16S rDNA analyses of the cyanobacterial microbiota through the water-column in a boreal lake with a metalimnic Planktothrix population

The Planktothrix population in Lake Steinsfjord has attracted particular attention, due to the potential development of toxic blooms. This population is special in the sense that mass developments of Planktothrix occur in the metalimnion. We investigated the distribution of Planktothrix, as well as other cyanobacteria, through the water-column during a Planktothrix mass development at 10-16 m depth. The analyses were done by chlorophyll measurements, microscopy, and by a recently developed 16S rDNA array-based method.These analyses showed that Planktothrix dominated the cyanobacterial community at 11 m, while cyanobacteria belonging to the order Nostocales were predominant at 4 m. The combination of analytical methods presented in this work provides a powerful tool to analyze cyanobacterial communities. We have developed a concept that enables both relative (16S rDNA array analyses) and absolute quantification (chlorophyll a measurements) of cyanobacteria through water-columns. Such approaches will be important in better understanding cyanobacterial microbiota and bloom dynamics.     

Growth and production of heterotrophic nanoflagellates in a meso-eutrophic lake

The growth of heterotrophic nanoflagellates (HNF) in mesotrophicLake Constance was measured in situ during a 13 month period.Experiments were conducted with 10 µm pre-filtered lakewater incubated in diffusion chambers at 3 m water depth atthe sampling location for 24 h. Growth rates were calculatedfrom changes in cell numbers occurring during the period ofincubation. Growth rates of all dominant taxa showed pronouncedseasonal variation (–0.13 to 1.76 day^–1 and weregenerally highest in summer at high water temperatures. In situgrowth rates were well below maximum growth rates known forthe respective and similar species from laboratory experiments.While water temperature was a key parameter positively relatedto the growth of all HNF species, the effect of various potentialfood items was taxon specific and less clear. Bacterial abundancewas equally important as temperature for growth in the smallbactenvorous Spumella sp., but was insignificant for growthrates of the larger omnivorous Kathablepharis sp. In Spuniellasp., 84% of the observed seasonal variation of its growth ratecould be explained by temperature and bacterial food supply.Based on these results, a multiple linear regression equationwith temperature and bacterial concentration as dependent variableswas calculated for the growth rate of Spumella. Taxon-specificproduction rates were derived from growth rates and averagebiomass of these two species, and compared to total HNF productionestimated from previously measured community growth rates andbiomass in Lake Constance. Production peaks of Spumella sp.and Kathablepharis sp. alternated seasonally. Total HINF productionranged from –0.01 to 10 mg C m^–3 day^–1. Theaverage seasonal production varied between 1.4 and 33 mg C m^–3day^–1 over 6 consecutive years. These small protozoa thuscontribute a substantial amount to total zooplankton productionin Lake Constance.     

Seasonal plankton dynamics in a Mediterranean hypersaline coastal lagoon: the Mar Menor

The seasonal distribution of plankton in a Mediterranean hypersalinecoastal lagoon has been studied through a dataset, comprisingthe taxonomic composition and the size–abundance distributionof both phyto- and zooplankton, measured by image analysis techniquesduring a one-year time series of weekly samplings. The studiedorganisms ranged from small nanoplanktonic heterotrophic flagellates(2 µm diameter) to fish larvae (>2 µm). The phytoplanktonannual succession was characterized by a winter period dominatedby Rhodomonas spp. and Cryptomonas spp. with Cyclotella spp.as the main diatom represented, a spring phase where diatoms(mainly Cyclotella) were the dominant group with some monospecificblooms of other diatoms (mainly of Chaetoceros sp.), a summerphase characterized by diatoms with blooms of Niztschia closterium,and a post-summer phase where dinoflagellates increased withpeaks of Ceratium furca. High densities of the microbial foodweb elements, flagellates and ciliates, indicate the importanceof the microbial loop in the ecosystem. Meroplankton contributedwidely to the seasonal character of the zooplankton distribution.Copepods, represented by Oithona nana, Centropages ponticusand Acartia spp. (mainly latisetosa), remained relatively constantthroughout the year, exhibiting a lower density in the warmerwater period (July–September). At the end of the samplingperiod, a massive proliferation of copepods (>1000 ind l^–1), mainly due to O. nana, took place. The autotrophsto heterotrophs biovolume ratio (A:H) remained lower than 1throughout the year except when, occasionally, large phytoplanktoncells bloomed. Persistent very low values of A:H suggest thatadditional sources of energy, such as the microbial loop ordetrital pathways, would be needed to sustain the high heterotrophicbiovolume found in the lagoon.     

