The deep mechanisms (deterministic and/or stochastic processes) underlying community assembly are a central challenge in microbial ecology. However, the relative importance of these processes in ...shaping riverine microeukaryotic biogeography is still poorly understood. Here, we compared the spatiotemporal and biogeographical patterns of microeukaryotic community using high-throughput sequencing of 18S rRNA gene and multivariate statistical analyses from a subtropical river during wet and dry seasons.
Our results provide the first description of biogeographical patterns of microeukaryotic communities in the Tingjiang River, the largest river in the west of Fujian province, southeastern China. The results showed that microeukaryotes from both wet and dry seasons exhibited contrasting community compositions, which might be owing to planktonic microeukaryotes having seasonal succession patterns. Further, all components of the microeukaryotic communities (including total, dominant, always rare, and conditionally rare taxa) exhibited a significant distance-decay pattern in both seasons, and these communities had a stronger distance-decay relationship during the dry season, especially for the conditionally rare taxa. Although several variables had a significant influence on the microeukaryotic communities, the environmental and spatial factors showed minor roles in shaping the communities. Importantly, these microeukaryotic communities were strongly driven by stochastic processes, with 89.9%, 88.5%, and 89.6% of the community variation explained by neutral community model during wet, dry, and both seasons, respectively. The neutral community model also explained a large fraction of the community variation across different taxonomic groups and levels. Additionally, the microeukaryotic taxa, which were above and below the neutral prediction, were ecologically and taxonomically distinct groups, which might be interactively structured by deterministic and stochastic processes.
This study demonstrated that stochastic processes are sufficient in shaping substantial variation in river microeukaryotic metacommunity across different hydrographic regimes, thereby providing a better understanding of spatiotemporal patterns, processes, and mechanisms of microeukaryotic community in waters.
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•ARG richness and abundance in waters increased from rural to urban regions.•ARGs profiles showed a temporal stability in downstream waters over five years.•Deterministic and ...stochastic processes mainly shaped ARGs and OTUs, respectively.•Bacterial function and taxonomy community were decoupled in urbanizing watershed.•Progressive urbanization promotes the enrichment and stability of ARGs in waters.
Although the occurrence and distribution of antibiotic resistance genes (ARGs) in various aquatic ecosystems are well explored, understanding of the ecological processes and mechanisms governing the composition and dynamics of bacterial ARGs still remains limited across space and time. Here, we used high-throughput approaches to detect spatial patterns of bacterial ARGs and operational taxonomic units (OTUs) in an urbanizing subtropical watershed, Xiamen, southeast China over a five-year period. At watershed scale, the OTU profiles were undergoing a directional change, but the ARG profiles showed a high stability or stochastic change over time. Compared with the upstream and midstream, the richness, absolute abundance, normalized abundance and diversity of ARGs were significantly higher in the downstream waters. Our results revealed a clear rural-urban disparity in ARG and OTU profiles which were mainly governed by deterministic and stochastic assembly processes, respectively. With the increase of urban building area along the river, the ecological processes of ARG profiles shifted from stochastic to deterministic. In downstream waters, the bacterial ARG profiles were much more stable than bacterial OTUs. Further, our results indicated that both human-dominated environment (e.g., land use) and mobile genetic elements (MGEs) played an important role in shaping the ARG profiles and dynamics. Overall, this was a response to spatially extensive human-landscape interactions that included urban development in the river downstream region, which were common across subtropical coastal cities of China and can alter the ARG profile dynamics along rural-urban gradient. Therefore, watershed management actions aiming at reducing threats posed by ARGs in urbanizing watershed should first consider the surrounding urbanization level and the mode and intensity of human activity. Our findings also imply that due to the decoupling of bacterial function and taxonomy, both aspects should be studied separately.
Understanding the influence of land use/land cover (LULC) on water quality is pertinent to sustainable water management. This study aimed at assessing the spatio-seasonal variation of water quality ...in relation to land use types in Lake Muhazi, Rwanda. The National Sanitation Foundation Water Quality Index (NSF-WQI) was used to evaluate the anthropogenically-induced water quality changes. In addition to Principal Components Analysis (PCA), a Cluster Analysis (CA) was applied on 12-clustered sampling sites and the obtained NSF-WQI. Lastly, the Partial Least Squares Path Modelling (PLS-PM) was used to estimate the nexus between LULC, water quality parameters, and the obtained NSF-WQI. The results revealed a poor water quality status at the Mugorore and Butimba sites in the rainy season, then at Mugorore and Bwimiyange sites in the dry season. Furthermore, PCA displayed a sample dispersion based on seasonality while NSF-WQI's CA hierarchy grouped the samples corresponding to LULC types. Finally, the PLS-PM returned a strong positive correlation (+ 0.831) between LULCs and water quality parameters in the rainy season but a negative correlation coefficient (- 0.542) in the dry season, with great influences of cropland on the water quality parameters. Overall, this study concludes that the lake is seasonally influenced by anthropogenic activities, suggesting sustainable land-use management decisions, such as the establishment and safeguarding protection belts in the lake vicinity.
