Water Residence Time Research Articles

Disentangling the effects of climate change and reoligotrophication on primary production in a large lake

Climate change and reduction in nutrient loads have significant effects on primary production and phytoplankton growth dynamics. Since in the last few decades in many regions, nutrients in lakes were reduced simultaneously as the climate changed. Yet, it remains unclear which of the two has impacted primary production the most. In this study, we couple the General Ocean Turbulence Model with the Ecological Regional Ocean Model to disentangle the effects of climate change and reoligotrophication on primary production (PP) in Lake Geneva, Switzerland–France. We apply a data assimilation method to calibrate the model with the observations from the past (1981–1990) and validate it against the in situ data from the present decade (2011–2019). Both decades represent different climate conditions and trophic states of the lake. We show that the model is skilful to reproduce assimilated and unassimilated observations from both periods. According to our results, the effect of reoligotrophication on PP is marginally higher than that of warming, leading to a net decrease in primary production by 10% from the past to the present. The areal phosphorus supply in Lake Geneva, in spite of a decrease by $$\sim$$ 70%, is still characteristic of a meso-to-eutrophic ecosystem. This points towards an incomplete reoligotrophication of the lake. The effects of future climate change on winter mixing and PP dynamics have also been studied. Although there would be a significant reduction in deep mixing, the autotrophic production in Lake Geneva is expected to increase by $$\sim$$ 20% by the end of twenty-first century, largely due to stimulation in biomass build-up of temperature-dependent algae (e.g. dinoflagellates and cyanobacteria). Considering our results to represent other large temperate lakes with similar trophic status and water residence time as Lake Geneva, future climate scenarios are expected to bring back symptoms of eutrophication.

Influence of social and environmental drivers on nutrient concentrations and ratios in lakes: A comparison between China and Europe

Global anthropogenic flows of nitrogen (N) and phosphorus (P) have exceeded planetary boundaries with significant implications for pollution of the freshwater resources in the world. Understanding the global patterns and drivers of N and P concentrations and their ratios in the lakes could help design more effective management and remediation strategies to mitigate the impacts of eutrophication. While a suite of drivers are associated with the sources of nutrients, their transport and internal processes that control concentrations of N and P in the lakes, much less is known about the relative importance of different drivers in explaining spatial variations of lake nutrients and ratios. In this study, we compared N and P concentrations and their ratios in the lakes across China and Europe and examined the differences of dominant environmental and social drivers on lake nutrients. Our comparison showed that total nitrogen (TN) and total phosphorus (TP) concentrations were much higher in the lakes in China compared to those in Europe (i.e., TN: 1.13 mg/L in China vs. 0.64 mg/L in Europe; TP: 35.83 μg/L in China vs. 19.38 μg/L in Europe, the median value). However, lake N/P ratios for both regions were not statistically different. Concentrations of TN and TP showed decoupling in both regions, with the majority of lakes having high N/P mass ratios when evaluated by the commonly accepted threshold of 23 (i.e., 61% in China and 68% in Europe), indicating that phytoplankton are more P limited relative to N. Agricultural activity in the lake catchment is an important predictor for both nutrient concentrations and their ratio in Europe. This reflects successful investments in infrastructure and policy prescriptions in addressing point sources of pollution. In comparison, lake depth and water residence time are important in the decoupling of N and P concentrations in China. The regional difference between the dominant drivers can provide important insights into development of effective water pollution control measures. It is necessary for policy makers and water resource managers to be aware of large-scale imbalance of nutrients in lake due to the potential environmental consequences. A set of spatially flexible policies for water quality controls would be beneficial for sustaining the ecological integrity and future health of lakes.

Beaver dams overshadow climate extremes in controlling riparian hydrology and water quality

Hydrologic extremes dominate chemical exports from riparian zones and dictate water quality in major river systems. Yet, changes in land use and ecosystem services alongside growing climate variability are altering hydrologic extremes and their coupled impacts on riverine water quality. In the western U.S., warming temperatures and intensified aridification are increasingly paired with the expanding range of the American beaver—and their dams, which transform hydrologic and biogeochemical cycles in riparian systems. Here, we show that beaver dams overshadow climatic hydrologic extremes in their effects on water residence time and oxygen and nitrogen fluxes in the riparian subsurface. In a mountainous watershed in Colorado, U.S.A., we find that the increase in riparian hydraulic gradients imposed by a beaver dam is 10.7–13.3 times greater than seasonal hydrologic extremes. The massive hydraulic gradient increases hyporheic nitrate removal by 44.2% relative to seasonal extremes alone. A drier, hotter climate in the western U.S. will further expand the range of beavers and magnify their impacts on watershed hydrology and biogeochemistry, illustrating that ecosystem feedbacks to climate change will alter water quality in river systems.

