Linking silver carp habitat selection to flow and phytoplankton in the Mississippi River

Invasive silver carp (Hypothalmichthys molitrix) occurs throughout much of the Mississippi River and threatens the Laurentian Great Lakes. To quantify habitat selection relative to river flow and potential phytoplankton food, 77 adult silver carp were implanted with ultrasonic transmitters during spring 2008 through spring 2009 in adjacent upstream dammed and downstream undammed reaches (48 km total) of the Mississippi River. Sixty-seven percent of the fish were located. Selection of major river habitat features (dammed vs. undammed, backwaters, channel border, wing dikes, island side channels, and the main channel) was quantified. Flow rates and chlorophyll a concentration were compared between silver carp locations and random sites. Foregut chlorophyll a concentrations plus presence of macrozooplankton and detritus of 240 non-tagged silver carp were quantified. About 30% of silver carp moved upstream into the dammed reach, where average flow was slower and chlorophyll a concentration was higher. Silver carp selected wing dike areas of moderate flow (about 0.3 m/s) and elevated chlorophyll a (about 7 μg/L) relative to random sites. No silver carp occurred in areas where flow was absent. Wing dikes were preferred while the main channel was avoided. Chlorophyll a concentrations in guts were positively related to temperature and were unrelated to flow or river chlorophyll a concentration. Macrozooplankton and detritus were rare in guts. Silver carp seek areas of low flow and successfully forage across a range of temperatures, flows, and chlorophyll a concentrations that occur in rivers and large lakes.     

Retention of suspended sediment and phosphorus on a freshwater delta, South Lake Tahoe, California

The Upper Truckee River and Trout Creek, two major tributaries inflowing to Lake Tahoe, join to form what was historically the largest wetland in the Sierra Nevada mountain range that separates California and Nevada (USA). In the 1950s the delta floodplain of the Upper Truckee River was greatly reduced in area (38%) by urban development and the diversion of the river into a single excavated channel. Conversely, Trout Creek still flows through a wide marsh system with significant overbank flooding before entering Lake Tahoe. This study hypothesized that river channel reaches that are not incised within the delta floodplain retain more sediment and nutrients as a result of greater floodplain connectivity, compared to more incised and excavated reaches. Suspended sediment (SS) and total phosphorus (TP) load data from the delta formed by the Upper Truckee River and Trout Creek were collected using flow stage sensors, turbidometers and depth-integrated samples. During the spring snowmelt flow events monitored in 2003, SS load was reduced by 13–41% for the Upper Truckee River and by 68–90% for Trout Creek. Similar reductions in TP load were observed: 13–32% for the Upper Truckee River and 61–84% for Trout Creek. Monitoring of Trout Creek indicated a reduction in load per unit volume of 20–34% in a moderately incised reach versus a reduction of 51–77% in a non-incised marsh reach containing lagoons, braided channels and backwater areas created by a beaver dam. Smaller particle sizes, <10 μm, were retained in the lower marsh reach with similar efficiencies as larger particle sizes. If retention rates from the Trout Creek portion of the marsh are applied to the Upper Truckee River, sediment loading to Lake Tahoe for 2003 would have been reduced by 917 tons of SS.     

Multistate models of bigheaded carps in the Illinois River reveal spatial dynamics of invasive species

Knowledge of the spatial distributions and dispersal characteristics of invasive species is necessary for managing the spread of highly mobile species, such as invasive bigheaded carps (Bighead Carp [Hypophthalmichthys nobilis] and Silver Carp [H. molitrix]). Management of invasive bigheaded carps in the Illinois River has focused on using human-made barriers and harvest to limit dispersal towards the Laurentian Great Lakes. Acoustic telemetry data were used to parameterize multistate models to examine the spatial dynamics of bigheaded carps in the Illinois River to (1) evaluate the effects of existing dams on movement, (2) identify how individuals distribute among pools, and (3) gauge the effects of reductions in movement towards the invasion front. Multistate models estimated that movement was generally less likely among upper river pools (Starved Rock, Marseilles, and Dresden Island) than the lower river (La Grange and Peoria) which matched the pattern of gated versus wicket style dams. Simulations using estimated movement probabilities indicated that Bighead Carp accumulate in La Grange Pool while Silver Carp accumulate in Alton Pool. Fewer Bighead Carp reached the upper river compared to Silver Carp during simulations. Reducing upstream movement probabilities (e.g., reduced propagule pressure) by?≥?75% into any of the upper river pools could reduce upper river abundance with similar results regardless of location. Given bigheaded carp reproduction in the upper Illinois River is presently limited, reduced movement towards the invasion front coupled with removal of individuals reaching these areas could limit potential future dispersal towards the Great Lakes.     