Traditionally, composition and biomass of phytoplankton communities are determined by microscopy, but this method is time-consuming and so does not allow for high-frequency data acquisition across ...space and time. Pigment-based chemotaxonomy (CHEMTAX) is now widely applied to study of phytoplankton community structure on broader spatial and temporal scales of oceans, but the ability of this approach to provide estimates of phytoplankton assemblage in freshwater ecosystems is yet underdeveloped. To investigate the efficiency of the high-performance liquid chromatography (HPLC)-CHEMTAX in quantifying the different phytoplankton groups in inland freshwater, we present a comparison between phytoplankton pigment analyses by HPLC with CHEMTAX and microscopic counting of phytoplankton samples from four subtropical reservoirs in January and July 2014, respectively. The correlation between pigment and phytoplankton abundance detected by microscopy was stronger than that between pigment and phytoplankton biomass. The published marker pigments and their revised ratios can be used to describe phytoplankton abundances in a mixed community of freshwater phytoplankton, and pigment-based CHEMTAX can successfully describe the overall pattern of phytoplankton community dynamics during different seasons. The use of pigment-based CHEMTAX for quick surveys of phytoplankton communities can be recommended as a useful supplement or alternative tool to microscopy for freshwater ecosystem management.
Urbanization often exerts multiple effects on aquatic and terrestrial organisms, including changes in biodiversity, species composition and ecosystem functions. However, the impacts of urbanization ...on river phytoplankton in subtropical urbanizing watersheds remain largely unknown. Here, we explored the effects of urbanization on phytoplankton community structure (i.e., biomass, community composition and diversity) and function (i.e., resource use efficiency) in a subtropical river at watershed scale in southeast China over 6 years. A total of 318 phytoplankton species belonging into 120 genera and 7 phyla were identified from 108 samples. Bacillariophyta biomass showed an increasing trend with increasing urbanization level. The phytoplankton community shifted from Chlorophyta dominance in rural upstream waters to Bacillariophyta dominance in urbanized downstream waters. Furthermore, phytoplankton diversity and resource use efficiency (RUE = phytoplankton biomass/total phosphorus) were significantly decreased with increasing urbanization level from upstream to downstream. Phytoplankton RUE exhibited a significant positive correlation with species richness, but a negative correlation with phytoplankton evenness. The variation in environmental factors (turbidity, total nitrogen, NH4+-N, total phosphorus, PO43−-P and percentage urbanized area) was significantly correlated with phytoplankton diversity and RUE. Overall, our results revealed the influence of urbanization on phytoplankton community structure and ecosystem function was due to its altering the environmental conditions. Therefore, human-driven urbanization may play crucial roles in shaping the structure and function of phytoplankton communities in subtropical rivers, and the mechanism of this process can provide important information for freshwater sustainable uses, watershed management and conservation.
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Land use without adequate soil erosion control measures is continuously increasing the risk of soil erosion by water mainly in developing tropical countries. These countries are prone to ...environmental disturbance due to high population growth and high rainfall intensity. The aim of this study is to assess the state of soil erosion by water in Uganda at national and district levels, for various land cover and land use (LCLU) types, in protected areas as well to predict the impact of support practices on soil loss reduction. Predictions obtained using the Revised Universal Soil Loss Equation (RUSLE) model indicated that the mean rate of soil loss risk in Uganda’s erosion‐prone lands was 3.2 t∙ha−1∙y−1, resulting in a total annual soil loss of about 62 million tons in 2014. About 39% of the country’s erosion‐prone lands were comprised of unsustainable mean soil loss rates >1 t∙ha−1∙y−1. Out of 112 districts in Uganda, 66 districts were found to have unsustainable estimated soil loss rates >1 t∙ha−1∙y−1. Six districts in Uganda were found to have mean annual soil loss rates of >10 t∙ha−1∙y−1: Bududa (46.3 t∙ha−1∙y−1), Kasese (37.5 t∙ha−1∙y−1), Bundibugyo (28.9 t∙ha−1∙y−1), Bulambuli (20.9 t∙ha−1∙y−1), Sironko (14.6 t∙ha−1∙y−1) and Kotido (12.5 t∙ha−1∙y−1). Among the LCLU types, the highest soil loss rates of 11 t∙ha−1∙y−1 and 10.6 t∙ha−1∙y−1 were found in moderate natural forest and dense natural forest, respectively, mainly due to their locations in highland areas characterized by steep slopes ranging between 16% to 21% and their high rainfall intensity, ranging from 1255 mm∙y−1 to 1292 mm∙y−1. Only five protected areas in Uganda were found to have high mean estimated mean soil loss rates >10 t∙ha−1∙y−1: Rwenzori Mountains (142.94 t∙ha−1∙y−1), Mount Elgon (33.81 t∙ha−1∙y−1), Bokora corridor (12.13 t∙ha−1∙y−1), Matheniko (10.39 t∙ha−1∙y−1), and Nangolibwel (10.33 t∙ha−1∙y−1). To manage soil erosion in Uganda’s protected areas, there is an urgent need to control wildfires and human‐induced disturbances such as timber harvesting and soil compaction from domestic animals. Our study analysis revealed that well‐established terraces and strip‐cropping could significantly reduce soil loss rates in Uganda’s croplands by 80% (from 1.5 t∙ha−1∙y−1 to 0.3 t∙ha−1∙y−1) and by 47% (from 1.5 t∙ha−1∙y−1 to 0.8 t∙ha−1∙y−1), respectively, well below the sustainable soil erosion tolerance rate (1 t∙ha−1∙y−1) for land and water conservation.
