Climate change-predicted shifts in the temperature regime of shallow groundwater produce rapid responses in ciliate communities

The ciliates living in a shallow groundwater system in southern Ontario, Canada were subjected to an in situ temperature manipulation over 14 months. Ciliates were collected from the bed surface of a small springbrook and from interstitial water collected at five depths beneath its surface. Mean temperature elevations established at each depth (?20, ?40, ?60, ?80, and ?100 cm) between the experiment's control and treatment blocks were 1.9, 3.5, 3.9, 3.8, and 3.6 °C, respectively, and were based on global warming projections for the region. In total, 160 species of ciliate belonging to 85 genera were identified. Overall, the treatment block had a higher density (6510±342 cells L^?1; ±1 SE) than the control (5797±237 cells L^?1), but densities were both vertically and longitudinally variable. Control densities decreased with depth, whereas treatment densities were more equal among depths. Total species richness showed no significant difference between blocks when combining all sampling dates and depths, although species composition changed. The ciliate community was dominated by small (15–50 μm), followed by medium (50–200 μm), and only a few large‐sized (>200 μm) species. Small ciliates contributed 82–97% of the total density. Small ciliates also contributed more to the treatment (94%) than the control block (88%). The most common ciliate feeding groups were bacterivores, omnivores, predators, and algae‐diatom feeders, with bacterivores being most dominant (83–99% of the total numbers collected). Ordination analyses revealed that ciliate distribution was strongly correlated with groundwater temperature, although dissolved oxygen level, concentrations of ammonia and nitrate, and depth also appeared to be influential. Peak densities of many species occurred in either the control or treatment blocks, but not in both. The benefits of using ciliates as a proxy for higher, much longer‐lived, eukaryotes in climate change studies are discussed.     

A model for predicting the temporal evolution of dissolved oxygen concentration in shallow estuaries

The design of sewage discharge systems in estuaries needs to consider the dissolved oxygen concentration among other water quality indicators. Due to the great number of factors affecting the dissolved oxygen, the prediction of the temporal evolution of this element requires the use of mathematical tools. In the case of shallow estuaries with extensive intertidal zones, the complexity of this task increases since the water domain varies continuously. This work describes a numerical model which solves the vertical integrated transport equation including the effect of extensive tidal flats. The model makes use of the tidal velocities computed by a previous run of a hydrodynamic model. This procedure allows the dissolved oxygen modelling to be performed using a larger time step than that used for the velocity field calculation. The numerical scheme developed for the model guarantees that even in the drying and wetting of intertidal areas the water column dissolved oxygen concentration is not modified by numerical errors. The representations of the processes included in the model (advection, dispersion and reaction) have been validated successfully in several theoretical cases. An application to the Urdaibai Estuary, a shallow estuary in the Basque Country, Northern Spain, is also presented.     

商丘市浅层地下水位下降对区域气候的影响

地下水包括浅层地下水和深层地下水,是陆地水循环的重要组成部分,其中浅层地下水受气候条件和植被地形及人类活动的影响大。浅层地下水埋深对气候变化的响应研究是气候变化影响研究的前沿和热点之一,对于水资源管理及其相关研究与应用具有重要意义。商丘市所在的黄淮平原是我国第一大地下水富集区,由于气候变化和人类活动的影响,地下水资源量与1984年相比下降了20%以上。地下水作为黄淮平原重要的水资源,研究气候变化和人类活动的影响与地下水的相应关系有着重要的现实意义。以几个典型城市的气温与地下水位的变化情况为基础,全面分析讨论了商丘市浅层地下水对水温、蒸发量和气温的影响关系,为商丘市以及黄淮平原地下水资源变化提供科学的依据。     

