Random array of drifting acoustic receivers (RADAR'07)

Rep 04/07 - SiPLAB 15/December/2007This report describes the complete set of data acquired during the RADAR’07 sea trial, that took place aboard the NRP D. Carlos from July 9 - 15, 2007, off the west coast of Portugal, in the Set´ubal area

Low flow controls on stream thermal dynamics

Water level fluctuations in surface water bodies, and in particular low flow drought conditions, are expected to become more frequent and more severe in the future due to the impacts of global environmental change. Variations in water level, and therefore in-channel water volume, not only have the potential to directly impact stream temperature, but also aquatic vegetation coverage which, in turn, may affect stream temperature patterns and dynamics. Manipulation experiments provide a systematic approach to investigate the multiple environmental controls on stream temperature patterns. This study aims to use temperature data loggers and fibre optic distributed temperature sensing (FO-DTS) to investigate potential drought impacts on patterns in surface water and streambed temperature as a function of change in water column depth. To quantify the joint impacts of water level and associated vegetation coverage on stream temperatures, investigations were conducted in outdoor flumes using identical pool-riffle-pool features, but with spatially variable water levels representative of different drought severity conditions. Naturally evolved vegetation growth in the flumes ranged from sparse vegetation coverage in the shallow flumes to dense colonization in the deepest. Observed surface water and streambed temperature patterns differed significantly within the range of water levels and degrees of vegetation coverage studied. Streambed temperature patterns were more pronounced in the shallowest flume, with minimum and maximum temperature values and diurnal temperature variation being more intensively affected by variation in meteorological conditions than daily average temperatures. Spatial patterns in streambed temperature correlated strongly with morphologic features in all flumes, with riffles coinciding with the highest temperatures, and pools representing areas with the lowest temperatures. In particular, the shallowest flume (comprising multiple exposed features) exhibited a maximum upstream-downstream temperature warming of 3.3 °C (T in = 10.3 °C, T out = 13.5 °C), exceeding the warming observed in the deeper flumes by ∼ 2 °C. Our study reveals significant streambed and water temperature variation caused by the combined impacts of water level and related vegetation coverage. These results highlight the importance of maintaining minimum water levels in lowland rivers during droughts for buffering the impacts of atmospheric forcing on both river and streambed water temperatures

The effects of a sediment flushing on Alpine macroinvertebrate communities

Sediment flushing from dams can help desilting reservoirs and reinstate the longitudinal sediment transfer continuity in rivers, but negative ecological impacts might arise. We monitored the ecological effects of a flushing event conducted from 27th May to 14th June 2019 in the Rienz River, in South Tyrol (NE Italy). Using a multi-habitat scheme approach, we collected macroinvertebrates 10 days before, and then 40 and 74 days after the completion of the operations. We selected seven biological traits—organism size, life cycle duration, mobility (dispersal and locomotion), feeding type, substrate and current velocity preference—to characterize and compare the invertebrate communities before and after the sediment pulse disturbance. Turbidity was recorded continuously for the entire duration of the event. Results indicate that invertebrate assemblages exhibited a general decrease in taxonomic richness and Shannon diversity 40 days after the event, but density and richness recovered over time. Shifts in species composition were observed in post flushing samples, with a reduction in density of sensitive species (Heptageniidae) and shredders. Post-flushing samples were generally characterized by sediment-tolerant taxa, able to cope with the new habitat conditions. Altered taxonomic and functional community composition following the flushing prevented the full functional recovery to pre-disturbance conditions

Woody debris is related to reach-scale hotspots of lowland stream ecosystem respiration under baseflow conditions

Stream metabolism is a fundamental, integrative indicator of aquatic ecosystem functioning. However, it is not well understood how heterogeneity in physical channel form, particularly in relation to and caused by in‐stream woody debris, regulates stream metabolism in lowland streams. We combined conservative and reactive stream tracers to investigate relationships between patterns in stream channel morphology and hydrological transport (form) and metabolic processes as characterized by ecosystem respiration (function) in a forested lowland stream at baseflow. Stream reach‐scale ecosystem respiration was related to locations (“hotspots”) with a high abundance of woody debris. In contrast, nearly all other measured hydrological and geomorphic variables previously documented or hypothesized to influence stream metabolism did not significantly explain ecosystem respiration. Our results suggest the existence of key differences in physical controls on ecosystem respiration between lowland stream systems (this study) and smaller upland streams (most previous studies) under baseflow conditions. As such, these findings have implications for reactive transport models that predict biogeochemical transformation rates from hydraulic transport parameters, for upscaling frameworks that represent biological stream processes at larger network scales, and for the effective management and restoration of aquatic ecosystems

Stream solute tracer timescales changing with discharge and reach length confound process interpretation

International audienceImproved understanding of stream solute transport requires meaningful comparison of processes across a wide range of discharge conditions and spatial scales. At reach scales where solute tracer tests are commonly used to assess transport behavior, such comparison is still confounded due to the challenge of separating dispersive and transient storage processes from the influence of the advective timescale that varies with discharge and reach length. To better resolve interpretation of these processes from field-based tracer observations, we conducted recurrent conservative solute tracer tests along a 1 km study reach during a storm discharge period and further discretized the study reach into six segments of similar length but different channel morphologies. The resulting suite of data, spanning an order of magnitude in advective timescales, enabled us to (1) characterize relationships between tracer response and discharge in individual segments and (2) determine how combining the segments into longer reaches influences interpretation of dispersion and transient storage from tracer tests. We found that the advective timescale was the primary control on the shape of the observed tracer response. Most segments responded similarly to discharge, implying that the influence of morphologic heterogeneity was muted relative to advection. Comparison of tracer data across combined segments demonstrated that increased advective timescales could be misinterpreted as a change in dispersion or transient storage. Taken together, our results stress the importance of characterizing the influence of changing advective timescales on solute tracer responses before such reach-scale observations can be used to infer solute transport at larger network scales