Factors promoting persistence of the bloom-forming Gonyostomum semen in temperate lakes

The nuisance large flagellate Gonyostomum semen (Raphidophyceae) is typical for brown-water lakes in temperate and boreal regions. This study is focused on two annual cycles of G. semen in shallow temperate lakes (one of which stratifies) with a particular emphasis on the species’ ecology and its potential impact on the alternation of phytoplankton assemblages. Temperature and phosphorus were the main forces driving seasonal growth of the raphidophyte. G. semen occurred at a broad temperature range and prevailed for the extended period from spring to late autumn at maximum biovolume and forming up to 98.7% of total phytoplankton in June–July and/or September. The species exclusively dominated under phosphate-replete conditions and caused heavy blooms in the stratified lake. The species outcompeted other flagellates in the lakes; considerable fluctuations in phytoplankton species composition occurred during the periods at low G. semen biovolume.     

Motile phytoplankton species such as Gonyostomum semen can significantly reduce CO2 emissions from boreal lakes

Boreal lakes typically have high levels of allochthonous dissolved organic matter (DOM), causing light limitation of photosynthetic CO₂ consumption while stimulating CO₂ production. They are therefore considered as important sources of atmospheric CO₂. However, boreal lakes are also experiencing a marked expansion of bloom-forming motile phytoplankton organisms that can circumvent the shading effect of DOM by performing phototaxis and that thus might have an impact on the lakes’ CO₂ balance. We tested this idea in a DOM-rich lake using the widespread raphidophyte flagellate Gonyostomum semen as model organism. Employing continuous field measurements, we found that G. semen can reduce the partial pressure of CO₂ even at low algal densities. Periods with high algal densities were associated with CO₂ undersaturation and invasion of atmospheric CO₂. The mean daily net losses of CO₂ to the atmosphere during and after a G. semen bloom were estimated at 12.9 and 70.4 mmol C m^–2 day^–1, respectively. G. semen caused steep pCO₂ gradients in space and time, which make it difficult to unveil the species’ impact without employing continuous pCO₂ vertical profiling. This suggests that the effect of G. semen on the CO₂ balance of boreal lakes might have been overlooked in the past. Taken together, our data suggest that G. semen can significantly reduce the CO₂ emissions from boreal lakes despite high concentrations of allochthonous DOM. G. semen and other motile phytoplankton species should therefore be considered when estimating CO₂ emissions from boreal lakes, especially if these organisms continue their expansion.     

Limnology of the neotropical high elevation shallow lake Yahuarcocha (Ecuador) and challenges for managing eutrophication using biomanipulation

Temperate and tropical shallow lakes differ in several fundamental aspects with respect to management of eutrophication. High altitude tropical shallow lakes are a special case, showing similarities with temperate and tropical lakes. We studied the ecology of the eutrophic high-altitude tropical lake Yahuarcocha in the Ecuadorian Andes and evaluated the potential of biomanipulation to control eutrophication. With a toxin-producing Cylindrospermopsis bloom, low Secchi depth and low submerged macrophyte cover, Yahuarcocha is clearly in a turbid ecosystem state. Relatively low nutrient concentrations should theoretically allow for a shift to a clear water state through biomanipulation. Top-down control of phytoplankton by zooplankton, however, is complicated by the (1) absence of predatory fish, (2) fish community dominated by small poecelid species, (3) lack of a refuge for zooplankton from fish predation within the macrophytes, and (4) persistent, grazing resistant bloom of the cyanobacterium Cylindrospermopsis. In these aspects, lake Yahuarcocha is more similar to tropical shallow lakes, probably because water temperature is high relative to the mean air temperature and because of the absence of a cold season. The fish and macrophyte communities consisted almost entirely of exotic species. The exotic fish species probably stabilized the turbid state in the lake.     

