Project 15 (WP4): Climate change impacts on riverine nutrient transfer and delivery to coastal sea

Project lead: Vincent Thieu

Post-doctoral researcher: Mélanie Raimonet

Project Start/End: October 2015 – March 2017

Context: Ecological functioning of aquatic ecosystems is directly impacted by increasing human activities and climate changes. Altering these functions sometimes results in environmental damages affecting the whole aquatic continuum from headwaters to coastal sea (such as eutrophication, green tides, anoxia, fish mortality …). While regional prospective analyses are still greatly supported by statistical approaches, the newly developed biogeochemical model pyNuts-Riverstrahler, rises up the challenge of a mechanistic representation of microscopic processes operating in the aquatic continuum, scalable and compatible with regional (and up to continental) domains. PyNuts-Riverstrahler modeling framework (which include the biogeochemical RIVE model) is now operational for all the north-east Atlantic rivers (more than 350 000 km of drainage network) for assessing present nutrient transfer, including the two past decades.

The proposed research aims at further developing the model to be used in climate change impact studies. Two regional domains will be studied, namely: the Seine-Normandy river basins flowing into the Bay of Seine, and the Adour-Garonne systems associated with small Cantabrian rivers flowing into the Bay of Biscay. This work will benefit from the recent developments brought to the pyNuts modeling framework, nevertheless, it requires substantial improvements to enable the analysis of climate change impacts: (i) implementation of a hydrological module, (ii) regionalization of several biogeochemical model’s parameters under changing climatic conditions, (iii) selection of relevant ecological indicators using existing estuarine model to assess the impacts of climate change on estuary and coastal area biogeochemical functioning.

Objectives

The first task is to add a hydrological module to the pyNuts-Riverstrahler modeling framework. Priority will be given to simple model structures like GR4J, as it only requires few parameters to be defined and show reasonable performances (compared to more complex and physically based models). Such an implementation also implies the use of downscaling methods to retrieve the climatic inputs (rainfall, temperature) at a finer space-scale than climate models projections (Euro-Cordex database).

In a second step the pyNuts-Riverstrahler model will be used to assess the quality of water resources forced by changing hydrologic conditions (potential changes in seasonality, prolongation of low flow periods, extreme events ...), and will incorporate the impacts of climate change on the biogeochemical functioning of river systems (response of biological processes to higher temperatures, e.g. development of bacterial, phytoplankton and zooplankton biomass, taken into account in the RIVE model). A prerequisite to the use of the pyNuts-Riverstrahler model will be the identification of parameters potentially affected by a change of climatic conditions.

Finally, the impact of climate change will be evaluated at the scale of coastal area. Riverine export fluxes from all major river systems associated with smaller coastal rivers will be analyzed. Existing estuarine model will be used to assess changes in N, P, Si, C fluxes along the salinity gradients, and to quantify deliveries to coastal area. Ecological indicator (like the existing ICEP: Indicator of coastal eutrophication potential or other indicator to be developed) will be used to point out eutrophication by nutrients and possible harmful algal blooms, hypoxia events and water acidification.

Position offer and results (January 2017 update)