Oxygen supply and the adaptations of animals in groundwater

1. The first part of this review focuses on the oxygen status of natural groundwater systems (mainly porous aquifers) and hyporheic zones of streams. The second part examines the sensitivity of groundwater organisms, especially crustaceans, to low oxygen concentrations (< 3.0 mg L^?1 O₂). 2. Dissolved oxygen (DO) in groundwater is spatially heterogeneous at macro- (km), meso- (m) and micro- (cm) scales. This heterogeneity, an essential feature of the groundwater environment, reflects changes in sediment composition and structure, groundwater flow velocity, organic matter content, and the abundance and activity of micro-organisms. Dissolved oxygen also exhibits strong temporal changes in the hyporheic zone of streams as well as in the recharge area of aquifers, but these fluctuations should be strongly attenuated with increasing distance from the stream and the recharge zone. 3. Dissolved oxygen gradients along flow paths in groundwater systems and hyporheic zones vary over several orders of magnitude (e.g. declines of 9 × 10^?5 to 1.5 ×10^?2 mg L^?1 O₂ m^?1 in confined aquifers and 2 × 10^?2 to 1 mg L^?1 O₂ m^?1 in parafluvial water). Several factors explain this strong variation. Where the water table is close to the surface, oxygen is likely to be consumed rapidly in the first few metres below the water table because of incomplete degradation of soil-generated labile dissolved organic carbon (DOC) in the vadose zone. Where the water table is far from the surface, strong oxygen depletion in the vicinity of the water table does not occur, DO being then gradually consumed as groundwater flows down the hydraulic gradient. In unconfined groundwater systems, oxygen consumption along flow paths may be compensated by down-gradient replenishment of DO, resulting either from the ingress of atmospheric oxygen or water recharge through the vadose zone. In confined groundwater systems, where replenishment of oxygen is impossible, the removal time of DO varies from a few years to more than 10 000 years, depending mainly on the organic carbon content of the sediment. Comparison of the hyporheic zones between systems also revealed strong differences in the removal time and length of underground pathways for DO. This strong variability among systems seems related to differences in contact time of water with sediment. 4. Although groundwater macro-crustaceans are much more resistant to hypoxia than epigean species, they cannot survive severe hypoxia (DO < 0.01 mg L^?1 O₂) for very long (lethal time for 50% of the population ranged from 46.7 to 61.7 h). In severe hypoxia, none of the hypogean crustaceans examined utilized a high-ATP yielding metabolic pathway. High survival times are mainly a result of the combination of three mechanisms: a high storage of fermentable fuels (glycogen and phosphagen), a low metabolic rate in normoxia, and a further reduction in metabolic rate by reducing locomotion and ventilation. It is suggested here that the low metabolic rate of many hypogean species may be an adaptation to low oxygen and not necessarily result from an impoverished food supply. 5. An interesting physiological feature of hypogean crustaceans is their ability to recover from anaerobic stress and, more specifically, rapidly to resynthesize glycogen stores during post-hypoxic recovery. A high storage and rapid restoration of fermentable fuels (without feeding) allows groundwater crustaceans to exploit a moving mosaic of suboxic (< 0.3 mg L?1 O₂), dysoxic (0.3–3.0 mg L^?1 O₂) and oxic (> 3 mg L^?1 O₂) patches. 6. It is concluded that although hypogean animals are probably unsuited for life in extensively or permanently suboxic groundwater, they can be found in small or temporarily suboxic patches. Indeed, their adaptations to hypoxia are clearly suited for life in groundwater characterized by spatially heterogeneous or highly dynamic DO concentrations. Their capacity to survive severe hypoxia for a few days and to recover rapidly would explain partly why ecological field studies often reveal the occurrence of interstitial taxa in groundwater with a wide range of DO.     

Influence of groundwater characteristics on the survival of enteric viruses

AIMS: This study was undertaken to further understand the processes affecting the persistence of enteric viruses in groundwater. METHODS AND RESULTS: Varying temperature, oxygen and nutrient levels were tested in the presence and absence of groundwater micro-organisms to determine which of the factors tested had dominant influence on the decay of Escherichia coli, the bacteriophage MS2, poliovirus and coxsackievirus. The results indicated that the most influential factor affecting the decay of the viruses and E. coli was the presence of groundwater micro-organisms. The results also implied that temperature, the presence of oxygen and nutrient levels indirectly influence viruses and E. coli decay by influencing the activity of the groundwater micro-organisms. CONCLUSIONS: E. coli and the viruses displayed maximum decay under aerobic conditions, at 28 degrees C without the addition of nutrients in the presence of groundwater micro-organisms. SIGNIFICANCE AND IMPACT OF THE STUDY: The results suggest that if the mode of action of the groundwater micro-organisms could be determined then the decay of viral pathogens in recharged waters may be more easily predicted.     