Effects of fish manipulation on the plankton community in small hypertrophic lakes from the Pampa Plain (Argentina)

Trophic cascade hypotheses predict that fish will affect the structure and biomass of pelagic plankton communities. In order to investigate trophic cascade effects from fish down to phytoplankton, whole-lake studies were performed in five hypertrophic (mean total phosphorus (TP) concentrations higher than 1000 mg m⁻³) shallow lakes located in the Pampa Plain. The main climatic characteristic of this region is the alternation between periods of drought and flood, with corresponding changes of lake depth and conductivity of lake water. All lakes were studied from April to December 2000. Samples were taken of their physical and chemical characteristics and biotic communities, focusing on the zooplankton community. Fish were manipulated in four lakes (Capurro, Longinotti, Vedia 1, Vedia 2), while the fifth (Lake Vedia 3) was left undisturbed as a reference system. High abundance of planktivorous minnows (Jenynsia multidentata and Cheirodon interruptus) dominated the fish community in the reference lake. In the manipulated lakes, fish stocks were largely reduced in late autumn (May). During winter, Capurro, Longinotti and Vedia 1 were stocked with a visual planktivore, the pampean silverside (Odontesthes bonariensis, Atherinidae). Fish stocking was 24, 33 and 19 kg ha⁻¹, respectively. In contrast, no fish were stocked in Lake Vedia 2. Following fish removal, large Daphnia appeared in these lakes and grazed intensively on the phytoplankton. In contrast, no Daphnia were found in the reference lake (Vedia 3). The stocking of O. bonariensis in lakes Capurro, Longinotti and Vedia 1 led to a decrease in the percentage of large cladocerans and a rise in the phytoplankton biomass:TP ratio. Moreover, the lakes mentioned were stocked with different quantities of silversides over different periods of time. These differences were reflected temporarily in the plankton dynamics, affecting mainly larger sized zooplankton. Nevertheless, the presence of Daphnia was short lived in the lake where fish had been removed and no O. bonariensis were stocked. Competition for resources and recruitment of remaining fish probably caused a collapse in the zooplankton biomass. Our results support the idea that fish predation on zooplankton and its effect on phytoplankton is very intense in small pampean lakes. Fish removal was short lived, however. This could be because in small pampean lakes fish recolonization is favored, and minnows are highly prolific. Moreover, if manipulation of the trophic structure of lakes is undertaken in the pampean region, high nutrient loading from the watershed, climate and hydrology should also be taken into account.     

城市湖泊春季绿藻水华特征及其影响因素——以宁波月湖为例

城市湖泊富营养化问题日趋严峻,以往对水华的研究多集中于大型自然淡水湖库,而对小型城市浅水湖泊的水华动态相对较少.以宁波月湖为研究对象,探讨水华暴发期间浮游植物变化特征及与影响因子之间的关系,以期判别影响城市湖泊水华的主控因子.月湖水华期间营养盐水平处于中富营养至极端富营养之间,此次共检出浮游植物8门61属,藻种组成以绿藻门（51.79%）和硅藻门（21.43%）为主,各点位浮游植物生长主要受水温、光照驱动,经历了隐藻门→硅藻门→绿藻门→蓝藻门的演替.水华种为雷氏衣藻（Chlamydomonas reinhardtii）,总藻密度最高达到1.55×10⁸ cells/L,水华暴发后各点位衣藻属比例升高（最高达到81.10%）,群落结构呈现单一化特征.通过Pearson相关性分析和RDA分析发现衣藻属生长与水温、pH、总磷浓度均呈显著正相关,春季气温回升、天气持续晴好,城市浅水湖泊高营养盐负荷、水体流动性差等特点为带鞭毛的衣藻属提供了适宜的生存条件,在环境条件均适宜的情况下拥有最大生长潜力的衣藻属在营养盐、光照等竞争中生长速率明显优于其他藻种,从而发生绿藻水华.     