Genetic analysis on Dolichospermum (Cyanobacteria; sensu Anabaena) populations based on the culture-independent clone libraries revealed the dominant genotypes existing in Lake Taihu,China

Lake Taihu has been severely eutrophied during the last few decades and dense cyanobacterial blooms have led to a decrease in phytoplankton diversity. The cyanobacterial blooms in Lake Taihu were mainly composed of unicellular colony-forming Microcystis and filamentous heterocystous Dolichospermum (formerly known as planktonic species of Anabaena). In contrast to that of Microcystis spp., the fundamental knowledge about diversity, abundance and dynamics of Dolichospermum populations in Lake Taihu is lacking. The present study was conducted to understand genotypic distribution, dynamics and succession of Dolichospermum populations in Lake Taihu. By sequencing 688 internal transcribed spacer (ITS) regions between the 16S and 23S rRNA genes of Dolichospermum, we were able to confirm that all the sequences were Dolichospermum rather than Aphanizomenon. 118 different genotypes were identified from the obtained sequences, and two genotypes (W-type and L-type) were found to dominate in the lake, representing 36.6% and 26.2% of the total sequences, respectively. These two dominant genotypes of Dolichospermum displayed the significant seasonal pattern. Stepwise regressions analysis revealed that water temperature was associated with the two dominant genotypes. The combined results implied the possible existence of ecotypes in bloom-forming cyanobacteria, probably triggered by water temperature in the lake.     

Dinoflagellate bloom development and collapse in Lake Kinneret: a sediment trap study

Warm monomictic Lake Kinneret, Israel, is characterized by awinter–spring water bloom of the large (~50 µm diameter)dinoflagellate Peridinium gatunense Nygaard. Usually the P.gatunense bloom declines in May–June and a less prominentbloom of smaller dinoflagellates (mostly Peridiniopsis spp.of ~20–30 µm diameter) develops. Water column abundancesand sedimentation losses to those dinoflagellates were followedthroughout 1994 and 1995. The objective was to quantify thevariables that describe population dynamics, that in turn willshed more light on the seasonal patterns of bloom dynamics.Sedimentation losses were measured by means of sediment trapswith and without a preservative (formaldehyde) that were exposedfor 24 h once every 2–3 weeks. Annual sedimentation lossesof Peridinium (hypolimnetic trap catches) were 209 g wet wtm^-2 year^-1 in 1994 and 187 g wet wt m^-2 year^-1 in 1995, whichconstituted 16 and 23% of Peridinium production in those years,respectively. This study revealed that increased death ratespreceded a mass sedimentation flux of Peridinium and causedthe decline of the bloom in Lake Kinneret. Annual sedimentationlosses of Peridiniopsis were 55 g wet wt m^-2 year^-1 in 1994and 34 g wet wt m^-2 year^-1 in 1995. In contrast to live Peridiniumcells, Peridiniopsis cells continued to swim to the lower trapafter the onset of thermal stratification, possibly taking advantageof the higher nutrient concentrations below the thermocline,at a time when the lake is already stratified and the epilimnionis nutrient depleted. This could be an important factor allowingPeridiniopsis spp. to peak after the decline of Peridinium.     

Prediction and elucidation of the population dynamics of Microcystis spp. in Lake Dianchi (China) by means of artificial neural networks