Integrating Hydrological Connectivity in a Process–Response Framework for Restoration and Monitoring Prioritisation of Floodplain Wetlands in the Ramganga Basin, India

Floodplain wetlands are critical for sustaining various ecological and hydrological functions in a riverine environment. Severe anthropogenic alterations and human occupation of floodplains have threatened these wetlands in several parts of the world. A major handicap in designing sustainable restoration and monitoring strategies for these wetlands is the lack of scientific process-based understanding and information on the basin-scale controls of their degradation. Here, we offer a novel approach to integrate the connectivity of the wetlands with the surrounding landscape along with other attributes such as stream density, hydrometeorological parameters, and groundwater dynamics to explain their degradation and then to prioritise them for restoration and monitoring. We hypothesise that the best possible connectivity scenario for the existence of a wetland would be if (a) the wetland has a high connectivity with its upslope area, and (b) the wetland has a low connectivity with its downslope region. The first condition ensures the flow of water into the wetland and the second condition allows longer water residence time in the wetland. Accordingly, we define four connectivity-based wetland health scenarios—good, no impact, bad, and worst. We have implemented the proposed method in 3226 wetlands in the Ramganga Basin in north India. Further, we have applied specific selection criteria, such as distance from the nearest stream and stream density, to prioritise the wetlands for restoration and monitoring. We conclude that the connectivity analysis offers a quick process-based assessment of wetlands’ health status and serves as an important criterion to prioritise the wetlands for developing appropriate management strategies.

Patterns and Drivers of Dissolved Gas Concentrations and Fluxes Along a Low Gradient Stream

AbstractFreshwater ecosystems are globally significant sources of greenhouse gases (GHGs) to the atmosphere. Previous work has indicated that GHG flux in headwater streams is dominated by terrestrially derived gases, with in situ production limited by short organic matter residence times and high dissolved oxygen concentrations due to turbulent reaeration. However, low‐gradient headwater streams that contain pool structures with longer residence times may be conducive to the in situ production of GHG. These streams, and the longitudinal heterogeneity therein, are seldom studied. We measured continuous ecosystem metabolism alongside concentrations and fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in a low‐gradient third order stream in the North Carolina Piedmont. From autumn to the following spring, we characterized spatial and temporal patterns of GHG along an 8 km segment in the context of channel geomorphology, hydrology, and ecosystem metabolic rates using linear mixed effects models. We found that stream metabolism was responsible for most of the CO2 flux over this period, and that in‐channel aerobic metabolism was a primary predictor of both CH4 and N2O fluxes as well. Long water residence times, limited reaeration, and substantial organic matter from terrestrial inputs foster conditions conducive to the in‐stream accumulation of CO2 and CH4 from microbial respiration. Streams like this one are common in landscapes with low topographic relief, making it likely that the high contribution of instream metabolism to GHG fluxes that we observed is a widespread yet understudied behavior of many small streams.

Hydrologic Connectivity and Residence Time Affect the Sediment Trapping Efficiency and Dissolved Oxygen Concentrations of the Atchafalaya River Basin

AbstractLittle is known about water movement, volume, or residence time (RT), and how those characteristics affect sediment trapping efficiency (TE) and dissolved oxygen concentrations (DO) in the United States' largest remaining bottomland hardwood swamp, the Atchafalaya River Basin. To better understand these dynamics, this study used bathymetry, lidar, and stage records to determine volumes in the Basin's hydrologically distinct water management units (WMUs). Discharge measurements determined flow distribution and RT. Residence time was compared with DO to identify conditions that coincided with DO increases or decreases. Suspended sediment concentrations (SSC) were used to determine TE relative to calculated and measured discharge and RT. Discharge through units (85–2,200 m3/s) and RT (0.37–231 d) depended on connectivity and river stage. At high stages, with water temperatures >20°C, DO in the largest WMU declined by −0.21 mg/l/day. DO trends indicated less well‐connected areas of the WMU contributed hypoxic waters as the flood wave lengthened and stages fell. In the two WMUs examined for TE, TE (−266% to 99% and up to 38 Gg/day) correlated with hydrologic connectivity, SSC, RT, water volume, and, in one WMU, discharge losses. Long RT and high TE indicated a high potential to process nutrients. These relationships varied among WMUs. Large volumes of sediment‐laden water moving over the floodplain combined with long RT, high TE, and hypoxia indicate that this ecosystem has continental‐scale importance in reducing nutrient loads to the northern Gulf of Mexico. Reports from other systems suggest similar processes may be operating on other large river floodplains globally.