Zooplankton response to the lake restoration in the drinking-water source in Meiliang Bay of subtropical eutrophic Lake Taihu, China

A 3-year ecological engineering project was carried out from 2003 to 2005 in the drinking water source in Meiliang Bay, Lake Taihu, China. The project area was divided into three zones: Zone C (outside the area), Zone B (pelagic part within the area) and Zone A (littoral part within the area). The community composition, density and biomass of zooplankton were studied as a part in the lake restoration study. Also, the abundance, biomass and composition of fish in the engineering area were annually analyzed in the littoral zone (Zone A). The average density of cladocerans decreased from the outer part to the inner part of the lake, i.e., from Zone C to Zone A, while the average density of copepod did not significantly vary among the 3 zones. The average densities of rotifer and ciliate increased from Zone C to Zone A. RDA analysis indicated that Microcystis biomass more impacted the zooplankton compositions after temperature. In Zone A, many planktivorous fishes, e.g., Coilia nasus, Hemiculter leucisculus, Pseudorasbora parva, dominated the fish community, which caused a marked decrease in the abundance of cladocerans, especially large-sized cladoceran Daphnia galeata and Daphnia obtusa. Our results suggested that both fish predation and other environmental factors such as the Microcystis bloom greatly affected the zooplankton community during the restoration of subtropical eutrophic lakes.     

The success of Gonyostomum semen (Raphidophyceae) in a boreal lake is due to environmental changes rather than a recent invasion

The freshwater microalgal species, Gonyostomum semen, has increased in abundance and distribution in boreal lakes during the past few decades, concerning ecologists and water managers. Due to its rapid spread, G. semen has often been referred to as an invasive species, although it was first described in the 1800s. We hypothesized that G. semen is not an invasive species in Norwegian lakes, and that the increasing success is due to beneficial changes in environmental conditions for this species during the past century. We tested these hypotheses by performing a paleolimnological study of a Norwegian Lake, Skjeklesjøen, with known mass occurrence of G. semen. A specific G. semen pigment biomarker, heteroxanthin, was used to detect this species in layers of a sediment core with known age determinations. Environmental factors in both lake and catchment were further investigated and the relationships with the amounts of G. semen was tested. Our results suggested that G. semen was in fact not an invasive species in this lake the past decades. Several factors were identified as plausible drivers for G. semen in this boreal lake. Between 1874–2016, the increasing levels of G. semen in Lake Skjeklesjøen was most closely correlated with Carbon (C), lake color (measured as absorbance of sediment extracts), Nitrogen (N) and spring temperature. Our results suggest that the rapid increase in G. semen population in this boreal lake over the past 70 years was probably due to a combination of climate change and local anthropogenic activities in the catchment, causing increased browning and increased inputs of organic matter and nutrients.     

热带浅水湖泊后生浮游动物群落结构对生态修复的响应

热带浅水湖泊后生浮游动物群落以轮虫和小型枝角类为主,往往缺乏大型浮游动物,其影响因子还存在争议,因此通过减少鱼类,提高大型后生浮游动物密度,从而增强对浮游植物的下行控制、修复热带富营养化湖泊的方法受到质疑.暨大南湖位于广州暨南大学校园内,是热带富营养化浅水湖泊,于2014年1—2月实施以鱼类去除和水生植被重建为主的生态系统修复工程,以期改善水质.通过对南湖修复前后水质、后生浮游动物以及修复后鱼类群落的调查分析,研究了后生浮游动物群落对修复的响应.结果表明:与修复前相比,总氮、总磷和叶绿素a浓度显著降低,透明度显著上升.枝角类丰度和生物量显著增加,修复后初期出现较高密度的大型枝角类蚤状溞(Daphnia pulex),桡足类丰度和生物量没有显著变化,轮虫丰度下降而生物量上升;后生浮游动物体长和生物多样性显著增加.随着鱼类密度的增加,蚤状溞丰度快速降低,后期鱼类减少后蚤状溞丰度又有所升高.因此,本研究显示鱼类是控制热带浅水湖泊大型浮游动物种群的主要因素,由于控制鱼类密度以维持较高大型浮游动物密度较为困难,因此单纯生物操纵在热带浅水湖泊修复中难以取得显著效果.     

