Seasonal variability in the relationship between body length and individual dry weight as related to food abundance and clutch size in two coexisting Daphnia species

Dry weights of individual Daphnia galeata and D. hyalina, whichcoexist in Lake Constance, were determined weekly during a twoyear investigation. From each sample and each species a length–weightrelationship (LWR) was calculated resulting in 54 (65) regressionequations based on N=2466 (3032) in D. galeata (D. hyalina).The constants (Ln(A)) and (B) of the LWRs showed a marked seasonalvariability, ranging from Ln(A)=1.0784–2.4740(1.0152–2.3860)and B=2.14–4.20 (2.11–4.15). The seasonal variabilityof the LWR was estimated with four different models. Condensingthe 54 (65) equations into one seasonal cycle resulted in amodel with an explained variance of r²=0.873 (0.869). Threemore models were established by calculating multiple regressionswith take stratification, average fecundity (E), and food concentrationas determining variates. Differences in LWRs of the two Daphniaspecies were significant (p <0.001). As the most useful approximationduring the period of stratification, LWRs are recommended forD. galeata: Ln(W)=(1.5674+0.0287^*(E))+Ln(L)^*(3.3611+0.0111^*(E)) as a generalized LWR for an epilimnetic daphnid, and for D.hyalina: Ln(W)=(1.5593+0.0613^*(E))+Ln(L)^*(3.2709+0.0017^*(E)) as LWR for a daphnid which migrates diurnally between epi andhypolimnion. The explained variance of this model is r²=0.843(0.826). The influence of food concentration in the epilimnionon LWRs was found to be significant on Ln(A) but not on (B). ^*This research project was supported by the Deutsche Forschungsgemeinschaft     

Production primaire en baie de Concarneau. Relations algues-bacteries et filtration differentielle

A study of the phytoplankton in Concarneau Bay (Bretagne, France)was carried out on 6–8 June, 1978. Autotrophy was measuredby the ¹⁴C method and heterotrophic activity was estimated by¹⁴C labelled glucose and protein hydrolysate uptake. The measurementswere made on the whole of the sample as well as on the bacterialfraction, isolated by size-fractionation on Nuclepore 3 µmfilters. Moreover, a study of the relationships between naturalphytoplanktonic and bacterial populations was possible becauseof cell counts. The phytoplankton shows coastal characteristicsand diatoms are dominant. The distribution of the genus Chaetoceros,Nitzschia and Leptocylindrus seems to have an influence on therelationship between algae and bacteria so that environmentalconditions not only interfere directly at the bacterial levelon heterotrophic activity, but also through the specific compositionand abundance of the phytoplankton.     

DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

Horizontal distribution and dominant species of heteropods in the East China Sea

This article discusses the horizontal distribution and dominantspecies of heteropods in the East China Sea. The ecologicalcharacteristics of heteropods and their adaptability to theenvironments were also considered. Oceanographic census wascarried out in the East China Sea (23°30 '33° N and118°30'128° E) in four seasons from 1997 to 2000. Itwas found that the total abundance showed obvious seasonal variations.It peaked in autumn with a mean value of 21.03 ind. (100 m³)^–1,followed by summer (4.89 ind. (100 m³)^–1). The lowestabundance occurred in winter and spring. As to the horizontaldistribution, abundance in summer and autumn was higher in thenearshore than in the offshore of the East China Sea. In winterand spring, heteropods were barely found in the northern nearshore.Three dominant species were observed in four investigated seasons,in which only Atlanta rosea dominated in winter, spring andsummer while Atlanta peroni and Atlanta lesueuri mainly dominatedin autumn. These two dominant species observed in autumn exhibiteda rather higher occurrence frequency than A. rosea. Temperaturewas found to be a major influencing factor whereas salinitywas a minor one. Comparing their adaptability, A. rosea cansurvive in a wider temperature range (1628°C), which enablesit to dominate in four seasons, while A. peroni survives ina relatively narrow temperature range (1928°CC) and therange for A. lesueuri was even more narrow (2128°CC). Moreover,A. rosea was also adapted to a wider range of salinity. However,the abundance of A. rosea in autumn was lower than those ofA. peroni and A. lesueuri. It can be thereby inferred that themultiplication speed of A. rosea was lower than the other twospecies within the same favorable temperature range. Due totheir adaptability to high salinity, the distribution of heteropodswas closely related to the domain of the Taiwan Warm Currentand Kuroshio. Especially for A. peroni and A. lesueuri, theirhigh abundance areas are always indicative of the lasting existenceof strong warm currents. The negative values of aggregationindices indicated relatively even distribution of heteropodsin the East China Sea. The high abundance area (31°00' N,126°00' E) of heteropods in autumn is on the migration pathwayof the mackerel (Scomber japonicus Houttuyn). Thus, there isalso a fishing ground of the mackerel. This suggested that thehigh abundance area of heteropods in autumn is important tothe fisheries in the East China Sea. Comparing with the historicalrecords, the abundance of heteropods appeared to increase inthe past 40 years. This may be a result of strengthened warmcurrents due to global warming.     

