Functioning Of Aquatic Ecosystems Research Articles

Compound hot temperature and high chlorophyll extreme events in global lakes

An emerging concern for lake ecosystems is the occurrence of compound extreme events i.e. situations where multiple within-lake extremes occur simultaneously. Of particular concern are the co-occurrence of lake heatwaves (anomalously warm temperatures) and high chlorophyll-a extremes, two important variables that influence the functioning of aquatic ecosystems. Here, using satellite observations, we provide the first assessment of univariate and compound extreme events in lakes worldwide. Our analysis suggests that the intensity of lake heatwaves and high chlorophyll-a extremes differ across lakes and are influenced primarily by the annual range in surface water temperature and chlorophyll-a concentrations. The intensity of lake heatwaves is even greater in smaller lakes and in those that are shallow and experience cooler average temperatures. Our analysis also suggests that, in most of the studied lakes, compound extremes occur more often than would be assumed from the product of their independent probabilities. We anticipate compound extreme events to have more severe impacts on lake ecosystems than those previously reported due to the occurrence of univariate extremes.

Invasive forest pathogens affect the characteristics, microbial colonisation, and decomposition of leaf litter in streams

Abstract Invasive tree pathogens threaten forests worldwide, but their effects on streams are poorly understood. Nevertheless, tree infections that lead to changes in the characteristics of litter inputs to streams may affect stream communities and ecosystem processes. We studied cross‐ecosystem effects derived from Phytophthora cinnamomi, Phytophthora ×alni, and Ophiostoma novo‐ulmi infection on Castanea sativa (chestnut), Alnus lusitanica (alder), and Ulmus minor (elm) trees, respectively, by assessing physical and chemical characteristics of senescent leaves from healthy, symptomatic, and highly symptomatic individuals. Leaf litter from the three health statuses per tree species was then incubated in laboratory microcosms and the effects of tree infection on microbial decomposers and leaf litter decomposition were assessed. Tree infection significantly affected leaf litter characteristics, microbial decomposers and leaf litter decomposition, and the health status of trees conditioned these effects differently depending on the tree species. In C. sativa, leaf litter of highly symptomatic trees had higher toughness, higher polyphenolic concentration and slower decomposition than leaf litter of symptomatic and healthy trees. In A. lusitanica, leaf litter of highly symptomatic trees had higher phosphorus concentration, lower carbon:phosphorus ratio and faster decomposition than leaf litter of symptomatic and healthy trees. Finally, in U. minor, leaf litter of highly symptomatic trees had higher nitrogen concentration and lower carbon:nitrogen ratio than leaf litter of healthy trees, and faster decomposition than leaf litter of symptomatic and healthy trees. Effects of changes in litter characteristics on litter decomposition were mediated by changes in microbial decomposer colonisation and activity. The composition of the aquatic hyphomycetes communities associated with C. sativa and U. minor litter varied depending on the tree health status. Most striking was the two‐fold higher aquatic hyphomycetes species richness in litter of U. minor trees infected with O. novo‐ulmi than in litter of healthy U. minor trees. Tree infection alters the nutritional quality of leaf litter, potentially affecting the functioning of aquatic ecosystems strongly dependent on riparian litter inputs.

Seasonal and temporal variation in the effects of forest thinning on headwater stream benthic organisms in coastal British Columbia

Removal of riparian trees can alter aquatic ecosystem structure and function by influencing factors such as light availability, sediment input, and stream temperature. Contemporary forest practices such as variable retention harvest are used to mitigate the effects of clear-cut forest harvest on stream communities, but few studies have examined the effects of these techniques on aquatic ecosystems. We examined the effects of variable retention harvest on light, temperature, biofilm biomass and macroinvertebrate consumers in three coastal headwater streams in British Columbia, Canada and compared them to three nearby reference streams with unlogged riparian stands. Variable retention harvest increased light and stream temperature variability. Harvested catchments had higher stream biofilm biomass in all seasons except winter and higher invertebrate abundance in summer. Variable retention harvest altered invertebrate community composition, largely driven by increasing Chironomidae abundance and decreasing Simuliidae abundance. In conclusion, we found that variable retention harvest modified stream benthic communities, but responses varied seasonally and among taxa. This is one of few studies to investigate the impacts of variable retention harvest on multiple trophic levels over multiple seasons and years. Understanding the cascading effects of forest harvest over multi-year time scales is important for management decisions.

