Alcian Blue-stained particles in a eutrophic lake

We used a neutral solution of Alcian Blue to stain transparentparticles in eutrophic Lake Frederiksborg Slotss0, Denmark.Alcian Blue-stained particles (ABSP) appeared to be similarto the so-called transparent exopolymer particles (TEP) identifiedwith an acidic solution of Alcian Blue. Our results on the abundance,size distribution and bacterial colonization of ABSP thereforereflect general patterns of TEP. The abundance of ABSP in thesize range 3–162 µm and retained by 3mu;m pore sizefilters averaged 3.62.4910⁵ ml^–1 (SD), which is amongthe highest concentrations reported for comparable size spectraof TEP. On average, 35 % of ABSP (by number) were colonizedby bacteria and 8.610⁵ bacteria ml^–1 lake water wereattached to ABSP, which corresponds to 7% of the total bacterialabundance.     

Abundance, size distribution and bacterial colonization of transparent exopolymeric particles (TEP) during spring in the Kattegat

The abundance, size distribution and bacterial colonizationof transparent exopolymeric particles (TEP) were monitored inthe Kattegat (Denmark) at weekly intervals throughout the spring(February-May) encompassing the spring diatom bloom. These recentlydiscovered particles are believed to be formed from colloidalorganic material exuded by phytoplankton and bacteria, and mayhave significant implications for pelagic flux processes. Duringthis study, the number concentration of TEP (>1 µm)ranged from 3 x 10³ to 6 x 10⁴ ml^–1 and the volume concentrationbetween 0.3 and 9.0 p.p.m.; they were most abundant in the surfacewaters subsequent to the spring phytoplankton bloom. The rangeof TEP (encased) volume concentration was similar to that ofthe phytoplankton, although at times TEP volume concentrationexceeded that of the phytoplankton by two orders of magnitude.The TEP size distribution followed a power law, with the abundanceof particles scaling with particle diameter^–(ß+1).The seasonal average estimate of ß (2.3) was not significantlydifferent from three, consistent with TEP being formed by shearcoagulation from smaller particles. However, date-specific estimatesof ß differed significantly from three, probably becauseTEP are fractal. All TEP were colonized by bacteria, and bacteriawere both attached to the surface of and embedded in TEP. Yetthe number of attached bacteria per TEP was related neitherto the surface area nor the volume, but rather scaled with TEPsize raised to an exponent of     

Autotrophic picoplankton in southern Lake Baikal: abundance, growth and grazing mortality during summer

Autotrophic picoplankton were highly abundant during the thermalstratification period in late July in the pelagic area (waterdepth 500–1300 m) of southern Lake Baikal; maximum numberswere 2 x 10⁶ cells ml^–1 in the euphotic zone ({small tilde}15m). Unicellular cyanobacteria generally dominated the picoplanktoncommunity, although unidentified picoplankton that fluorescedred under blue excitation were also abundant (maximum numbers4 x 10⁵ cells ml^–1) and contributed up to {small tilde}40%of the total autotrophic picoplankton on occasions. Carbon andnitrogen biomasses of autotrophic picoplankton estimated byconversion from biovolumes were 14–84 µg C l^–1and 3.6–21 µg N l^–1. These were comparableto or exceeded the biomass of heterotrophic bacteria. Autotropicpicoplankton and bacteria accounted for as much as 33% of paniculateorganic carbon and 81% of nitrogen in the euphotic zone. Measurementsof the photosynthetic uptake of [^l4C]bicarbonate and the growthof picoplankton in diluted or size-fractionated waters revealedthat 80% of total primary production was due to picoplankton,and that much of this production was consumed by grazers inthe <20 µ.m cell-size category. These results suggestthat picoplankton-protozoan trophic coupling is important inthe pelagic food web and biogeochemical cycling of Lake Baikalduring summer.     