Seasonal variation in the species composition of tintinnid cilates in Hiroshima Bay, the Seto Inland Sea of Japan

Seasonal changes in the species composition of tintinnid ciliateswere examined based on time-series samples taken at 2 week intervalsover a 3 year period in Hiroshima Bay, the Seto Inland Sea ofJapan. The maximum abundance of total tintinnids over the entireperiod was 5.7 x 10³ indi viduals l^– Among 32 speciesidentified, a consistent seasonal occurrence was recognizedin 22 species. The relationships between various environmentalfactors and the abundance of each species of tintinnids wereanalyzed using principal component analysis From this analysis,the abundance of many tintinnids was revealed to be associatedwith temperature, the <20 µm size fraction of chlorophylla and water column stability, but not with the <20 µmsize fraction of chlorophyll a, nor with salinity. From theseresults, tintinnid species were divided into five associationtypes: species whose abundance increased with increasing temperature,decreasing temperature, nanophytoplankton abundance, increasingwater column mixing, or increasing water stratification coupledwith low temperature.     

Population biomass, feeding, respiration and growth rates, and carbon budget of the scyphomedusa Aurelia aurita in the Inland Sea of Japan

We investigated the seasonal occurrence, wet : dry : carbon: nitrogen weight ratios, population biomass, gastric pouchcontents, and rates of feeding, growth and respiration of thescyphomedusa Aurelia aurita in the central part of the InlandSea of Japan. Aurelia aurita medusae began to appear in January/Februaryas ephyrae, reached annual maximum body size in July/August,and disappeared, presumably due to death, by November. Initialslow growth in early spring was followed by a period of exponentialgrowth (mean growth rate: 0.069 d^–1) between April andJuly. In the Ondo Strait, which is characterized by strong tidalmixing, the A. aurita population (mean carbon biomass: 66.0mg C m^–3) overwhelmingly dominated the zooplankton-communitybiomass (mean biomass of micro- and mesozooplankton: 23.7 mgC m^–3) between May and early August The gastric contentanalysis revealed that A. aurita ate almost all micro- and mesozooplankters,of which small copepods were most important. On the basis ofdigestion time for small copepods (60 min) and their abundancein the gastric pouch of field-collected A. aurita, we determinedthe weight specific feeding rates and clearance rates. The formerincreases linearly with increasing copepod abundance, but thelatter was relatively constant irrespective of the food supply.We also measured the respiration rates of A. aurita and expressedthem as functions of body weight and temperature. These physio-ecologicalparameters enabled us to construct the carbon budget of theA. aurita population typical of early summer in the Ondo Strait.Predicted population-feeding rate (6.07 mg C m^–3 d^–1)was higher than the population-food requirement for both metabolismand growth (4.55 mg C m^–3 d^–1), indicating thatfood supply was sufficient to sustain the observed growth rate.This feeding rate was equivalent to 26% of micro- and mesozooplanktonbiomass, a significant impact on zooplankton.     