Warming soil temperature and increasing baseflow in response to recent and potential future climate change across northern Manitoba, Canada

AbstractThis study investigates the impacts of climate change on the hydrology and soil thermal regime of 10 sub‐arctic watersheds (northern Manitoba, Canada) using the Variable Infiltration Capacity (VIC) model. We utilize statistically downscaled and bias‐corrected forcing datasets based on 17 general circulation model (GCM) ‐ representative concentration pathways (RCPs) scenarios from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to run the VIC model for three 30‐year periods: a historical baseline (1981–2010: 1990s), and future projections (2021–2050: 2030s and 2041–2070: 2050s), under RCPs 4.5 and 8.5. Future warming increases the average soil column temperature by ~2.2°C in the 2050s and further analyses of soil temperature trends at three different depths show the most pronounced warming in the top soil layer (1.6°C 30‐year−1 in the 2050s). Trend estimates of mean annual frozen soil moisture fraction in the soil column show considerable changes from 0.02 30‐year−1 (1990s) to −0.11 30‐year−1 (2050s) across the study area. Soil column water residence time decreases significantly (by 5 years) during the 2050s when compared with the 1990s as soil thawing intensifies the infiltration process thereby contributing to faster conversion to baseflow. Future warming results in 40%–50% more baseflow by the 2050s, where it increases substantially by 19.7% and 46.3% during the 2030s and 2050s, respectively. These results provide crucial information on the potential future impacts of warming soil temperatures on the hydrology of sub‐arctic watersheds in north‐central Canada and similar hydro‐climatic regimes.

Key factors determining water quality, fish community dynamics, and the ecological health in an Asian temperate lotic system

Identifying the key determinants and their impacts on water quality and fish community structure is imperative to environmental assessment and ecosystem conservation. The main objectives of this study were to assess the factors driving water quality, fish assemblages, and ecological health in a temperate lotic ecosystem using a long-term (12 years) dataset. The results indicate that sewage treatment plants significantly negatively impact river water quality and are primary pollution sources. Temporal fluctuations showed that the summer monsoon adversely influenced TN and EC due to dilution effects while increasing the COD and TSS in the water. Annual variations and the Mann–Kendall test demonstrated increasing trends in COD, TN:TP ratio, and CHL-a but a decreasing trend of TP with the impoundment. Artificial barriers (weirs) in the river created more favorable algal growth conditions due to increased water residence time. COD and TSS levels in the river were significantly affected by soil erosion. The environmental disturbance measures index indicated that all sites were severely degraded. Fish composition analysis suggested that exotic fish species such as, Lepomis macrochirus and Micropterus salmoides, dominated the community structure and negatively impacted native species after the installation of weirs. Our findings indicate that fish guilds and the IBI model are controlled by nutrient enrichment, organic matter, algal production, and land use/land cover. Water quality governs the biological health of the river. Overall, evaluation of the river's ecological health based on the multi-metric WPI and the IBI revealed that the river has a “poor” to “very poor” ecological status. The outcomes of this study may aid policymakers in managing and restoring the river.

Contribution of water rejuvenation induced by climate warming to evapotranspiration in a Siberian boreal forest