The diel vertical migration of the nuisance alga Gonyostomum semen is controlled by temperature and by a circadian clock

Gonyostomum semen is a bloom-forming freshwater raphidophyte that is currently on the increase, which concerns stakeholders and ecologists alike. Much indicates that the success of G. semen is linked to its diel vertical migration (DVM). The DVM was therefore studied in its natural setting using environmental sensors with data logging. DVM happened partly in the absence of light detectable by sensors, suggesting that the DVM involved gravitaxis, although an involvement of phototaxis could not be fully excluded. G. semen migrated downwards the earlier the warmer the water was. The upwards migration showed features of endogenous rhythms, implying that the temporal control of the DVM also has a circadian component that ensures synchronization with the day-night cycle. It is hypothesized that by causing an early descent, exposure to warm water may give G. semen more time to exploit the nutrient-rich lower part of the water column before the circadian control initiates upwards migration. The warming of lakes, caused by global warming or by the more effective conversion of light energy into heat due to the brownification of lakes, may therefore be of advantage to G. semen. In addition, the steep temperature gradient, which is typical for humic lakes during summer stratification, may help avoid the desynchronizing effect that turbulence can have on gravitactic DVMs. Above features of the DVM of G. semen therefore provide putative explanations for why the alga is particularly successful in humic lakes and why it profits from lake brownification and global warming.     

Differences in zooplankton feeding rates and isotopic signatures from three temperate lakes

Bacteria and phytoplankton are integral in the mobilization and transfer of organic matter to higher trophic levels. Hence, we examined their role in zooplankton diets and assessed trends in their nitrogen isotopic variability. We performed feeding experiments with natural particulate organic matter (POM) and four zooplankton groups (Daphnia, Holopedium, large calanoids and small calanoids) to (1) examine whether there are differences in consumption (presented as clearance and ingestion rates) of phytoplankton and bacteria, and (2) determine whether differences in zooplankton clearance and ingestion rates are correlated with their δ¹⁵N isotopic signatures. In general, phytoplankton and bacteria clearance rates and biomass ingested per animal varied significantly among different zooplankton groups within lakes and between lakes for a given zooplankton group. Within a given lake, Daphnia and Holopedium had the highest phytoplankton and bacteria clearance and ingestion rates, followed by large calanoids, and then small calanoids. For a given zooplankton group, bacteria and phytoplankton clearance rates varied among lakes. In contrast, phytoplankton ingestion rates were consistently highest in Dickie Lake for all taxa, whereas bacteria ingestion rates were more variable among lakes for the different zooplankton taxa. The percentage contribution of different phytoplankton taxa to the biomass of phytoplankton ingested also varied significantly among lakes for a given taxa, but there were few differences within a given lake among zooplankton. Zooplankton δ¹⁵N_DOMC values were correlated with their size adjusted phytoplankton and bacteria clearance and ingestion rates. The correlations were stronger with (1) phytoplankton compared to bacteria, and (2) clearance rates compared to ingestion rates of biomass. Together our results suggest that zooplankton taxa with low phytoplankton and bacteria clearance and ingestion rates and higher δ¹⁵N_DOMC are likely exploiting food sources from higher trophic levels.     

Zooplankton response to long-term liming: Comparison of 15 limed and 15 reference lakes in Sweden

Taxonomic composition and variation in density of zooplankton were studied since 1989 to investigate possible long-term effects of anthropogenic acidification and liming in Sweden (the Integrated Studies of the Effects of Liming in Acidified Water (ISELAW) programme). In the programme, 15 long-term (18-31 years) limed lakes were studied in parallel with 8 untreated circumneutral lakes (representing target conditions) and 7 acid lakes. Five-year means from these lakes were used to evaluate differences between the three lake groups. Water chemistry (including trace metals), phytoplankton, invertebrate predators and fish were also monitored. Older records from 8 of the limed lakes obtained during the acid period preceding the initial liming were compared with records from the limed period. The number of taxa more than doubled in 6 lakes after liming. Total present-day zooplankton biovolume showed insignificant differences between the neutral and limed groups, i.e., conformed with the target conditions. Trace metals, including inorganic aluminium, appeared in low concentrations in limed and neutral lakes whereas zooplankton in one acid lake was Al-intoxicated. Daphnid populations in 4 other lakes may suffer from effects of high Cd or Cu concentrations. “Bottom-up” regulation of the zooplankton biovolumes was indicated by positive regressions between total phosphorus-phytoplankton and phytoplankton-zooplankton. Corresponding tests showed no “top-down” regulation of zooplankton by fish and of phytoplankton by zooplankton. Biomasses of planktivorous fish were higher in the neutral reference lakes than in both the acid and the limed lake groups, but neither zooplankton density and biovolume nor mean body size were lower in the neutral references. A higher production and turnover of zooplankton in neutral reference lakes may explain similar densities and structure of zooplankton in spite of a heavier predation pressure.     