Observations of diet patterns of photosynthesis in cyanobacteria and nanoplankton in the Santa Barbara Channel during 'el Nino'

During the 1983 ‘el Niño’, filter fractionationshowed that over 80% of the chlorophyll-based phytoplanktonbiomass in the Santa Barbara Channel was <5 µm. Largernanoplankton (5–30 µm) accounted for the chlorophyllin the remaining fraction but, unlike other years, no significantquantities of net plankton <30 µm were detected. Thepopulation as a whole was dominated by chroococcalean cyanobacteriawhich were two times were abundant (123±24x10³ cellsml^–1 than previously reported for the California CurrentSystem (Krempin and Sullivan, 1981). Numbers of other typesof bacteria were uniformly low (277±44x10³ cells ml^–1Cyanobacteria and larger nanoplankton exhibited similar diurnalpatterns of photosynthesis, i.e., maximal rates of light-saturatedphotosynthesis (P_max) occurred mid-day and day-night amplitudeswere >2.0. In both size fractions the onset of the rise andfall in P_max preceded sunrise and sunset, respectively, andthe photosynthetic periodicity was independent of both chlorophyllcontent and dark fixation of inorganic carbon. Unlike previousstudies on diel periodicity in phytoplankton, no significantoscillations in light-limited rates of photosynthesis (     

Spatial distribution of transparent exopolymer particles in the Bransfield Strait, Antarctica

Transparent exopolymer particles (TEP) are recognized to playan important role in the flux of exported carbon to the deepocean. However, there is little information on how TEP standingstocks are affected by different hydrographic conditions andother relevant ecological factors in situ. This lack of knowledgeis particularly serious for the Southern Ocean. During Australsummer 1999, the Strait of Bransfield presented high mesoscalevariability. Two fronts were present, the Bransfield hydrographicfront and a slope front along the South Shetland Islands andseveral mesoscale anticyclonic eddies and/or frontal meanders.The spatial distributions of biological properties were largelyaffected by this complex hydrography. Chlorophyll a (Chl a)(0.05–4.81 µg L^–1), TEP (from undetectableto 346 µg GXeq L^–1) and heterotrophic bacteria (HB)(1.7–9.4 x 10⁵ cells mL^–1) were positively correlateddespite the wide hydrographic heterogeneity of the BransfieldStrait. Higher abundances of autotrophic biomass, and correspondlyhigher TEP and heterotrophic bacteria (HB), were found in themore stratified waters. TEP spatial distribution was mostlyrelated to the abundance of autotrophic biomass although localhigh TEP concentrations were not matched by similarly high valuesof Chl a in some areas where diatoms were relatively abundant.     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

House Production by Oikopleura dioica (Tunicata, Appendicularia) Under Laboratory Conditions

We examined the generation time and the house renewal rate ofOikopleura dioica under various conditions. Animals were fedtwo flagellates, Isochrysis galbana and Tetraselmis sp., withconcurrent determination of the carbon contents of body andhouse to estimate house production. The generation time was6 days at 15°C, 4 days at 20°C and 3 days at 25°Cat both 25 and 30 p.s.u. with a food concentration of 4 x 10⁴cells ml^–1. The carbon content of newly secreted housesranged from 0.5 to 0.8 µg, corresponding to 15.3 ±4.8% of body carbon. The house renewal rates increased withincreasing temperature and decreasing salinity. Food concentrationsranging from 100 to 16 x 10⁴ cells ml^–1, body size andlight condition had no effect on house renewal rate. Cloggingof the inlet filter by adding the large diatom Ditylum sol causedan increase in house renewal rates. The total number and carboncontent of houses during an animal's lifetime ranged from 46to 53 houses and from 6.5 to 10.4 µg, respectively. Sincedaily house production calculated for the O. dioica populationcorresponded to 130–290% of its biomass and daily discardedhouse materials corresponded to 490–1100% of the biomass,this organism must play an important role as a producer of macroscopicaggregates.     