Lake Dianchi is a shallow and turbid lake, located in Southwest China. Since 1985, Lake Dianchi has experienced severe cyanabacterial blooms (dominated by Microcystis spp.). In extreme cases, the algal cell densities have exceeded three billion cells per liter. To predict and elucidate the population dynamics of Microcystis spp. in Lake Dianchi, a neural network based model was developed. The correlation coefficient (R²) between the predicted algal concentrations by the model and the observed values was 0.911. Sensitivity analysis was performed to clarify the algal dynamics to the changes of environmental factors. The results of a sensitivity analysis of the neural network model suggested that small increases in pH could cause significantly reduced algal abundance. Further investigations on raw data showed that the response of Microcystis spp. concentration to pH increase was dependent on algal biomass and pH level. When Microcystis spp. population and pH were moderate or low, the response of Microcystis spp. population would be more likely to be positive in Lake Dianchi; contrarily, Microcystis spp. population in Lake Dianchi would be more likely to show negative response to pH increase when Microcystis spp. population and pH were high. The paper concluded that the extremely high concentration of algal population and high pH could explain the distinctive response of Microcystis spp. population to + 1 SD (standard deviation) pH increase in Lake Dianchi. And the paper also elucidated the algal dynamics to changes of other environmental factors. One SD increase of water temperature (WT) had strongest positive relationship with Microcystis spp. biomass. Chemical oxygen demand (COD) and total phosphorus (TP) had strong positive effect on Microcystis spp. abundance while total nitrogen (TN), biological oxygen demand in five days (BOD₅), and dissolved oxygen had only weak relationship with Microcystis spp. concentration. And transparency (Tr) had moderate positive relationship with Microcystis spp. concentration.     

Seasonal variation of microbial eukaryotic community composition in the large,shallow, subtropical Taihu Lake,China

Microbial eukaryotic community (0.8–20 μm) composition and its seasonal variation were investigated in large, shallow, subtropical Lake Taihu located in Southeast China. The water samples were collected monthly from August 2006 to July 2007 at two sites distantly located in Meiliang Bay and Lake Center, which differed strongly in their trophic status and sediment resuspension. The microbial eukaryotic community compositions (MECC) were measured using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified eukaryotic 18S rRNA gene fragments. Seasonal variations were related to environmental factors by means of redundancy analysis (RDA) in order to reveal the most important regulating factors. At both sites, the structures and Shannon–Wiener indices of the microbial eukaryotic communities displayed apparent seasonal variations, which were significantly related to environmental variables. There were significant intrahabitat differences in environmental factors regulating MECC, as well as in the seasonal dynamics of the two sites during the study period. At Meiliang Bay, the variations of total phosphorus concentration and cladocera abundance had the most profound impacts on the microbial eukaryotic community composition, while at Lake Center, the total nitrogen concentration and conductivity were most influential. Our results suggest that the DGGE method is a cost-effective way to analyze the seasonal dynamics of MECC and their interaction with environmental variables, which will provide new insights into the diversity and dynamics of MECC.     

Species composition and seasonal abundance of zooplankton in two Ethiopian Rift Valley lakes — Lakes Abiata and Langano

The composition and annual cycle of the zooplankton of two Ethiopian Rift Valley soda lakes is described. Lake Langano has a conductivity of 1 400 to 1800 µS cm^–1 and a permanent mineral turbidity. Lake Abiata is more concentrated (conductivity 19 000 to 23 000 µS cm^–1) and more alkaline but less turbid; it is characterised by dense phytoplankton blooms, mainly cyanophytes.The zooplankton assemblage is typically tropical, with relatively few species of Cladocera and Copepoda. There was a marked difference in zooplankton between the two lakes, Lake Abiata showing much higher concentrations and greater wet season/ dry season differences. The species composition was also different. Lake Abiata lacked Cladocera, and calanoid copepods occurred only during the wet season with lower conductivities. These two phenomena were attributed to high sodium bicarbonate concentration and to dense cyanophyte blooms. Eleven species of rotifers occurred in Lake Abiata, including six Brachionus spp. but B. rubens was the only rotifer found in Lake Langano. The seasonal variation of the zooplankton is discussed in relation to seasonal fluctuations in conductivity and Chl a concentration.     

Seasonal dynamics of phyto- and zooplankton and their interactions in the hypereutrophic reservoir

The seasonal dynamics of phyto- and zooplankton, as well as their trophic interactions, were studied for the shallow freshwater hypereutrophic Curonian Lagoon in the Baltic Sea in 2007–2009. The water temperature and high concentrations of nutrients were the limiting factors in the seasonal dynamics of the studied communities. Decreases in the B _z /B _ph ratio and the grazing pressure of phytophagous zooplankton were observed over the last few decades due to the eutrophication process and cyanobacteria blooms. The large zooplankton consumes a significant amount of primary production (Chlorococcales and Bacillariophyta) when the predator pressure of the fish fry is absent. This grazing pressure may precondition cyanobacteria scum.     

