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Groundwater effects on coastal ecosystems

Periodic Reporting for period 1 - GRECO (Groundwater effects on coastal ecosystems)

Reporting period: 2017-04-01 to 2019-03-31

Coastal lagoons host some of the most dynamic, diverse and productive ecosystems on Earth, which are subject to significant pressure from human activities. Hydrological land-ocean connectivity is a universally accepted important driver of coastal ecosystems, but the ecological effects of groundwater and associated solute fluxes to coastal systems remain poorly understood for most of the world’s shores. The overall goal of GRECO was to evaluate the role that groundwater fluxes play in the functioning and vulnerability of coastal lagoon ecosystems, by studying its effects on primary producers. This project encompassed a suite of innovative and interdisciplinary investigations aimed at (a) quantifying groundwater-derived freshwater and nutrient inputs and documenting the spatio-temporal distribution of these inputs to lagoons; and (b) evaluating the groundwater-derived effects on lagoon primary productivity by identifying the nutrient sources for primary production and the role of groundwater in lagoon ecological functioning. The novel understanding of groundwater-ecological interactions derived from this project will significantly advance the state-of-the art of groundwater studies in the coastal zone by providing direct evidences of ecological effects of groundwater discharge and by adding new methods to study groundwater implications. The results of GRECO will allow closing a current gap in the fundamental understanding of coastal processes, improving our capacity to sustainably manage hydrological and ecological resources in the coastal.
Two French lagoons with contrasting hydroecological conditions have been investigated, combining current methods to quantify groundwater discharge and associated nutrient inputs (use of tracers such as radioisotopes and salinity) with the evaluation of new tools (application of the stable isotope signature in primary producers).

Two economically and ecologically important French lagoons with contrasting hydroecological conditions were investigated by combining current methods to quantify groundwater discharge and associated nutrient inputs (use of tracers such as radioisotopes, heat and salinity) with the evaluation of new tools to groundwater studies (nutrient stable isotope analysis in primary producers to resolve the nutrient contribution of groundwater fluxes).

The main scientific progress and achievements derived from the GRECO project include:
- Quantifying fluxes of water and dissolved nutrients driven by groundwater and recirculation inputs in the studied coastal lagoons.
- Demonstrating the importance of recirculation of lagoon waters as a conveyor of dissolved nutrients to coastal ecosystems.
- Improving the understanding on the temporal and spatial scales of lagoon water recirculation through sediments.
- Advancing the application of radionuclides as tracers of groundwater and recirculation inputs to coastal environment (development of new modeling approaches & improvement of currently applied mass balance approaches).
- Evaluating the dynamics of water fluxes across the sediment-water interface in response to temporally variable driving forces by using the inversion of thermal and salinity time series observations in porewaters.
- Providing direct evidence for the role of groundwater and recirculation fluxes in sustaining primary production in coastal lagoons.
- Applying nitrogen stable isotope analyses to coastal groundwater studies and demonstrating its potential to trace the transfer of groundwater-driven nutrient to primary producers (Objective B).

Scientific advances from the GRECO project have resulted in 11 peer-review publications in international highly ranked journals, all of them available in open access. These project outcomes have also been presented at international scientific conferences and workshops. The newly acquired knowledge from this project has also been disseminated through outreach activities, mainly including the realization of two stakeholder information meetings with agents involved in the use and management of the studied lagoons and two workshops for scholars.
GRECO has significantly advanced the state-of-the-art of coastal groundwater studies by i) demonstrating the relevance of water recirculation through sediments as a net nutrient source, ii) documenting groundwater-ecological links in coastal ecosystems, and iii) adding new tools (stable isotope signature) and improving current approaches (radon, salinity and temperature modeling) for studies of groundwater processes and their ecological implications. The results obtained in GRECO concern thus a broad scientific community, including hydrologists, biogeochemists, ecologists, oceanographers and other scientists working on coastal processes.

Outcomes of GRECO are also of great interest for agents involved in lagoon management and use (either for leisure or commercial purposes), and have been directly communicated to these end users through regular interactions and through the participation of resource managers in the project. The newly acquired understanding of coastal processes will allow better responding the urgent need of developing integrated sustainable management of coastal hydrological and ecological resources, in particular with regards to EU regulatory frameworks (specifically the Water Framework and Habitat Directives).