Leaf-like Structure in the Photosynthetic, Succulent Stems of Cacti

This research examined the hypothesis that as cacti evolve tothe leafless condition, the stem epidermis and cortex becomemore leaflike and more compatible with a photosynthetic role.All cacti in the relict genus Pereskia have non-succulent stemsand broad, thin leaves. All members of the derived subfamilyCactoideae are ‘leafless’, having an expanded cortexthat is the plant's only photosynthetic tissue. In Pereskia,leaves have a high stomatal density (mean: 50.7 stomata mm^–2in the lower epidermis, 38.1 mm^–2 in the upper epidermis),but stems have low stomatal densities (mean: 11.3 mm ₂, threeof the species have none). Stems of Cactoideae have a high stomataldensity (mean: 31.1 mm^–2, all species have stomata). Theouter cortex cells of stems of Cactoideae occur in columns,forming a palisade cortex similar to a leaf palisade parenchyma.In this palisade cortex, the fraction of tissue volume availablefor gas diffusion has a mean volume of 12.9%, which is identicalto that of Pereskia leaf palisade parenchyma. Pereskia stemcortex is much less aerenchymatous (mean: 5.3% of cortex volume).Cactoideae palisade cortex has a high internal surface density(0.0207 cm₂ cm^–2 which is higher than in Pereskia stemcortex (0.0150 cm₂ cm^–3) but not as high as Pereskia leafpalisade parenchyma (0.0396 cm₂ cm^–3). Pereskia stem cortexhas no cortical bundles, but Cactoideae cortexes have extensivenetworks of collateral vascular bundles that resemble leaf veins. Cactaceae, cactus, intercellular space, stomatal density, internal surface/volume, evolution     

Rapid separation of carbonic anhydrase isozymes using cellulose acetate membrane electrophoresis

A method is described for the rapid separation of carbonic anhydrase(CA) isozymes by cellulose acetate membrane electrophoresisin which CA activity is detected using the pH-indicating dye,bromcresol purple. This method can detect bovine erythrocyteCA in a 0.3 mm³ sample applied at a concentration of 100 ngcm^–3 (total of 30 pg applied) while at higher concentrationsthree isozymes were observed. It was found, using a potentiometrictechnique, that intact cells of Anabaena flos-aquae (Cyanophyceae)and Chlorella ellipsoidea had no detectable activity while C.saccharophila and Chlamydomonas reinhardtii (Chlorophyceae)had external CA activity. CA activity of the extracts suggestedthe presence of internal CA in all species. After electrophoresisit was found that C. saccharophila and C. reinhardtii had twoisozymes while A. flos-aquae and C. ellipsoidea had only a singledetectable band. Spinach had up to five detectable isozymesthat were difficult to resolve. Incubation of spinach extractwith the CA inhibitor ClO₄^– (500 mol m^–3) inhibitedCA activity by 90% using the potentiometric technique, but afterelectrophoresis had no detectable effect. This technique isuseful in identifying isozymes that are substantially differentin electrical charge and in monitoring CA isozyme activity duringenzyme purification. Key words: Carbonic anhydrase, isozymes, cyanobacteria, microalgae, spinach     

A rapid Utermohl method for estimating algal numbers

The standard method for counting phytoplankton requires 24–48h of settling (Utermöhl, 1931). The modified Utermöhlmethod (plunger) described here reduces settling time to 2 h.Estimates of counts for phytoplankton using the plunger methodplotted against counts from the Utermöhl method yield agood correlation (r² = 0.97), and 95–100% of species (>2µm) recorded in the Utermöhl method were recordedwith the plunger method.     

Chemical composition and biomass parameters of a population of Craspedacusta sowerbii Lank1880 (Cnidaria: Limnomedusa)

Until now, very little information about the ecology of thefreshwater jellyfish, Craspedacusta sowerbii, has been available.Although many publications deal with Craspedacusta, most ofthem contain only observations. Detailed analyses are rare.In this study, investigations on size-dependent fresh and dryweights, and carbon, nitrogen and phosphorus, are presented.The water content of the medusae ranged between 96.74 and 99.87%.Fresh and dry weights ranged from 0.06 to 331.86mg FW ind.^0–1and 0.01 to 2.50 mg DW ind.^–1, respectively. The molarC:N:P relationship was calculated as 39:9:1. The C:P ratio waslow in contrast to other freshwater zooplankton species, andimplied a high demand for P. A comparison with the C:P stoichiometryof their prey indicates the possibility of P limitation of Craspedacustaat a gross growth efficiency above 25%. The results are discussedin relation to medusae abundance, bioaccumulation and theirimpact on food web structure.     

