lakes. Where this layer is thick enough, it acts as a hydrological barrier between the lakes and the underlying karst. However, heavily fractured and uncovered limestone outcrops, which are present in both lakes, still efficiently connect the lakes with the underlying karst water system and facilitated the rapid lake-level decline observed during 2019. This study generated baseline information for a subsequent proposal with the goal to better understand the dynamics of this karst system, and thus possible causes, effects and recurrence rates of seasonal lake desiccation events, such as the one observed in July 2019, during the Holocene. The potential of a larger follow-up project is clearly shown by the high number of bachelor and master thesis, scientific publications and congress contributions that were achieved during the short funding period of one year. We plan to continue gaining knowledge through a genuine collaborative endeavor among geoscientists, government agencies, and local Lacandon Maya to contribute to the management and conservation of freshwater ecosystems in the mountains of the Lacandon Forest.
Publications
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(2020). Ecological effects of sudden drainage of large karst lakes in the Lacandon Maya region, southern Mexico. EGU General Assembly 2020, Session SSP3.1. Limnogeology: reading the geological record of lakes. European Geoscience Union
Pérez, L., and the Selva Lacandona Science Team
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(2021) Water- and land-borne geophysical surveys before and after the sudden water-level decrease of two large karst lakes in southern Mexico, Solid Earth 12: 439-461
Bücker, M., Flores Orozco, A., Gallistl, J., Steiner, M., Aigner, L., Hoppenbrock, J., Glebe, R., Morales Barrera, W., Pita de la Paz, C., García García, E., Razo Pérez, J. A., Buckel, J., Hördt, A., Schwalb, A., and Perez, L.
