Eco-hydrological tools based on segmented Concentration-Discharge relationships
Several lines of evidence suggest that C-Q response at low flows can be influenced by biogeochemical processes, but that hydrology becomes dominant during high flows. We proposed splitting the hydrograph at the median daily flow (Q50) and calculating separate logC-logQ slopes for low and high flows. This segmentation accommodated nonlinearity and potential shifts in hydrological and biogeochemical controls at different hydrological states. Three conceptually simple indices (b50low, b50high and CQ50) of ecohydrological behaviour can be used as sensitive indicators of interactions between hydrology, biology and catchment characteristics.
- b50low, logC-logQ slope for Q < Q50, a proxy of “biogeochemical processes”
- b50high, logC-logQ slope for Q > Q50a proxy of “catchment export regimes” and used to characterize the flashiness of solute and sediment exports
- CQ50, average solute or particulate concentration at the median daily flow (Q50), a proxy of catchment characteristics.
The quantification of solute and sediment export from drainage basins is challenging. A large proportion of annual load of most constituents is exported during relatively short periods of time, a “hot moment”, which vary between constituents and catchments. We developed a new framework based on C-Q relationship to characterize the export regime of stream particulates and solutes during high water periods when the majority of annual and inter-annual load is transported. We evaluated the load flashiness index (percentage of cumulative load that occurs during the highest 2% of daily load, M2), a function of flow flashiness (percentage of cumulative Q during the highest 2% of daily Q, W2) and export pattern (slope of the logC-logQ relationship for Q higher than the daily median Q, b50high). We established this relationship based on long-term water quality and discharge datasets of 580 streams sites of France and USA, corresponding to 2507 concentration time series of total suspended sediments (TSS), total dissolved solutes (TDS), total phosphorus (TP), nitrate (NO3) and dissolved organic carbon (DOC), generating 1.5 million data points in highly diverse geologic, climatic and anthropogenic contexts. Load flashiness (M2) increased with b50high and/or W2. Also, M2 varied as a function of the constituent transported. M2 had the highest values for TSS and decreased for the other constituents in the following order: TP, DOC, NO3, TDS. Based on these results, we constructed a load-flashiness diagram to determine optimal monitoring frequency of dissolved or particulate constituents as a function of b50high and W2.
Representation of the load flashiness M2 indicator (load flashiness) as a function of W2 (flow-flashiness) and b50high (export regime). Each colour represents a class of flashiness. The colour of the points corresponds to the class given by the observed M2. The lines are derived from equation 9. B: Examples of M2 values for four water quality parameters (TSS, TDS, TP and NO3) of four Erie Lake tributaries of increasing areas. The colour of the points corresponds to the class given by the observed M2. The lines are derived from equation 9. C: Diagram of load flashiness and five classes of flashiness from very low flashiness with M2<8%, to very high flashiness with M2>64%. (Moatar et al, 2020, Frontiers in Ecology and Evolution, 7)
Eutrophication in the Loire River
Trends and seasonality analysis from 1980 onward and longitudinal distribution, from headwaters to estuary, of chlorophyll a, nitrate and phosphate were investigated in the eutrophic Loire River. The continuous decline of phosphate concentrations which has been recorded since 1991 both in the main river and in the tributaries has led to the conclusion that it was responsible for the significant reduction in phytoplanktonic biomass across the whole river system, although Corbicula spp. clams invaded the river during the same period and probably played a significant role in the phytoplankton decline.
The Loire River is a relevant case of a river recovering from severe eutrophication by controlling phosphorus direct inputs. (Minaudo et al, 2015)
The model QUALity-NETwork (QUAL-NET) was developed and tested on the Middle Loire River Corridor, a sub-catchment of the Loire River in France, prone to eutrophication. Hourly variations computed efficiently by the model helped disentangle the complex interactions existing between hydrological and biological processes across different timescales. Phosphorus (P) availability was the most constraining factor for phytoplankton development in the Loire River, but simulating bacterial dynamics in QUAL-NET surprisingly evidenced large amounts of organic matter recycled within the water column through the microbial loop, which delivered significant fluxes of available P and enhanced phytoplankton growth. This explained why severe blooms still occur in the Loire River despite large P input reductions since 1990. (Minaudo et al, 2018)
River network metabolism: spatiotemporal patterns and regime shifts (HOT project)
Estimates of ecosystem metabolism in freshwater systems like streams and rivers have become much cheaper and easier in recent years, but many knowledge gaps remain, particularly how metabolism scales from reaches (e.g., 100 m2) to river networks (e.g., 100,000 m2). To fill these gaps, we have two research foci:
1) Determine how river network structure influences stream metabolism in headwater systems, with a particular emphasis on tributary junctions and patterns of land use and geomorphology, and
2) Determine how observed shifts in primary producer communities from algae to macrophytes has influenced river metabolism and nutrient transport.
For both research foci, we estimate GPP, ER, and NEP in streams and rivers in the Loire River watershed, France using continuous observations of dissolved oxygen concentrations. For research focus 1, we employ a dense network of dissolved oxygen sensors in the headwaters of the Loire, and for research focus 2, we rely on thirty years of hourly dissolved oxygen and related physicochemical data from the Middle Loire, a large shallow river.
This project is the continuation of the “Eutrophication – trend project” (2012-2016)
Climate change and human impacts on discharge and stream temperature (HOT project)
In this project, we use the semi-distributed hydrological model EROS and the physical-based T-NET model (Beaufort et al, 2016, Loicq et al, 2018) to quantify the impacts of climate change, vegetation and human alterations (ponds, dams) on discharge and water temperature at a regional scale. We also perform reconstruction of water temperature during the last 50 years overall Loire Catchment and simulations under future climate change scenarios.
Stream temperature monitoring stations
Geostatistical regionalization of stream temperature metrics at a national level (TIGRE Project)
This project made possible to collect around 2300 monitoring stations of the stream temperature across France on an hourly basis over 2 to 10 years. We develop now geostatistical models for summer stream temperature metrics, based on hydroclimatic and morphological factors.
Transposing C-Q curves onto unmonitored catchments to estimate nutrient loads
Many lakes and reservoirs in Europe suffer from severe eutrophication. Accurate quantification of nutrient loads are critical for effective mitigation measures, but this information is often unknown. For example, in France, only 50 out of 481 lakes and reservoirs have national monitoring allowing estimation of interannual nitrogen and phosphorus loads, and even these loads are computed from low-frequency data. To address this lack of data, we developed a straightforward method to predict seasonal loads in lake tributaries. First, we analyzed concentration-discharge curves in monitored catchments and identified key metrics. Then, we used stepwise multiple linear regression models to empirically link the characteristics of C-Q curves with a set of catchment descriptors such as land use, lithology, morphology indices, climate, and hydrological indicators. Modeled C-Q relationships were then used to estimate annual nutrient loads in nearby and similar unmonitored catchments. We implemented this approach on a large dataset from France where stream flow was surveyed daily and water quality (nitrate and total phosphorus concentrations) was measured on a monthly basis at 233 stations over the past 20 years in catchments from 10 to 3000 km².