Spatio-temporal patterns in the copepod community in Malangen, Northern Norway

The objective of this study was to identify the key copepodspecies and their life cycles, and provide evidence for anyseasonal and spatial changes in the copepod community in Malangen,a fjord located 30 km to the south of Tromsø in NorthernNorway (69°30'N, 18°21'E). As a result of high levelsof freshwater run-off in May, the fjord became highly stratifiedwith a sharp pycnocline at 10–30 m depth from May to August.The generation patterns of six copepod species are described.Calanusfinmarchicus produced one generation during the spring thatyear, whereas two generations appeared to be produced by bothPseudocalanus acuspes and P.minutus: one in spring (March-June)and the other in autumn (August-December). However, it is uncertainto what extent P.minutus regularly produces a second generation.Two peaks of CI-CIII Metridia spp. were found; there were differencesalong the length of the fjord in the timing of these, but therelative contributions of M.longa and M.lucens are uncertain.Chiridius armatus CI-CIII peaked in abundance in the spring,which indicates that one main generation was produced at theouter station of the fjord. The copepod community in Malangencould be grouped into three entities according to their numericalabundance during the year one group of highly abundant forms,generally with maxima >50 000 individuals m^–3 (C.finmarchicus,Microcalanus sp., Oithona similis , Oithona spinirostris, Acartiasp. and Pseudocalanus spp.), a second group of less abundantspecies with a clear seasonality in abundance, varying from500 to 50 000 individuals m^–3 (M.longa, M.lucens, Calanushyperboreus, Carmatus, Tenwra longicornis, Oncaea sp., Euchaetanorvegica and Scolecithrwella minor), and a third group of 14holoplanktonic species, sporadically occurring in the fjord.The study demonstrates clear gradients in the abundance of fivespecies along the length of the fjord: the recruiting generationof C.finmarchicus occurred in higher abundances at the outerstation in May and June compared to the other inner sites. Laterin the season, the reverse situation appeared, in which thepopulation was more abundant in the inner part of the fjorcCalanw hyperboreus increased abruptly in abundance from lowwinter levels to a maximum in April-May, and declined steadilyduring the season (except at the innermost station). Metridialucens, M.longa and C.armatus demonstrated different distributionpatterns in Malangen that matched their preferred areas of distribution.Both M.lucens and C.armatus are known as oceanic and deep-waterspecies, respectively, and these were prevalent at the two outersites in Malangen. Metridia longa is a more nentic species andwas found in highest numbers at the two innermost sites. Themechanisms for the differences in abundance among these specieswithin the fjord are discussed.     

DENSITY FLUCTUATIONS, GROWTH AND DRY TISSUE PRODUCTION OF HYDROCOCCUS BRAZIERI (TENISON WOODS, 1876) AND ARTHRITICA SEMEN (MENKE, 1843) IN PEEL INLET, WESTERN AUSTRALIA

Hydrococcus brazieri and Arthritica semen on a sandflat at Coodanup,Peel Inlet, Western Australia, accounted for 89.5% of totalmollusc numbers. The mean density of H. brazieri was 9487 m^–2and A. semen averaged 8105 m^–2. H. brazieri grew at 0.5mm month^–1 and reached maturity in 4 months; A. semengrew at 0.3 mm month^–1 and reached maturity in 6 months.Somatic production in the Peel-Harvey estuarine system was estimatedto be 0.5 g m^–2yr^–1for H. brazieri and 4.1 g m^–2yr^–1for A. semen. The positions of these two species in the estuarinefoodweb is discussed. (Received 26 June 1981;     

Presence of Gymnodinium catenatum (Dinophyceae) in a coastal Mediterranean lagoon

The occurrence and abundance of the toxic, chain-forming dinoflagellateGymnodinium catenatum in a Tyrrhenian coastal lagoon, the Fusaro,during an annual sampling cycle are reported. Peak abundanceswere observed from late spring until early autumn Although veryhigh cell numbers were recorded, up to 1 5 x 10⁶ cells l^–1,no monospecific bloom of this species occurred. The first observationof G.catenatum in the Mediterranean occurred in the Fusaro andthe appearance of this species in a traditional shellfish farmingarea, where no shellfish intoxication has been reported to date,is discussed in relation to human interventions in the basin.In particular, intensive dredging in recent years with resuspensionof bottom sediments may have seeded the water body with cysts.A Gymnodinium n d species, illustrated using scanning electronmicroscopy, caused a monospecific bloom in concomitance withmaximum abundances of G.catenatum, apparently outcompeting thislatter species     

Separation of marine seston and density determination of marine diatoms by density gradient centrifugation