The distribution of large mixotrophic ciliates (Stentor) in deep North Patagonian lakes (Chile): First results

This study showed first results on the geographical distribution of mixotrophic ciliates of the genus Stentor in the Chilean North Patagonian Lake District. Thirteen deep North Patagonian lakes were sampled during 2004 and 2005, and important ecological parameters as nutrients, light climate, chlorophyll a and all plankton groups were analysed. The data were evaluated using correlation, cluster and discriminant function analyses.In four of 13 lakes, Stentor was an important constituent of the plankton community and contributed significantly to the total zooplankton biomass. It coinhabited lakes with calanoid copepods (Boeckella, Tumeodiaptomus) and small cladocerans (Eubosmina, Ceriodaphnia). Stentor was negatively correlated with cyclopoid copepods and large cladocerans. Cyclopoid copepods were a very good predictor to discriminate between lakes with and without Stentor. It is suggested that cyclopoid copepods have top-down impact on Stentor.     

The effects of nutrients and zooplankton community structure on phytoplankton growth in a subtropical Australian reservoir: An enclosure study

Patterns of zooplankton–phytoplankton interactions in subtropical lakes of the Southern Hemisphere may deviate from those established for north-temperate lakes. We tested the responses of phytoplankton growth to different community structures of zooplankton and nutrient enrichment in a subtropical Australian reservoir for the prediction of potential outcomes of lake biomanipulation. Two zooplankton communities were created in lake enclosures over 4 weeks: a rotifer-dominated community developed in the presence of planktivorous fish (Hypseleotris spp.) and a Ceriodaphnia-dominated community developed in the absence of fish. Biomass gradients of both communities were established in 20 L containers and several separate containers received no additions (controls) or were enriched with nitrogen and/or phosphorus. The growth rate of total phytoplankton significantly increased in response to nutrient enrichment, indicating nutrient limitation. Most phytoplankton taxa were not markedly affected by grazing of either zooplankton community. However, both communities had significant stimulatory effects on the growth of inedible chlorophytes. The ability of zooplankton grazing to negatively affect phytoplankton growth during the summer was counteracted regardless of zooplankton community structure, possibly by nutrients regenerated by zooplankton. We hypothesise that in the subtropical system studied, changes in food web nutrient recycling may be more important for the outcome of biomanipulation than grazing impacts.     

富营养化初期湖泊放养鲢(Hypophthalmichthys molitrix)、鳙(Aristichthys nobilis)控藻生态效果的初步评估

放养鲢（Hypophthalmichthys molitrix）、鳙（Aristichthys mobilis）控制富营养化湖泊蓝藻水华暴发是当前最有效的手段之一.为评估富营养化初期湖泊鲢、鳙控藻的生态效果,2016年在洱海红山湾开展原位围栏鲢、鳙控藻实验,对围栏内外的浮游生物群落结构及水环境因子的变化进行对比分析,同时测定了围栏内鲢、鳙的生长及摄食节律.结果表明：围栏内鲢、鳙呈现匀速生长,围栏生存环境适宜.根据Egger''s模型估算鲢、鳙的日摄食量分别为24.96%和18.18%,鲢的日粮高于鳙;滤食率结果表明,鲢对浮游植物的摄食率高于鳙（分别为3.01和2.19 L/（g·h）,而鳙对浮游动物的滤食率高于鲢（分别为18.61和13.54 L/（g·h））.研究期间,围栏内外水体理化因子无显著差异;但围栏内外微囊藻生物量差异明显,并且在7月达到生物量峰值（围栏内为1.7 mg/L,围栏外为2.4 mg/L）.鲢、鳙对浮游植物的优势种类——微囊藻和隐藻有明显的削减效果,6-8月逐渐上升并在8月达到最高（64%）;当浮游植物优势种由微囊藻替换为硅藻门和绿藻门的种类时,鲢、鳙的削减效率不明显（10-11月为负值）.此外,鲢、鳙放养显著降低了浮游甲壳动物的总生物量和枝角类生物量.因此,结合实验结果得出,鲢具有更高的控藻能力,尤其对微囊藻水华,但同时具备了较高对浮游甲壳动物的削减能力,需要进一步分析富营养化初期湖泊——洱海中实施非经典生物操纵的适宜性.     

