Picoplankton community structure at a coastal front region in the northern part of the South China Sea

Abundances of picoplankton groups were determined by flow cytometryin the Northern South China Sea (SCS) in winter 2004 to studythe dynamics of picoplankton at a coastal front region. Prochlorococcusis more abundant in relatively high temperature and salinitywaters than in nearshore area. Heterotrophic bacteria dominatein total picoplanktonic biomass but keep rather stable in biomassand surface/bottom biomass ratio on both sides of the front.Increases of picophytoplanktonic biomass and their surface/bottombiomass ratio are remarkable mainly owing to the contributionof Synechococcus on the offshore open SCS waters. Temperatureis found to limit the growth of Synechococcus and Prochlorococcus.Picoeukaryotes and heterotrophic bacteria are less sensitiveto the change in hydrographic conditions across the front. Theautotrophic/heterotrophic biomass ratio of picoplankton is lowerin eutrophic coastal waters on the nearshore side relative tothe offshore and oligotrophic open SCS.     

The role of picoplankton (cyanobacteria and plastidic picoflagellates) in the diet of tintinnids

Food vacuole contents in naturally occurring populations oftintinnids in surface waters at the mouth of the Bay of Villefranchewere examined from November 1988 to December 1989. Three typesof prey were considered: cyanobactena (Synechococcus spp.),plastidic pico- and nanoflagel lates. The relative frequency(%) of these were observed in the diet of 18 tintinnid species.Results confirm that nanoflagellates constitute the dominantfraction in the diet of most of the tintinnids examined. Theyare preferentially ingested by some species, such as Amphoridesquadrilineata and Favella sp. Nevertheless, picoplankton isingested throughout the year by all the tintinnids, and canreach up to 100% of the ration mainly during winter and summer.Cyanobacteria are the prey most ingested by Rhabdonella spiralisand the only prey observed in food vacuoles of Salpingella acuminaia.As indicated by the Strauss linearity index values, picoflagellatesare actively grazed by some species (e.g. Dictyocysta elegans,Eutintinnus spp.). The implications of the ingestion of picoplanktonby tintinnids in the food webs are discussed.     

Productivity of picoplankton compared with that of larger phytoplankton in the subarctic region

We determined the productivity (µg C µg^–1Chi a h^–1) of size-fractionated phytoplankton in the northernNorth Pacific and the Bering Sea in summer and winter. Picoplankton(<2 µm) were more productive than larger sized phytoplankton(2–10 and 10–200 µm) in the subtropical region,where the in situ temperature was >10°C; whereas picoplanktonin the subarctic region were similar in productivity or lessproductive than larger sized plankton, where the in situ temperaturewas <10°C. The result from the subtropical region inthis study agrees with previous results from tropical and subtropical waters, which indicate that phytoplankton productivitytends to decrease with increasing cell size. The result fromthe subarctic region, however, differs from previous results.We observed a positive linear regression for in situ temperatureand picoplankton productivity, but this trend was not seen inthe larger sized phytoplankton. The results show that the productivityof picoplankton is markedly influenced by in situ temperaturecompared with that of larger sized plankton. Low tem peratureappears to account largely for the observation that the productivityof picoplankton is not significantly higher than that of largersized phytoplankton in the subarctic region.     

Effects of UV radiation on grazing by two marine heterotrophic nanoflagellates on autotrophic picoplankton

Phytoplankton <4-3 µm in diameter, or autotrophic picoplankton,can constitute the majority of the biomass and productivityof photosynthetic organisms in marine and freshwater systems.Indirect evidence has indicated that mortality of autotrophicpicoplankton occurs principally at night in the open ocean,but continuously in coastal water. The predominant view of thefate of autotrophic picoplankton production in the ocean isthat they are consumed by heterotrophic nanoflagellates. A possiblemechanism to explain these observations is that grazing of heterotrophicnanoflagellates on autotrophic picoplankton is inhibited byultraviolet radiation (W), at least in clear open-ocean environments.A series of laboratory experiments was conducted to examinethe effects of UV radiation on the grazing impact of two heterotrophicnanoflagellates on Synechococcus spp., a commonly occurringgenus of autotrophic picoplankton. The two nanoflagellates usedwere Paraphysomonas bandaieensis and Paraphysomonas imperforata.For both nanoflagellates, there was an inverse relationshipbetween the grazing mortality of Synechococcus and UV irradiance.The grazing mortality of Synechococcus was reduced less withP.imperforata than with P.bandaiensis. In some experiments,the effect of UV on the grazing impact of the nanoflagellatepopulations was caused in part by UV-related reductions in nanoflagellatesurvival. However, UV reduced the grazing impact of nanoflagellatesprimarily by reducing the rates of consumption of Synechococcusby individual nanoflagellates, to a degree directly relatedto UV irradiance. The results suggat that UV radiation may bean important selection factor in clear open-ocean water, andthat in order to predict the effect of increasing UV radiationon marine microbial plankton communities, we must consider interactionsbetween trophic levels as well as effects on single trophiclevels.     