Phytoplankton communities in water bodies at Dungeness,U.K.: analysis of seasonal changes in response to environmental factors

The species richness and the dynamics of the ciliatecommunitieswere studied at three stations along a streamlet formed bywaterdischarged from a geothermal sulphur spring. Altogether, 73specieswere identified. Ciliate coenoses were composed primarily offreshwater forms, but they were numerically dominated byeuryhalinespecies. The great fluctuation in both number of species andindividuals observed along the streamlet were due to vigorouschemical changes in the physico-chemical characteristics ofthewater. These changes were caused mainly by the sulphide andsaltgradient observed along the water course. The maximum value ofsulphide was observed in August with 391 µM l^–1, andthehighest conductivity value was observed in April with 18 mScm^–1. The most characteristic and abundant species weresmall-sized bacterivorous scuticociliates which have a rapidecological adaptability and whose tolerance limits toenvironmentalchanges are wide. Bacterivorous scuticociliates such as Cyclidium citrullus, C. glaucoma, Cinetochilummargaritaceum, and Uronema nigricans were numericallydominant when the thermal water contained the highestconcentrationof sulphide. The diversity index values changed sharply intimeboth inside and among sampling stations, indicating unstablecommunities which depended on highly fluctuatingconditions.     

Effects of aquatic macrophytes on water quality and phytoplankton communities in shallow lakes

We investigated aquatic macrophytes, water quality, and phytoplankton biomass and species composition in three shallow lakes with different levels of vegetation cover and nutrient concentration in Kushiro Moor, during August 2000. Trapa japonica can live in a wide range of nutrient levels. This species forms an environment with a steeper extinction of light, higher concentrations of dissolved organic carbon (DOC), lower concentrations of dissolved oxygen (DO) near the bottom, and lower concentrations of nitrate+nitrite and soluble reactive phosphorus (SRP) than other vegetation types. The pH was much higher in a Polygonum amphibium community, and the DO near the bottom did not decrease compared to a T.japonica community in the summer. The relationship between chlorophyll a and the limiting nutrient (total phosphorus (TP) when total nitrogen (TN):TPis 10 and TN/10 when TN:TP is <10) significantly differed between lakes with and without submerged vegetation. The chlorophyll a concentrations at a given nutrient level were significantly lower in water with submerged macrophytes than in water without them. Correspondence analysis showed that the difference in phytoplankton community structure across sites was largely due to the presence or absence of submerged macrophytes, and the ordination of phytoplankton species in the lakes with submerged macrophytes is best explained by environmental gradients of TN, chlorophyll, pH and SRP.     

四川盆地西缘浅层地下水铁、锰含量的空间变异特征

以四川盆地西缘低山丘陵区典型县--名山县为例,采用地统计分析方法,研究了四川盆地西缘低山丘陵区浅层地下水铁、锰含量的空间变异特征.结果表明,研究区浅层地下水铁含量变异较大,局部区域超标现象较为严重,42.86%的样点未达到欧盟生活饮用水水质标准(<0.2 mg/L),25%的样点未达到我国和美国生活饮用水水质标准(<0.3 mg/L);锰含量总体水平较低,其中94.64%的样点达到我国的生活饮用水水质标准(<0.1 mg/L),所有样点均达到世界卫生组织规定的生活饮用水水质标准(<0.4 mg/L).研究区浅层地下水中铁、锰含量空间分布特征基本一致,均呈现出从东北向西南逐渐降低,至名山县城附近又有所上升的趋势,高值区主要出现在东北部一带,低值区主要出现在中南部一带.研究区地下水中铁、锰含量分布特征与区域土壤母质、土壤类型、人为活动等因素密切相关, 其中受土壤母质和土壤类型的影响尤为明显.统计结果表明,第四系更新统冰碛物及冰水沉积物(Q2fgL)分布区域地下水铁、锰含量显著高于白垩系夹关组(K2j)、灌口组(K2g),第三系名山群(K1～2mn)等紫色岩分布区域;黄壤下浅层地下水铁、锰含量显著高于紫色土.     