Microzooplankton herbivory and bacterivory in Newfoundland coastal waters during spring, summer and winter

Grazing by microzooplankton on autotrophic and heterotrophicpicoplankton as well as >0.7 µm phytoplankton (as measuredby chlorophyll a) was quantified during July, August, October,January and April in the surface layer of Logy Bay, Newfoundland(47°38'14'N, 52°39'36'W). Rates of growth and grazingmortality of bacteria, Synechococcus and >0.7 µm phytoplanktonwere measured using the sea water dilution technique. Microzooplanktoningested 83–184, 96–366 and 64–118% of bacterial,Synechococcus and >0.7 µm phytoplankton daily potentialproduction, respectively and 34–111, 25–30 and 16–131%of bacterial, Synechococcus and >0.7 µm phytoplanktonstanding stocks, respectively. The trends in prey net growthrates followed the seasonal cycles of prey biomass, suggestingthat microzooplankton are important grazers in Newfoundlandcoastal waters. Ingestion was lowest during January and October(~2 µg C l^–1 day^–1) and highest in August(~20 µg C l^–1 day^–1). Aside from April when>0.7 µm phytoplankton represented the majority (~80%)of carbon ingested, bacterioplankton and <1 µm phytoplanktonrepresented most of the carbon ingested (~40–100%). Althoughmicrozooplankton have here-to-fore been unrecognized as an importantgrazer population in Newfoundland coastal waters, these resultssuggest that they play an important role in carbon flow withinthe pelagic food web, even at low temperatures in Logy Bay.     

Community structure, biomass and productivity of size-fractionated summer phytoplankton populations in lower Narragansett Bay, Rhode Island

Seventeen size-fractionation experiments were carried out duringthe summer of 1979 to compare biomass and productivity in the< 10, <8 and <5 µm size fractions with that ofthe total phytoplankton community in surface waters of NarragansettBay. Flagellates and non-motile ultra-plankton passing 8 µmpolycarbonate filters dominated early summer phytoplankton populations,while diatoms and dinoflagellates retained by 10 µm nylonnetting dominated during the late summer. A significant numberof small diatoms and dinoflagellates were found in the 10–8µm size fraction. The > 10 µm size fraction accountedfor 50% of the chlorophyll a standing crop and 38% of surfaceproduction. The <8 µm fraction accounted for 39 and18% of the surface biomass and production. Production by the< 8 µm fraction exceeded half of the total communityproduction only during a mid-summer bloom of microflagellates.Mean assimilation numbers and calculated carbon doubling ratesin the <8 µm (2.8 g C g Chl a^–1 h^–1; 0.9day^–1)and<5 µm(1.7 g C g Chl a^–1h^–1; 0.5day^–1)size fractions were consistently lower than those of the totalpopulation (4.8 g C g Chl a^–1 h^–1; 1.3 day^–1)and the <10 µm size fraction (5.8 g C g Chl a^–1h^–1; 1.4 day ^–1). The results indicate that smalldiatoms and dinoflagellates in fractionated phytoplankton populationscan influence productivity out of proportion to their numbersor biomass. ¹Present address: Australian Institute of Marine Science, P.M.B.No. 3, Townsville M.S.O., Qld. 4810, Australia.     

The energetics of Calanus euxinus: locomotion, filtration of food and specific dynamic action

The effect of locomotor activity on respiration rate was studiedin the food-deprived copepod Calanus euxinus tethered to a forcesensor. The power generated by mouth appendages during cruisinglocomotion, with a frequency of 40 Hz, accounted for 0.026 and0.0031 W for metabolic and mechanical processes, respectively.To overcome total hydrodynamic drag during foraging with a meanswimming speed of 3.2 cm s^–1, the copepods need 0.4 x10^–3 W, equating to 1.3% of total metabolism. The lossesof mechanical energy for body propulsion amounted to 1.3 x 10^–3W, whilst the cost of feeding current generation run up to 1.8x 10^–3 W, or 58% of the total. Changing of locomotor activityand respiration rate during feeding was examined separatelyin tethered and free-swimming copepods. At algal concentrationof 300 µg C L^–1, the magnitude of specific dynamicaction (SDA) averaged 1.2 ± 0.44 nL O₂ µg C^–1h^–1 in copepodites V and females, with similar movingactivity before and during feeding. The contribution of SDAinto total metabolism varied from 23 to 85% in C. euxinus withlow activity level and constituted only 10% in active animals.In starved copepods, with low locomotor activity, feeding eventsstimulated the increase in frequency and total duration of locomotionwhich resulted in elevated energy expenditure enhancing the‘apparent SDA’.     

