Structural parameters of biofilm and bacterioplankton are better indicators of urbanization than photosynthetic functional parameters in low-order streams

Abstract

The aim of this study was to assess urbanization effects on microbial communities from low-order streams. Artificial substrata were placed upstream (control) and downstream (urban) of the cities of the selected streams. Photosynthetic parameters derived from chlorophyll-a fluorescence were measured using a Pulse of Amplitude Modulated Fluorometer, and the bacterial biofilm and bacterioplankton were counted by microscopy after staining with 4′,6-diamidino-2-phenylindole. We found higher bacterial biofilm biomass together with higher concentration of nutrients in urban reaches. The biofilm total density of bacteria was negatively correlated with the humic acid concentration, while the bacterioplankton total density was positively correlated with soluble reactive phosphorus and nitrite concentration. Autotrophic Index reflected the predominance of heterotrophs in the three streams. The concentration of chlorophyll-a, the minimum fluorescence, and the other photosynthetic parameters showed variations between the streams. These would respond to environmental factors at local scale not included in this study and may be influenced by the low development of autotrophic biomass at least in two of the streams studied. The bacterial morphotypes Small rod, Large rod, and Vibrio shaped (large) allowed the differentiation of urban reaches and would be useful as indicators of urbanization effects in both biofilm and bacterioplankton of lowland streams.