Spatial Heterogeneity of Water Quality in a Highly

Authors: Luis Zambrano Victoria Contreras Marisa MazariHiriart Alba E ZarcoArista

Publish Date: 2008/10/21

Volume: 43, Issue: 2, Pages: 249-263

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Abstract

Awareness of environmental heterogeneity in ecosystems is critical for management and conservation We used the Xochimilco freshwater system to describe the relationship between heterogeneity and human activities This tropical aquatic ecosystem south of Mexico City is comprised of a network of interconnected canals and lakes that are influenced by agricultural and urban activities Environmental heterogeneity was characterized by spatially extensive surveys within four regions of Xochimilco during rainy and dry seasons over 2 years These surveys revealed a heterogeneous system that was shallow 11 m SD = 04  warm 17°C SD = 29 well oxygenated 50 mg l−1 SD = 3 turbid 457 NTU SD = 2696 and extremely nutrientrich NO3–N = 159 mg l−1 SD=137 NH4–N = 288 mg l−1 SD = 424 and PO4–P =  83 mg l−1 SD = 24 Most of the variables were not significantly different between years but did differ between seasons suggesting a dynamic system within a span of a year but with a high resilience over longer periods of time Maps were produced using interpolations to describe distributions of all variables There was no correlation between individual variables and land use Consequently we searched for relationships using all variables together by generating a combined water quality index Significant differences in the index were apparent among the four regions Index values also differed within individual region and individual water bodies eg within canals indicating that Xochimilco has high local heterogeneity Using this index on a map helped to relate water quality to human activities and provides a simple and clear tool for managers and policymakersWe thank PASPA and Posgrado de Ciencias Biologicas de la UNAM This project was supported by Instituto Nacional de Ecología SEMARNAT and CONACYT Project No 32505T 1999/2001 We would also like to thank Pilar Islas Rosa I Amieva Josué Sandoval Elsa Valiente and Roberto Altamirano for their help with the data gathering and lab work We are grateful to Emily Stanley for her invaluable suggestions about the manuscriptInterpolations from physicochemical and nutrient variables using the index built based on more suitable to less suitable categories Higher values darker regions on the map give less suitable conditions of the water quality based on each variable a dissolved oxygen b turbidity c electrical conductivity d nitrate concentration e ammonium concentration f phosphate concentration Numbers relate to the name of the canals or areas 1 ecological park PEX 2 El Bordo canal 3 Paso del Aguila canal 4 Apatlaco canal 5 Puente de Urrutia canal 6 Santa Cruz canal 7 Laguna del Toro lake

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