Interaction between Physical Heterogeneity and Microbial Processes in Subsurface Sediments: A Laboratory-Scale Column Experiment.

Physical heterogeneity determines interstitial fluxes in porous media. Nutrients and organic matter distribution in depth influence physicochemical and microbial processes occurring in subsurface. Columns 50 cm long were filled with sterile silica sand following five different setups combining fine and coarse sands or a mixture of both mimicking potential water treatment barriers. Water was supplied continuously to all columns during 33 days. Hydraulic conductivity, nutrients and organic matter, biofilm biomass, and activity were analyzed in order to study the effect of spatial grain size heterogeneity on physicochemical and microbial processes and their mutual interaction. Coarse sediments showed higher biomass and activity in deeper areas compared to the others; however, they resulted in incomplete denitrification, large proportion of dead bacteria in depth, and low functional diversity. Treatments with fine sediment in the upper 20 cm of the columns showed high phosphorus retention. However, low hydraulic conductivity values reported in these sediments seemed to constraint biofilm activity and biomass. On the other hand, sudden transition from coarse-to-fine grain sizes promoted a hot-spot of organic matter degradation and biomass growth at the interface. Our results reinforce the idea that grain-size disposition in subsurface sandy sediments drives the interstitial fluxes, influencing microbial processes.

Phosphorus use by planktonic communities in a large regulated Mediterranean river.

The regulation of large rivers to meet human requirements (e.g. hydroelectricity production, flood prevention, recreation activities) alters the longitudinal distribution of plankton communities and may affect their capacity to use nutrients and organic matter. Here we analyzed phosphorus (P) availability and use by phytoplankton and bacterioplankton in 6 upstream and 5 downstream sites from a reservoir system in the Ebro River (N Spain). Alkaline phosphatase activity (APA) was related to nutrient availability and biomass of both phytoplankton and bacterioplankton. During dry periods phytoplankton and bacterioplankton APA was inversely correlated to P availability in the water, but these patterns became less clear during wet periods. The phosphorus-APA patterns were more consistent in the upstream sites and especially during dry periods. Although phytoplankton APA was 6-40 times greater than that of bacterioplankton, APA per unit of biomass suggested that bacterioplankton was more efficient at utilizing dissolved organic phosphorus (DOP) in the upstream section during dry periods. Imbalanced N:P ratios in the particulate (N:P ranging 133-170) and dissolved (N:P ranging 301-819) water fractions confirmed the strong P limitation in these upstream communities. The phosphorus-APA patterns were weaker in the downstream section and during wet periods. The reservoirs caused a change in the downstream dynamics, where bacterioplankton biomass was positively correlated to APA but APA per unit of biomass decreased. Our findings reveal that river regulation drives changes in plankton use of organic phosphorus, especially during extreme dry periods.