Land use changes affect water quality and chlorophyll concentration in tropical streams

Background: Forest conversion to agriculture and pasture that has occurred in the last decades in southern Mexico can affect the ecological functioning of rivers and streams. Objective: to investigate how land use change at different scales (catchment and riparian zone) influences water quality and benthic chlorophyll in streams of the Chiapas and Tabasco mountains. Methods: Over a period of 13 months, water physicochemi-cal variables were measured, and samples were collected for the determination of nutrients, suspended so-lids, and benthic chlorophyll in eight streams encompassing different land use and cover in the Puyacatengo, Pichucalco, and Teapa sub-basins of the Grijalva watershed. The study variables were statistically compared among land use categories and seasons and the relationship with land use at different scales was analyzed. Results: Streams draining forested areas showed lower water temperature, conductivity, pH, and phos-phorus and benthic chlorophyll concentrations than streams in pasture and mixed land use categories. Forest streams also showed a greater proportion of suspended organic matter and dissolved oxygen concentration than the more disturbed streams. Temporal dispersion of values for stream variables at the forest sites was lower than at pasture and mixed-land use streams. Variation in water quality was related to land use variables at the catchment (forest, pasture, and secondary vegetation) and riparian (pasture and secondary vegetation) scales. Conclusions: Land use changes at the catchment and riparian scales can affect water quality and the functioning of streams in the mountains of Tabasco and Chiapas, and thus conservation and restoration actions, including reforestation, must be implemented at both scales.