Quantitative reconstruction of eutrophication histories in central Mexican lakes
The primary aim of this study was to develop a diatom based nutrient transfer function for central México, and apply it to a sediment core from Laguna Zirahuén, Michoacán Mexico, (19⁰ 26' N, 101⁰ 44' W) reconstruct change in nutrient availability over the last ca. 200 years. The principle techniques employed were analysis of diatoms, stable isotopes of carbon and nitrogen and lipid biomarkers. These were supplemented by magnetic susceptibility and isotopic measurements of modern aquatic and terrestrial organic matter. In order to interpret the diatom record and quantitatively reconstruction the change in the diatom flora a calibration data set, from 30 lakes, was collected during both the wet and the dry seasons. Detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) were used to explore species-environment relationships and to establish which environmental variables made independent and significant contributions to species variance. Results showed that electrical conductivity and chlorophyll-a caused a significant and independent proportion of variance in the species data (12.54%, p = 0.001), this was supported by variance partitioning and the ratio of CCA axis one to CCA axis two. Weighted averaging regression and calibration was then used to create an inference model to reconstruct changes in Chlorophyll-a. The strength of the model was assessed using the squared coefficient of determination (r2 = 0.83) and root mean square of errors (RMSE = 0.32). Application of cross validation resulted in a substantial drop in r2 and increase in RMSEP, this is most likely associated with the small and heterogeneous nature of the data set. Numerical reconstruction of the change in nutrient status was complicated by a high number of non-planktonic species and a lack of modern analogues. In the upper 3 cm the trajectory of change inferred by the reconstruction was, however, in line with other existing contemporary nutrient data from the lake. There is a marked decrease in productivity between 24 and 9 cm, shown by declining concentrations of 013C, lipid biomarkers, 013C and diatom inferred Chl-a and corresponding increase C/N. This may correspond to the decline in the copper smelting industry in the basin the in the late 19th early 20th century. A rise in productivity is noted over the top 3 cm of the core, possibly the last 15-20 years. The majority of proxies showed a marked response to the deposition of the Paricutín tephra. Despite the fact that electrical conductivity is the primary variable in controlling diatom species variation, it is evident that human impact on lake basins though increased levels of nutrients in having an impact on the ecology of the lake. In the case of Laguna Zirahuén its sensitivity and the current trajectory of change may be an important consideration in its future management.