In situ observations of temporary rivers dry sediments: chemical quality and microbial properties

Temporary rivers are widespread in arid and semiarid climatic regions where they represent a direct source of water for human purposes. These systems are characterised by drastic changes of hydrological conditions where the duration of the dry phase is expected to increase under climate change. The hydrological regime regulates the deposition and re-suspension of the streambed benthic detritus as well as the biogeochemical processes. Sediments play an important role in these processes representing the element of continuity between flowing and dry period. In temporary rivers drying period is the most crucial as the suspended materials settle on the sediment (including pollutants) and the aquatic biota must pass through a "bottle neck" until the first flow arrives. Heterotrophic microbial communities play an important role in the food web channelling the C-flux from dissolved organic matter to biomass. Benthic microbial communities can adopt specific strategies to survive the drought, affecting organic matter quality and bioavailability with the potential to change the water quality when flow resumes. Drying imposes such physiological constraints that microbial communities from different temporary rivers may adopt similar functional strategies to survive i.e. the conservation of key processes useful for carbon and nutrient uptake. The first objective of this experimental work was to provide a cross view of hydrological regime together with chemical and microbial properties of dry sediments. To achieve this objective, sediments from five temporary rivers (Candelaro River basin, southern Italy) were analysed at the end of the dry period (early autumn) before flow resumption. Sediments were analysed for physical-chemical parameters (temperature, texture, water content, pH, conductivity, mineral composition, ash free dry weight, total organic carbon and nitrogen, pesticides and heavy metals concentrations) and microbial descriptors (prokaryotic biomass, DAPI stain; fungi biomass, ergosterol; total live biomass, ATP, luminescence; four extracellular enzyme activities, fluorescence technique; community respiration rates ETS; Biolog(TM)analyses). The second objective was to predict the impact of microbial processes onto carbon and nutrient fluxes at given dry conditions by analysing microbial properties of collected and historical data. This could help in elucidating biogeochemical processes active under these extreme conditions and their potential implications for the aquatic environment.