Small artificial impoundments have big implications for hydrology and freshwater biodiversity

Headwater streams are critical for freshwater ecosystems. Global and continental studies consistently show major dams as dominant sources of hydrological stress threatening biodiversity in the world’s major rivers, but cumulative impacts from small artificial impoundments concentrated in headwater streams have rarely been acknowledged. Using the Murray Darling River basin (Australia)and the Arkansas River basin (USA) as case studies, we examine the hydrological impact of small artificial impoundments. The extent of their influence is significant, altering hydrology in 280 - 380% more waterways when compared to major dams alone. Hydrological impacts are concentrated in smaller streams (catchment area < 100 km2), raising concerns that the often diverse and highly endemic biota found in these systems may be under threat. Adjusting existing biodiversity planning and management approaches to address the cumulative effects of many small and widely distributed artificial impoundments presents a rapidly emerging challenge for ecologically sustainable water management.

Flooding Causes Dramatic Compositional Shifts and Depletion of Putative Beneficial Bacteria on the Spring Wheat Microbiota

Flooding affects both above- and below-ground ecosystem processes, and it represents a substantial threat for crop and cereal productivity under climate change. Plant-associated microbiota play a crucial role in plant growth and fitness, but we still have a limited understanding of the response of the crop-microbiota complex under extreme weather events, such as flooding. Soil microbes are highly sensitive to abiotic disturbance, and shifts in microbial community composition, structure and functions are expected when soil conditions are altered due to flooding events (e.g., anoxia, pH alteration, changes in nutrient concentration). Here, we established a pot experiment to determine the effects of flooding stress on the spring wheat-microbiota complex. Since plant phenology could be an important factor in the response to hydrological stress, flooding was induced only once and at different plant growth stages (PGSs), such as tillering, booting and flowering. After each flooding event, we measured in the control and flooded pots several edaphic and plant properties and characterized the bacterial community associated to the rhizosphere and roots of wheat plant using a metabarcoding approach. In our study, flooding caused a significant reduction in plant development and we observed dramatic shifts in bacterial community composition at each PGS in which the hydrological stress was induced. However, a more pronounced disruption in community assembly was always shown in younger plants. Generally, flooding caused a (i) significant increase of bacterial taxa with anaerobic respiratory capabilities, such as members of Firmicutes and Desulfobacterota, (ii) a significant reduction in Actinobacteria and Proteobacteria, (iii) depletion of several putative plant-beneficial taxa, and (iv) increases of the abundance of potential detrimental bacteria. These significant differences in community composition between flooded and control samples were correlated with changes in soil conditions and plant properties caused by the hydrological stress, with pH and total N as the soil, and S, Na, Mn, and Ca concentrations as the root properties most influencing microbial assemblage in the wheat mircobiota under flooding stress. Collectively, our findings demonstrated the role of flooding on restructuring the spring wheat microbiota, and highlighted the detrimental effect of this hydrological stress on plant fitness and performance.

Future Summer Drying in the U.S. Corn Belt and the Role of Midlatitude Storm Tracks

During the summer, the Midwest United States, which covers the main US corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO2 and the associated warming increases evapotranspiration. while precipitation reduces in the region leading to further reduction in precipitation minus evaporation (P-E) in the future. At the same time, the poleward transient moisture flux increases leading to enhanced atmospheric moistures export from the corn belt region. However, storm track intensity is generally weakened in the summer due to reduced north-south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region due to the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future, the US Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export leading to drier soils in the region.

The past, present, and future of multiple wheat and maize breadbasket shocks

Simultaneous yield shocks in multiple breadbaskets pose a potential threat to global food security, yet the historical risks and causes of such shocks are poorly understood. Here, we compile a dataset of subnational maize and wheat yield anomalies in 25 countries dating back to 1900 to better characterize the past, present, and future risk of multiple breadbasket shocks. We find that years in which at least half of all maize or wheat breadbaskets fall 10% (5%) below expected yields has occurred in ~2-3% (~14-16%) of years over the last century. Importantly, multiple breadbasket shocks have been decreasing in frequency from 1930 to 2017. The El Niño Southern Oscillation (ENSO) most strongly affects the probability of multiple maize breadbasket shocks, while the North Atlantic Oscillation (NAO) most strongly affects the probability of multiple wheat breadbasket shocks, each influencing the probability by up to 40%. The effect of climate change on climate stress in maize and wheat breadbaskets is mixed; extreme heat will increase uniformly, agricultural soil moisture stress will remain constant or increase, but hydrological stress (as measured by runoff) will remain constant or decrease in breadbasket regions.