长江水系钱草鱼遗传结构及变异性的RAPD研究

采用随机扩增多态性DNA（RAPD）技术对长江中游的湖北嘉鱼与江西瑞昌两个地理群体和中游的汉江和湘江两大水系的鲢和草鱼的群体进行了遗传学研究。发现长江水系链的遗传变异要高于草鱼,与现今生物量成反比的反常现象。鲢遗传多样性从大到小的分布是嘉鱼→湘江→瑞昌→汉江。草鱼遗传多样性从大到小的分布是瑞昌→汉江→湘江→嘉鱼,鲢和草鱼的遗传多样性地理分布并不一致。遗传分化指数Gst分别为12.3%和17.5%,表明鲢和草鱼的四个地理群体的遗传分化度较低,可能与地理较近和基因交流频繁有关。     

The Invasion of the Chinese Mitten Crab (<Emphasis Type="Italic">Eriocheir sinensis</Emphasis>) in the United Kingdom and Its Comparison to Continental Europe

Owing to its catadromous lifestyle, the Chinese mitten crab, Eriocheir sinensis, allows comparison between a coastal and an inland biological invasion of the same species. Information about the distribution of this species in the United Kingdom has been collected from sightings made by governmental agencies, The Natural History Museum (London) collection, literature, and from the general public. This information indicated that the range of the species has expanded since the species’ arrival in 1973. The spread has been most marked along the east coast northwards to the river Tyne, on the south coast westwards to the river Teign. The expansion range was quantified and compared using geographic information software, and then compared to recorded spread in Europe. Mitten crabs dispersed along the coast at an average rate of 78 km per year (1976–1999), with a recent sharp increase to 448 km per year (1997–1999). These values are comparable with the historic outbreak in continental Europe where the average rate of dispersion along the Baltic Sea coast (1928–1935) was 416 km per year. Comparable figures for the North Sea coast (1923–1954) were 75 km per year with a peak of 168 km per year in 1927–1937. The upstream spread along rivers in the United Kingdom was 16 km per year in 1973–1998 with a marked increase since 1995 to 49 km per year (1995–1998). These data, in combination with population data published for the river Thames, indicate that the population has been increasing since the early 1990s, causing further range expansion into previously uninvaded river systems. The comparison of the spreading behaviour of the ongoing invasion in the United Kingdom with the historic invasion in northern Europe suggests that E. sinensis in future has the potential to establish itself in all major UK estuaries.     

Nitrogen attenuation in the Connecticut River,northeastern USA; a comparison of mass balance and N<Subscript>2</Subscript> production modeling approaches

Two methods were used to measure in-stream nitrogen loss in the Connecticut River during studies conducted in April and August 2005. A mass balance on nitrogen inputs and output for two study reaches (55 and 66 km), at spring high flow and at summer low flow, was computed on the basis of total nitrogen concentrations and measured river discharges in the Connecticut River and its tributaries. In a 10.3 km subreach of the northern 66 km reach, concentrations of dissolved N₂ were also measured during summer low flow and compared to modeled N₂ concentrations (based on temperature and atmospheric gas exchange rates) to determine the measured “excess” N₂ that indicates denitrification. Mass balance results showed no in-stream nitrogen loss in either reach during April 2005, and no nitrogen loss in the southern 55 km study reach during August 2005. In the northern 66 km reach during August 2005, however, nitrogen output was 18% less than the total nitrogen inputs to the reach. N₂ sampling results gave an estimated rate of N₂ production that would remove 3.3% of the nitrogen load in the river over the 10.3 km northern sub-reach. The nitrogen losses measured in the northern reach in August 2005 may represent an approximate upper limit for nitrogen attenuation in the Connecticut River because denitrification processes are most active during warm summer temperatures and because the study was performed during the annual low-flow period when total nitrogen loads are small.     

