The main objective of this study was to determine the effect of Fe in Font de Ferro’s water on the leaf litter decomposition mostly from Quercus pubescens. The field work was performed between December 2016 and January 2017, in each sampling a bag of oak leaves was collected from each point as well as pH, conductivity, dissolved O2, temperature and water velocity data. In the laboratory leaf weight loss, alkalinity, absorbance, fluorescence index, metal content (Al, B, Fe, Mn, Rb, Sr, Cr, Co, Ni, Cu, Zn, As, Sn, Sb i Ba), nutrient content (NH4+, PO43-, P org, Cl-, NO2-, NO3- and SO42-), observation of hydromycetes diversity and concentration of ergosterol (to know fungal biomass) were measured. The decomposition rates varied (points 1-4) from -0,0021 to -0,0234 days-1, and this variation fitted with the Fe concentrations in the water going (points 1-4) from 4972,53 to 78,64 ppb, also with water conductivity values varying from 6,71 to 6,11 mS/cm and with ergosterol values (from 6,65 to 56,75 μg ergosterol/g dry weight between points 1-4). The 5th point results follow a completely different trend from the rest of points. Statistical analyses were ANCOVA to observe if the evolution of the percentage of organic matter remaining varies significantly depending on the days that the leaf bags in the water have passed and the distance of the sampling point from the source, a Tukey test to observe if there were significant differences between the decomposition rates of the different points and 2 PCA’s (one with physicochemical variables and Fe, and another with metals, pH and dissolved O2) to analyse the variability of the data in function of two new variables that explained about 60% of the variability. These results suggest that Fe affects the rate of decomposition and allow the division of the source in 3 parts: 1st (points 1-2) where the Fe concentration is high, the massive Fe precipitation and high water conductivity inhibited the decomposition and photosynthesis; 2nd (points 3-4) where these inhibitory factors did not act; and 3rd (point 5) where all tendencies were broken because the water passed under the sediments and this changed their characteristics
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