Eastern Lake Research Articles

Humification levels of dissolved organic matter in the eastern plain lakes of China based on long-term satellite observations

Under the influence of intensive human activities and global climate change, the sources and compositions of dissolved organic matter (DOM) in the eastern plain lake (EPL) region in China have fluctuated sharply. It has been successfully proven that the humification index (HIX), which can be derived from three-dimensional excitation-emission matrix fluorescence spectroscopy, can be an effective proxy for the sources and compositions of DOM. Therefore, combined with remote sensing technology, the sources and compositions of DOM can be tracked on a large scale by associating the HIX with optically active components. Here, we proposed a novel HIX remote sensing retrieval (IRHIX) model suitable for Landsat series sensors based on the comprehensive analysis of the covariation mechanism between HIX and optically active components in different water types. The validation results showed that the model runs well on the independent validation dataset and the satellite-ground synchronous sampling dataset, with an uncertainty ranging from 30.85 % to 36.92 % (average ± standard deviation = 33.6 % ± 3.07 %). The image-derived HIX revealed substantial spatiotemporal variations in the sources and compositions of DOM in 474 lakes in the EPL during 1986–2021. Subsequently, we obtained three long-term change modes of the HIX trend, namely, significant decline, gentle change, and significant rise, accounting for 74.68 %, 17.09 %, and 8.23 % of the lake number, respectively. The driving factor analysis showed that human activities had the most extensive influence on the DOM humification level. In addition, we also found that the HIX increased slightly with increasing lake area (R2 = 0.07, P < 0.05) or significantly with decreasing trophic state (R2 = 0.83, P < 0.05). Our results provide a new exploration for the effective acquisition of long-term dynamic information about the sources and compositions of DOM in inland lakes and provide important support for lake water quality management and restoration.

Effects of nitrogen on benthic diatom assemblages in high-elevation central and eastern alpine lakes

ABSTRACT We explored patterns of benthic diatom composition across sixty-two high-elevation alpine lakes spanning a wide range of nitrogen (N) concentrations due to atmospheric deposition and background variation in lake and watershed characteristics. Our goals were to (1) assess the effect of lake water N concentration on benthic diatom composition during late summer or fall conditions and (2) identify policy-relevant response thresholds. The analyses were carried out on a large set of diatom and water chemistry data, integrated with new data. Multivariate and correlation analyses revealed associations between pH, N concentration, and benthic diatom composition, but the effects of pH and N were confounded. However, partial correlation analysis allowed us to identify “N-responsive diatoms”; that is, diatom taxa with nonspurious associations with N. Focusing on these taxa, we detected a decline in the abundance of taxa preferring low N concentrations and an increase in the abundance of taxa preferring high N concentrations starting at NO3 concentrations of approximately 5 µmol L−1. We interpreted this shift as an effect of watershed N saturation due to atmospheric deposition. Based on the results, we suggest a late-summer or fall concentration threshold of 5 µmol NO3 L−1 to prevent change in benthic diatoms in high-elevation alpine lakes.

Dynamics of dissolved organic phosphorus in the nearshore of eastern Lake Erie

Abstract The east basin of Lake Erie continues to suffer from blooms of filamentous green algae (primarily Cladophora). Potential management action through reduced phosphorus loadings have and continue to focus on the dissolved inorganic phosphorus pool but other potentially bioavailable phosphorus pools are not always considered. In this study, we describe the dissolved organic phosphorus pool in eastern Lake Erie, with an extensive sampling of four transects along the north shore of the eastern basin from May to September 2019. The dissolved organic phosphorus pool was characterized and quantified using sequential enzymatic hydrolysis to provide information on the enzymatically hydrolysable phosphorus fraction and component monoester P, diester P and a phytase hydrolysable component. These dissolved organic phosphorus fractions were compared to the soluble reactive phosphorus and total dissolved phosphorus pools. Results from this study revealed that a significant fraction (up to 63%) of the dissolved organic phosphorus pool is potentially bioavailable via enzymatic hydrolysis. Vertical differences in soluble reactive phosphorus, total dissolved phosphorus, and phosphate diester pools were also observed when comparing water column with near lake-bed samples, suggesting that the new benthic ecosystem since dreissenid colonization may be a greater source of dissolved inorganic phosphorus to the east basin than prior to colonization.

