PALEOLIMNOLOGICAL EVIDENCE OF HOLOCENE TRANSITIONS BETWEEN MEROMICTIC AND HOLOMICTIC MIXING REGIMES IN A MID ALTITUDE TROPICAL LAKE IN WESTERN MEXICO

La Alberca de Tacambaro (19°12’40”N 101°27’30°W, 1,460 m asl) is a relatively deep lake (30 m) compared to its area (0.08 km²), located within a volcanic crater with steep sides (170 m high) to the W lowering gradually towards the E. Due to its altitude this lake lies at the ecotone between the tropical climates dominated by tropical forests and the temperate highland climates dominated by pine and oak forests. For this reason this lake was selected for paleoclimatic and paleolimnological studies and an 8.5 m core was retrieved. Chronology is given by 12 AMS 14-C dates. Diatom data give evidence of a increasing nutrient levels between the bottom of the sequence (ca. 8000 BP) and up to ca. 5000 BP, with an Aulacoseira dominated assemblage and gradually increasing numbers of Urosoleia and Discostella stelligera. This assemblage is indicative of effective mixing as Aulacoseira species require turbulence to remain in the phytoplankton. By ca. 5000 BP the high nutrient level forced the lake through an ecological threshold with a drastic change in diatom assemblage with Fragillaria construens-Fragillaria tenera becoming dominant. This is interpreted as a change in the mixing pattern of the lake as the Fragillaria spp. are better competitors for phosphorous and their lighter valves do not require turbulent waters to remain in the phytoplankton. We infer that the lake became nutrient limited as meromictic conditions developed that restricted nutrient recirculation. Meromixis is common in nutrient rich deep tropical lakes with small surface areas and protected basins such as La Alberca de Tacambaro. Pollen data from this core suggest that warmer than present conditions were coeval with this change in mixing pattern as tropical taxa were more abundant between ca. 5000 BP and AD 1200. After this date a similar to modern diatom assemblage, dominated by Discostella stelligera was established, indicating the onset of the modern tropical monomictic mixing regime.