50年来长江鲢、鳙形态特征的变迁(英文)

采用多元分析方法,对采自长江中游的上世纪中期(下称早期)和本世纪初(下称当前)的121尾1龄鲢、鳙的30项形态度量性状进行了分析。依据对判别贡献最大的5个参数所建立的判别公式,对鲢、鳙早期和当前群体的平均判别准确率分别达94.2%和98.%,显示鲢或鳙的当前与早期群体间在总体上存在着显著差异。单因素方差分析显示,在早期和当前群体间,鲢有12个特征差异极显著、8个差异显著,鳙分别有8个特征差异极显著或显著。其中,鲢和鳙的6个显著或极显著变大的特征参数集中在头部,而14个鲢和10个鳙的显著或极显著变小的特征参数则主要集中在躯干部与尾部。研究表明,最近50年来长江鲢、鳙的头及头部特征有相对变大的趋势,而躯干部与尾部的许多特征参数则有相对变小的趋势,这些形态演变趋势符合人们喜食鱼头、追求高经济效益的期望。     

Preliminary evidence of delayed spawning and suppressed larval growth and condition of the major carps in the Yangtze River below the Three Gorges Dam

Commercial catches of the ‘major carps’ (grass carp Ctenopharyngodon idella, silver carp Hypophthalmichthys molitrix, black carp Mylopharyngodon piceus and bighead carp Aristichthys nobilis) in the middle and lower reaches of the Yangtze River have declined precipitously since construction of the Three Gorges Dam (TGD). To assess specific environmental effects, young-of-the-year major carps were collected below TGD in the Yangtze River at Jianli (larvae) and E’zhou (juveniles) and in the Dongting Lake (juveniles) during June–August 2007. Lapillar otoliths were analyzed to determine early growth. There were no discernible growth increments in otoliths for a majority of the larvae (38 of 63 grass carp and 40 of 47 silver carp), while increments were evident in otoliths formed in the larval stage for all the juveniles, indicating that larvae without discernible increments failed to recruit to the juvenile stage. Back-calculated hatch dates of major carps were between 31 May and 24 July. The initiation of the spawning season was delayed about 1 month compared to pre-TGD records. Larval growth rates, as reflected by otolith-increment width, were lower at Dongting Lake (closer to TGD) than at E’zhou (farther from TGD), indicating that TGD-moderated effects on early growth and development of carps are more apparent for fishes hatched nearer to the dam. The delay in spawning and decreased early growth may partly explain the recent decline of the major carp resources in the middle and lower Yangtze River.     

An experimental test and models of drift and dispersal processes of pallid sturgeon (<Emphasis Type="Italic">Scaphirhynchus albus</Emphasis>) free embryos in the Missouri River

Free embryos of wild pallid sturgeon Scaphirhynchus albus were released in the Missouri River and captured at downstream sites through a 180-km reach of the river to examine ontogenetic drift and dispersal processes. Free embryos drifted primarily in the fastest portion of the river channel, and initial drift velocities for all age groups (mean = 0.66–0.70 m s⁻¹) were only slightly slower than mean water column velocity (0.72 m s⁻¹). During the multi-day long-distance drift period, drift velocities of all age groups declined an average of 9.7% day⁻¹. Younger free embryos remained in the drift upon termination of the study; whereas, older age groups transitioned from drifting to settling during the study. Models based on growth of free embryos, drift behavior, size-related variations in drift rates, and channel hydraulic characteristics were developed to estimate cumulative distance drifted during ontogenetic development through a range of simulated water temperatures and velocity conditions. Those models indicated that the average free embryo would be expected to drift several hundred km during ontogenetic development. Empirical data and model results highlight the long-duration, long-distance drift and dispersal processes for pallid sturgeon early life stages. In addition, results provide a likely mechanism for lack of pallid sturgeon recruitment in fragmented river reaches where dams and reservoirs reduce the length of free-flowing river available for pallid sturgeon free embryos during ontogenetic development.     

