Periodic Reporting for period 1 - RIPARIONS (Transferring hydrological and biogeochemical concepts from boreal to Mediterranean RIPARian zones: developing a broad knowledge framework across ecoregIONS)
Période du rapport: 2020-09-01 au 2022-08-31
The Water Framework Directive (WFD) refers to the riparian zone as a “key hydromorphological quality element for ecological status assessment”. Riparian zones lie on the banks of streams and rivers and connect terrestrial and aquatic ecosystems. Their role in controlling surface water quality is especially important in forest headwaters (i.e. small streams), which make up ca. 90% of the total river network. Advancing in our understanding of riparian zone function in forest headwaters across European ecoregions was the central aim of this project. To fullfill this aim, a novel conceptual framework developed for boreal forest headwaters was tested and applied in Mediterranean and temperate forest headwaters across Europe. This framework includes the idea of a ‘Domiant Source Layer’ (DSL), which explains location, timing, and amount of water and solute transfer from riparian zones to streams in forest headwaters. Overall, the results of this project have helped to develop a unified knowledge framework on riparian zone function that can be implemented across different ecoregions and thereby support both scientific assessments and management of river networks.
The DSL framework was used in two contrasting headwaters in the Mediterranean ecoregion (one with sub-humid climate and the other one with semi-arid climate) and in a temperate forest headwater in central Germany. This work has resulted in three Open Access peer-reviewed publications in scientific journals, one for each of the first three out of four planned work packages. In the first publication we show that during rainfall events of low intensity, hydrological responses at the semi-arid Meditearranean site were more delayed and more variable than at the sub-humid Mediterranean site. By contrast, during rainfall events of moderate and high rainfall intensities (including an extratropical cyclone), hydrological responses were similar in the two sites. Furthermore, at both sites, stream responses were controlled by riparian groundwater table responses, which directly relates to the DSL idea. The results of the second publication suggest that, at the sub-humid Mediterranean site, (i) increased supply of limited resources during storms can sustain the activity of aquatic microorganisms in the stream during high flows, especially during large storm events preceded by dry conditions, and (ii) riparian zones provide organic matter and carbon inputs that are needed for the metabolism of aquatic microorganisms in the stream. Furthermore, in this publication we proposed a conceptual model synthesizing the complex processes driving the mobilization of resources for aquatic microorganisms from riparian zones to streams during storm events based on the DSL framework. In the third publication, we simulated future potential changes in the DSL at the sub-humid Mediterranean site based on future climate scenarios. Nearly all scenarios projected that, together with reductions in stream flow and water exports, the DSL will move down in the future. Consequently, terrestrial organic matter inputs to streams will likely decrease, potentially disrupting stream ecological status. As part of the last work package, work is currently being done to synthetize the understanding of the riparian zone function in the Mediterranean, the temperate, and the boreal catchments based on the DSL framework.
The first publication contributes to our mechanistic understanding on how riparian-stream runoff generation takes place in forest headwaters with contrasting climate in the Mediterranean region, particularly during exceptionally large storms, which have been difficult to study in the past and which are predicted to become more frequent in the future. Overall, the second publication integrates hydrological and ecological approaches and expertise; an integration that is rarely found in the literature. The third publication provides strong support for the usefulness of the DSL approach beyond the boreal ecoregion, and shows the high value of combining empirical and modelling approaches in climate impact studies, something that is relatively uncommon in the literature. Finally, once the last work package is completed, all results put together will make up a unified knowledge framework on riparian zone function based on the DSL and across different ecoregions, supporting both scientific assessments and management of river networks.