Characteristics of Hydrocarbon-Generating Pressurization in Shale Series of Fengcheng Formation in the West and South of Mahu Sag, the Junggar Basin, China

Formation overpressure is an important controlling factor for the formation of high-yield oil reservoirs in the shale series of the Permian Fengcheng Formation in the west and south of Mahu Sag, the Junggar Basin. Hydrocarbon-generating pressurization (HGP) is an important cause of overpressure in the shale series of Fengcheng Formation, but the evidence for this viewpoint is insufficient. There is still no systematic study on the quantitative calculation and distribution characteristics of HGP in the Fengcheng Formation shale series. The control effect of HGP on the formation of high-pressure and high-yield reservoirs is still unclear. Therefore, by using the data of mudstone logging and measured formation pressure, the causes of overpressure in the Fengcheng Formation shale series are clarified. The predicted organic matter abundance, the predicted maturity and thickness of source rocks, and the statistical ratios of mudstone thickness to formation thickness in each section of Fengcheng Formation are used for HGP of shale series. Combined with the physical characteristics of reservoir rocks and the geochemical characteristics of oil, the control effect of HGP on the formation of high-pressure and high-yield reservoirs is analyzed. The results indicate the following: (1) The organic matter abundance and the thickness of source rocks predicted by the logging data gradually decrease from the eastern lake basin area to the western slope area. (2) The HGP of shale series in Fengcheng Formation is related to the hydrocarbon generation capacity and the overpressure preservation conditions of the source rocks. The HGP can be quantitatively predicted by comprehensively using the organic matter abundance, the maturity, the thickness of source rocks, and the ratios of mudstone thickness to formation thickness. (3) The HGP in the Fengcheng Formation shale series also gradually decreases in distribution characteristic from the eastern lake basin area to the western slope area. (4) The oil accumulation mode of the Fengcheng Formation shale series is that, with the drive of overpressure, the oil migrates slightly within the layer, and finally accumulates to form the oil reservoir. The research results are helpful to understand the distribution characteristics of HGP and the formation mechanism of high-pressure and high-yield reservoirs in the shale series of Fengcheng Formation in the west and south of Mahu Sag, and are of great significance to guide the exploration and development of shale oil and tight oil.

Changing water levels and sandy barrier morphodynamics, eastern lake Ontario

Changing water levels and sandy barrier morphodynamics, eastern lake Ontario

Land cover and water quality of drowned river mouths: Evidence of an environmental gradient along the eastern Lake Michigan shoreline

Lake Michigan’s drowned river mouths (DRM) are hydrologically unique systems with both riverine and large-lake influences. Serving as focal points for human development, DRMs have experienced a history of industrialization, urbanization, and are now moving towards an era of restoration and revitalization. The goal of this study was to examine water quality in 12 DRMs along Lake Michigan’s eastern shoreline. We hypothesized that there is a latitudinal gradient in indicators of water quality in these DRM lakes, which is the result of natural land cover, anthropogenic land use, and underlying geology. We identified a latitudinal gradient in land use/land cover with developed land area more abundant in southern DRMs and forest more abundant in northern land area; this distinction was evident at both the local and whole watershed geographic scales but was more distinct at the local scale. Water quality followed suit with specific conductance, chlorophyll-a, and total phosphorus (TP) concentrations higher in southern DRMs and lower in northern DRMs; whereas, Secchi disk depth showed the opposite trend. Multivariate analysis results were consistent with the relationship of water quality and latitudinal gradient. Thermal stratification and low dissolved oxygen (DO) concentrations in the hypolimnion were more common in deeper (i.e., greater than 4 m) areas of DRMs; low DO in the hypolimnion was more likely in southern DRMs than in northern DRMs. These results provide a foundation for future research initiatives in helping separate anthropogenic vs. natural stress in these systems, which provide critical ecosystem services for their surrounding communities.