The Relationship between the Community Structure and Function of Bacterioplankton and the Environmental Response in Qingcaosha Reservoir

Phytoplankton and bacterioplankton play a vital role in the structure and function of aquatic ecosystems, and their activity is closely linked to water eutrophication. However, few researchers have considered the temporal and spatial succession of phytoplankton and bacterioplankton, and their responses to environmental factors. The temporal and spatial succession of bacterioplankton and their ecological interaction with phytoplankton and water quality were analyzed using 16S rDNA high-throughput sequencing for their identification, and the functions of bacterioplankton were predicted. The results showed that the dominant classes of bacterioplankton in the Qingcaosha Reservoir were Gammaproteobacteria, Alphaproteobacteria, Actinomycetes, Acidimicrobiia, and Cyanobacteria. In addition, the Shannon diversity indexes were compared, and the results showed significant temporal differences based on monthly averaged value, although no significant spatial difference. The community structure was found to be mainly influenced by phytoplankton density and biomass, dissolved oxygen, and electrical conductivity. The presence of Pseudomonas and Legionella was positively correlated with that of Pseudanabaena sp., and Sphingomonas and Paragonimus with Melosira granulata. On the contrary, the presence of Planctomycetes was negatively correlated with Melosira granulata, as was Deinococcus-Thermus with Cyclotella sp. The relative abundance of denitrifying bacteria decreased from April to December, while the abundance of nitrogen-fixing bacteria increased. This study provides a scientific basis for understanding the ecological interactions between bacteria, algae, and water quality in reservoir ecosystems.

Ditch the low flow: Agricultural impacts on flow regimes and consequences for aquatic ecosystem functions

AbstractLarge‐scale, intensive agriculture is a critical activity supporting global food production, yet it has taken a significant toll on the equally critical ecosystem services supplied by global biodiversity. This is particularly true for the planet's most threatened ecosystems: freshwaters. As one of the world's largest agricultural producers, Canada is also home to much of the world's freshwater. As Canada's agricultural capacity expands under climate warming into more northerly latitudes—and in some cases regions with large carbon sinks—it is imperative that this sectoral shift is accompanied by careful management to avoid exacerbating ecosystem service losses. Across Canada, agricultural practices vary in terms of their impact on freshwater ecosystems. Agricultural water extraction, storage behind dams, diversions, dredging and clearing of riparian vegetation can impact more naturalized flow regimes. This review explores the influence of managed low flows on ecosystem functioning in man‐made drainage/irrigation ditch systems. We examine how low flows in these systems can impact ecosystem functions in agricultural watersheds with fragmented natural capital. We provide management options to protect ecosystem functions under a changing climate, recognizing that in agro‐ecosystems, drainage/irrigation ditch systems provide a critical remnant habitat to support biodiversity in otherwise depauperate landscapes.

Nutrient Removal in Eutrophic Water Promotes Stability of Planktonic Bacterial and Protist Communities.

Nutrient (nitrogen and phosphorus) removal by using bioremediation technologies in eutrophic water alters bacterial and protist community structure and function, but how it changes the stability of community remains unclear. To fill this gap, in this study, bacterial and protist communities were investigated using 16S and 18S rRNA gene high-throughput sequencing during the nutrient removal by using ecological floating beds of Canna indica L. Our results showed that both bacterial and protist community compositions in the treatment group were similar to those in the control group at the beginning of the experiment (day 1 to day 11), but then bacterial and protist community compositions became more stable with the removal of nutrients in the treatment group than those in the control group (day 12 to day 18). We further explored the mechanisms for this increased stability and found that the contribution of the stochastic process to bacterial and protist community variations was higher in the control group than that in the treatment group. This suggests that the high nutrient concentration in the control group might increase the random colonization or extinction, and therefore resulted in the high temporal variability (i.e., unstable) of bacterial and protist communities. Our findings suggest that bioremediation for eutrophication can promote the stability of aquatic communities, and therefore potentially maintain aquatic ecosystem functions and services to humanity.