Evidence for predatory control of the invasive round goby

We coupled bioenergetics modeling with bottom trawl survey results to evaluate the capacity of piscivorous fish in eastern Lake Erie to exert predatory control of the invading population of round goby Neogobius melanostomus. In the offshore (>20 m deep) waters of eastern Lake Erie, burbot Lota lota is a native top predator, feeding on a suite of prey fishes. The round goby invaded eastern Lake Erie during the late 1990s, and round goby population size increased dramatically during 1999–2004. According to annual bottom trawl survey results, round goby abundance in offshore waters peaked in 2004, but then declined during 2004–2008. Coincidentally, round goby became an important component of burbot diet beginning in 2003. Using bottom trawling and gill netting, we estimated adult burbot abundance and age structure in eastern Lake Erie during 2007. Diet composition and energy density of eastern Lake Erie burbot were also determined during 2007. This information, along with estimates of burbot growth, burbot mortality, burbot water temperature regime, and energy densities of prey fish from the literature, were incorporated into a bioenergetics model application to estimate annual consumption of round goby by the adult burbot population. Results indicated that the adult burbot population in eastern Lake Erie annually consumed 1,361 metric tons of round goby. Based on the results of bottom trawling, we estimated the biomass of yearling and older round goby in offshore waters eastern Lake Erie during 2007–2008 to be 2,232 metric tons. Thus, the adult burbot population was feeding on round goby at an annual rate equal to 61% of the estimated round goby standing stock. We concluded that the burbot population had high potential to exert predatory control on round goby in offshore waters of eastern Lake Erie.     

Experimental study of the impacts of silver carp on plankton communities of eutrophic Villerest reservoir (France)

We examined the impact of five silver carp biomass levels (0, 8, 16, 20, and 32 g m⁻³) on plankton communities and water quality of Villerest eutrophic reservoir (France). We realized the experiments using outdoor mesocosms. The presence of silver carp led to changes in zooplankton and phytoplankton assemblages. High fish biomass strongly reduced cladoceran abundance (through predation). Silver carp inefficiently grazed down particles < 20 μm. More importantly, however, the suppression of herbivorous cladocerans resulted in the increase of small size algae which were relieved from grazing and benefit from high nutrient concentrations. In contrast, in mesocosms without fish, the dominance of cladocerans (mainly Daphnia) controlled small size algae and probably also larger size algae (colonial chlorophytes, cyanobacteria). Thus, the Secchi disc transparency increased markedly. Through cascade effects, the modification of grazers communities led to changes in the utilization patterns of the added nutrients by phytoplankton communities. In high fish biomass treatments, nutrients were more efficiently accumulated into particulate fractions compared with no-fish and low-fish biomass treatments that were characterized by higher dissolved nutrients concentrations. Zooplankton was an essential source of food for silver carp. The productivity of zooplankton sustained a moderate silver carp biomass (up to 16 g m⁻³). In the presence of the highest fish biomass, the productivity of zooplankton was not large enough and silver carps fed on additional phytoplankton. Although mesocosms with high fish biomass were characterized by a slight cyanobacteria development compared with other fish mesocosms, silver carp was not effective in reducing cyanobacteria dominance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Degradation of icefishes (Salangidae) in the Yangtze River basin of China: threats and strategies

The Yangtze River basin has been a major area in terms of icefish diversity conservation and fisheries. In the past two to five decades, however, the wild icefish resources have degraded continuously in both the brackish estuary and freshwater portions of the basin. Stocks have declined in all icefish fisheries, resulting in an annual fishing yield decrease from the maximum of 4,000 metric tons to less than 1,300 metric tons in this river basin. The stocks decline significantly (t = −6.74, df = 1, p < 0.001) correlated with icefish fishing stress in the estuary. Meanwhile, species composition changed markedly with smaller sized icefish species increased relative to the number of larger ones in terms of their relative abundances. Following the stocks decline and size reduction, icefish commercial value decreased to the point that 91.9% of the traditional icefish-producing waters (57 out of 62) terminated their icefish fisheries and some species became locally extinct. Based on the degradation process, we suggest that these changes in stock size and species diversity are the result of overfishing and habitat loss and degradation. We propose a number of measures aimed at the conservation and restoration of the wild icefish resources in the Yangtze River basin.     

Phytoplankton and zooplankton development in a lowland, temperate river

The longitudinal and seasonal patterns of plankton developmentwere examined over 2 years in a lowland, temperate river: theRideau River (Ontario, Canada). Following an initial decreasein phytoplankton and zooplankton biomass as water flowed fromthe headwaters into the Rideau River proper, there was an increasein chlorophyll a (chl a) and zooplankton biomass with downstreamtravel. At approximately river km 60, both phytoplankton andzooplankton reached their maximum biomass of 27 µg l^–1(chl a) and 470 µg l^–1 (dry mass), respectively.Downstream of river km 60, the biomass of both planktonic communitiesdeclined significantly despite increasing nutrient concentrationsand favorable light conditions. These downstream declines maybe due to the feeding activity of the exotic zebra mussel (Dreissenapolymorpha) which was at high density in downstream reaches(>1000 individuals m^–2). There was no evidence forlongitudinal phasing of phytoplankton and zooplankton, as increasesand decreases in chl a and zooplankton biomass appeared to coincide.Overall, chl a was best predicted by total phosphorus (R²=0.43),whereas zooplankton biomass was best predicted by chl a (R²=0.20).There was no evidence for significant grazing effects of zooplanktonon phytoplankton biomass.     