Effect of carbon addition and predation on acetate-assimilating bacterial cells in groundwater

Groundwater microbial community dynamics are poorly understood due to the challenges associated with accessing subsurface environments. In particular, microbial interactions and their impact on the subsurface carbon cycle remain unclear. In the present project, stable isotope probing with uniformly labeled [¹³C]-acetate was used to identify metabolically active and inactive bacterial populations based on their ability to assimilate acetate and/or its metabolites. Furthermore, we assessed whether substrate availability (bottom–up control) or grazing mortality (top–down control) played a greater role in shaping bacterial community composition by separately manipulating the organic carbon supply and the protozoan grazer population. A community fingerprinting technique, terminal restriction fragment length polymorphism, revealed that the bacterial community was not affected by changes in acetate availability but was significantly altered by the removal of protozoan grazers. In silico identification of terminal restriction fragments and 16S rRNA gene sequences from clone libraries revealed a bacterial community dominated by Proteobacteria, Firmicutes , and Bacteroidetes . Elucidation of the factors that structure the bacterial community will improve our understanding of the bacterial role in the carbon cycle of this important subterranean environment.     

浅层地下水对华北地区河岸杨树林树干液流的影响

华北地区河岸杨树林生长季的用水特征对杨树人工林经营管理具有重要意义.本文以北京市潮白河畔的杨树人工林为研究对象,采用热扩散式探针(TDP)测定树干液流、管式TDR土壤含水量法测定土壤体积含水量、开路涡度相关系统测定环境因子数据,对2014年6—7月的杨树树干液流及其影响因素进行系统研究,以探究浅层地下水对树干液流的可能影响.结果表明: 杨树树干液流日变化随太阳辐射呈现单峰或双峰变化,在土壤水分相对亏缺时,杨树树干液流密度明显减小,达到峰值的时间从14:00提前到12:30,但树干液流对太阳辐射的时滞效应没有明显变化.在表层土壤水分相对充足时,太阳辐射和空气饱和水汽压差是杨树树干液流变化的主导因子,但在表层土壤水分亏缺时,土壤水会成为杨树树干液流的限制因子;当表层土壤水分相对亏缺时,杨树单株耗水与100 cm土层土壤体积含水量呈显著负相关,与其余各土层体积含水量呈显著正相关.浅层地下水(相对稳定的150 cm及更深处)在土壤表层土壤水分含量相对亏缺时,可在毛管力作用下对上层土壤水进行补充,供给杨树生长需要.     

Heavy metals contamination in shallow groundwater of a coal-mining district and a probabilistic assessment of its human health risk

To manage public health and make better use of groundwater resources, the concentration characteristics and a health risk assessment of eight heavy metals in shallow groundwater were studied. Besides this, this paper introduced triangular fuzzy numbers into the USEPA health risk assessment model to assess the health risk posed to local children and adults through different exposure pathways. The results showed that Mn levels exceeded the WHO’s guideline values of 100?µg/L with the proportion of 27.98% and Sr were over the health reference level (HRL) of 1500?µg/L with the proportion of 56.25%, while other heavy metals were below the corresponding standard. The results of the HRA showed that the non-carcinogenic risks from Sr and Mn in the district were relatively higher, while those from the remaining six heavy metals were relatively lower. All hazard index (HI) values did not exceed the safety level of 1 for either age group. The average carcinogenic risk from Cr was slightly higher than the acceptable level of 1?×?10^?6 for adults. Sensitivity analyses conducted using Monte Carlo simulation indicated that Sr and Cr concentrations were the most influential variables contributing to the non-carcinogenic and carcinogenic risk values, respectively, while body weight had a minor contribution.     

Weedbeds and big bugs: the importance of scale in detecting the influence of nutrients and predation on macroinvertebrates in plant‐dominated shallow lakes

1. The scale of investigations influences the interpretation of results. Here, we investigate the influence of fish and nutrients on biotic communities in shallow lakes, using studies at two different scales: (i) within‐lake experimental manipulation and (ii) comparative, among‐lake relationships. 2. At both scales, fish predation had an overriding influence on macroinvertebrates; fish reduced macroinvertebrate biomass and altered community composition. Prey selection appeared to be size based. Fish influenced zooplankton abundance and light penetration through the water column also, but there was no indication that fish caused increased resuspension of sediment. 3. There were effects of nutrients at both scales, but these effects differed with the scale of the investigation. Nutrients increased phytoplankton and periphyton at the within‐lake scale, and were associated with increased periphyton at the among‐lake scale. No significant effect of nutrients on macroinvertebrates was observed at the within‐lake scale. However, at the among‐lake scale, nutrients positively influenced the biomass and density of macroinvertebrates, and ameliorated the effect of fish on macroinvertebrates. 4. Increased prey availability at higher nutrient concentrations would be expected to cause changes in the fish community. However, at the among‐lake scale, differences were not apparent in fish biomass among lakes with different nutrient conditions, suggesting that stochastic events influence the fish community in these small and relatively isolated shallow lakes. 5. The intensity of predation by fish significantly influences macroinvertebrate community structure of shallow lakes, but nutrients also play a role. The scale of investigation influences the ability to detect the influence of nutrients on the different components of shallow lake communities, particularly for longer lived organisms such as macroinvertebrates, where the response takes longer to manifest.     