An attempt has been made to separate constituents of marineseston samples: inorganic material, detritus and the algal species,by density gradient centrifugation, without affecting the physiologicalstate of the algae. A relatively inert gradient material, consistingof Percoll, salt and sucrose, was composed. Since the densitiesof detritus and algae as well as those of different algal speciesoften overlapped, only 10 of the 100 samples processed in thecourse of the year showed a reasonable separation. However,an enrichment with respect to one or more species was oftenachieved. Densities of eleven species of marine diatoms andof one dinoflagellate have been determined at different timesof the year. For eight diatom species and for the dinoflagellatethe following specific density ranges were established: Bidduiphiaaurita: 1.18–1.23 g cm^–3, Biddulphia sinensis: 1.03–1.08g cm^–3, Cerataulina bergonii: 1.03–1.06 g cm^–3,Ditylum brightwellii: 1.07–1.13 g cm^–3, Rhizosoleniadelicatula: 1.04–1.09 g cm^–3, Skeletonema costatum:1.12–1.17 g cm^–3, Streptotheca thamensis: 1.04–1.10g cm^–3 , Thalassiosira rotula: 1.05–1.10 g cm^–3,Peridinium sp.: 1.08–1.12 g cm^–3. No seasonal variationin density was demonstrated. Gradients of different compositiondid not influence density measurements.     

The annual cycle of planktonic ciliates in nearshore waters at Signy Island, Antarctica

The abundance and biomass of marine planktonic ciliates in BorgeBay, Signy Island, were determined at monthly intervals betweenApril 1990 and June 1991. At least 24 different ciliate taxawere recorded from samples preserved in Lugol's iodine, includingthe tintinnids Codonellopsis balechi, Cymalocylis convallaria,Laackmaniella naviculaefera and Salpingella sp., and the aloricatetaxa Didinium sp. and Mesodinium rubrum. Ciliate abundance andbiomass exhibited a clear seasonal cycle with high values duringthe austral summer and low values in the austral winter. Abundanceranged from 0.3 10³l^–1 in September to 2.3 10³l^–1in January, while biomass ranged from 0.5 µg C l^–1in October to 12.6 µg C l^–1 in December. Small ciliatesdominated abundance throughout the year, and biomass duringwinter. Larger ciliates contributed most to biomass during summer.Aloricate ciliates were common throughout the year, while tintinnidscontributed substantially to abundance and biomass only duringsummer. Salpingella sp. was the commonest tintinnid, but C.convallariacontributed most to tintinnid biomass. The seasonal patternof ciliate abundance and biomass matched that of chlorophylla concentration and bacterial biomass, suggesting tight trophiccoupling between ciliates and other components of the pelagicmicrobial community. ¹Present address: Scott Polar Research Institute, Universityof Cambridge, Lensfield Road, Cambridge CB2 1ER, UK     

Temporal variability in the night-time distribution of epipelagic siphonophores in the North Atlantic Ocean at Bermuda

The temporal variability of siphonophores was compared on night-to-night,month-to-month and year-to-year scales in open-net collectionsmade at or near the Bermuda Atlantic Time Series (BATS) site.These collections were made as oblique tows to 100 m on (i)one summer night in each of 1986, 1989 and 1991, (ii) one nightper month on 13 cruises between April 1991 and August 1992,and (iii) three consecutive summer nights of 1992. The meanwet displacement volume (WDV) of sipho nophores in the thesecollections was 1.4 ± 0.6 ml per 100 m³ or 20 ±10% of the total catch biovolume. The most common species belongedto the Calycophorae families Diphyidae and Abylidae: the combinednumerical abundance of Chelophyes appendiculala, Eudoxoidesmitra, Lensia subtilis. Eudox-oides spiralis, Bassia bassensisand Abytopsis eschscholtzi averaged 300–400 per 100 m³.Combined numbers of these common species varied little whensampled hour by hour at the same location during consecutivesummer nights or at different night-time sampling locations10¹–10² km apart when these were sampled within 3–5h. In contrast, most of these six species exhibited 2- to 3-folddifferences in night-time abundance between summers of differentyears and between seasons. Three seasonal groups were detected,with a winter group being the most distinct from the others.Changes in the abundance of several species were correlatedwith seasonal changes in the depth of the mixed layer.     