Co-occurrence of planktonic bacteria and archaea affects their biogeographic patterns in China\u2019s coastal wetlands

Planktonic bacteria and archaea play a key role in maintaining ecological functions in aquatic ecosystems; however, their biogeographic patterns and underlying mechanisms have not been well known in coastal wetlands including multiple types and at a large space scale. Therefore, planktonic bacteria and archaea and related environmental factors were investigated in twenty-one wetlands along China’s coast to understand the above concerns. The results indicated that planktonic bacteria had different biogeographic pattern from planktonic archaea, and both patterns were not dependent on the wetland's types. Deterministic selection shapes the former’s community structure, whereas stochastic processes regulate the latter’s, being consistent with the fact that planktonic archaea have a larger niche breadth than planktonic bacteria. Planktonic bacteria and archaea co-occur, and their co-occurrence rather than salinity is more important in shaping their community structure although salinity is found to be a main environmental deterministic factor in the coastal wetland waters. This study highlights the role of planktonic bacteria-archaea co-occurrence on their biogeographic patterns, and thus provides a new insight into studying underlying mechanisms of microbial biogeography in coastal wetlands.

Plant invasion affects vegetation structure and sediment nitrogen stocks in subtropical mangroves

Plant invasion can primarily affect the structure and functioning of terrestrial and aquatic ecosystems. Although there is evidence that plant invasion can modify organic matter dynamics in mangroves, it is uncertain whether and to which extent these changes can affect carbon (C) and nitrogen (N) dynamics in the sediment-plant system. Here, we measured: (i) the structure of native vegetation and C and N in the sediment-plant system in subtropical mangroves subjected to aquatic macrophytes invasion in southeastern Brazil. We answered the following questions: i) Do invaded mangroves differ in aboveground biomass compared to non-invaded mangroves?; ii) Are there C4 macrophytes in these sites? iii) What are the C and N stocks in sediment of invaded mangroves? We quantified C and N concentrations and the isotopic signature of such elements (δ13C and δ15N) in the sediment-plant system, the C and N stocks in the sediment (0–20 cm depth), and mangrove aboveground biomass. Mangrove aboveground biomass was lower at invaded compared to non-invaded sites reflecting the species displacement in invaded sites. The sediment at invaded mangroves did not significantly contribute to C4 sources because of the large predominance of both mangrove and invasive C3 plants. While sediment C stocks were similar among study sites (∼47 Mg ha−1), N stocks were lower at invaded (2.7 Mg ha−1) comparing to non-invaded (3.2 Mg ha−1) mangroves. The lower N stocks at invaded sites can reflect the higher leaf N concentrations and lower C:N ratios of invasive plants compared to mangroves. Thus, the effects of macrophytes invasion in subtropical mangroves are more apparent for vegetation structure and N stocks. C stocks alteration is expected the be detectable in the future.

Dissolved organic compounds with synchronous dynamics share chemical properties and origin

AbstractThe temporal dynamics of dissolved organic matter (DOM) are inherently linked with the functioning of aquatic ecosystems. Because DOM represents a complex mixture of millions of different compounds, the statistical analysis of DOM dynamics poses a huge challenge. Here, we present a statistical approach based on hierarchical clustering of time series that groups DOM compounds with synchronous dynamics. We applied this approach to time series of Fourier‐transform ion cyclotron resonance mass spectrometry data of DOM sampled over a period of 26 months near Helgoland, an island in the Southern North Sea. We identified three DOM clusters, which represented a total of 1392 different molecular formulae and showed distinct chemical properties and noticeably compound matches within the PubChem database. Correlations of the three DOM clusters with abundance data of prokaryote and phytoplankton species and with environmental parameters provided consistent indications on the potential origin of the clustered compounds. The first cluster integrated terrestrial DOM originating from riverine discharge reaching Helgoland waters. The second cluster was attributed to DOM related to phytoplankton and microbial activity, whereas the third cluster was interpreted as representing the marine refractory DOM background. Accordingly, while further partitioning divided each of the first two clusters into five sub‐clusters with distinct temporal dynamics and molecular characteristics, the third cluster persisted as a stable feature. Applying a purely mathematical approach, we thus confirmed the differential dynamics of individual DOM compounds and compound groups and showed that temporal dynamics of dissolved molecules are linked to their origin and transformation history.