Invasion Success of Bunias orientalis (Warty Cabbage) in Grasslands: A Mesocosm Experiment on the Role of Hydrological Stress and Disturbance

Climate change is altering precipitation patterns, with higher frequency and magnitude of extreme events. Specifically, longer and more pronounced waterlogged conditions are predicted after rain spells as well as more frequent droughts, especially in Central Europe. Such hydrological changes can severely affect species performance and alter the function of ecosystems, as well as favor plant invasions. Competition with native communities may change depending on water stress. Bunias orientalis is an invasive plant that may benefit from disturbance or precipitation changes. We conducted a 3-year mesocosm experiment in a common garden to investigate how invasion success of B. orientalis in native German grassland communities is affected by varying hydrological conditions (from very dry to waterlogged). We measured the establishment and growth of B. orientalis in varying water table depths in bare soil (simulating disturbance) vs. in the community. Establishment and biomass of B. orientalis was generally highest under non-stress conditions. The species was also highly tolerant to dry conditions, but only when growing in bare soil. However, performance of B. orientalis was generally low, whereby interspecific competition in communities greatly limited invasion success. This might be due to the low competitive ability of the species in conditions of hydrological stress and the near-natural grassland communities with an extensive mowing regime used in our experiment. Our results suggest that invasion success of B. orientalis in grasslands will not increase if precipitation patterns change toward more extreme events. However, disturbance that creates bare soil patches might favor B. orientalis under drought conditions.

The Chennai flood (India): preliminary results using CETEMPS Hydrological Model (CHyM) stress index

&lt;p&gt;An extreme weather event hit the coastal city of Chennai, India, in November-December 2015 causing severe damage to infrastructure worth billions of dollars, people&amp;#8217;s lives and their livelihood. Nearby districts to Chennai, such as Cuddalore, Kancheepuram and Tiruvallur were also affected by rainfall over 300mm during the first week of December. This was caused by the unusual wind surges in the troposphere providing favorable environmental conditions for the extensive rainfall and the formation of a deep depression in the Bay of Bengal on 30 November 2015, which was blocked by Eastern Ghats that inhibited the movement of the synoptic system. Electricity and telecommunication lines were suspended and some hospitals were shut down for a few days. It brought the whole city into a state of emergency and National Disaster Rescue Force were deployed in an effort to take care of the evacuation of people.&lt;/p&gt;&lt;p&gt;In this work, we present the estimation of the hydrological stress caused by the extreme rainfall event in Chennai and the nearby river basins during the course of this northeastern monsoon event in India. The hydrological stress is given through the application of Best Discharge based Drainage (BDD)&amp;#160; Index, calculated by the CETEMPS Hydrological Model (CHyM). Hydrological simulation is carried out by forcing the model with the 3-hourly NASA IMERG 0.1x0.1 grid precipitation dataset. Preliminary results show a spatial coherence between the hydrological stress detected by the index and the most impacted river segments, due to heavy precipitation. The application of hydrological stress indices is helpful for forecasting fluvial floods in the river network with minimum calibration requirements, providing a useful tool for warning the respective authorities for minimal losses due to natural calamities.&lt;/p&gt;

Linking Demographic Transitions to Population Dynamics in a Fluctuating Environment

Recruitment has been linked to decreases in the ratio of age-specific mortality (M’) to mass-specific growth (G’), and year-class strength may be predicted by the age when M’/G’=1. Hydrological stress adversely affects these parameters for species inhabiting floodplains; however, the relationship between M’ and G’ in hydrologically variable environments is poorly understood. We evaluated age-specific mortality for six species from a 20-year time series, and growth curves from otolith length-at-age data. We assessed the effect of hydrology on the transitional age (age M’/G’=1) at 21 sites representing a hydrological gradient. Disturbance intensity influenced age-specific mortality but had no effect on mass-specific growth. The transitional age was inversely correlated with annual density, but weakly associated with population biomass. Hydrological disturbance shifted the transition age to older ages, reducing recruitment overall. We demonstrated that the M’/G’ transition was affected adversely by hydrological stress and can be applied to a diverse group of taxa. Growth, survivorship, and the transitional age should be evaluated to improve population modelling efforts used to predict the influence of future restoration actions.