Microcystins risk assessment in lakes from space: Implications for SDG 6.1 evaluation

Cyanobacterial blooms release a large number of algal toxins (e.g., Microcystins, MCs) and seriously threaten the safety of drinking water sources what the SDG 6.1 pursues (to provide universal access to safe drinking water by 2030, United Nations Sustainable Development Goal). Nevertheless, algal toxins in lake water have not been routinely monitored and evaluated well and frequently so far. In this study, a total of 100 large lakes (>25 km2) in densely populated eastern China were studied, and a remote sensing scheme of human health risks from MCs based on Sentinel-3 OLCI data was developed. The spatial and temporal dynamics of MCs risk in eastern China lakes since OLCI satellite observation data (2016–2021) were first mapped. The results showed that most of the large lakes in eastern China (80 out of 100) were detected with the occurrence of a high risk of more than 1 pixel (300×300 m) at least once. Fortunately, in terms of lake areas, the frequency of high human health risks in most waters (70.93% of total lake areas) was as less as 1%. This indicates that drinking water intakes can be set in most waters from the perspective of MCs, yet the management departments are required to reduce cyanobacterial blooms. This study highlights the potential of satellite in monitoring and assessing the risk of algal toxins and ensuring drinking water safety. It is also an important reference for SDG 6.1 reporting for lakes that lack routine monitoring.

Spatial and temporal pattern of benthic macroinvertebrate assemblages in two large Chinese freshwater lakes subjected to different degrees of eutrophication

Freshwater lakes are subjected to increasing eutrophication and algal blooms worldwide, which have dramatically impacted the aquatic biodiversity. This situation is especially true in the Yangtze River Floodplain, China. However, few researches have been focused on differences in the spatial and temporal patterns of benthic macroinvertebrates assemblages between the two adjacent lakes in this floodplain. In this study, we conducted seasonal investigations in 2021 to compare the spatial and temporal patterns of macroinvertebrates in Lakes Taihu and Lake Chaohu. Non-metric multidimensional scaling (NMDS) analyses showed Lake Taihu was found to have differences in assemblages within the lake, but Lake Chaohu was found to have uniform assemblage across the entire lake. Macrophyte-dominated lake regions (eastern Lake Taihu) were characterised by the highest species richness and diversity indices than those in other lake regions. The pollution sensitive species (e.g. EPT: Ephemeroptera, Plecoptera and Trichoptera) were only distributed in the macrophytic areas of Lake Taihu. Corbicula fluminea, polychaetes Nephtys oligobranchia and Capitella sp. were dominant in the middle central lake regions in Lake Taihu. Lake Chaohu and the northern Lake Taihu, with turbid algal-dominated state, were characterised by dominance of Oligochaetes and Chironomids. These differences were attributed to varying habitats and environmental parameters among different lake regions. Canonical correspondence analysis (CCA) demonstrated that macrophytes, chemical oxygen demand (CODMn), temperature, ammonium (NH3-N) and electric conductivity (EC) were detected as important factors influencing such macroinvertebrate assemblage patterns between these two lakes. This study contributes to the scientific knowledge of within lake habitats, community assemblages, nutrients and pollution within these lakes.

Intra-lake trends and inter-lake comparisons of Mysis diluviana life history variables and their relationships to food limitation

The opossum shrimp, Mysis diluviana, is an important member of the offshore food webs of the Laurentian Great Lakes, but its response to ecosystem changes that have occurred over the past several decades is not well understood. We combined the data of four long-term sampling programs, adding several years of data (post and prior) to previously published analyses to offer a longer-term, cross-basin analysis of M. diluviana populations in the Great Lakes from 1997 to 2019. Densities were high in lakes Superior and Ontario (summer values 100–300/m2), high and variable but declining (from 200–300/m2 in 1997–2004 to less than 100/m2 in 2017–2019) in Lake Michigan, low (∼20–50/m2 since 2005) in Lake Huron, and very low in shallower eastern Lake Erie (<1/m2). Biomass showed similar trends. Life history parameters (mortality, fecundity, and growth) were consistently highest in eastern Lake Erie, followed by lakes Ontario, Michigan, Huron, and Superior. Generation time was 1 year in Lake Erie and 2 years in the other lakes. Cross-basin relationships between annual M. diluviana areal densities and food indices (chlorophyll-a concentration and zooplankton biomass) were non-linear, increasing with food levels up to about 250 mysids/m2 and about 650 mg dry wt/m2. Annual growth rates were also positively correlated to both food indices in the four deep lakes, but fecundity and mortality rates were not. Our results suggest food availability is a primary factor predicting M. diluviana density and biomass. Density-dependent mortality and fish predation could explain some of the inter-lake differences, but these relationships could benefit from further investigations.

