Microbial carbon processing along a river discontinuum

Abstract

The hydrological continuum in rivers can be altered by the presence of small dams that modify the water residence time (WRT) and prevailing habitat, turning lotic river sections into lentic ones and influencing downstream reaches. The structure and activity of the microbial community occurring in the benthic and planktonic compartments can be modified by these small dams. We studied the microbial community processing of organic C along a sequence of 4 lentic-lotic sections in a medium-size Mediterranean river during base flow (spring) and low flow (summer). We hypothesized that longitudinal anomalies in WRT would influence the relative contribution of benthic vs planktonic compartments and their relevance in C processing along the river network, particularly during low flows. The biomass of free-living and particle-associated bacterioplankton was higher in the lentic sections, which had longer WRT, resulting in higher organic C processing (enzymatic activities and respiration). Microbial aggregates occurred in the lentic sections especially during the low-flow period and resulted in hotspots of organic C processing. The lotic reaches received a significant contribution of C in the form of bacterio- and phytoplankton. The small dams subsidized the lotic sections downstream and increased their respiration activity. Our results reveal the influence of small dams on organic C processing along the river network. Accounting for their effect, together with that of large dams, may be essential for accurate estimations of organicmatter transformation in river networks