Stratification by cyanobacteria in lakes: a dynamic buoyancy model indicates size limitations met by Planktothrix rubescens filaments

The ability of the Planktothrix rubescens to stratify in Lake Zürich is related to the size and shape of the cyanobacterial filaments. Detailed measurements made in the lake are used in a dynamic computer model of buoyancy regulation to investigate the vertical movements of filaments tracking the depth at which the irradiance would support neutral buoyancy. The movement of the filament lags behind the constantly changing target depth owing to (a) the time taken for the filament to respond to the irradiance by changing its density and (b) the time it takes to move by sinking down or floating up through the water column. The model simulates the stratification depth over a 5-month period of the summer from the continuous measurements of irradiance and weekly measurements of light attenuation and temperature, without any further adjustment over the period. Models using filaments of the size observed in Lake Zürich explain several details of the observed depth changes: smaller planktonic cyanobacteria (e.g. Limnothrix sp.) are unable to migrate fast enough and larger ones (e.g. Anabaena spp.) will overshoot and become entrained in the epilimnion. The model can be used to simulate recruitment of Planktothrix filaments from different depths after vernal stratification. Recruitment of filaments from depths down to 45 m will contribute to the metalimnetic population increase in early July.     

Seasonal occurrence and vertical distribution of appendicularians in Toyama Bay, southern Japan Sea

To investigate seasonal variation in the community structureof appendicularians, vertical hauls (0–500 m) with a Norpacnet were made at an offshore station in Toyama Bay at intervalsof 2–4 weeks from February 1990 to January 1991. Additionalsamples were collected with MTD nets at 12–17 differentdepth layers between the surface and a depth of 600–700m at the same position in June, September and December 1986,and March 1992, to examine the vertical distribution of appendicularians.Twenty-one species (including two unidentified species) belongingto five genera were found, and the dominant species were dividedinto three groups by their occurrence period. Oikopleura longicauda,Fritillaria borealis f. typica and F. borealis f. sargassi occurredthroughout most of the year. Fritillaria pellucida, O. fusiformisand O. rufescens were found in summer and autumn. Oikopleuradioica was found in spring and winter. Oikopleura longicaudawas overwhelmingly the most abundant species throughout theyear. This species was always distributed in the upper 100 mdepth, with a peak at a depth of 0–50 m that correspondedto the peak of chlorophyll a concentration during the day andnight in all seasons. The day–night vertical distributionpatterns of F. borealis f. typica, F. pellucida, O. fusiformisand O. rufescens were similar to that of O. longicauda. Seasonalvariations in abundance of appendicularians are considered tobe the result of biological factors rather than physical factorssuch as temperature and salinity. In particular, O. dioica seemsto be affected by food availability.     

Comparative analysis of nutrients,chlorophyll and transparency in two large shallow lakes (Lake Taihu,P.R. China and Lake Okeechobee,USA)

This article compares limnological attributes of two of the world’s largest shallow lakes—Lake Okeechobee in Florida, USA and Lake Taihu in P.R. China. Both the systems support an array of ecological and societal values including fish and wildlife habitat, public water supply, flood protection, and recreation. Both have extensive research programs, largely because of concern regarding the lakes’ frequent cyanobacterial blooms. By evaluating these systems together, we compare and contrast properties that can generally advance the understanding and management of large shallow lowland lakes. Because of shallow depth, long fetch, and unconsolidated mud sediments, water chemistry, and transparency in both the lakes are strongly influenced by resuspended sediments that affect light and nutrient conditions. In the central region of both the lakes, where depth is the greatest, evaluation of limiting factors by a trophic state index approach indicates that light most often limits phytoplankton biomass. In contrast, the more sheltered shoreline areas of both the lakes display evidence of nitrogen (N) limitation, which also has been confirmed in nutrient assays conducted in earlier studies. This N limitation most likely is a result of excessive levels of phosphorus (P) that have developed in the lakes due to high external loads over recent decades and the currently high internal P recycling. Comparisons of these lakes show that Lake Taihu has higher N than, similar total phosphorus (TP) and similar light conditions to that of Lake Okeechobee, but less chlorophyll a (CHL). The latter may be as a result of lower winter temperatures in Lake Taihu (around 5°C) compared to Lake Okeechobee (around 15°C), which could reduce phytoplankton growth and abundance through the other seasons of the year. In these systems, the important role of light, temperature, and nutrients in algal bloom dynamics must be considered, especially due to possible adverse and unintended effects that might occur with projects such as sediment removal, and in the long term, in regard to buffering lake responses to external load reduction. Handling editor: D. Hamilton     