Zooplankton community structure and ecology of the tropical-highland Lake Hayq, Ethiopia

Lake Hayq, a highland lake in Ethiopia, was stocked with Tilapia fish (Oreochromis niloticus) in late 1970s, offering an opportunity to study the effect of fish predation in a natural lake. Since 1930s, some limnological surveys have been done sporadically documenting a change in zooplankton composition including the disappearance of cladocerans, hypothesizing the stocked planktivorous fish could be a cause. Nevertheless, no detailed research was conducted to identify potential effects of fish stocking predominantly due to its remote location. The article presents data about zooplankton composition, abundance and biomass done between October 2007 and January 2009 on short-time intervals including the underlying limnological variables. The zooplankton community was depauperate comprising two copepods, three cladocerans, and six rotifers taxa, as typical for tropical lakes. Total mean standing biomass of all crustacean zooplankton was 237 mg dry mass m⁻³, which gave Lake Hayq an intermediate position when compared with other tropical lakes. Of copepods, Thermocyclops ethiopiensis was almost an exclusive species, and its temporal variation was influenced by food supply and water temperature. We refute the hypothesis that Tilapia was the cause for the seasonal disappearance of cladocerans, and attribute it to the adverse effect of episodic mixing. Nevertheless, the planktivorous fish probably plays a key role in structuring the cladocerans in particular the large-sized Daphnia magna. In January 2008, we observed a massive planktivorous fish mortality that triggered high algal biomass, which was later grazed by large-sized D. magna demonstrating the trophic cascade hypothesis in a natural ecosystem.     

Abiotic controls of potentially harmful algal blooms in Santa Monica Bay,California

Despite the increasing occurrence of harmful phytoplankton blooms along the North American west coast, records of phytoplankton populations and related environmental conditions are uncommon. In this study, twice monthly measurements in the upper 50 m are used to assess physico-chemical conditions contributing to the growth of potentially harmful bloom taxa over two annual cycles (2004–2005) in the Santa Monica Bay, California. Results were compared to the predictions of the Intaglio model [Smayda, T.J., Reynolds, C.S., 2001. Community assembly in marine phytoplankton: application of recent models to harmful dinoflagellate blooms. Journal of Plankton Research 23, 447–461.] of phytoplankton community assembly. Potentially harmful taxa were present in every surface sample and were numerically dominant during the largest observed blooms, contributing up to 92% of the total phytoplankton abundance >5 μm. Large interannual variation was observed in the dominant taxa and bloom seasonality; Pseudo-nitzschia sp. dominated blooms in early 2004 (February and April), whereas Prorocentrum micans and Lingulodinium polyedrum blooms occurred in May and September of 2005, respectively. The Pseudo-nitzschia sp. blooms were associated with elevated nitrate, dissolved silicon and phosphate concentrations throughout the euphotic zone; the first bloom followed a strong upwelling and the second occurred during the onset of seasonal stratification. In contrast, the blooms of P. micans were associated with highly stratified, low nutrient waters. Multivariate analysis supports the roles of temperature, mixed-layer depth and nutrient concentrations as primary controls of bloom growth, following the conceptual Intaglio model. The strong presence of potentially harmful bloom species in the Santa Monica Bay during this study appears unusual in comparison to limited studies over the last several decades.     