Remote detection of algae by copepods: responses to algal size, odors and motility

Selective feeding on large algae by copepods involves remotedetection of individual particles and subsequent active captureresponses In this study we use radiotracer experiments to quantifythe clearance rates of five coexisting freshwater copepods andto investigate the relative merits of the chemoreception andmechanoreception hypotheses of remote detection Tropocyclopsand three diaptomid copepods exhibited relatively high clearancerates when feeding on low concentrations of large algae, suggestingthat most previous studies with freshwater copepods have underestimatedmaximal clearance rates and the degree of size selectivity.All five species of copepods exihibited strong selection foran intermediate-sized flagellate (25 µm Cartena) or alarge-sized nonmotile alga (80 µ.m Pediastrum) over asmall-sized flagellate (6 µ.m Chlamydomonas). The weight-specificclearance rate for Tropocylops prasmus feeding on motile Cartena(271 ml mg^–1 h^–1) was about twice that of threediaptomid copepods and more than an order-of-magnitude higherthan the estimate for Epischura lacustris feeding on its preferredalga, Pediastrum Assuming that distance chemoreception (‘smell’)is important in remote detection, we predicted that the additionof high concentrations of ‘algal odors’ would obscureany chemical gradients emanating from individual algal cellsand would thereby hinder the remote detection and active captureof large algae Contrary to this hypothesis, the addition ofamino acids, sucrose, and algal extracts had no effect on theclearance rates and selectivity of Diaptomus birgei. These results,together with recent cinematographic studies (Vanderploeg etal.,1990), suggest that mechanoreception is the primary mechanismfor the remote detection of large particles by diaptomid copepods.A raptorial cyclopoid, Tropocyclops prasinus, exhibited strongpreferences for motile algae, whereas a suspension-feeding calanoid,D birgei, did not select between motile and nonmotile cells.Motility appears to be an important factor in algal detectionfor small cyclopoid copepods but not for suspension-feedingdiaptomids     

Bacterivory in seawater samples estimated by a dual radioactive-labelling technique

A dual radioactive-labelled bacteria technique using Vibrio(DRLV), developed for laboratory studies on bacterivory, hasbeen refined for use at the concentrations of prey and predatorstypcially found at sea. Experiments with estuarine water collectedin spring and in autumn showed that bacterivorous nanoflagellates(HNF) (concentration 1.38±0.35x10³ HNF ml^–1) ingested2.7±0.96 DRLV flagellate1^–1 h^–1 at concentrationsof 0.8–2.2x10⁶ DRLV ml^–1 in the presence of 2.04±0.68x10⁶natural bacteria ml^–1. The method was also applied tosamples collected in October in the Celtic Sea, when on average1 ml of water from the surface layer contained 1.41±0.16x10⁶natural bacteria, 14.6x10³ cyanobacteria, 530±170 HNF,7.3±3.0x103 phototrophic nanoflagellates (1.5–4µm), 49.0±26.5 phototrophic dinoflagellates, 36.3±12.6heterotrophic dinoflagellates and 21.3±9.5 Leucocryptosmarina. Under these conditions the grazing rate in most samplesdid not exceed the coefficient of variation of the method (2%),although we estimate the grazing rate was -1.6 DRLV HNF^–1h^–1 and on one occasion a rate of 2.45 was recorded. Thegross growth efficiency for protein of -30% displayed by naturalHNF means that they could release about     

Studies on the Growth in Culture of Plant Cells: X. FURTHER STUDIES ON THE CONDITIONING OF CULTURE MEDIA BY SUSPENSIONS OF ACER PSEUDOPLATANUS L. CELLS

