Interplay between sediment properties and stream flow conditions influences surface sediment organic matter and microbial biomass in a Mediterranean river

Abstract

River sediment physical properties are linked to flow and are important for the attachment of microorganisms. The objective of this study was to assess the relationship between physical characteristics of surface sediments in a Mediterranean river and their organic matter content and microbial biomass. To do this, we analyzed particle-size distribution, organic matter content, chlorophyll-a, and bacterial density in sediments collected along a 54 km reach under three flow conditions (i.e., drought, low-flow, and base-flow). Multiple regression analysis revealed that during the drought condition, sediment heterogeneity and porosity regulated bacterial density and organic matter content and that bacterial density tended to be lower as the proportion of mud increased. However, under the low-flow and base-flow conditions, bacterial density was related to percent mud, which may provide more surface area for colonization than cobbles. Algal biomass was affected by sediment particle-size distribution only under the base-flow condition, when chlorophyll-a content was enhanced by sediment heterogeneity and a higher relative abundance of sand, suggesting that when biomass declines due to increased shear stress, sediment particle-size distribution becomes more determinant for algal colonization. Our results highlight the importance of considering the interplay of sediment particle-size distribution and flow regime when studying microbial communities in river sediments.