Whether bacterioplankton are assembled in the same way as microeukaryotes is a key question that has been answered only partially in microbial ecology. In particular, relating distribution patterns ...to the underlying ecological processes for plankton communities in highly dynamic ecosystems, such as river–reservoirs subjected to anthropogenic impacts, remains largely unstudied. Here, we analyzed taxonomic distribution patterns, and unraveled community assembly processes underlying the core and random bacterioplankton and microeukaryotes from a subtropical river–reservoir system. These plankton domains were modelled using the spatial abundance distributions (SpADs) of the operational taxonomic units (OTUs) as a proxy for abundant and rare taxa delineation. Both bacterioplankton and microeukaryote plankton communities exhibited significant distance–decay relationships, and samples were grouped depending on reservoir or river habitats. The neutral community model showed that 35–45% of the plankton community variation could be explained by neutral processes. The phylogenetic null model revealed that dispersal limitation accounted for the largest percentage of pairwise comparisons (42–68%), followed by environmental selection (18–25%). We concluded that similar prevalence of ecological processes acting on particular subsets of the bacterioplankton and microeukaryotes might have resulted from similar responses to environmental change, potentially induced by human activities in the watershed.
The size-plasticity hypothesis posits that larger size organisms are less plastic in their metabolic rates and, therefore, are more strongly environmental-filtered than smaller organisms. Many ...studies have supported this hypothesis by evaluating the relative roles of environmental filtration and dispersal for different taxonomic groups of soil organisms. Most observations are made at large spatial scales, which are assumed to have a wide array of varying habitats. However, since urbanization causes habitat fragmentation at smaller regional scales, testing the size-plasticity hypothesis at this scale would help better understand the spatial assortment of urban soil organisms which, in turn, would help to develop improved management and conservation strategies for urban soil health. Here, we used DNA metabarcoding on five groups of soil biota (bacteria, fungi, protists, nematodes, and invertebrates) to assess the relative importance of dispersal and environmental filters to examine the size-plasticity hypothesis at this spatial scale in an urban environment. We observed strong distance-decay of community similarities associated with higher levels of stochastic changes in bacteria, nematode, and protist communities but not fungal or invertebrate communities. Bacterial communities occupied the widest niche followed by protists and nematodes, potentially because of their higher dispersal abilities compared to the larger soil organisms. Null deviation of communities varied with taxonomic groups where bacteria and nematodes were mainly driven by homogenizing dispersal, protists and fungi by drift, and soil invertebrates by environmental selection. We further identified a small percentage of locally-adapted taxa (2.1%) that could be focal taxa for conservation and restoration efforts by, for example, restoring their habitats and enhancing their regional connectivity. These results support the size-plasticity hypothesis at the relatively unexplored regional scale in an urbanization context, and provide new information for improving urban soil health and sustainable city models.
Following publication of the original article 1, the authors reported an error in Figure 1 and a text on page 13. In Figure 1, some areas are missing and the correct figure is presented here. On page ...13, the sentence should be updated to the following (change has been indicated in bold italics).
Although the influence of microbial community assembly processes on aquatic ecosystem function and biodiversity is well known, the processes that govern planktonic communities in human-impacted ...rivers remain largely unstudied. Here, we used multivariate statistics and a null model approach to test the hypothesis that environmental conditions and obstructed dispersal opportunities, dictate a deterministic community assembly for phytoplankton and bacterioplankton across contrasting hydrographic conditions in a subtropical mid-sized river (Jiulong River, southeast China). Variation partitioning analysis showed that the explanatory power of local environmental variables was larger than that of the spatial variables for both plankton communities during the dry season. During the wet season, phytoplankton community variation was mainly explained by local environmental variables, whereas the variance in bacterioplankton was explained by both environmental and spatial predictors. The null model based on Raup-Crick coefficients for both planktonic groups suggested little evidences of the stochastic processes involving dispersal and random distribution. Our results showed that hydrological change and landscape structure act together to cause divergence in communities along the river channel, thereby dictating a deterministic assembly and that selection exceeds dispersal limitation during the dry season. Therefore, to protect the ecological integrity of human-impacted rivers, watershed managers should not only consider local environmental conditions but also dispersal routes to account for the effect of regional species pool on local communities.
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•Riverine bacterio- and phytoplankton were studied during the dry and wet seasons.•Deterministic assembly underlies both plankton groups in the dry season.•Landscape and hydrology act together to dictate deterministic assembly.•Watershed management could consider dispersal routes for regional species pool.
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