EFFECTS OF HYDROLOGICAL INTERMITTENCY ON THE FUNCTIONING OF LOTIC ECOSYSTEMS

The hypotheses underlying this work concern the marked impact of the increased hydrological intermittence phenomenon on lotic ecosystem metabolism, as the specific aim was to analyze the effects of hydrological intermittency on river ecosystem processes and functioning. In the first chapter of this thesis, I studied the effects of stream drying on the organic matter decomposition and nutrient turnover. A microcosm experiment was conducted to investigate the interactive effect of water intermittency, macrofauna and leaf size on nutrient recycling. Leaf disks (1 or 5 cm diameter) were incubated for 40 days with or without the leaf-consumer, Potamophylax cingulatus larvae (Trichoptera, Limnephilidae) and with or without an intervening, 10-days simulation of stream drying and subsequent rewetting. Nutrient fluxes, residual leaf biomass and leaf elemental composition were measured to evaluate how intermittency, macrofauna and leaf size affect organic matter mineralization rates and stoichiometry. Results suggest that drying slows decomposition rates, impacting both the microbial and macrofauna activities. Macrofauna increases mineralization and nutrient (C, N and P) regeneration rates. During the experiment, the C:N:P ratios of residual litter changed, as the leaf material became enriched with N and P. The second chapter is aimed to investigate the effects of sediments desiccation on microbial activities in hyporheic zone, focusing on how nutrient regeneration changes in response to sediment desiccation. I also expected that intermittent and perennial microbial communities differ in their capacity to recover after drying due to the adaptation to intermittent conditions. A flow-through experiment was carried out using mesocosms packed with sediments collected from 10 perennial and 10 intermittent streams in Austria and incubated for a total of 11 weeks under controlled conditions. All mesocosms were exposed to three hydrological conditions: flowing phase; drying phase and rewetting phase. Overall, five samplings were carried out: after the flowing period and after the rewetting phase (1, 3, 7, 14 days after rewetting). Oxygen respiration, denitrification and nutrient fluxes were measured by reactors inlet and outlet samplings. Results suggested that intermittency significantly affects aerobic metabolism rates and produce a rapid nutrient release immediately upon rewetting. No significant differences were found for intermittent and inundated systems, suggesting high plasticity of hyporheic communities. Thereafter, metabolic rates and nutrient processing returned to pre-desiccation values. Nitrate tended to be consumed in the hyporheic zone with high rates but not via denitrification. In the last chapter I moved the attention to agricultural canals, regularly subjected to wet and dry annual cycles linked to agricultural practices. The aim was to analyse how intermittency affects carbon metabolism and the trends of CO2 emissions along water saturation gradients and to understand which factors are involved in CO2 emissions regulation. Carbon dioxide measurements were performed in five canals within the Po River basin (Northern Italy). In each canal, three sampling zones along a humidity gradient were selected. Besides dark CO2 flux measurements, sediments were collected for sediment characterization and determination of net potential nitrification and denitrification. I hypothesized an inverse correlation between water content and CO2 emission, due to increasing oxygen penetration under desiccation. Results suggest that CO2 emissions tended to increase along water saturation gradients, with site-specific regulation, depending on water and organic matter content and on microbial N transformations. Net potential nitrification produces little effects on CO2 fluxes, whereas potential denitrification may be responsible for variable fractions (up to 100%) or CO2 production.