Water age is a useful metric to evaluate the influence of anthropogenic and natural forcings on the terrestrial water cycle. Current climate warming is enhancing the warming of permafrost soil in the Arctic. Although permafrost is a crucial component of the Arctic terrestrial water cycle, its influence on processes regulating the fluxes and ages of Arctic terrestrial water, particularly soil storage and evapotranspiration, is not well understood. In this study, a water age calculation scheme was implemented into the coupled hydrological and biogeochemical model (CHANGE) to assess the mechanisms through which climate warming affects the soil water storage–evapotranspiration–water age feedback cycle in a boreal forest. Continuous air temperature increase from 1980 to 2016 caused earlier snowmelt and soil thawing, inducing decreasing age trends in snow- and rain-sourced water. The younger water contributed to higher spring evapotranspiration. In summer, the higher evapotranspiration dried the surface soil layer. In turn, the drier surface layer increased the loss of fresh rainwater. Autumn precipitation, preserved in the frozen winter soil until the following spring, became an additional source of water and enhanced plant transpiration in the following summer. This increase accounted for 4.2% of the annual total transpiration. These results suggest that permafrost warming, characterized by earlier soil thawing and later freezing, induced higher evapotranspiration, thereby shortening the residence time of precipitation-sourced water in the active layer and further rejuvenating water in soil layers and in evapotranspiration. Under future climate warming conditions, this effect is expected to intensify and the water cycle will accelerate.

Seasonal variability of water residence time in the Subei Coastal Water, Yellow Sea: The joint role of tide and wind

Assessing the coastal water export rate is important for better solving the coastal environmental problems and developing environmental management strategies. To investigate the water export rate and its spatiotemporal variation in the Subei coastal water (SCW), a subregion of the Yellow Sea, this study applied water residence time (RT) as an indicator and studied the roles of tides, wind, and river discharge on the water export rate using numerical modeling and the RT adjoint method. RT had an annual and spatial mean of ∼80 days but presented highly spatial and seasonal heterogeneity. Spatially, high RT (∼100–300 days) occurred in the nearshore region, and its location varied in seasons. Temporally, RT was the shortest in the spring (∼55 days), longest in the summer (∼91 days), and median (∼81 days) in autumn and winter. Furthermore, the seasonal RT mismatched the strength of residual transport velocity (RTV) at the current season but instead relied on the combined effects of RTVs in the current and next seasons. Additional sensitivity experiments revealed that tides greatly increased the RT, the Changjiang River runoff slightly decreased the RT, while monsoon wind dominated the RT seasonal variation. As highlighted by an idealized analytical framework, increasing the magnitude ratio of the wind- and tidal-induced RTVs could amplify the RT seasonal variability. Ecologically, the shortest RT in spring might promote the rapid expansion of macroalgal blooms originating from the SCW into offshore waters, thereby contributing to green tide occurrence in the Yellow Sea. Therefore, the seasonality of RT should be considered in coastal management and scientific planning.

Bacterial communities in a subarctic stream network: Spatial and seasonal patterns of benthic biofilm and bacterioplankton.

Water-column bacterial communities are assembled by different mechanisms at different stream network positions, with headwater communities being controlled by mass effects (advection of bacteria from terrestrial soils) while downstream communities are mainly driven by environmental sorting. Conversely, benthic biofilms are colonized largely by the same set of taxa across the entire network. However, direct comparisons of biofilm and bacterioplankton communities along whole stream networks are rare. We used 16S rRNA gene amplicon sequencing to explore the spatiotemporal variability of benthic biofilm (2 weeks old vs. mature biofilm) and water-column communities at different network positions of a subarctic stream from early summer to late autumn. Amplicon sequence variant (ASV) richness of mature biofilm was about 2.5 times higher than that of early biofilm, yet the pattern of seasonality was the same, with the highest richness in midsummer. Biofilm bacterial richness was unrelated to network position whereas bacterioplankton diversity was negatively related to water residence time and distance from the source. This pattern of decreasing diversity along the network was strongest around midsummer and diminished greatly as water level increased towards autumn. Biofilm communities were phylogenetically clustered at all network positions while bacterioplankton assemblages were phylogenetically clustered only at the most downstream site. Both early and mature biofilm communities already differed significantly between upstream (1st order) and midstream (2nd order) sections. Network position was also related to variation in bacterioplankton communities, with upstream sites harbouring substantially more unique taxa (44% of all upstream taxa) than midstream (20%) or downstream (8%) sites. Some of the taxa that were dominant in downstream sections were already present in the upmost headwaters, and even in riparian soils, where they were very rare (relative abundance <0.01%). These patterns in species diversity and taxonomic and phylogenetic community composition of the riverine bacterial metacommunity were particularly strong for water-column communities, whereas both early and mature biofilm exhibited weaker spatial patterns. Our study demonstrated the benefits of studying bacterioplankton and biofilm communities simultaneously to allow testing of ecological hypotheses about biodiversity patterns in freshwater bacteria.

