Contribution of fungi and bacteria to leaf litter decomposition in a polluted river

Abstract

The contribution of fungi and bacteria to decomposition of alder leaves was examined at two reference and two polluted sites in the Ave River (Northwest Portugal). Leaf mass loss, microbial production from incorporation rates of radiolabeled compounds into biomolecules, fungal biomass from ergosterol concentration, sporulation rates and diversity of aquatic hyphomycetes associated with decomposing leaves were determined. The concentration in organic nutrients, and inorganic nitrogen and phosphorous in the stream water was elevated and increased at downstream sites. Leaf decomposition rates were high (0.013 d-1 < k < 0.042 d-1) and the highest value was estimated at the most downstream polluted site, where maximum values of microbial production and fungal biomass and sporulation were found. The slowest decomposition occurred at the other polluted site where, along with the nutrient enrichment, the lowest values of current velocity and dissolved oxygen in water were observed. At this site, fungal production, biomass and sporulation were depressed, suggesting that stimulation of fungal activity by increased nutrient concentrations might be offset by other factors. Although bacterial production was higher at polluted sites, fungi accounted for more than 94% of the total microbial net production. Fungal yield coefficients varied from 10.2 to 13.6%, while those of bacteria were less than 1%. Fungal contribution to overall leaf carbon loss (29.0-38.8%) greatly exceeded that of bacteria (4.2-13.9%).