Growth, production and losses of phytoplankton in the lowland River Spree. I. Population dynamics

The seasonal and longitudinal dynamics of dominant phytoplanktonpopulations were observed in the lower Spree (Germany), a lowlandriver draining several lakes. Centric diatoms and cyanobacteriawere the chief components of the total phytoplankton biomassof 16 mg^–1 at the end of the river course (average March-October,1988–89). The cyanobacteria required a retention timeof several weeks in a drained lake to establish stable populations,whereas the other algal populations originated from upstreamimpoundments and slow-flowing river stretches. In spring, thedominant populations increased (the diatoms Nitzschia acicularis,Asterionella formosa, Stephanodiscus hantzschii. Cyclostephanosdubius, Cyclotella radiosa, C.meneghiniana) or kept their abundancesnearly constant (cyanobacterium Limnothrix redekei. diatom Synedraacus), but the biomass of the ‘summer species’ (diatomActinocyclus normanii, cyanobacteria Planktothrix agardhii andAphanizomenonsp.) was halved along the river course. These seasonal differencesin algal growth are probably caused by the changed mixing patternin the regions of origin and thus different pre-adaptation toturbulent mixing. Pronounced dear-water phases occurred regularlyat the end of May. They were intensified along the river coursedespite the lack of mesozooplankton. The dynamics of phytoplanktonwere reflected in the proportions of dissolved and particulatenutrients. The nutrient supply was always high enough to makelimitations of algal growth improbable.     

Field and experimental studies on the combined impacts of cyanobacterial blooms and small algae on crustacean zooplankton in a large,eutrophic, subtropical,Chinese lake

Field and experimental studies were conducted to evaluate the combined impacts of cyanobacterial blooms and small algae on seasonal and long-term changes in the abundance and community structure of crustacean zooplankton in a large, eutrophic, Chinese lake, Lake Chaohu. Seasonal changes of the crustacean zooplankton from 22 sampling stations were investigated during September 2002 and August 2003, and 23 species belonging to 20 genera were recorded. Daphnia spp. dominated in spring but disappeared in mid-summer, while Bosmina coregoni and Ceriodaphnia cornuta dominated in summer and autumn. Both maximum cladoceran density (310 ind. l⁻¹) and biomass (5.2 mg l⁻¹) appeared in autumn. Limnoithona sinensis, Sinocalanus dorrii and Schmackeria inopinus were the main species of copepods. Microcystis spp. were the dominant phytoplankton species and formed dense blooms in the warm seasons. In the laboratory, inhibitory effects of small colonial Microcystis on growth and reproduction of Daphnia carinata were more remarkable than those of large ones, and population size of D. carinata was negatively correlated with density of fresh large colonial Microcystis within a density range of 0–100 mg l⁻¹ (r = −0.82, P< 0.05). Both field and experimental results suggested that seasonal and long-term changes in the community structure of crustacean zooplankton in the lake were shaped by cyanobacterial blooms and biomass of the small algae, respectively, i.e., colonial and filamentous cyanobacteria contributed to the summer replacement of dominant crustacean zooplankton from large Daphnia spp. to small B. coregoni and C. cornuta, while increased small algae might be responsible for the increased abundance of crustacean zooplankton during the past decades.     

Effects of Salinity and Temperature on Survival and Reproduction of Boeckella hamata (Copepoda: Calanoida) from a Periodically Brackish Lake

The freshwater calanoid copepod, Boeckella hamata Brehm, occursperiodically in Lake Waihola, New Zealand, a shallow, tidally-influencedlake that experiences daily and seasonal fluctuations in salinity,and seasonal changes in temperature. To determine whether theseenvironmental changes might contribute to the copepod's disappearancefrom the lake in summer, we tested the survival and egg productionof B. hamata at a range of salinities (20-3000 mg^–1 Cl)and two temperatures (10°C, 20°C). Adults survived for20 days or more only at salinities     

Geographical and seasonal variations in taxonomic composition, abundance and biomass of microzooplankton across a brackish-water lagoonal system of Japan