DISTRIBUTION AND ABUNDANCE OF GYMNOSOMATA (GASTROPODA: OPISTHOBRANCHIA) IN THE SOUTHWEST ATLANTIC

The distribution and abundance of gymnosome gastropods in theArgentine Sea and Brazil—Malvinas Confluence during 1978–1979and 1988 were studied. The collections analyzed included 768quantitative samples obtained between 48°W and the coast,and from 35°S to 55°S. Two species were found. Spongiobranchaeaaustralis was the most frequent and abundant (up to 730 per1000 m3); its presence in the area was associated with the coreof the Malvinas Current. Clione antarctica was less abundant(maximum abundance: 230 per 1000 m³) and was also associatedwith the Malvinas Current. The geographic ranges of both speciesin the area are wider than previously described. Since the rangeof S. australis in the area extends far from the range of itsprey Clio, it is not clear whether S. australis can feed onthe thecosomatous pteropod Limacina (and not only on Clio, asdescribed in the bibliography) or it starves in that area. Duringthe 1978–1979 annual cycle, the abundance of both speciesfollowed neither the abundance patterns of their prey nor ofthe total zooplankton, and differed from each other. The residencetime of swarms of both gymnosomes were shorter than one month.As a general pattern, the aggregates are rapidly transportednorthward by the Malvinas Current and also penetrate the outershelf water, but they remain there only during a short periodand cannot preclude the final expatriation. So, the abundanceof gymnosomes in the area depends on passive migration morethan intrinsic population factors. (Received 9 July 1997; accepted 15 December 1997)     

Grazing of echinoderm larvae (Paracentrotus lividus and Arbacia lixula) on naturally occurring particulate matter

Larvae (72 hr old) of P. lividus and A. lixula grazed on varioussuspensions of natural particulate matter with a size rangeof 2 to 30 microns, and on two species of algae (Phaeodactylumtricor-nutum and Nitzschia sp.) — Larvae graze most in the size range where the particleconcentration is highest. — If larvae deplete certain size categories of particlesthey then graze other size ranges in which the concentrationis still high. — The grazing rate of the two species varied between 988and 91.949 µm³ per pluteus per hour. — For A. lixula larvae the grazing rate increases withincreasing temperature to a maximum at 22°C.     

Effects of net, pump and bottle sampling on the abundance estimates of planktonic Rotifera

Eighty-seven freshwater plankton samples collected by meansof two different nets, a submersible centrifugal pump and a5.81 water bottle were compared with respect to their abilityto catch the rotifer Keratella cochlearis. Sample size, illumination(day/night), the presence of bridles ahead of the net mouths(versus unbridled nets), and different mouth diameters (0.2and 0.5 m) did not affect abundance estimates. Slight differenceswere found between the yields of pushed nets versus towed nets;these are probably due to uneven distribution of the animalsin the 0–3 m layer. Both pump and bottle volumes stronglyaffected abundance estimates (K. cochlearisl^–1 in pumpsamples=164.84 volume of water filtered^{–0 817}; K. cochlearisl^–1in bottle samples=84.74+2336.6 volume of water filtered^–1)Net sample results were always considerably higher (4.6–12.3times) than pump and bottle estimates; these differences aremost probably due to evading reactions of the rotifer.     

Protozooplankton and bacterioplankton in a large oligotrophic lake--Loch Ness, Scotland

The seasonal changes in the abundance of protozoan and bacterialplankton in a large, coloured, oligotrophic lake. Loch Ness(Scotland), were investigated between August 1991 and January1993. The coloured water supported only low concentrations ofchlorophyll a (<1.6 µgl^–1). with the highestvalues occurring in summer. Mean bacterial abundance rangedbetween 2.3 x 10⁸ and 7.1 x 10⁸ l^–1 in the 100 m watercolumn. Maximum abundance did not correlate with maximum chlorophylla concentrations, but appeared to be related to the input ofallochthonous carbon from the catchment, which in turn was influencedby rainfall levels. Consequently, the highest bacterioplanktonconcentrations occurred in autumn and winter. The pattern ofheterotrophic nanoflagellate abundance tended to follow thatfor bacteria, with mean concentrations in the top 100 m of thewater column of between 12 x 10³ and 273 x 10³ l^–1. Ciliateabundance showed no seasonal trends over the study period andprobably mirrored the fluctuating availability of various foodresources. Oligotrichs, particularly mixotrophic taxa, werea prominent element of the community throughout the year. Aggregatesof detrital material were a regular feature in the plankton.When these occurred, they formed foci for bacteria and nanoflagellates.The evidence suggests that the dynamics of the microbial planktonin Loch Ness may be driven by allochthonous carbon inputs ratherthan by the more usual dominance of carbon fixed within thesystem. ¹Present address: School of Zoology, La Trobe University, Bundoora,Melbourne, Victoria 3083, Australia ²Present address: Loch Ness & Morar Project, Loch Ness Centre,Drumnadrochit, Invernesshire, UK