Application of a regional climate model on autumn dust events over the Urmia Basin

Dust events have occurred over the Urmia Lake Basin in northwestern Iran for many years. In this study, we identified the mechanism and source of dust events in the region and assessed the ability of the Regional Climate Model (RegCM) to simulate these events. The observation data includes horizontal visibility and present weather phenomena achieved in Tabriz, Urmia, and Saqez stations. The ERA5, MERRA-2, and CAMS data were prepared for this research. The long-term dusty day analysis was conducted from 2000 to 2019. The synoptic-dynamic analysis and RegCM simulations were done during dust events on 28–31 October 2017—the HYSPLIT model and MODIS image usage to consider more detail. The results show that the Urmia Basin experiences two peaks, dusty days in the May and autumn months, including September and October. The persistent and widespread dust event in Urmia resulted from the approaching trough at 500 hPa to the north of Saudi Arabia and Iraq, trigged low pressure and increasing wind speed over the area. Based on the synoptic-dynamic analysis, MODIS satellite images, and the outputs of HYSPLIT, Iraq deserts were the primary source of dust on 28–31 October 2017. The dust distribution in the RegCM model, MERRA-2 data, and MODIS image agree. The RegCM aerosol optical depth (AOD) output time series agreed with MERRA-2 data, with a correlation coefficient of 0.84 and an index of agreement (IOA) of 0.56 in Urmia stations. The local dust source detected by the surface emission flux in the eastern Lake Urmia close to Tabriz can be a reason for the dustiest day in Tabriz. The AOD diurnal variation obtained from RegCM, MERRA-2, and CAM are similar in Tabriz and Urmia. The AOD of RegCM is underestimated compared to MERRA-2 and CAM in three selected stations.

Pollution and health risk assessment of heavy metals in the surface sediments of Timsah Lake, Suez Canal, Egypt

Urbanization and industrial activities around Timsah Lake have led to discharging of pollutants into the lake. The heavy metals pollution level in the lake sediments and implications for living organisms and human health were estimated using several ecological indices. The spatial variations of metals (Cd, Cu, Pb, Mn, Zn, Ni, and Fe) and organic matter content demonstrate that most concentrations increase toward the western part of the lake. The geoaccumulation index (Igeo) arranged the metals as next; Cd > Pb > Zn > Cu > Mn > Fe > Ni, where the lake sediments are extended between moderately-to-extremely contaminated by Cd. Moreover, the contamination factor (CF) for Cd and Pb indicates very high contamination for 100 % of stations and moderate contamination for 58.3 % of the stations, respectively. Spatially, the degree of contamination (Cd) classified lake sediments into three categories; moderately, considerable and very highly contaminated, distributed in the lake's eastern, middle and western parts, respectively. Regarding ecological risk, the potential ecological risk factor (Eri) arranged the metals risks as follows: Cd > Pb > Cu > Ni > Zn, where Cd concentrations suggest high and very high risk for 16.7% and 83.3 % of stations, respectively. The potential ecological risk index (RI) divided lake sediments into moderate, considerable (most of the eastern part), and very high risk (most of the western part). According to sediment quality guidelines (SQGs), Cd values indicate that there have been negative effects on the biota ranging between occasional and frequent. A dermal exposure-based health risk evaluation revealed no non-carcinogenic impact on human health, while Cd indicated a tolerable risk of cancer for children. Moreover, 58.3 and 83.3 % of the stations indicated a tolerable risk of cancer by Ni for adults and children, respectively. The study recommended necessary proactive measures to monitor and manage heavy metal pollution, especially Cd, Pb, and Ni caused by human activities in the Lake.

Genetic diversity and population structure of Vernonia amygdalina Del. in Uganda based on genome wide markers.

Determining the extent and distribution of genetic diversity is an essential component of plant breeding. In the present study, we explored the genetic diversity and population structure of Vernonia amygdalina, a fodder, vegetable and medicinal species of Africa and some parts of Yemen. Most empirical studies demonstrate that populations that are separated by geographic or ecological factors may experience genetic differentiation resulting from restricted gene flow between populations. A total of 238 individuals were sampled from two populations: i) Lake Victoria crescent (LVC) and ii) Southern and Eastern Lake Kyoga basin (SEK) agroecological zones of Uganda and genotyped using DArT platform. Of the two populations, the overall mean observed heterozygosity (Ho) was low to medium (Ho = 0.07[silicoDArTs] and 0.2[SNPs]). Inbreeding levels were also very low (-0.04 to -0.08) suggesting the presence of random mating. Partitioning of genetic structure in the two populations indicated that SEK exhibited a higher genetic diversity than LVC. The principal coordinates analysis (PCA) showed no geographical structuring, consistent with the low genetic differentiation (Fst = 0.00) and the low Euclidean genetic distance (1.38-1.39) between the LVC and SEK populations. However, STRUCTURE analysis with admixture models revealed weak possible genetic clusters with very small genetic distance among them. Overall, the results suggest low genetic diversity and weak genetic differentiation between the two populations. One possible explanation of the results could be the presence of human assisted gene flow over long distances.