Aquatic Eddy Covariance: The Method and Its Contributions to Defining Oxygen and Carbon Fluxes in Marine Environments

Aquatic eddy covariance (AEC) is increasingly being used to study benthic oxygen (O2) flux dynamics, organic carbon cycling, and ecosystem health in marine and freshwater environments. Because it is a noninvasive technique, has a high temporal resolution (∼15 min), and integrates over a large area of the seafloor (typically 10-100 m2), it has provided new insights on the functioning of aquatic ecosystems under naturally varying in situ conditions and has given us more accurate assessments of their metabolism. In this review, we summarize biogeochemical, ecological, and biological insightsgained from AEC studies of marine ecosystems. A general finding for all substrates is that benthic O2 exchange is far more dynamic than earlier recognized, and thus accurate mean values can only be obtained from measurements that integrate over all timescales that affect the local O2 exchange. Finally, we highlight new developments of the technique, including measurements of air-water gas exchange and long-term deployments.

An integrated approach for chemical water quality assessment of an urban river stretch through Effect-Based Methods and emerging pollutants analysis with a focus on genotoxicity

The impact of emerging chemical pollutants, on both status and functionality of aquatic ecosystems is worldwide recognized as a relevant issue of concern that should be assessed and managed by researchers, policymakers, and all relevant stakeholders. In Europe, the Reach Regulation has registered more than 100.000 chemical substances daily released in the environment. Furthermore, the effects related to the mixture of substances present in aquatic ecosystems may not be predictable on the basis of chemical analyses alone. This evidence, coupled with the dramatic effects of climate changes on water resources through water scarcity and flooding, makes urgent the application of innovative, fast and reliable monitoring methods. In this context, Effect-Based Methods (EBMs) have been applied in the urban stretch of the Tiber River (Central Italy) with the aim of understanding if detrimental pressures affect aquatic environmental health. In particular, different eco-genotoxicological assays have been used in order to detect genotoxic activity of chemicals present in the river, concurrently characterized by chemical analysis. Teratogenicity and embryo-toxicity have been studied in order to cover additional endpoints. The EBMs have highlighted the presence of diffuse chemical pollution and ecotoxicological effects in the three sampling stations, genotoxicological effects have been also detected through the use of different tests and organisms. The chemical analyses confirmed that in the aquatic ecosystems there is a diffuse presence, even at low concentrations, of emerging contaminants such as pharmaceuticals, not routinely monitored pesticides, personal care products, PFAS. The results of this study can help to identify an appropriate battery of EBMs for future studies and the application of more appropriate measures in order to monitor, mitigate or eliminate chemical contamination and remediate its adverse/detrimental effects on the ecosystem health.

Livestock as vectors of organic matter and nutrient loading in aquatic ecosystems in African savannas.

Populations of large wildlife have declined in many landscapes around the world, and have been replaced or displaced by livestock. The consequences of these changes on the transfer of organic matter (OM) and nutrients from terrestrial to aquatic ecosystems are not well understood. We used behavioural data, excretion and egestion rates and C: N: P stoichiometry of dung and urine of zebu cattle, to develop a metabolism-based estimate of loading rates of OM (dung), C, N and P into the Mara River, Kenya. We also directly measured the deposition of OM and urine by cattle into the river during watering. Per head, zebu cattle excrete and/or egest 25.6 g dry matter (DM, 99.6 g wet mass; metabolism) - 27.7 g DM (direct input) of OM, 16.0–21.8 g C, 5.9–9.6 g N, and 0.3–0.5 g P per day into the river. To replace loading rates OM of an individual hippopotamus by cattle, around 100 individuals will be needed, but much less for different elements. In parts of the investigated sub-catchments loading rates by cattle were equivalent to or higher than that of the hippopotamus. The patterns of increased suspended materials and nutrients as a result of livestock activity fit into historical findings on nutrients concentrations, dissolved organic carbon and other variables in agricultural and livestock areas in the Mara River basin. Changing these patterns of carbon and nutrient transport and cycling are having significant effects on the structure and functioning of both terrestrial and aquatic ecosystems.

Functional groups in piscivorous fishes.

Piscivory is a key ecological function in aquatic ecosystems, mediating energy flow within trophic networks. However, our understanding of the nature of piscivory is limited; we currently lack an empirical assessment of the dynamics of prey capture and how this differs between piscivores. We therefore conducted aquarium‐based performance experiments, to test the feeding abilities of 19 piscivorous fish species. We quantified their feeding morphology, striking, capturing, and processing behavior. We identify two major functional groups: grabbers and engulfers. Grabbers are characterized by horizontal, long‐distance strikes, capturing their prey tailfirst and subsequently processing their prey using their oral jaw teeth. Engulfers strike from short distances, from high angles above or below their prey, engulfing their prey and swallowing their prey whole. Based on a meta‐analysis of 2,209 published in situ predator–prey relationships in marine and freshwater aquatic environments, we show resource partitioning between grabbers and engulfers. Our results provide a functional classification for piscivorous fishes delineating patterns, which transcend habitats, that may help explain size structures in fish communities.