Effect of groundwater depth on riparian plant diversity along riverside-desert gradients in the Tarim River

AimsRiparian plant diversity is sensitive to changes in groundwater in arid regions. However, little is known about how plant diversity responds to changes in environment along riverside-desert gradients in riparian ecosystem. Our objectives were to (i) identify riparian plant diversity along riverside-desert gradients in Tarim desert riparian forests, (ii) analyze the impact of environment variables on plant diversity, (iii) determine the optimum groundwater depth for different plant life-forms.     

Mixed aerobic and anaerobic microbial communities in benzene-contaminated groundwater

Aims:  To investigate the factors affecting benzene biodegradation and microbial community composition in a contaminated aquifer.
Methods and Results:  We identified the microbial community in groundwater samples from a benzene-contaminated aquifer situated below a petrochemical plant. Eleven out of twelve groundwater samples with in situ dissolved oxygen concentrations between 0 and 2·57 mg l⁻¹ showed benzene degradation in aerobic microcosm experiments, whereas no degradation in anaerobic microcosms was observed. The lack of aerobic degradation in the remaining microcosm could be attributed to a pH of 12·1. Three groundwaters, examined by 16S rRNA gene clone libraries, with low in situ oxygen concentrations and high benzene levels, each had a different dominant aerobic (or denitrifying) population, either Pseudomonas , Polaromonas or Acidovorax species. These groundwaters also had syntrophic organisms, and aceticlastic methanogens were detected in two samples. The alkaline groundwater was dominated by organisms closely related to Hydrogenophaga .
Conclusions:  Results show that pH 12·1 is inimical to benzene biodegradation, and that oxygen concentrations below 0·03 mg l⁻¹ can support aerobic benzene-degrading communities.
Significance and Impact of the Study:  These findings will help to guide the treatment of contaminated groundwaters, and raise questions about the extent to which aerobes and anaerobes may interact to effect benzene degradation.     

Latitudinal gradients in Atlantic reef fish communities: trophic structure and spatial use patterns

Trophic strategies and spatial use habits were investigated in reef fish communities. The results supported the hypothesis of differential use of food resources among tropical and higher latitude reef fishes, i.e . the number of species and relative abundance of fishes relying on relatively low‐quality food significantly decreased from tropical to temperate latitudes. The species : genus ratio of low‐quality food consumers increased toward the tropics, and was higher than the overall ratio considering all fishes in the assemblages. This supports the view that higher speciation rates occurred among this guild of fishes in warm waters. It was also demonstrated that density of herbivorous fishes (the dominant group relying on low‐quality food resources) in the western Atlantic decreased from tropical to temperate latitudes. Spatial use and mobility varied with latitude and consequently reef type and complexity. Fishes with small‐size home ranges predominated on tropical coral reefs.     

Extensive spatial analysis of the light environment in a subtropical shallow lake, Laguna Iberá, Argentina

Variations in the attenuation of photosynthetically available radiation were analysed using extensive spatial sampling in two seasons in a subtropical wetland lake. Simultaneously with the attenuation measurements, the principal absorption and scattering components of the water column were also measured. The elevated spatial resolution used in the study allowed the determination of spatially distinct optical water classes within the lake. Changes in dissolved organic matter, phytoplankton and tripton concentrations led to a wide variation in the vertical attenuation coefficients. These changes depended on local characteristics of the ecosystem and time of year. The spatial distribution of the attenuation coefficients was examined in relation to the hydrological and geomorphological characteristics of the littoral area of the lake. The impacts of two small rivers on the light environment and attenuation components are shown. Finally, the resulting model was used to examine the possible impacts of changes in light availability at the lake bottom in relation to recent changes in lake water level.     