Comparison of the Seasonal Variations of Synechococcus Assemblage Structures in Estuarine Waters and Coastal Waters of Hong Kong

Seasonal variation in the phylogenetic composition of Synechococcus assemblages in estuarine and coastal waters of Hong Kong was examined through pyrosequencing of the rpoC1 gene. Sixteen samples were collected in 2009 from two stations representing estuarine and ocean-influenced coastal waters, respectively. Synechococcus abundance in coastal waters gradually increased from 3.6 × 10³ cells ml⁻¹ in March, reaching a peak value of 5.7 × 10⁵ cells ml⁻¹ in July, and then gradually decreased to 9.3 × 10³ cells ml⁻¹ in December. The changes in Synechococcus abundance in estuarine waters followed a pattern similar to that in coastal waters, whereas its composition shifted from being dominated by phycoerythrin-rich (PE-type) strains in winter to phycocyanin-only (PC-type) strains in summer owing to the increase in freshwater discharge from the Pearl River and higher water temperature. The high abundance of PC-type Synechococcus was composed of subcluster 5.2 marine Synechococcus, freshwater Synechococcus (F-PC), and Cyanobium. The Synechococcus assemblage in the coastal waters, on the other hand, was dominated by marine PE-type Synechococcus, with subcluster 5.1 clades II and VI as the major lineages from April to September, when the summer monsoon prevailed. Besides these two clades, clade III cooccurred with clade V at relatively high abundance in summer. During winter, the Synechococcus assemblage compositions at the two sites were similar and were dominated by subcluster 5.1 clades II and IX and an undescribed clade (represented by Synechococcus sp. strain miyav). Clade IX Synechococcus was a relatively ubiquitous PE-type Synechococcus found at both sites, and our study demonstrates that some strains of the clade have the ability to deal with large variation of salinity in subtropical estuarine environments. Our study suggests that changes in seawater temperature and salinity caused by the seasonal variation of monsoonal forcing are two major determinants of the community composition and abundance of Synechococcus assemblages in Hong Kong waters.     

Abundance, frequency of dividing cells and growth rates of Synechococcus sp. (cyanobacteria) in the stratified Northwest Mediterranean Sea

The abundance, frequency of dividing cells and growth ratesof the planktonic cyano bacteria Synechococcus sp. during thesummer of 1995 and 1996 were estimated in the Northwest MediterraneanSea to test whether depth-dependent growth rates of this speciesexplain its dominance in the deep chlorophyll maximum (DCM)layer formed during summer thermal stratification in the NWMediterranean, compared to the surface layer. Abundance at theDCM layer (50–70 m) was up to two orders of magnitudegreater than that at the surface, with values ranging from 1.7to 13x10⁶ cells I-¹ and from 4 to 175 x 10⁶ cells I-¹ at thesurface and in DCM waters, respectively. Gross growth rates,however, were much higher at the surface than in the DCM layer(surface: 0.76–1.07 day DCM: 0.30–0.47 day-¹ Thehigher gross growth rates at the surface layer were supportedby a higher frequency of dividing cells (surface: 0.09–0.24;DCM: 0.01–0.12). The negative correlation between theabundance or standing stock and growth rates of these planktonicpicocyanobacteria points to losses, and not growth rate, asthe main control on the abundance of Synechococcus. Althoughwe provide some evidence that grazing alone may be able to accountfor these losses, further, direct determinations are clearlyneeded to elucidate the regulation of the abundance of Synechococcusin the NW Mediterranean.     