非经典生物操纵驱动从水华向非水华的稳态转化——来自武汉东湖的全湖验证实验

对藻类水华的生态控制区分为欧美学者提出的经典生物操纵和我国学者提出的非经典生物操纵,前者依赖浮游动物,可用来控制小型藻类,后者依赖滤食性鱼类,可用来控制形成群体的蓝藻（特别是微囊藻）。武汉东湖是一个富营养化城市湖泊,30余年未发生水华,2021年突然暴发微囊藻水华,一度覆盖湖面近87%。东湖生态站通过对长期监测数据的分析发现,东湖目前的营养水平仍然适合蓝藻的暴发,而短期内又无法进一步有效削减营养盐负荷,因此,基于非经典生物操纵的理论（合理配置本土的滤食性鲢、鳙）,实施了定量化的生物控藻方案,结果控藻成效显著,在极端高温干旱的2022年并未发生蓝藻水华,即将东湖生态系统从水华（浊水）稳态迅速切换到了非水华稳态,完成了一种新的稳态转换,获得了经典生物操纵从未有过的成功。非经典生物操纵理论的有效性通过全湖实验得到了验证,证明它提供了一种对环境友好、成本极低且颇为有效的控制富营养湖泊中形成群体蓝藻水华的方法,具有广阔的应用前景。     

鳑鲏与河蚌交互作用对浮游生物和底栖生物的影响分析

放养河蚌,提高水体透明度以促进沉水植物生长,是湖泊生态修复中的常用手段之一.而小型杂食性鱼类鳑鲏依赖河蚌繁殖,河蚌放养可能会促进鳑鲏种群的发展;而鳑鲏与河蚌交互作用对水生态系统的影响仍研究较少.于2018年11 12月通过原位受控实验,设置对照组、河蚌组、鳑鲏组和河蚌+鳑鲏组,研究了鳑鲏(大鳍鱊Acheilognathus macropterus)与河蚌(背角无齿蚌Sinanodonta woodiana)对水质、浮游生物和底栖生物的影响.结果表明:鳑鲏显著增加了上覆水总氮、总磷、悬浮质和叶绿素a(Chl.a)浓度;但并未显著影响无机悬浮质的浓度,说明鳑鲏对沉积物的扰动作用较弱.此外,鳑鲏显著增加了浮游植物生物量和蓝藻的比例.浮游动物总生物量并未受鳑鲏的影响,但显著增加了浮游动物的丰度,其中以轮虫为主.鳑鲏组的底栖动物总丰度(主要为水丝蚓)显著高于对照组,说明鳑鲏对水丝蚓的摄食压力较低.鳑鲏与河蚌交互作用对总磷、Chl.a、浮游植物和浮游动物丰度具有显著影响:具体表现为河蚌虽然抵消了部分鳑鲏对水质(如氮、磷和悬浮质浓度,以及Chl.a浓度和浮游植物生物量等)的负面影响,但其对浮游动物和水丝蚓生物量的影响不显著.在湖泊生态修复和管理中,需要关注和重视鳑鲏等此类小型杂食性鱼类对水生态系统可能产生的负面影响,通过物理(网簖等)或生物(放养肉食性鱼类)等方式将其控制在较低密度水平,从而降低这些鱼类对水质、浮游生物和底栖生物群落可能产生的负面影响,维持生态修复效果的长效与稳定运行.     

Cladoceran community composition in tropical semi-arid highland reservoirs in Tigray (Northern Ethiopia): A metacommunity perspective applied to young reservoirs

A field survey of zooplankton communities was carried out in 32 recently established tropical semi-arid reservoirs in the highlands of Northern Ethiopia with the aim to identify to what extent environmental factors determine species composition of the cladoceran community in such isolated and young reservoirs. To address seasonal variation, the survey was carried out both at the beginning and the end of the dry season. A total of 15 species of cladocerans were identified. Daphnia was the most abundant cladoceran genus, and was present in all reservoirs. Using presence-absence data, no association between cladoceran community composition and geographic distance was found. RDA results indicate that the set of environmental variables that explained cladoceran community composition differed among seasons. Depth, altitude and fish biomass showed a significant association with cladoceran community composition during the wet season, whereas variation in cladoceran community structure was associated with phytoplankton biomass in the dry season. The relative abundance of Daphnia was much higher in the pelagic than in the littoral zone of our study systems. Two key groups of pelagic filter-feeding cladocerans, Diaphanosoma and Daphnia, showed a clear pattern, in which one or the other tended to strongly dominate the community. In addition, we observed a negative association between dominance of Daphnia in the zooplankton community and dominance of cyanobacteria in the phytoplankton community.