Swimming performance of adult bighead carp Hypophthalmichthys nobilis (Richardson, 1845) and silver carp H. molitrix (Valenciennes, 1844)

Although the movement of invasive bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix) in the Upper Mississippi River system is dependent on their ability to swim through its numerous lock‐and‐dams, the swimming performance of adults of these species is at present unknown. Using a large (2,935‐L) mobile swim tunnel, the swimming performance of adult bighead and adult silver carp was quantified at water velocities that challenged them to exhibit either prolonged and/or burst swimming (76–244 cm/s) with fatigue times of less than 10 min. Simple log‐linear models best described the relative swim speed to fatigue relationships for both species. Under these conditions, the swimming performances of adult bighead and silver carp were similar to several species of adult fishes native to the Mississippi River system, but relatively low (<3 total body lengths per second, TL/s) compared to previously studied juveniles and sub‐adult bigheaded carps (3–15 TL/s). The decline in endurance with water velocity was three times greater in bighead carp (slope = ?2.98) than in silver carp (slope = ?1.01) and the predictive ability of the bighead model was appreciably better than the silver carp model. The differences in adult swimming performance between the two species were coincident with behavioral differences (e.g. breaching in silver carp but not in bighead carp). The swimming performance data of adult bighead and silver carp can now be used to evaluate whether their passage through manmade river structures including the gates of lock‐and‐dams in the Upper Mississippi River might be reduced.     

Population genetic structure and evolutionary history of grass carp <Emphasis Type="Italic">Ctenopharyngodon idella</Emphasis> in the Yangtze River,China

Ample studies have been conducted to investigate the population genetic structure of grass carp Ctenopharyngodon idella in the Yangtze River, China. However, samples from the upper reaches were not included. In this study, we collected samples from the entire river, including three locations in the upper reaches: Yibin, Banan and Yunyang, two locations in the middle reaches: Shishou and Ruichang and one location in the lower reaches: Hanjiang, and sequenced three mitochondrial coding genes (ND5, ND6 and Cytb) and one control region (i.e., the D-loop). Nineteen haplotypes were observed in grass carp of the Yangtze River through the analysis of combined sequence data sets (around 4428 bp). Haplotype diversity indices (0.6000 ∼ 0.9333) and nucleotide diversity indices (0.0002 ∼ 0.0020) demonstrated low genetic diversity in the Yangtze grass carp. The analysis of molecular variance and the fixation index (F _ST = 0.0202) revealed insignificant genetic difference between samples from different reaches. Two monophyletic lineages of haplotypes were identified, with the lineage A experiencing potential expansion events. Along with previous findings, this study provides a better understanding of genetic diversity and variation of grass carp in the Yangtze River and will be served as an important baseline to evaluate the long-term impact of the Three Gorges Dam and other hydroelectric facilities on fish biodiversity.     

Impact of moderate silver carp biomass gradient on zooplankton communities in a eutrophic reservoir. Consequences for the use of silver carp in biomanipulation.

We examined the impacts of moderate gradient silver carp biomass (five levels from 0 to 36 g.m-3, i.e. about 0-792 kg.ha-1) on zooplankton communities of the eutrophic Villerest reservoir (France). During our mesocosm experiment changes in zooplankton assemblages were dependent on silver carp biomass. In the fishless and low fish biomass treatments, zooplankton abundance increased through time, owing to a peak in cladoceran density, but decreased (mainly cladocerans) at highest fish biomass. Copepods and rotifers were less affected at the highest fish biomass and dominated zooplankton communities. We highlighted that the presence of high silver carp biomass could lead to changes in phytoplankton assemblage via the impact on herbivorous zooplankton. Since silver carp efficiently graze on particles > 20 microns, the suppression of herbivorous cladocerans could result in an increase in small size algae (< 20 microns) abundance since these species would be released from grazers as well as competitors (large algae grazed by silver carp) and nutrients levels would be enhanced by fish internal loading. Our results showed that the use of low silver carp biomass (< 200 kg.ha-1) would allow us to minimize these negative effects.     