The water bloom of Cyanobacterial picoplankton in Lake Biwa,Japan

Unicellular autofluorescent picoplankton ranging from 0.4 to 1.5 µm in diameter were found to be a significant component of phytoplankton in the North Basin of Lake Biwa during early summer in 1989 and 1990. The abundance of these picoplankton varied seasonally by about three orders of magnitude with one maximum of up to 10⁶ cells ml^–1. Bloom-forming picoplankton were isolated by dilution and further cultivated in liquid medium. Three clones were found to be representative species of the bloom. Using epifluorescence and electron microscopy as well as absorption and fluorescence emission spectroscopy, we examined these clones according to shape and pigment composition. They have ringlike thylakoids, are photosynthetically active and have no nuclear envelope. The cyanobacterial clones isolated represent three types containing phycobilisomes with either phycocyanin or phycoerthrin as the dominant accessory pigment. They are described here as three new species, two phycoerythrin-rich types and one phycocyanin-rich type, all of them belonging to the Synechococcus group. The differences found by fluorescence emission of isolated clones are discussed with respect to in situ strain identification.     

Influence of the Changjiang River Flood on Synechococcus Ecology in the Surface Waters of the East China Sea

Synechococcus spp. have been suggested as the primary component of picophytoplankton in the East China Sea (ECS). However, the influences of sudden environmental changes on Synechococcus assemblage composition have not yet been investigated. In the summer of 2010, a disastrous flood occurred in the Changjiang River basin. To improve our understanding of how this flood affected the Synechococcus ecology on the ECS surface, their assemblages and distributions have been described using two-laser flow cytometry and phylogenetic analysis of the phycocyanin operon. During the nonflooding summer of 2009, phycoerythrin-rich (PE-rich) Synechococcus thrived near the outer boundary of the Changjiang River diluted water (CDW) coverage, while phycocyanin-rich (PC-rich) Synechococcus predominated inside the turbid CDW with a transparency of <80 %. During the 2010 summer, flooding expanded the CDW coverage area to over half of the ECS. PE-rich cells showed a homogeneous distribution and a decline in abundance, while the spatial pattern of the PC-rich Synechococcus resembled the pattern from 2009. Based on the phycocyanin operon phylogeny, the Synechococcus in the ECS were categorized into five groups, ECS-1 to ECS-4 and ECS-PE, comprising a total of 19 operational taxonomic units. In the summer of 2009, ECS-2 dominated in the coast, and the ECS-3 and ECS-PE clades prevailed in the offshore waters. However, during the summer of 2010, ECS-4 and ECS-PE became the dominant strains. The injection of abundant anthropogenic pollutants and the enhancement of transparency within the CDW expansion area appear to be the factors needed to transiently alter the ecology of Synechococcus after flooding.     

Phytoplankton population dynamics in perennially ice-covered Lake Fryxell, Antarctica

Phytoplankton were collected over five austral summers (1987–88through 1991–92) to examine seasonal and annual fluctuationsin species composition and biovolume in Lake Fryxell, a perenniallyice-covered lake located in the Dry Valleys of South VictoriaLand, Antarctica. Lake Fryxell has perennial gradients of salinity,dissolved oxygen and nutrients. We found that algal speciesdiversity was low (56 taxa were collected), confirming the resultsof previous short-term studies. The phytoplankton consistedprimarily of cryptophyte and chlorophyte flagellates, and filamentouscyanobacteria. The presence of filamentous cyanobacteria, whichhave not been reported as abundant in this lake by previousworkers, may represent a significant ecological change. Eachaustral summer, one dominant species contributed >70% ofthe total biovolume; Chroomonas lacustris was dominant in 1987–88,while Cryptomonas sp. dominated the phytoplankton in the remaining4 years. No species succession occurred during the austral summer.Some common taxa were vertically stratified (Oscillatoria limnetica,Phormidium angustissimum, Pyramimonas sp., Oscillatoria sp.),while others showed no distinct vertical stratification (Chlamydomonassubcaudata, Cryptomonas sp.). The stratification of the phytoplanktonreflects the gradients of nutrients and light, and the stabilityof the water column.     

Quantitative Tracing, by Taq Nuclease Assays, of a Synechococcus Ecotype in a Highly Diversified Natural Population