Vertical Migration of Adult Plecoptera and Trichoptera above Forested Headwater Streams.

Simple SummaryStream insects are essential components of aquatic and terrestrial ecosystems, but fewer studies have examined the terrestrial stage compared to the aquatic stage. Most adult stream insects are flight capable, which allows for short- and long-distance movements away from the stream channel into riparian or upland habitats. This study examined if adult stream insects migrate vertically into riparian forest canopies above the stream. We found a meaningful number of adult Trichoptera and Plecoptera in the forest canopy above the stream that was comparable to horizontal migrations. This result demonstrated that adult stream insects utilize riparian forest canopies and that numerous avenues for additional basic and applied research on terrestrial–aquatic linkages exist. Discovering how riparian canopy habitats are important for stream insects and how stream insects are linked to canopy ecosystems can inform restoration and conservation actions.Stream insects are essential components of aquatic and terrestrial ecosystem structure and function. Terrestrial stages are important components of terrestrial food webs, and flight-capable individuals are responsible for long-distance dispersal. Horizontal migrations by flying or crawling adults away from stream channels that link insects to riparian food webs and movements across catchment boundaries are well established through empirical research, but studies examining vertical migration of adult stream insects into forest canopies are generally lacking. This study focused on differences in adult Plecoptera and Trichoptera abundance at ground level versus the riparian canopy and differences in abundances among summer and autumn sampling periods to empirically demonstrate use of canopy ecosystems by stream insects. Malaise traps at ground level and canopy traps placed 8 to 10 m above the stream at four sites in the Mosquito Creek watershed (Pennsylvania) were used to examine vertical migration. Larval assemblages were collected and compared to adult assemblage to investigate patterns of local migration in the catchment. We found significantly more stream insects at ground level than in the forest canopy for Trichoptera, Plecoptera, and all individual plecopteran families, but a meaningful number of individuals were found in the riparian canopy. Canopy abundances were similar to abundances captured in adjacent ground-level habitats in other studies. Comparisons of adult and larval abundances among sites, taxa, and stages indicated site- and taxon-specific patterns for vertical movement into riparian canopies. Demonstrating that adult stream insects utilize riparian forest canopies indicates that riparian forest conservation should be prioritized over reforestation and that several potential research questions exist to inform riparian management.

Capturing the spatial variability of algal bloom development in a shallow temperate lake

Abstract Algal blooms can have profound effects on the structure and function of aquatic ecosystems and have the potential to interrupt valuable ecosystem services. Despite the potential ecological and economic consequences of algal blooms, the spatial dynamics of bloom development in spatially complex ecosystems such as shallow lakes remain poorly characterised. Our goal was to evaluate the magnitude and drivers of spatial variability of algal biomass, dissolved oxygen, and pH over the course of a season, in a shallow lake in order to better understand the spatial dynamics of algal blooms in these ecosystems. We sampled 98 locations in a small eutrophic lake on a 65‐m grid for several parameters (chlorophyll a, phycocyanin, dissolved oxygen, pH, and temperature), weekly over 122 days. This was done to estimate the dynamics of variability and spatial autocorrelation during the course of multiple bloom events. We also compared the spatial measurements to a high frequency sensor deployed at a fixed station and estimated the optimal spatial sampling resolution by performing a rarefaction analysis. Spatial heterogeneity of algal pigments was high, particularly during bloom events, and this pattern and the overall severity of the bloom were not well captured with the fixed station monitoring. The pattern of algal pigments and other limnologically important variables (dissolved oxygen and pH) was related to the direction of prevailing winds 24 hr prior to sampling, the shallow northern basin where the main surface inlet is located, and heavy precipitation. Additionally, a dense bed of floating‐leaf macrophytes contributed to local patchiness in all variables. Finally, from the rarefaction analysis we found that minimal information about the mean state of the ecosystem was gained after c. 30 locations had been sampled. This study revealed how spatially heterogeneous shallow lakes are over the course of a single season, and that the magnitude of variability was highest during biologically intensive periods such as algal blooms. As such, continued research is needed across a range of trophic conditions to better understand the structure of horizontal variability in lakes. Overall, these data demonstrate the need for spatially explicit monitoring to better understand the dynamics and drivers of algal blooms in shallow lakes and to better manage ecosystem services.