MODIS observations reveal decrease in lake suspended particulate matter across China over the past two decades

Variations in the concentrations and distribution of suspended particulate matter (SPM) of lakes can be used to show the responses of lake environment to climate and landscape change. However, the shifts and trends of SPM and potential drivers have not been well investigated across large spatial and temporal dimensions. This study developed a robust machine learning model to generate SPM time series in 269 lakes across China larger than 30 km2 from 2002 to 2021 using MODIS/Aqua imagery. The support vector regression model showed satisfactory performance on SPM retrievals over four orders of magnitude (0.1–1000 mg L−1) (mean absolute percentage error = 26%). The model performance was shown to be insensitive to changes in environmental and observing conditions (e.g., aerosol type and thickness, viewing geometry), based on a radiative transfer simulation model. The long-term MODIS record showed a spatial pattern of lower SPM in the western lakes compared to the shallow lakes of east China. Importantly, the SPM showed a significant decrease in the 21st century (average rate of change of −0.2 mg L−1/decade). The interannual variations in SPM were aggregated into five categories, ranging from lakes with continuous changing patterns to those with reversed changing patterns. The driving factors behind the changing patterns vary between different climate zones and ecoregions. A warmer and wetter climate was associated with decreasing SPM in western lakes, while the decrease in wind speed and reduced possibility of soil erosion were the primary drivers of progressively lower SPM in the eastern shallow lakes. These results not only show a comprehensive picture of the SPM dynamics of lakes in China but also provide new insights into the complex mechanisms that drive SPM spatiotemporal dynamics.

Assessing ecosystem health of floodplain lakes using an Integrated Bioassessment Index

Lakes within river-floodplain ecosystems are key biodiversity refuges majorly threatened by eutrophication and hydrologic river-lake fragmentation. However, an integrative index, specifically designed to comprehensively assess the health of floodplain lakes, considering multiple biological taxa and major stressors, remains unavailable. Based on biological data collected from 68 isolated lakes in the eastern plain lake region of China, we constructed a reliable index for assessing floodplain lake ecosystem health: Integrated Bioassessment Index (L-IBAI). The L-IBAI consists of components of overall health and species diversity, eutrophication, and hydrological connectivity and water level regimes. To construct the L-IBAI, we firstly used the observed species (SO)-area (A) model to predict the expected species richness (SE), and then developed observed to expected indices by calculating the SO/SE ratio for fish (F-O/E-SA) and benthos (B-O/E-SA). The health level thresholds for the other four indices, namely the ratio of Secchi depth and water depth (ZSD/ZM), chlorophyll a in phytoplankton (Chl a), the percent of rheophile fish species, and emergent macrophytes coverage, were determined using cumulative frequency curves. The final L-IBAI score was obtained by summing the weighted values of the three components mentioned earlier. The L-IBAI can significantly discriminate rural from urban lakes, and is a reliable index for assessing lake ecosystem health. Moreover, we developed three simplified three-index integrated bioassessment indices, which consists of three indices belonging to the three elements are also effective. The developed L-IBAI is flexible to be extended and applied to other floodplain lakes, and outcomes of health assessments for the eastern plain lakes in China helped to identify conservation and restoration priorities.

Unlocking the potential of Dongting Lake-grown Miscanthus lutarioriparius biomass: A comprehensive quality analysis and bioproduct application study