Quantitative Taq nuclease assays (TNAs) (TaqMan PCR), nested PCR in combination with denaturing gradient gel electrophoresis (DGGE), and epifluorescence microscopy were used to analyze the autotrophic picoplankton (APP) of Lake Constance. Microscopic analysis revealed dominance of phycoerythrin (PE)-rich Synechococcus spp. in the pelagic zone of this lake. Cells passing a 3-μm-pore-size filter were collected during the growth period of the years 1999 and 2000. The diversity of PE-rich Synechococcus spp. was examined using DGGE to analyze GC-clamped amplicons of a noncoding section of the 16S-23S intergenic spacer in the ribosomal operon. In both years, genotypes represented by three closely related PE-rich Synechococcus strains of our culture collection dominated the population, while other isolates were traced sporadically or were not detected in their original habitat by this method. For TNAs, primer-probe combinations for two taxonomic levels were used, one to quantify genomes of all known Synechococcus-type cyanobacteria in the APP of Lake Constance and one to enumerate genomes of a single ecotype represented by the PE-rich isolate Synechococcus sp. strain BO 8807. During the growth period, genome numbers of known Synechococcus spp. varied by 2 orders of magnitude (2.9 × 10³ to 3.1 × 10⁵ genomes per ml). The ecotype Synechococcus sp. strain BO 8807 was detected in every sample at concentrations between 1.6 × 10¹ and 1.3 × 10⁴ genomes per ml, contributing 0.02 to 5.7% of the quantified cyanobacterial picoplankton. Although the quantitative approach taken in this study has disclosed several shortcomings in the sampling and detection methods, this study demonstrated for the first time the extensive internal dynamics that lie beneath the seemingly arbitrary variations of a population of microbial photoautotrophs in the pelagic habitat.     

Comparative phosphorus nutrition of the marine cyanobacterium Synechococcus WH7803 and the marine diatom Thalassiosira weissflogii

Phosphate uptake kinetics of Synechococcus sp. WH7803 and Thalassiosiraweissflogii were studied in axenic batch culture. Phosphate-repleteSynechococcus sp. WH7803 cells have a lower affinity for inorganicphosphate (Pi) (K_s = 67 µmol l^–1) than Pi-starvedcells (K_s = 3.1 µmol l^–1). The K_s of Pi-starvedcells increased     

Seasonal variations of size-fractionated phytoplankton along the salinity gradient in the York River estuary, Virginia (USA)

The dynamics of phytoplankton size structure were investigatedin the freshwater, transitional and estuarine zones of the YorkRiver over an annual cycle. The contribution of large cells(microplankton, >20 µm) to total concentrations ofchlorophyll a increased downstream during winter, whereas thatof small cells (nanoplankton, 3–20 µm; picoplankton,<3 µm) increased downstream during summer. In the freshwaterregion, the contribution of micro phytoplankton to total concentrationsof chlorophyll a was significant during warm seasons (springand summer) but not during colder seasons (winter), whereasthe contribution of small-sized cells (especially picoplankton)increased during cold seasons. Temperature, light and high flushingrate appear to control phytoplankton community structure inthe freshwater region. In the transitional region, nano-sizedcells dominated the phytoplankton population throughout allseasons except during the spring bloom (April) when the chlorophylla concentration of micro phytoplankton increased. Size structurein the transitional region is most likely regulated by lightavailability. In the mesohaline region, nano- and pico-sizedcells dominated the phytoplankton population during the summerbloom, whereas micro-sized cells dominated during the winterbloom. Factors controlling phytoplankton community size structurein the mesohaline zone may be riverine nitrogen input, temperatureand/or advective transport from up-river. Based on these results,the spatial and seasonal variations in size structure of phytoplanktonobserved on the estuarine scale may be determined both by thedifferent preferences for nutrients and by different light requirementsof micro-, nano- and picoplankton. The results suggest thatanalyses of phytoplankton size structure are necessary to betterunderstand controls on phytoplankton dynamics and to bettermanage water quality in river-dominated, estuarine systems.     

Water-mass influences on summer Antarctic phytoplankton biomass and community structure

The association between the variability of phytoplankton biomass and community structure and the distribution of water masses around the Antarctic Peninsula were examined during austral summer 1993. Phytoplankton biomass showed high variability, and was dominated by an autotrophic flagellate (Cryptomonas sp.) that represented, on average, 91% of total phytoplankton biomass. The lowest phytoplankton biomasses were associated with the strongly mixed, saline, cold waters characteristic of the Weddell Sea water mass, and with the waters influenced by ice melt from the Bellingshausen Sea. The highest biomasses were found in the confluence of these water masses, where a front develops. Community composition also differed among water masses, with eukariotic picoplankton and diatoms having their highest relative contribution to community biomass in stations with Bellingshausen Sea and Weddell Sea water masses, whereas the abundance of Cryptomonas sp. was highest at the confluence of these waters. These results indicate that mesoscale processes, that determine water mass distribution, are of paramount importance in controlling the time and space variability of Antarctic phytoplankton.     