The taxonomic composition, abundance and biomass of microzooplanktonwere studied at eight stations in Lake Shinji–Ohashi River–LakeNakaumi brackish-water system, Japan, monthly from April 1998to March 1999. Over the entire area, naked ciliates numericallydominated the microzooplankton community (annual mean 39.6%)followed, in order, by tintinnids (30.3%), copepod nauplii (24.6%)and rotifers (5.5%). The abundance of each taxonomic group ofmicrozooplankton varied geographically due to large salinityvariations (range 1.5–33.3 p.s.u.). It was notable thatnaked ciliates occurred overwhelmingly in Lake Shinji (54.9%of total microzooplankton) and rotifers were relatively numerousin Lake Shinji (8.8%) and Ohashi River (11.1%), where the salinitywas lower (annual mean 4.1 and 13.6 p.s.u., respectively) thanin Lake Nakaumi and Sakai Strait (26.3 and 29.8 p.s.u., respectively).Owing to large seasonal temperature variation (range 5.4–29.8°C),the abundance of microzooplankton showed marked seasonal variations,being higher in spring and summer than in the remaining seasons.A total of 49 species of tintinnids were identified, and 15of these species reached concentrations >500 individualsl^-l. The occurrence of most tintinnid species was confined tocertain months or locations, closely associated with species-specifictemperature and salinity preference and/or tolerance. In thiseutrophic system, food supply for microzooplankton might besufficient due to extremely high chlorophyll a concentration(annual mean 8.8 µg l^-lin Sakai Strait to 22.6 µgl^-l in Lake Shinji). However, microzooplankton biomass remainedmoderate (range 0.19–18.7 µg C l^-l) due probablyto heavy predation by mesozooplankton, which inhabit this brackish-watersystem at extremely high biomass.     

Increased abundance of the non-indigenous zooplanktivore, <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis>, is strongly correlated with greater spring prey availability in Canadian Shield lakes

The non-indigenous zooplanktivore, Bythotrephes longimanus, is a large Palaearctic cladoceran that is spreading rapidly in the Great Lakes watershed in North America. As a voracious predator, Bythotrephes can reduce herbivorous cladoceran abundance and diversity; however, the variables that affect its abundance are not well understood. To determine what bottom-up factors are associated with the abundance and seasonal dynamics of established Bythotrephes populations, two Bythotrephes datasets from lakes in south-central Ontario, Canada, were analysed using multiple regression and multivariate analyses: a multi-lake dataset of nine lakes sampled in 2003 and a multi-year dataset of one of these lakes, Harp Lake, sampled from 1994–1998 and 2001–2004. Bottom-up variables tested were Secchi disk depth, epilimnetic temperature, cladoceran (prey) density, total phosphorus, dissolved organic carbon and Chlorophyll a, as well as maximum depth for the multi-lake dataset. In both analyses and datasets, springtime abundance of herbivorous cladocerans was consistently found to be a significant factor associated with Bythotrephes (June–September) abundance; Bythotrephes annual abundance was significantly and positively associated with mean May and June prey abundance, along with mean Secchi disk depth for the multi-lake dataset, and groups of lakes or years with similar Bythotrephes seasonal abundance patterns were predicted by June prey abundance. Additionally, prey availability was the dominant contributor towards changes in weekly Bythotrephes birth rates calculated for two of the study lakes. Our study suggests that prey availability influences Bythotrephes abundance, which provides evidence that Bythotrephes establishment success is affected by the abundance of its prey.     

Experimental and Field Studies of Mesocyclops notius (Copepoda: Cyclopoida) from a Subtropical Chinese Lake, Lake Donghu