The standard synthetic culture medium (Stuart and Street, 1969)has been modified by adjustment of its initial pH to 6.4 andby the addition of gibberellic acid (0.25 mg/l) and of a mixtureof 15 L-amino acids formulated from an analysis of the conditionedmedium. The minimum effective density for the growth of sycamorecell suspensions in the standard medium is 9–15 x 10³cells ml^–1, for the modified synthetic medium it is 2.0x 10³ cells ml^–1, and for conditioned medium 1.0–1.25x 10³ cells ml^–1. Using either conditioned medium (Stuart and Street, 1969) orthe modified synthetic medium it is demonstrated that the growthof cultures initiated at low density is enhanced by a volatilefactor released from actively growing cell suspensions. In presenceof conditioned medium and this volatile factor cultures canbe established from stationary-phase cells at a density of 6x 10² cells ml^–1. The volatile factor can be absorbedin 40 per cent w/v KOH but attempts to replace the factor byair containing carbon dioxide at concentrations up to 5 percent have so far been unsuccessful.     

Specific growth rates of heterotrophic plankton organisms in a eutrophic lake during a spring bloom

The in situ growth of the dominating pelagic organisms at severaltrophic levels was investigated during a spring bloom characterizedby well-mixed cold water. The study includes primary productionand the carbon flow through the nano-, micro- and mesozooplanktonpopulations based on population dynamics and specific growthrates. The phytoplankton biomass and production were totallydominated by small algae <20 µm. of which {small tilde}5%were <3µm. potentially a food source for the nano-and microzooplankton. The mean carbon-specific primary productionwas 0.15 day^–1 and was regulated solely by light. Themean volume-based specific growth rate of bacterioplankton wasmodest. 0.1 day^–1. and probably controlled by the lowtemperature. The volume-based specific growth rates of heterotrophicnanoflagellates. ciliates. rotifers and copepods were 0.35.0.13. 0.16 and 0.03 day^–1, respectively. The observedgrowth of the heterotrophic plankton was generally not foodlimited, but was controlled by temperature. The stable temperatureduring the experiment therefore allows a cross-taxonomic comparisonof specific growth rates. The b exponent in the allometric relationship(G = aV^th) between volume-specific growth rate (G) and individualbody size (V) was –0.15 ± 0.03 for all filtratingzooplankton. indicating an in situ scaling not far from thephysiological principles onginally demonstrated for laboratorypopulations.     

Zooplankton feeding ecology: does a diet of Phaeocystis support good copepod grazing, survival, egg production and egg hatching success?

The bloom-forming alga Phaeocystis is ingested by a varietyof zooplankton grazers, but is thought to be a poor source offood. We examined copepod grazing on solitary Phaeocystis cellsby adult females of Temora stylifera, and survival, fecal pelletproduction, egg production and egg hatching success in Calanushelgolandicus and T. stylifera over periods of 15 consecutivedays. Phaeocystis cell concentrations were high (1.2–3.6x 10⁴ cells ml^–1 for C. helgolandicus and 2.5–7.9x 10⁴ cells ml^–1 for T. stylifera), but within the rangeof maxima recorded for natural blooms. Both copepods survivedwell and continuously produced fecal pellets (indicating continuousgrazing) on a diet of Phaeocystis. However, egg production ratesfor both copepods were low, even though hatching success ofthe few eggs produced was high. Clearance rates for T. styliferawere higher than for most previous measurements of other copepodsfeeding on Phaeocystis solitary cells at lower cell concentrations.We conclude that even though copepods feed well upon Phaeocystis,resulting poor fecundity on this diet may inhibit copepod populationincreases during blooms, thereby contributing to the perpetuationof blooms. However, the high egg hatching success on this dietargues against Phaeocystis containing chemical compounds thatact as mitotic inhibitors reducing copepod egg viability, suchas those found in some other phytoplankters.     