Miscanthus lutarioriparius grown in Dongting Lake has an annual biomass yield potential of 1 million tons. However, with the shutdown of its previous utilization for paper-making, abandoning this huge amount of biomass has caused serious economic, ecological, and social problems. Constructing an industrial cluster to continuously convert biomass into various bioproducts is a win-win measure to address this dilemma. With the increasing confirmation of the importance of biomass quality affecting the conservation process, fully understanding the biomass characteristics of Dongting Lake-grown M. lutarioriparius is crucial for building a scientific industrial cluster. The present work is designed to explore the variation in biomass quality across the entire Dongting Lake area. Results show that the biomass contented with Cd, Mn, Zn, and Cr has significant geographical differences, with a general trend of Southern Dongting Lake-grown biomass having a higher concentration than that from Eastern and Western Dongting Lake areas. Moreover, significant differences are found in terms of biomass ash content, lignin content, and the degree of polymerization of cellulose (DP). The biomass with low ash content is generally from the entire Eastern Dongting Lake area and the northern part of the Western Dongting Lake area. Virtually all Western Dongting Lake-grown biomass has a low lignin content (approximately 18 %). Regarding the spatial variation of DP, Eastern Dongting Lake-grown biomass has a higher DP (average at 585.33) than that in Southern (575.15) and then Western Dongting Lake (529.16). Based on these quality indicators, the biomass production potentials for bioethanol, biochar, and xylo-oligosaccharide were calculated and visualized. Results show that biomass from almost the entire Western and Eastern Dongting Lake area is suitable for bioethanol and xylo-oligosaccharide production, while biomass from the Southern Dongting Lake area for biochar production. These results provide scientific guidance for the future utilization of Dongting Lake-grown M. lutarioriparius biomass.

Climate change drives rapid warming and increasing heatwaves of lakes

Climate change could seriously threaten global lake ecosystems by warming lake surface water and increasing the occurrence of lake heatwaves. Yet, there are great uncertainties in quantifying lake temperature changes globally due to a lack of accurate large-scale model simulations. Here, we integrated satellite observations and a numerical model to improve lake temperature modeling and explore the multifaceted characteristics of trends in surface temperatures and lake heatwave occurrence in Chinese lakes from 1980 to 2100. Our model-data integration approach revealed that the lake surface waters have warmed at a rate of 0.11 °C 10a−1 during the period 1980–2021, being only half of the pure model-based estimate. Moreover, our analysis suggested that an asymmetric seasonal warming rate has led to a reduced temperature seasonality in eastern plain lakes but an amplified one in alpine lakes. The durations of lake heatwaves have also increased at a rate of 7.7 d 10a−1. Under the high-greenhouse-gas-emission scenario, lake surface temperature and lake heatwave duration were projected to increase by 2.2 °C and 197 d at the end of the 21st century, respectively. Such drastic changes would worsen the environmental conditions of lakes subjected to high and increasing anthropogenic pressures, posing great threats to aquatic biodiversity and human health.

Spotty distributions: Spotted Gar (Lepisosteus oculatus) and Spotted Sucker (Minytrema melanops) range expansion in eastern Lake Erie

Natural range expansions in warm-water freshwater fishes are currently not well understood, but shifts in native species distributions can be influenced by many factors, including habitat restoration or degradation and climate change. Here, we provide empirical evidence of range expansions observed in two native freshwater fish species in Lake Erie: Spotted Gar (Lepisosteus oculatus) and Spotted Sucker (Minytrema melanops). We confirmed our field identifications of L. oculatus and M. melanops using mtDNA barcoding. Maximum likelihood phylogenetic analyses reveal that our samples confidently resolve in the L. oculatus and M. melanops clades respectively, with additional identification support from BLAST searches. Notably, we found no correlation between the increased detection rate of both species and an increase in sampling effort when compared to previous records. Historically, eastern Lake Erie experienced habitat degradation through channelization, siltation, dredging, and toxification of sediments. We hypothesize that recent habitat remediation efforts have provided suitable habitat for both species to recolonize shallow waters with densely vegetated habitat (>90% substrate coverage). Both species are likely to continue their northern expansion as habitats are restored and climatic changes favor warm-water fishes.

Long-term changes of nitrogen leaching and the contributions of terrestrial nutrient sources to lake eutrophication dynamics on the Yangtze Plain of China

