A 12,000 Year Diatom-Based Paleoenvironmental Record from Lago De Zirahuén, Mexico

The volcanic dammed Zirahuén Lake is one of the least eutrophic lakes in the state of Michoacán in the central Mexican highlands. Diatom-based records from two cores – one littoral in a water column of 15 m deep and one from the center of the ca. 40 m-deep lake – provide a sequence of environmental change spanning the last c. 12 ky. Diatom assemblages during the late Glacial are representative of shallow water or marsh-type environments, indicating lower lake levels in the lake margin. Fluctuating conditions occurred in the early Holocene, as evidenced by diatom assemblages in the central core, while an unconformity was observed in the littoral core between 12.1 and 7.2 ka BP associated with the damming of the lake as product of the development of the Magueyera volcano. The cores recorded a lake deepening and more stable lake levels between ca. 8.8 and 2 ka BP, with the planktonic Aulacoseira ambigua dominating the diatom assemblages. The lake deepening is associated with the last two La Magueyera lava flows that caused the final damming of the lake. These volcanic events increased hydrothermal activity and eutrophic conditions in the lake and favored increased silica availability for the Urosolenia eriensis blooms, with almost monospecific diatoms at c. 10.2 ka BP and again between 6.8 and 6.6 ka BP. Anthropogenic influence on the lake ecosystem is evident from c. 3.6 cal ka BP as increased catchment erosion. Significant changes from the mid-eighteenth century associated mainly with colonial copper smelting, deforestation, and recent intensification of land use in the catchment for avocado production are responsible for lake eutrophication.