Rotifer community structure in three shallow lakes: seasonal fluctuations and explanatory factors

The present work aimed at studying the rotifer communities of three shallow eutrophic lakes in Portugal (lakes Mira, Vela and Linhos). At the time of the study, Mira and Vela faced large inputs of allochthonous nutrients, while Linhos was facing terrestrialisation, with cycles of dominance-senescence of macrophytes. The three lakes differed in terms of their abiotic features, with Linhos presenting very high nutrient levels and low pH, while Vela and Mira shared most of the characteristics. The rotifer communities of these two lakes were poorly diversified but highly abundant (max. > 2000 ind l⁻¹), with a clear dominance of eurytopic euplanktonic species (mainly Keratella cochlearis). On the other hand, Linhos presented lower abundances (<1000 ind l⁻¹) but higher species richness, mainly due to macrophyte-associated taxa, such as the littoral genera Lepadella, Testudinella and Squatinella. In all lakes, summertime represented a peak in terms of abundance and diversity. Canonical correspondence analysis (CCA) identified two main environmental gradients that shape up the rotifer assemblages: a temporal gradient, mainly related to temperature, and a eutrophy gradient, associated with nitrogenous nutrients. The latter gradient is clearly dependent on between-lake variation, due to the high nutrient levels observed in lake Linhos. Variance partitioning using CCA revealed that the largest portion (27.5%) of the total variation explained (52.1%) was attributed to the interaction between lake and environmental variables.     

高砷含水层参与腐殖酸-铁矿物转化的关键微生物群落及其对砷迁移转化的影响

【目的】探究不同腐殖酸浓度下参与含砷水铁矿转化的微生物类群组成和丰度变化及对砷释放的影响,预测原位高砷含水层中功能微生物群参与有机质—含砷铁矿物转化过程对砷转化释放的作用。【方法】对河套平原高砷地下水和同深度高砷沉积物中的铁还原功能群落进行富集培养,构建室内厌氧微宇宙体系,将富集菌群分别加入到实验室条件下合成的不同浓度腐殖酸(0、1.5、7、14 mg C/L)-含砷水铁矿体系中,通过体系中砷、铁形态及浓度的变化分析,结合高通量测序技术、X-射线衍射(X-ray diffractometer, XRD),探究不同条件下砷的释放固定和群落的演替。【结果】高砷地下水组(G组)和沉积物组(S组)富集得到的铁还原功能群落具有明显差异,G组中以Aeromonadaceae为特殊优势菌群,而S组中以Shewanellaceae为特殊优势菌群。微宇宙实验结果显示,S组的铁还原量相对较高且速率较快;G组与S组中液相砷形态存在明显差异,整个培养期内G组均以As(Ⅴ)为主,而S组中前期以As(Ⅴ)为主,当反应到达20 d时液相As(Ⅲ)高达3.4μmol/L,推测此时具有砷还原功能的群落占优势地位。当反应...     

拉萨河中下游纤毛虫群落时空分布模式及其驱动机制

为了探究拉萨河中下游纤毛虫群落的组成模式、时空多样性格局及其维持机制, 本文于2015年5月和8月以及2016年10月在拉萨河中下游17个样点进行采样, 采用活体观察、鲁哥氏碘液固定染色以及Wilbert蛋白银法相结合的物种鉴定方法, 对纤毛虫群落结构进行了研究。通过Shannon多样性指数、Margalef丰富度指数、物种数分析群落结构时空上的差异性; 通过共现网络分析纤毛虫类群之间的相互作用; 通过冗余分析(redundancy analysis, RDA)探讨水体理化因子对纤毛虫群落结构的影响。结果表明, Shannon多样性指数在季节和河段间没有显著性差异; Margalef丰富度指数、物种数在河段间存在极显著性差异; 中游和下游河段共现网络节点间的相关关系均以正相关为主; 溶解氧(DO)、总氮(TN)、总磷(TP)、总溶解盐(TDS)是影响纤毛虫群落结构的关键因子。综上所述, 拉萨河中下游纤毛虫群落结构在季节间没有显著差异, 在空间上具有显著差异; 纤毛虫在纲级水平上类群间的相互作用以协同作用为主导, 不同类群间存在复杂的相互作用, 整体上互作关系在春季较为复杂、夏季较为简单; 影响拉萨河中下游纤毛虫群落结构是多个环境因子共同作用的结果。