Superconducting power cables represent a recent innovative development for highcapacity underground transmission. Their promise lies principally in their high efficiency associated with a small size and with potential advantages in terms of environmental impact. Within the BEST PATHS European project, the DEMO 5 demonstrator aims to illustrate the technological maturity of superconducting HVDC links for operation in the grid. At the same time, this demonstrator is also a first attempt to employ MgB 2 as a superconductor for HVDC cables. More concretely, DEMO 5 aims to develop a monopole superconducting cable designed to operate in helium gas at 10 kA and 320 kV, corresponding to a transferred power of up to 3.2 GW. The project is coordinated by leading cable manufacturer Nexans and encompasses expertise from transmission system operators, industry, and research organizations. Thus, in addition to the design, development, optimization, manufacturing and testing activities, special attention will be devoted to studying the integration of a superconducting link into the future transmission grid and to assessing the availability and economic viability of the system. An overview of the project will be presented at the meeting, including the main tasks and challenges ahead as well as preliminary results after one year of activity.

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.

Transient storage zones for water represent potential hot spots for metabolic activity in streams. In lowland rivers, the high abundance of submerged vegetation can increase water transient storage, bioreactive surface areas, and, ultimately, in‐stream metabolic activity. Changes in flow resulting from climatic and anthropogenic factors that influence the presence of aquatic vegetation can also, thereby, impact in‐stream metabolism and nutrient cycling. We investigated the effects of water column depth on aquatic vegetation cover and its implications on water transient storage and associated metabolic activity in stream mesocosms (n = 8) that represent typical conditions of lowland streams. Continuous injections of metabolically reactive (resazurin‐resorufin) tracers were conducted and used to quantify hydraulic transport and whole‐mesocosm aerobic respiration. Acetate, a labile carbon source, was added during a second stage of the tracer injection to investigate metabolic responses. We observed both higher vegetation coverage and resazurin uptake velocity, used as a proxy of mesocosm respiration, with increasing water column depth. The acetate injection had a slight, positive effect on metabolic activity. A hydrodynamic model estimated the water transport and retention characteristics and first‐order reactivity for three mesocosms. These results suggest that both the vegetated surface water and sediments contribute to metabolically active transient storage within the mesocosms, with vegetation having a greater influence on ecosystem respiration. Our findings suggest that climate and external factors that affect flow and submerged vegetation of lowland rivers will result in changes in stream respiration dynamics and that submerged vegetation is a particularly important and sensitive location for stream respiration.

Rivers and marginal wetlands contribute significantly to the carbon (C) exchange rate per unit area compared to adjacent terrestrial ecosystems, due to high C inputs, metabolic activity and CO 2 supersaturation. Within riverscapes, the contribution of emerging bottoms (i.e. parafluvial zones) and ephemeral vegetation (i.e. microphytobenthos or annual vascular plant communities) to the C metabolism is understudied. The aim of this study was to evaluate the CO 2 exchange rates at the water-and emergent sand bar-atmosphere interfaces within a lowland large temperate river stretch. CO 2 fluxes were measured seasonally, in summer (August 2007) and winter (March 2008) via static closed chambers, together with the primary producers' biomass (microphytobenthos and vascular macrophytes). Our results showed that the river was a CO 2 source (between 0.2 and 7.6 mmol CO 2 m -2 day -1 ), whilst the vegetated parafluvial zones acted as a net sink, with assimilation rates peaking at 623.4 mmol CO 2 m -2 day -1 . Emerging bare sediments were on the contrary net emitting CO 2 systems (2.7-60.1 mmol CO 2 m -2 day -1 ). Within lotic environments, seasonally emergent vegetated sand bars may represent important C fixation hot spots, with summer primary production particularly contributing to offsetting CO 2 emissions and counterbalancing the heterotrophic metabolism of the saturated zones. Based on these outcomes, we suggest that studies addressing the strictly aquatic fluvial environment, which is generally a CO 2 emitter, should be linked to those from transitional areas in order to better integrate terrestrial and aquatic C budgets.

Low ßow controls on stream thermal dynamics.Limnologica