Mobility of individual roach <Emphasis Type="Italic">Rutilus rutilus</Emphasis> (L.) in three weir-fragmented Belgian rivers

Adult roach Rutilus rutilus (L.) (N = 24; 19.9–36.1 cm FL) from three highly fragmented Belgian rivers were tagged with surgically implanted radio transmitters. Their seasonal movements were observed from March to August 2004 (circum reproduction period) in river stretches delimited by two physical barriers. In the three rivers, roach displayed similar patterns of movements which were mainly influenced by the date of observation (movements increased in late April–May) and water temperature (travel distances were more important when water temperature ranged between 10°C and 14°C). Roach sometimes cleared physical obstacles. The mean distances travelled in each river were relatively short (max. 2.5 km) and mainly influenced by the length of the study reach, which was delimited by physical barriers.     

Influence of drought on algal biofilms and meiofaunal assemblages of temperate reservoirs and rivers

The role of hydrological droughts in shaping meiofauna abundance through alterations in biofilm biomass and composition was investigated. In January 2005, continental Portugal was under a moderate to severe drought resulting from a 40% to 60% decrease in rainfall during the previous 12 months relative to the long-term average (1961–1990). Reservoir capacity was reduced by 30–50% relative to average values and the width of streams was reduced by 20–80% in the Zêzere River Basin (central Portugal). Algal biomass and algal class composition of biofilms was assessed through quantification of algal pigments in three reservoir and six river locations. During drought, habitat alterations are expected to be sharp in rivers while, in the absence of water quality deterioration, the habitat characteristics of reservoirs are expected to remain fairly unaffected. Chlorophylls and carotenoid pigments were extracted from biofilm samples and analysed using high performance liquid chromatography (HPLC). In the winter of 2003, during the period of average rainfall, biofilm biomass did not exceed 5 μg chlorophyll a cm⁻² at any location. River biofilm biomass was roughly half of that measured in the reservoirs. In the winter of 2005 (drought), biofilm biomass increased by more than 5-fold in river locations and remained low or decreased in the reservoirs. Algal biofilms were either dominated by Bacillariophyceae or by Chlorophyceae regardless of the existence of drought. The relative contribution of Bacillariophyceae to total biofilm biomass was higher during the drought than under average hydrological conditions. The abundance of harpacticoids, cladocerans and ostracods was favoured by the drought only in the reservoirs where an increase in diatom proportion in biofilms was observed. The increase in the abundance of cyclopoid copepods, turbellarians, nematodes and chironomids in rivers during the drought could be explained by algal class composition and biomass of biofilms and environmental variables (organic matter sediment content, phosphorus availability content and sediment granulometry). The hydrological drought appears to regulate meiofauna abundance only in river locations, possibly through the promotion of the growth of biofilms and the availability of organic matter deposited in rivers during the drought. Handling editor: D. Ryder     

Plankton composition and water chemistry in the mixing zone of the Selenga River with Lake Baikal

Chemical and biological components of the Selenga River waters, the largest tributary of Lake Baikal, differ significantly from the lake waters. Active transformation processes of river waters into the lake ones occur in the vast barrier-like zone in the river-sea boundary areas. This study presents results on the spatial distribution and dynamics of water chemistry as well as the quantity and diversity of phyto- and bacterioplankton at a distance of 14 km off the Selenga River mouth. The most representative tracers of river and lake waters are total amount of ions and sulphates. Principal changes of chemical and biological parameters were fixed at 1–3 km off the Selenga River mouth that was determined as a mixing zone. Intense development of phytoplankton and eukaryotic picoplankton causing the decrease of nitrate and phosphate concentrations and organic matter rise were registered in this area. Gradual replacement of river phytoplankton by the lake one, abundance reduction of microorganisms and organotrophic bacteria and percentage increase of oligotrophic and psychrotolerant bacteria occurred in the mixing zone. Replacement of PC-rich picocyanobacteria by PE-rich ones was also recorded here. At a distance of 5–7 km off the shore, nutrient concentration and plankton composition were similar to those of Lake Baikal.