Merging empirical and mechanistic approaches to modeling aquatic visual foraging using a generalizable visual reaction distance model

Visual encounter distance models are important tools for predicting how light and water clarity mediate visual predator-prey interactions that affect the structure and function of aquatic ecosystems at multiple spatial, temporal, and organizational scales. The two main varieties of visual encounter distance models, mechanistic and empirical, are used for similar purposes but take fundamentally different approaches to model development and have different strengths and weaknesses in terms of predictive accuracy, physical and biological interpretability of parameters, ability to incorporate outside information, and utility for knowledge transfer. To overcome weaknesses of existing mechanistic and empirical models and bridge the gap between approaches, we developed a generalized visual reaction distance model that relaxes assumptions of a widely-used mechanistic model that are violated in real predator-prey interactions. We compared the performance of the generalized visual reaction distance model to a widely used mechanistic model and an empirical visual encounter distance model by fitting models to data from four predator-prey experiments. The generalized visual reaction distance model substantially outperformed the other models in all cases based on fit to reaction distance data and presents an attractive alternative to prior models based on comparatively high predictive accuracy, use of interpretable parameters, and ability to incorporate outside information—characteristics that facilitate knowledge transfer.

Establishing the relationship between benthic macroinvertebrates and water level fluctuation in subtropical shallow wetlands.

Wetland water level fluctuations often influence benthic macroinvertebrate communities through changes in water quality, substrate, and macrophytes and, hence, affect the structure and function of aquatic ecosystems. However, there is lack of understanding on how water level fluctuations affect the structure and composition of benthic macroinvertebrates in subtropical shallow wetlands in Nepal. Here, we assessed the changes in benthic macroinvertebrate community composition in response to water level fluctuations and identified indicator taxa sensitive to such fluctuations. A study was conducted over 4 seasons covering one annual cycle of water level fluctuation in 4 wetlands of Koshi Tappu Wildlife Reserve, Nepal. The study revealed that benthic macroinvertebrate composition significantly differed across water levels. Dissimilarities in macroinvertebrate community composition were mainly attributed by families Atyidae, Dytiscidae, Baetidae, Planorbidae, Chironomidae, Bithyniidae, Sphaeriidae, and Thiaridae. Taxon specific richness, densities, and biomass varied across the water levels while no difference was documented for overall family richness, density, and biomass. Ephemeroptera and Trichoptera richness decreasedwhen water levels were low while Coleoptera and Diptera richness increased. Medium water level supported high benthic macroinvertebrate diversity. Indicator taxa analysis identified Coleoptera: Hydrophilidae and Dytiscidae, Hemiptera: Pleidae, Diptera: Muscidae and Mollusca: Sphaeriidae, Viviparidae, and Thiaridae, as indicators of low water level. Similarly, Coleoptera: Scirtidae, Hemiptera: Micronectidae and Oligochaeta: Tubificidae as indicators of medium water level, and Trichoptera: Polycentropodidae and Ephemeroptera: Caenidae as indicators of high water level. Redundancy analysis identified water level as one of the most influencing factors in benthic macroinvertebrate community variation. Considering the significant response of benthic macroinvertebrates to water level fluctuations, theyare important as ecological indicators in research aimed at developing environmental flow frameworks. Indicator speciesare likely to be a vital tool in environmental impact assessment and monitoring in relation to hydrological development. The outcomesof this research have important implications to conservation and management of wetlands to preserve the valuable ecosystem functions provided by wetlands.

Contrasting stream water temperature responses to global change in the Mid-Atlantic Region of the United States: A process-based modeling study

