Multitemporal Relationships Between the Hydroclimate and Exports of Carbon, Nitrogen, and Phosphorus in a Small Agricultural Watershed

Agriculture affects the biogeochemical cycles of carbon, nitrogen, and phosphorus, leading to a deterioration of surface water quality. The increasing magnitude of climate change raises questions about potential additional or mitigating effects of climate change on this deterioration. One way to understand these potential effects is to cross‐analyze the dynamics of nutrient concentrations and hydroclimatic variables at multiple time scales. Here, we used a 16‐year data set, from a 5 km2 agricultural watershed in France with a temperate oceanic climate, that contains a daily record of nutrient concentrations and hydroclimatic variables from 2002–2017. We calculated Mann‐Kendall and Theil‐Sen tests, Fourier transforms, and daily hydroclimatic distributions associated with extreme stream concentrations to investigate long‐term trends, seasonal dynamics and their interannual variations, and the daily time scale, respectively. Dynamics of dissolved organic carbon (DOC) and nitrate (NO3) concentrations displayed opposite patterns at the three temporal scales, while soluble reactive phosphorus concentrations showed decoupled dynamics, related more to extreme hydrological events. Climate and past agricultural changes seem to have a synergetic effect that leads to long‐term NO3 decrease and DOC increase. Precipitation and, to a greater extent, watershed wetness controlled seasonal and event‐driven dynamics.

Long-term impacts of nutrient control, climate change, and invasive clams on phytoplankton and cyanobacteria biomass in a large temperate river

Camille Roland Marie Minaudo

Recent studies suggest that climate change, with warmer water temperatures and lower and longer low flows, may enhance harmful planktic cyanobacterial growth in lakes and large rivers. Concomitantly, controlling nutrient loadings has proven effective in reducing phytoplankton biomass especially in North America and Western Europe. In addition, the impact of invasive benthic filter-feeder species such as Corbicula on phytoplankton has largely been overlooked in large rivers, leading to even more uncertainty in predicting future trajectories in river water quality. To investigate how nutrient control, climate change and invasion of benthic filter-feeders may affect phytoplankton biomass and composition, we assembled a large database on the entire water course of the River Loire (France) over three decades (1991–2019). We focus on cyanobacteria to provide an in-depth analysis of the 30-year trend and insights on future possible trajectories. Since 1991, total phytoplankton and cyanobacteria biomasses have decreased 10-fold despite warmer water temperature (+0.23 °C·decade−1) and lower summer flow (−0.25 L·s−1·km−2·decade−1). In the long-term, the contribution of planktic cyanobacteria to total biomass was on average 2.8%. The main factors driving total phytoplankton and cyanobacteria biomasses were total phosphorus (4-fold decrease), the abundance of Corbicula clams (from absence before 1998 to 250–1250 individuals·m−2 after 2010), the duration of summer low flows and the intensity of summer heatwaves. The River Loire constitutes an example in Europe of how nutrient control can be an efficient mitigation strategy, counteracting already visible effects of climate change on the thermal regime and flow pattern of the river. This may hold true under future conditions, but further work is needed to account for the climate trajectory, land and water use scenarios, the risk of enhanced benthic biofilm and macrophyte proliferation, together with the spread of invasive filter-feeding bivalves.

2020

The effects of land use and climate change on the carbon cycle of Europe over the past 500 years

Jed Oliver Kaplan, Kristen Marie Krumhardt

The long residence time of carbon in forests and soils means that both the current state and future behavior of the terrestrial biosphere are influenced by past variability in climate and anthropogenic land use. Over the last half-millennium, European terrestrial ecosystems were affected by the cool temperatures of the Little Ice Age, rising CO2 concentrations, and human induced deforestation and land abandonment. To quantify the importance of these processes, we performed a series of simulations with the LPJ dynamic vegetation model driven by reconstructed climate, land use, and CO2 concentrations. Although land use change was the major control on the carbon inventory of Europe over the last 500years, the current state of the terrestrial biosphere is largely controlled by land use change during the past century. Between 1500 and 2000, climate variability led to temporary sequestration events of up to 3Pg, whereas increasing atmospheric CO2 concentrations during the 20th century led to an increase in carbon storage of up to 15Pg. Anthropogenic land use caused between 25Pg of carbon emissions and 5Pg of uptake over the same time period, depending on the historical and spatial pattern of past land use and the timing of the reversal from deforestation to afforestation during the last two centuries. None of the currently existing anthropogenic land use change datasets adequately capture the timing of the forest transition in most European countries as recorded in historical observations. Despite considerable uncertainty, our scenarios indicate that with limited management, extant European forests have the potential to absorb between 5 and 12Pg of carbon at the present day.

2012

Spatio-temporal controls of C–N–P dynamics across headwater catchments of a temperate agricultural region from public data analysis

Camille Roland Marie Minaudo

Characterizing and understanding spatial variability in water quality for a variety of chemical elements is an issue for present and future water resource management. However, most studies of spatial variability in water quality focus on a single element and rarely consider headwater catchments. Moreover, they assess few catchments and focus on annual means without considering seasonal variations. To overcome these limitations, we studied spatial variability and seasonal variation in dissolved C, N, and P concentrations at the scale of an intensively farmed region of France (Brittany). We analysed 185 headwater catchments (from 5–179 km2) for which 10-year time series of monthly concentrations and daily stream flow were available from public databases. We calculated interannual loads, concentration percentiles, and seasonal metrics for each element to assess their spatial patterns and correlations. We then performed rank correlation analyses between water quality, human pressures, and soil and climate features. Results show that nitrate (NO3) concentrations increased with increasing agricultural pressures and base flow contribution; dissolved organic carbon (DOC) concentrations decreased with increasing rainfall, base flow contribution, and topography; and soluble reactive phosphorus (SRP) concentrations showed weaker positive correlations with diffuse and point sources, rainfall and topography. An opposite pattern was found between DOC and NO3: spatially, between their median concentrations, and temporally, according to their seasonal cycles. In addition, the quality of annual maximum NO3 concentration was in phase with maximum flow when the base flow index was low, but this synchrony disappeared when flow flashiness was lower. These DOC–NO3 seasonal cycle types were related to the mixing of flow paths combined with the spatial variability of their respective sources and to local biogeochemical processes. The annual maximum SRP concentration occurred during the low-flow period in nearly all catchments. This likely resulted from the dominance of P point sources. The approach shows that despite the relatively low frequency of public water quality data, such databases can provide consistent pictures of the spatio-temporal variability of water quality and of its drivers as soon as they contain a large number of catchments to compare and a sufficient length of concentration time series.

2021

Spatio-temporal controls of C–N–P dynamics across headwater catchments of a temperate agricultural region from public data analysis

Camille Roland Marie Minaudo

2021

Long-term impacts of nutrient control, climate change, and invasive clams on phytoplankton and cyanobacteria biomass in a large temperate river

Camille Roland Marie Minaudo

2020