Seasonal patterns of phytoplankton productivity and abundance in nearshore oligotrophic waters of the Levant Basin (Mediterranean)

A biweekly sampling program from two stations at the easternLevant Basin was carried out during a 1-year period (1983).The first station (neritic) was located 2 km offshore over theIsraeli continental shelf, while the second (pelagic) was located10 km offshore slightly beyond the continental shelf. It wasfound that during summer the relatively close pelagic watershad chlorophyll a concentrations comparable with the most oligotrophicdeep sea regions of the world's oceans. During winter and spring,profound fluctuations were observed in both phytoplankton standingcrop and primary productivity at the neritic station. This wasin response to weather phenomena, such as heavy rains or storms,which did not affect the pelagic Station to such an extent.The picoplanklon size fraction (<3 µm) dominated atthe neritic station during summer and fall, while the nanoplanktonfraction (3–20 µm) dominated during spring. At thepelagic station the picoplankton fraction dominated almost allyear round, but it is suspected that some portion of it wasphotosynthetically inactive.     

Nucleotide Sequence of Plasmid pAQ1 of Marine Cyanobacterium Synechococcus sp. PCC7002

We have determined the complete nucleotide sequence of pAQ1,the smallest plasmid of the unicellular marine cyanobacteriumSynechococcus sp. PCC7002. The plasmid consists of 4,809 bpand has at least four open reading frames that potentially encodepolypeptides of 50 or more amino acids. We found that a palindromicelement, the core sequence of which is G(G/A)CGATCGCC, is over-representednot only in plasmid pAQ1 but also in the accumulated cyanobacterialgenomic sequences from Synechococcus sp. PCC6301, PCC7002, PCC7942,vulcanus and Synechocystis sp. PCC6803 within GenBank and EMBLdatabases. It suggests that this sequence might mediate generearrangement, thus increasing genetic diversity, since recombinationevents are frequent in cyanobacteria.     

Picophytoplankton, nanophytoplankton, heterotrohpic bacteria and viruses in the Changjiang Estuary and adjacent coastal waters

Flow cytometry (FCM) was used to examine the abundances anddistributions of different picophytoplankton groups (i.e. Synechococcus,Prochlorococcus and picoeukaryotes), nanophytoplankton, heterotrophicbacteria and viruses were examined in the Changjiang Estuary,China and adjacent coastal waters during autumn 2004. Watertemperature and light availability were found to be criticalfactors for picophytoplankton growth. Positive correlationswere found between picophytoplankton, heterotrophic bacteriaand viruses, and a seaward-increasing trend in the V-I (thegroup yielding high green fluorescence according to FCM) populationwithin viruses was detected. The importance of nanophytoplanktonis progressively usurped by picophytoplankton with increasingdistance offshore. Picoeukaryotes are the most successful groupamong picophytoplankton in near-shore eutrophic waters, whereasProchlorococcus surpasses other groups within the pico- andnanophytoplankton community in offshore oligotrophic regionsof the East China Sea Shelf.     

Diurnal and seasonal variation in the contributions of autotrophic pico-, nano- and microplankton to the primary production of an upland lake