Sediment oxygen demand rate in a flow regulated lagoon (Venice, Italy)

From October 2020, the MOSE system went into operation in the three inlets to protect Venice and its lagoon from flooding caused by high tide events in the Adriatic Sea. While the MOSE increases the prospect of physical defense, how will the new status of a regulated lagoon affect the functioning of the waterbody? In particular, the dissolved oxygen balance in the water column can be affected by the expected increase in water residence times. Sediments play an important role in oxygen consumption in the water column and for this reason sediment oxygen demand (SOD) was studied in situ for the first time, in 16 sites at four lagoon areas. SOD rate, measured throughout 2021 with specially made benthic chambers, varied in the different areas and with seasons (from 0.63 to 24.00 g m−2 d−1), with higher values at the more confined sites and significantly lower values in those belonging to the open lagoon. Based on SOD rate, consumption times of dissolved oxygen in the water column were estimated at the MOSE closures, from saturation values to hypoxia and in the temperature range of 11.7–33.3°C. Especially in the confined sites, sediments can exert an important depleting action on dissolved oxygen already at water temperatures above 20°C, which are not particularly high for the lagoon.

Effects of a Volcanic-Fluid Cycle System on Water Chemistry of a Deep Caldera Lake: Lake Tazawa, Akita Prefecture, Japan

Lake Tazawa, the deepest lake (423.4 m depth at maximum) in Japan underwent drastically changed water quality in 1940, because volcanic water from two active volcanos was then drawn into the lake for power generation and irrigation. Thereby, the pH of lake water decreased from 6.7 to 4.2, which exterminated a land-locked type of sockeye salmon, Oncorhynchus nerkakawamurae (locally called Kunimasu trout). Additionally, the mean residence time of lake water changed from 195 years to 8.9 years by rapidly increasing the outflow for power generation and irrigation. In this study, long-term chemical fluxes controlling lake water chemistry were obtained, and a groundwater water cycle system between the lake and the volcano was explored by estimating hydrological and chemical budgets of the lake. In the chemical budget estimate, two ionic species, SO42− and Cl−, in volcanic fluids were chosen and each mass conservation equation was yielded. The hydrological budget estimate gave us the net groundwater inflow at −1.36 m3/s on average over three periods in 2020–2021, and then the simultaneous equation coupled with the chemical budget equation allowed us to separate into groundwater inflow and outflow at 6.01 m3/s and 7.37 m3/s, averaged over the three periods, respectively. The evaluated groundwater inflow and outflow were compared with those of the other crater or caldera lakes. The linear relationship between the lake volume and the magnitude of groundwater inflow or outflow suggests that the groundwater cycle scale in such a lake increases with the magnitude of the volcanic eruption to have formed the lake.

Recent changes of the thermal structure in Three Gorges Reservoir, China and its impact on algal bloom in tributary bays

Despite the fact that the water temperature of the Yangtze River has changed substantially due to the construction of Three Gorges Reservoir (TGR), little is known about how the thermal structure of TGR responded to the implementation of two large upstream reservoirs in 2013. We investigated the seasonal variations of water temperature and discharge in the middle reaches of the Yangtze River by analyzing multi-year (2004–2018) hydrological and water temperature data. The effects of upstream dams on the thermal regime, density currents and phytoplankton blooms in TGR were examined. The results show that water temperature in TGR increased in spring (+1.1 °C) and winter (+0.8 °C) seasons after the filling of upstream reservoirs compared to the years before. Longer duration of stratification is associated with a smaller discharge and a longer water residence time. Moreover, the increase of inflow, which was at least partially due to upstream dam construction, lead to the weakening of stratification during the spring season. As a consequence of the higher water temperature in TGR in spring, the density-driven exchange flows between the main reservoir and tributary bays have changed. Overflows became more frequent, which promoted the deepening of the surface mixing layer, and resulted in a dramatic reduction in the frequency and biomass of phytoplankton blooms in tributary bays of TGR.

Mitigating the impact of flip angle and orientation dependence in single compartment R2* estimates via2-pool modeling.