The accurate estimation of stream water temperature is essential for understanding environmental controls on the structure and functioning of aquatic ecosystems. Few studies have coupled soil and stream water temperatures to capture the synergy of thermal balances between terrestrial and riverine systems. As a result, little is known about how multiple environmental stresses have affected water temperature, particularly for different orders of streams. Here we incorporated a new water transport scheme into the Dynamic Land Ecosystem Model (DLEM) to predict water temperature in 1st order and higher-order streams (>1st order). Driven by a 4-km geo-referenced dataset of multiple environmental factors, our new water temperature model was utilized to predict the spatiotemporal variations of water temperature in the U.S. Mid-Atlantic Region during 1900–2015. Results revealed that water temperature during 1970–2015 increased significantly (p < 0.05), and the rate of increase of the 1st order streams 0.32 °C∙decade−1 is higher than that of higher-order streams 0.28 °C ∙ decade −1. The buffering effect of groundwater on water temperature in 1st order streams diminished under the context of climate warming. Factorial analysis showed that climate change and variability explain most of the changes (~80%) in stream water temperature since 1900. Land-use conversions (mostly from cropland to forest), CO2 fertilization, and land nitrogen management collectively explained a greater percent of change in water temperature in 1st order streams (24%) than higher-order streams (9%), implying that 1st order streams are particularly vulnerable to human activities.

Combined effects of warming and nutrient enrichment on water properties, growth, reproductive strategies and nutrient stoichiometry of Potamogeton crispus

Global warming and eutrophication are two important environmental issues facing freshwater ecosystems. Especially in winter and spring, freshwater ecosystems lacking large submerged plants are the most vulnerable. However, there is a paucity of research on the combined effects of global warming and eutrophication on the water quality of freshwater ecosystems and the growth characteristics of submerged plants in winter and spring. We therefore performed a mesocosm experiment examining the combined effects of nutrient (nitrogen and phosphorus) enrichment and warming (2.65 ± 0.07℃) on the submerged plant Potamogeton crispus. We found that warming affected the morphological characteristics of P. crispus, promoted plant height, accumulated more biomass, augmented the proportion of above-ground biomass and decreased the nitrogen content of the above-ground organs. Meanwhile, warming increased more resources for asexual reproduction (turions) and reduced the number of flowers. The addition of nitrogen or phosphorus alone could significantly augment the content of nitrogen or phosphorus in vegetative organs, respectively. In addition, the effects of different nutrient enrichment on the reproduction strategy of P. crispus were different. The addition of nitrogen reduced the number of flowers, while the addition of phosphorus augmented the number of flowers and turions. In contrast, the warming and co-addition of nitrogen and phosphorus treatments had a more negative effect on P. crispus, reducing biomass, maximum fluorescence and the number of ramets, flowers and turions. Although the leaf biomass allocation increased, the turion biomass allocation decreased. This may be due to the increase in algae, turbidity and total suspended solids caused by warming and the co-addition of nitrogen and phosphorus, which stressed the growth of P. crispus. Our results suggest that global warming and eutrophication can affect the structure and function of aquatic ecosystems by inhibiting the growth of submerged plants, which may lead to further fragile stability of freshwater ecosystems in winter and spring.

Changes in the concentration of sulfate ions in the Volga water (section of the Zhigulevsky hydroelectric complex)

In water bodies, sulfate ions play an important role in the sulfur cycle and affect the state and functioning of aquatic ecosystems. In the Volga water, sulfates rank second among the main anions in terms of their contribution to water mineralization. The concentration of sulfates does not remain constant, but changes continuously throughout the year. For a quantitative assessment of changes in the concentration of sulfates on the Volga River in the section of the Zhigulevsky hydroelectric complex, an analysis of long-term data of hydrological and hydrochemical observations was carried out. For the period 2001-2018 the average annual river flow was 7,7 thousand m3/s. The highest flow rate was observed in 2005 and amounted to 9,0 thousand m3/s, and the lowest in 2011 – 6,2 thousand m3/s. The average annual concentration of sulfates was 55 mg/dm3, the highest – 64 mg/dm3, and the lowest – 45 mg/dm3. The sulfate content was characterized by significant seasonal variability. During the winter low-water period, the highest concentrations of sulfates were observed with a maximum in April, before the beginning of the spring flood (67 mg/dm3). During the flood, the content of sulfates decreased, reaching the lowest values during the summer low-water period in August (44 mg/dm3), and then, from September, the concentration of sulfates gradually increased to 57 mg/dm3 at the beginning of the winter low-water period. Over the 18-year observation period, the amplitude of sulfate concentrations was 34-87 mg/dm3. Seasonal changes in the content of sulfates in the section of the Zhigulevskaya hydroelectric complex are mainly due to the change in water masses formed on the overlying reservoirs as a result of the interaction of the surface and underground components of the water runoff. In dry years, the content of sulfates increases, and in high water, it decreases. The difference in the content of sulfates in extremely high-water and low-water years can reach 15-20 mg/dm3.