This paper reports experimental and field studies on the cyclopoidMesocyclops notius from subtropical Lake Donghu close to theYangtze river. Mesocyclops notius, a dominant crustacean zooplankterthroughout tropical Australia, was previously considered tobe endemic to Australia, but recently, Mesocyclops leuckartiin Lake Donghu was re-identified as M. notius. Laboratory cultureexperiments were conducted to reveal the effect of temperature(15, 20, 25 and 30°C) on the development, growth and reproductionof M. notius. Temperature was inversely related to developmenttimes of eggs, nauplii and copepodites, body length and physiologicallongevity of adults, and brood size. Body length and physiologicallongevity of females were greater than those of males at thesame temperature. No reproduction took place at 15°C. Productionand seasonal cycles of M. notius during 1980–1982 werestudied at two sampling stations of Lake Donghu. At the mid-lakestation, the annual production and production/biomass (P/B)ratio of M. notius varied between 6.0 and18.0 g dry wt m^–2year^–1 and 74.6 and 95.5, respectively. Mesocyclops notiusreached their highest density peaks in the warm months (July–October),with a maximum density of 1256 individuals l^–1 at a littoralsite. No reproduction and recruitment by M. notius took placeduring the cold months (December–March) when the temperatureof the lake water was <15°C. Mesocyclops notius weremore abundant at a littoral station than at a pelagic station,possibly due to different food availability. The higher male:femalesex ratio of M. notius at the littoral station was most likelycaused by size-selective fish predation on larger females.     

Oligochaetes and water pollution in two deep Norwegian lakes

Analyses of the oligochaete fauna of two of the deepest lakes in Scandinavia — the Norwegian lakes Mjösa (450 m) and Tyrifjorden (295 m), revealed a totally different species composition in the deep profundal compared with the upper profundal - in contact with the nutrient-enriched epilimnion. In both lakes a pronounced thermal stratification develops in the summer, thus the epilimnion receiving gross organic pollution behaves differently from the profundal. The lakes are each effectively divided into two bodies of water with limited water exchange between them, i.e. one major oligotrophic body and one minor more nutrient-rich. Since the 1950s both lakes have been exposed to heavy pollution of various kinds. In Lake Mjösa in 1975 and 1976 unpleasant algal blooms of the blue-green alga Oscillatoria bornetii fa. tenuis occurred. Bottom samples obtained at the same time revealed that the deep central bottoms of the lake were totally dominated by oligotrophic oligochaete indicators, i.e. by Stylodrilus heringianus and Spirosperma ferox, while the fauna of the upper profundal in the vicinity of domestic and agricultural sewage outfalls, wood processing industries, etc. was dominated by Limnodrilus hoffmeisteri and Tubifex tubifex in great abundance, indicating enriched conditions. Several other species indicative of eutrophy, were absent, most of them belonging to the genus Potamothrix. A fairly similar situation exists in Lake Tyrifjorden, where, for instance, in the shallow bay of Steinsfjorden — heavily eutrophied by agricultural wastes — blooms of blue-green algae have caused problems from time to time. The same oligochaete communities as in Lake Mjösa distinguish the central oligotrophic bottoms from the regionally more enriched upper profundal. The likely reasons for an intact profundal oligochaete fauna are great volumes of oxygen-rich hypolimnic water of low temperature and a high bottom/lake surface area ratio.     

Structure and dynamics of zooplankton communities in the littoral zone of some North Carolina lakes

Species composition and seasonal, dynamics of zooplankton in the littoral zone were studied in three piedmont North Carolina lakes for one year. Nygaard's compound index indicated oligotrophic conditions in Belews Lake and eutrophic conditions in Salem Lake and Lake 150. A total of 32 species of zooplankton were found in the samples. Significantly fewer species of truely littoral zooplankton were present in the oligotrophic lake regardless of season (P < 0.05). Eutrophication appears to favor increased diversity in zooplankton of the littoral zone. Habitat heterogeneity, provided by a well developed littoral zone containing aquatic macrophytes, may be the major factor contributing to the maintenance of more taxa of zooplankton in eutrophic systems.Distinct taxa of Cladocera and Copepoda are present in the littoral zone, exhibiting seasonal dynamics that are largely different from the limnetic fauna. Such dynamics may result from predation by characteristically limnetic species of cyclopoid copepods which seek prey in the littoral zone. The numerical and seasonal abundance of predators and prey are inversely related and the true littoral taxa, represented in the present study by the genera Alona, Chydorus, Pleuroxus, Sida, Simocephalus, and Eucyclops, contained no predaceous species. The seasonal dynamics of these genera are very similar even in widely separated geographical regions, indicating that the factor(s) responsible operate independently of climatic and chemical variables. Intrazooplankton predation appears to be a more plausable explanation than segregation along depth which, although consistent with data from studies of limnetic waters, cannot be related to populations of zooplankton in shallow littoral areas.