The effective transverse relaxation rate ( ) is influenced by biological features that make it a useful means of probing brain microstructure. However, confounding factors such as dependence on flip angle (α) and fiber orientation with respect to the main field ( ) complicate interpretation. The α- and -dependence stem from the existence of multiple sub-voxel micro-environments (e.g., myelin and non-myelin water compartments). Ordinarily, it is challenging to quantify these sub-compartments; therefore, neuroscientific studies commonly make the simplifying assumption of a mono-exponential decay obtaining a single estimate per voxel. In this work, we investigated how the multi-compartment nature of tissue microstructure affects single compartment estimates. We used 2-pool (myelin and non-myelin water) simulations to characterize the bias in single compartment estimates. Based on our numeric observations, we introduced a linear model that partitions into α-dependent and α-independent components and validated this in vivo at 7T. We investigated the dependence of both components on the sub-compartment properties and assessed their robustness, orientation dependence, and reproducibility empirically. increased with myelin water fraction and residency time leading to a linear dependence on α. We observed excellent agreement between our numeric and empirical results. Furthermore, the α-independent component of the proposed linear model was robust to the choice of α and reduced dependence on fiber orientation, although it suffered from marginally higher noise sensitivity. We have demonstrated and validated a simple approach that mitigates flip angle and orientation biases in single-compartment estimates.

Attenuation of organic pollutants and the effects of salinity and seasonality in a Mediterranean micro-estuary

Micro-estuaries are small ubiquitous transitional water bodies that are often located in semi-arid zones. Unlike the vastly studied large estuaries, micro-estuaries lack the ability to dilute and contain pollution from point and non-point sources due to low natural water discharges. Therefore, these diverse ecological systems are susceptible to pollutant loads due to prolonged water residence time and complex geochemical dynamics. Although this elevated anthropogenic stress limits their potential to provide ecological and recreational services, micro-estuaries have some traits similar to those found in wetlands, which provide a natural potential to retain and mitigate organic pollutants. A two consecutive years study conducted at the Alexander micro-estuary tracked the influx and outflux of a large organic pollutant mixture during base-flow and flood events. During the research period, 165 kg of active ingredients entered the micro-estuary and 160 kg flowed out to the Mediterranean Sea, suggesting negligible net attenuation. However, this broad picture conceals inner shifts in pollutant mixture loads, which contained 46 pesticides and 19 pharmaceuticals. Only a handful of pollutants were actually balanced, whereas most compounds were either removed or added to the flow, with no observed correlation to chemical properties. A prominent observation was the load increase along the flow for some pollutants during base-flow conditions. This trend, which was correlated with salinity elevation and was verified in lab experiments, suggests that seawater intrusion to the bottom of the estuary may increase desorption rates of pollutants from the estuary bed, creating an estuarine desorption magnification effect. The combination of strong anthropogenic stress with increased desorption rates severely limits the estuary's potential to mitigate pollutants, frequently transforming it into a pollution source rather than a sink.

Development and performance evaluation of solar heating system for biogas production process

The breakdown of organic waste, through anaerobic digestion, is a temperature dependent process. Attaining the temperature required for this process, under normal conditions, is a challenging task. Hence, the development of an effective heating system is necessary. In this paper, a solar heater's computational fluid dynamics (CFD) model, with water as working fluid, is prepared to estimate the temperatures of the absorber plate and that of the water available at the outlet. The validations of the numerical results were done by conducting experimental trials and a fair agreement between them was observed. For the examined solar collector, with outlet temperature of 61°C, the maximum thermal efficiency was estimated as 71%. The effects of operating parameters (like transmissivity coefficient, ambient temperature, wind speed, inlet water temperature and sun's insolation), were explored. It was predicted that 30–60°C tank water temperature can be attained at flow rates of 0.00833, 0.0167, and 0.021 kg/s. The bent-tube Flat Plate Colector (FPC) model, proposed in the present work, increases the residence time of water in the collector. It also increases the outlet temperature. Hence, the developed FPC model may be considered as an alternative solution for growing thermophilic bacteria during the anaerobic digestion in biogas production.

