Major ions, nutrients and primary productivity in volcanic neotropical streams draining rainforest and pasture catchments at Los Tuxtlas, Veracruz, Mexico

Six streams in the Los Tuxtlas region, a volcanic area in southeastern Mexico, were characterized chemically and biologically. Temperature, pH, conductivity, ions (Ca2+, Mg2+, Na+, K+, CaCO3- and SO42-), nutrients (NO3-, NH4+, total P and PO4-3), and chlorophyll a from epilithon were measured every other month from September 1996 to July 1997. The streams studied had a consistent pattern of cation dominance (Na+> Ca2+> Mg2+>K+), and ionic concentrations varied little during the year of study; nutrients, however, showed strong temporal variability. The ion chemistry of the streams was influenced by bedrock weathering according to the Gibbs Model. The streams are chiefly mesotrophic, but their primary production may be limited by nitrogen based on the N:P ratio. Streams differed in chlorophyll a concentrations and their productivity changed temporally. They were among the most mineral-rich tropical streams, and both their ion concentration levels and cationic patterns coincided with other neotropical volcanic streams. Although there was a pattern in which ion concentrations of the streams were negatively related to the proportion of conserved vegetation and positively related to the proportion of pastures and croplands, the relationships were not statistically significant. We concluded that differences in the major ions of the streams studied were caused by the great heterogeneity in geology and soil types, as well as by geothermal activity in the area. Temporal changes in nutrients were related to biological processes in the streams that influenced primary productivity. Moreover, the influence of land use might be hidden by the strong effect of this heterogeneity on the streams studied.