Abstract. Over the past half-century, drastically increased chemical fertilizers have entered agricultural ecosystems to promote crop production on the Yangtze Plain, potentially enhancing agricultural nutrient sources for eutrophication in freshwater ecosystems. However, long-term trends of nitrogen dynamics in terrestrial ecosystems and their impacts on eutrophication changes in this region remain poorly studied. Using a process-based ecosystem model, we investigated the temporal and spatial patterns of nitrogen use efficiency (NUE) and nitrogen leaching on the Yangtze Plain from 1979–2018. The agricultural NUE for the Yangtze Plain significantly decreased from 50 % in 1979 to 25 % in 2018, with the largest decline of NUE in soybean, rice, and rapeseed. Simultaneously, the leached nitrogen from cropland and natural land increased, with annual rates of 4.5 and 0.22 kg N ha−1 yr−2, respectively, leading to an overall increase of nitrogen inputs to the 50 large lakes. We further examined the correlations between terrestrial nutrient sources (i.e., the leached nitrogen, total phosphorus sources, and industrial wastewater discharge) and the satellite-observed probability of eutrophication occurrence (PEO) at an annual scale and showed that PEO was positively correlated with the changes in terrestrial nutrient sources for most lakes. Agricultural nitrogen and phosphorus sources were found to explain the PEO trends in lakes in the western and central part of the Yangtze Plain, and industrial wastewater discharge was associated with the PEO trends in eastern lakes. Our results revealed the importance of terrestrial nutrient sources for long-term changes in eutrophic status over the 50 lakes of the Yangtze Plain. This calls for region-specific sustainable nutrient management (i.e., nitrogen and phosphorus applications in agriculture and industry) to improve the water quality of lake ecosystems.

Synergistic impacts of climate change and human activities on spatiotemporal organic nitrogen burial variation in a plateau lake in southwest China

ABSTRACT The concentration and sources of organic nitrogen (ON) in lake sediments affect lake nitrogen cycles. However, the factors influencing ON accumulation rate (ONAR) are unclear. We collected 3 sediment cores from northern, eastern, and southern Dianchi Lake (DC-N, DC-E, and DC-S, respectively) in July 2014, to study the effects of autochthonous and allochthonous sources of ON. The ON and ONAR increased 2.4–5.1 and 2.6–4.8 times, respectively, from 1900 to 2000, especially since the 1980s, when algal blooms have occurred more frequently. The ON decreased in the order DC-S > DC-N > DC-E, whereas the ONAR decreased in the order DC-N > DC-S > DC-E, suggesting that ONAR was influenced by ON content as well as the depositional environment. The total concentrations of n-alkanes (n-C12 to n-C34) ranged from 4719 to 61 960 ng g−1 in the 3 sediment cores, with proportions varying with vertical depth. The sources of ON were mainly allochthonous (soil erosion and terrestrial plants) and autochthonous (algal and aquatic plants) in DC-S and DC-N, respectively, and primarily mixed planktonic and terrestrial in DC-E. The stochastic impacts by regression on population, affluence, and technology (STIRPAT) model revealed that a 1% increase in air temperature and nitrogen fertilizer corresponded to an increase in ONAR by 23–33% and 20–79% in the Dianchi Lake basin, especially in DC-S and DC-E. However, a 1% increase in urban land area reduced ONAR by 2–11%, especially in DC-N. Our study suggests that the spatial and temporal ONAR in Dianchi Lake may increase in response to a warmer and wetter climate combined with increasing chemical nitrogen fertilizer application.

Composition and spore abundance of arbuscular mycorrhizal fungi in sweet potato producing areas in Uganda

IntroductionFarming systems influence composition and abundance of microbial communities.MethodologyA study was conducted using morphotyping and enumeration methods to determine the composition and spore abundance of Arbuscular Mycorrhizal Fungi (AMF) in sweet potato producing regions in eastern Uganda. Sampling was done from fields with crop types (CTs) including legumes (groundnuts, common beans, cowpea, soybeans, green grams), sorghum, sweet potato, and fallowed fields which were used as a control. Three agro-ecological zones (AEZs) i.e., Mt. Elgon High Farmlands (MEHF), Lake Victoria Crescent (LVC), and Southern and Eastern Lake Kyoga Basin (SELKB) were considered.Results and discussionA total of 6 AMF genera comprising of Glomus, Acaulospora, Scutellospora, Entrophospora, Archaeospora, and Gigaspora were isolated from the study sites. Agro-ecological zones had a significant (p&amp;lt;0.05) effect on Entrophospora spp. while crop types had a significant (p&amp;lt;0.05) effect on Gigaspora spp. although all the AMF genera were present in all AEZs and CTs. Spore abundance was similar across the AEZs except for MEHF (177) which was lower while spore abundance lowest in sweet potato (177) and largest in fallow (224), attributed to soil properties and similar crops included in the crop rotation program. The AMF can be isolated, identified, and multiplied to produce bioinoculants for the regions.