Water Budget, Biological Water Use, and the Soil Hydrological Cycle across Typical Ecosystems of the Heihe River Basin

Quantification of the water budget of an arid inland river ecosystem is essential but still a challenge for the sustainable development of water resources. In situ observed data were used to analyze the monthly and annual water budgets and the soil hydrological cycle for six typical ecosystems in the Heihe River Basin (HRB). The two-source model was used to partition evapotranspiration (ET) into transpiration (T) and evaporation, after which the validated model was applied to quantitatively analyze the biological water use fraction [T/Ecosystem Water Supply (WS)] for different ecosystems. There were differences in the water budgets of the different ecosystems due to differences in climate, vegetation, soil, and external inputs. Precipitation in the HRB decreased from upstream to downstream, whereas there was a gradual increase in ET. External sources of water (e.g., natural runoff from upstream, irrigation in the middle reaches, and groundwater recharge in the lower reaches) to soil layers played an important role in regulating the water budgets of HRB ecosystems. Cropland obtained the maximum biological water use fraction (0.50), followed by Populus euphratica (0.49), alpine meadow (0.49), alpine swamp meadow (0.44), Tamarix ramosissima (0.42), and Kalidium foliatum (0.4). The soil water residence time (at a depth of 40 cm) varied from 14 d to 97 d (average of 60 d). The order of plant species in terms of soil water residence time was: K. foliatum (88 d) &gt; T. ramosissima (72 d) &gt; alpine meadow (68 d) &gt; alpine swamp meadow (63 d) &gt; cropland (53 d) &gt; P. euphratica forest (20 d). Differences in the biological water use fraction and soil water residence time could be attributed to the characteristics of the water budget for each ecosystem. This study quantified the water budget, biological water use, and soil hydrological cycle across typical ecosystems in HRB, and can act as a reference for ecosystem management of the arid inland river basin.

Bacterioplankton seasonality in deep high-mountain lakes.

Due to global warming, shorter ice cover duration might drastically affect the ecology of lakes currently undergoing seasonal surface freezing. High-mountain lakes show snow-rich ice covers that determine contrasting conditions between ice-off and ice-on periods. We characterized the bacterioplankton seasonality in a deep high-mountain lake ice-covered for half a year. The lake shows a rich core bacterioplankton community consisting of three components: (i) an assemblage stable throughout the year, dominated by Actinobacteria, resistant to all environmental conditions; (ii) an ice-on-resilient assemblage dominating during the ice-covered period, which is more diverse than the other components and includes a high abundance of Verrucomicrobia; the deep hypolimnion constitutes a refuge for many of the typical under-ice taxa, many of which recover quickly during autumn mixing; and (iii) an ice-off-resilient assemblage, which members peak in summer in epilimnetic waters when the rest decline, characterized by a dominance of Flavobacterium, and Limnohabitans. The rich core community and low random elements compared to other relatively small cold lakes can be attributed to its simple hydrological network in a poorly-vegetated catchment, the long water-residence time (ca. 4 years), and the long ice-cover duration; features common to many headwater deep high-mountain lakes.

Modelling the removal of nitrogen and sediment by a constructed wetland system in north Queensland, Australia

This study of a multi-component 2.5 ha constructed wetland system near Mackay, north Queensland was established to determine the efficacy of these types of wetlands for improving water quality entering the Great Barrier Reef lagoon. To do this a water balance model was developed using water depth measurements made in the wetland inlet basin, between 25 December 2018 and 30 September 2020. The water balance model allowed daily values of run-in and drainage to be calculated, which were combined with a nitrogen balance model to derive a value for the amount of total nitrogen (TN) removed by the wetland of 83 kg ha−1 year−1, or 37% of the nitrogen input to the wetland. The nitrogen balance model used a mean inflow TN concentration of 5060 (± 1734) μg L−1 derived from TN concentration measurements (automatic and grab) made during four high flow events. The largest losses of nitrogen where via drainage with 49% leaving as outflow and 14% as groundwater seepage. Outflow carried 38% of the sediment and particulate nitrogen (PN) out of the wetland, which means that 62% settled to the bottom of the wetland. Dissolved nitrogen (DN) removal in Bakers Creek is mainly (89%) due to the relatively long residence time of water in its biodiversity pond component, the largest and deepest part of this wetland system which always contained water, even during the dry season. This illustrates the value of large, permanent deep pools in a wetland system, whether they are constructed or natural. Wetlands constructed downstream of areas generating sediments and nutrients should therefore be able to improve the quality of water entering the Great Barrier Reef lagoon. However, to do this they need to retain water for significant amounts of time, which they may do in comparatively low rainfall areas. In higher rainfall areas open, flow through wetlands may not be able to do this, so wetlands constructed in these regions need to be designed to retain water in order to have significant N removal capacity. Proper monitoring of future constructed wetlands is vital in order to derive robust design criteria for systems that can remove significant nitrogen from agricultural run-off water.