An investigation of the diurnal variation in productivity andcontribution to production of populations of autotrophic picoplankton(0.2–2.0 µm), nanoplankton (>2 <20 µm)and microplankton (>20 µm) was carried out at monthlyintervals, from May to October 1989, in Llyn Padarn a mesotrophicupland lake in North Wales. Maximum rates and contributionsto production of the lake by autotrophic picoplankton occurredduring mid-late summer, with the highest average daily contributionfrom picoplankton (64%) recorded in September at 4 m depth.Diurnal variation in contributions from picoplankton was pronounced,with greatest input, recorded at the end of the day, duringthe period of picoplankton dominance in mid-late summer. Maximumcontribution from picoplantkon (86% of total, 9.2 mg C m^–3h^–1) was recorded in September. Nanoplankton primary productionwas of greatest significance in June and July, although levelswere lower than for picoplankton in subsequent months. Contributionsvia nanoplankton increased with depth in the lake at this time,reaching a maximum of 78% of the total at the end of the dayat 9 m depth in early July. At this time, diurnal variationin contributions via nanoplankton was considerable, with maximumphotosynthesis generally at the end of the photoperiod at depthsof 4 and 9 m. Microplankton made the greatest impact on primaryproduction during the mixed water conditions of spring and autumn,and at these times did variation in production was less thanthose of both pico and nanoplankton during summer thermal stratification.Photosynthetic capacity was lower for picoplankton than fornanoplankton and microplankton; the highest values were 5, 33and 51 mg C (mg chl a)^–1) h^–1) for pico-, nano-and microplankton, respectively. The photosynthetic efficiencyof all three size categories of phytoplankton increased withdepth. Maximum values were similar for all phytoplankton groups,between 75 and 131 mg C (mg chl a)^–1) E^–1) m² butmean levels of photosynthetic efficiency for the 6 months werelower for picoplankton than for nano- or microplankton. Ratesof carbon fixation per cell for picoplankton spanned three ordersof magnitude, varied considerably diurnally and reached maximumvalues of 484 fg C(cell)^–1) h^–1) in the afternoonin near-surface waters in the early stages of exponential populationgrowth in July. During the population maximum of picoplanktonin August and September, maximum daily values of carbon fixationper cell, assimilation number and photosynthetic efficiencywere all recorded at the end of the day. The seasonal and diurnalpatterns of production of the three size categories of planktonicalgae in Llyn Padarn were distinct. During spring, microplankton(mainly diatoms) were the dominant primary producers. As thermalstratification developed, nanoplankton were the major contributorsto phytoplanktonic production, particularly in the deeper regionsof the euphotic zone. Picoplankton made the greatest contributionto production in August and September, exhibiting maximum inputtowards the end of the light cycle. Diatoms became the majorphotosynthetic plankton in the mixed water conditions prevalentin Uyn Padarn in October.     

Amphipods, isopods and surface currents: a case for passive dispersal in the Bay of Fundy, Canada

Six species of isopods and 18 species of amphipods were collectedin the neuston of the Bay of Fundy and adjacent waters. Collectionswere made over a grid of stations covering 2.4x10⁴ km² duringthree spring, three summer and two autumn surveys. No isopodsand only five species of amphipods were found in spring surveys.Isopods and amphipods were diverse and plentiful in the neustonin summer and autumn. Dominant isopods were Idotea baltica andI.metallica, and dominant amphipods were Calliopius laeviusculusand Parathemisto gaudichaudi. Amphipods and isopods reach theneuston of the Bay of Fundy in three ways. Idotea metallica,the only euneustonic species present, was probably advectedinto the Bay of Fundy from southern waters in summer, and didnot appear to overwinter in the Bay. Most species, includingI.baltica, were collected with drifting littoral vegetation,and we suggest that transport by surface currents is an importantfactor in dispersal of some shoreline crustaceans. Midwaterplankton, such as Parathemisto gaudichaudi, reached the neustoneither by advection in upwelling waters or by an extension oftheir normal diel vertical distribution.     

The picoplankton in Antarctic lakes of northern Victoria Land during summer 1989–1990

Summary Photoautotrophic picoplankton is reported from some lakes located near the Italian Antarctic station of Terra Nova. Observations, carried out by both flow cytometry on water samples and electron microscopy on micro-organisms in cultures from each lake, have confirmed the occurrence in all the environments studied of this fraction accounting, in several cases, for more than the 50% of the phytoplankton, measured as chlorophyll. Cultures of the picoplankton fraction from these waters contained known prokaryotic (Synechococcus) and eukaryotic (Chlorella) genera as well as two unidentified entities, possibly prochlorophytes.     

The distributions of autumn picoplankton in relation to environmental factors in the reservoirs along the Wujiang River in Guizhou Province,SW China

Autumn picoplankton (Synechococcus, picoeukaryotes and heterotrophic bacteria) and environmental factors have been investigated in a series of reservoirs along the Wujiang River in Guizhou Province, SW China. The average abundances of Synechococcus, picoeukaryotes and heterotrophic bacteria was 10⁴, 10² and 10⁶ cells ml⁻¹, respectively. In autumn meso-eutrophic reservoirs, thermal stratification was clear and abundances of different picoplankton groups in release water was low; whereas these phenomena were not obvious in autumn hypereutrophic reservoir. Picoplankton numbers decreased with increasing water depth and showed a positive correlation with water temperature, which reflected the importance of light and temperature on the picoplankton growth. Contribution of Synechococcus to total phytoplankton production and contribution of picoeukaryotes to total phytoplankton production asynchronous changed with varying trophic states. Synechococcus preferred meso-eutrophic reservoirs over hypereutrophic reservoir and picoeukaryotes showed no preference for the investigated reservoirs in autumn. Handling editor: L. Naselli-Flores