Feeding and metabolism of the siphonophore Sphaeronectes gracilis

The in situ predation rate of the siphonophore Sphaeronectesgracilis was estimated from gut content analysis of hand-collectedsiphonophores and from laboratory data on digestion rates ofprey organisms. At daytime prey densities of 0.25 copepods 1^–1,S. gracilis was estimated to consume 8.1 – 15.4 prey day^–1siphonophore^–1. From data on abundances of siphonophoresand copepods, S. gracilis was estimated to consume 2–4%of the copepods daily. In laboratory experiments, ingestionrates averaged 13.8 prey day^–1 siphonophore^–1 atprey densities of 5 copepods 1^–1 and 36.9 at 20 copeods1^–1. This was equivalent to a specific ingestion rate(for both carbon and nitrogen) of –17% day^–1 and45% day^–1, respectively, while specific ingestion in situwas only 2% day^–1. Ammonium excretion averaged 0.095 µg-atsiphonophore^–1 day^–1 at 5 prey 1^–1, and 0.162at 20 prey 1^–1. The specific respiration (carbon) andspecific excretion (nitrogen as ammonium) were calculated tobe 3% day^–1 at the lower experimental food level, and5% day^–1 at the higher food level. ¹Contribution from the Catalina Marine Science Center No. 66. ²Present address: Dept. of Biology, University of Victoria,Victoria, B.C., Canada V8W 2Y2.     

Copepod egg production, moulting and growth rates, and secondary production, in the Skagerrak in August 1988

Measurements of hydrography, chlorophyll, moulting rates ofjuvenile copepods and egg production rates of adult female copepodswere made at eight stations along a transect across the Skagerrak.The goals of the study were to determine (i) if there were correlationsbetween spatial variations in hydrography, phytoplankton andcopepod production rates, (ii) if copepod egg production rateswere correlated with juvenile growth rates, and (iii) if therewas evidence of food-niche separation among co-occumng femalecopepods The 200 km wide Skagerrak had a stratified water columnin the center and a mixed water column along the margins. Suchspatial variations should lead to a dominance of small phytoplanktoncells in the center and large cells along the margins; however,during our study blooms of Gyrodinium aureolum and Ceratium(three species) masked any locally driven differences in cellsize: 50% of chla was >11 µm, 5% in the 11–50µm fraction and 45% <50 µm. averaged for allstations. Chlorophyll ranged from 0.2 to 2.5 µg l^–1at most depths and stations. Specific growth rates of copepodsaveraged 0.10 day^–1 for adult females and 0.27 day^–1for juveniles The latter is similar to maximum rates known fromlaboratory studies, thus were probably not food-limited. Eggproduction rates were food-limited with the degree of limitationvarying among species: 75% of maximum for Centropages typicus, 50% for Calanus finmarchicus, 30% for Paracalanus parvus and 15% for Acartia longiremis and Temora longicornis. Thedegree of limitation was unrelated to female body size suggestingfood-niche separation among adults. Copepod production, summedover all species, ranged from 3 to 8 mg carbon m^–3day^–1and averaged 4.6 mg carbon m^–1 day^–1. Egg productionaccounted for 25% of the total.     

Annual variation in the abundance and size of heterotrophic nanoflagellates on the SW coast of Finland, the Baltic Sea

Annual variation and vertical distribution in the abundanceand cell volume of heterotrophic nanoflagellates (HNF) was studiedon the SW coast of Finland, the Baltic Sea. HNF cell numbersand mean cell volume varied annually from 90 to 10⁴ cells ml^–1,and from 3 to 32 µm³, respectively, with maxima in earlysummer. The proportion of choanoflagellates in the HNF communitywas 0–23%. Statistical analysis revealed the verticaldifferences in HNF abundance to be insignificant, but verticaldifferences in the size structure of HNF communities were found,especially during thermal stratification. The majority (>80%)of HNF were small (maximum dimension 2–4 µm); theproportion of large (>7 µm) cells were only 2–4%of the HNF abundance. An empirical equation for the relationshipbetween HNF cell length and volume is presented, and the measurementof flagellate volume by epifluorescence microscopy is discussed.     

Laboratory-measured grazing and ingestion rates of the salp, Thalia democratica Forskal, and the doliolid, Dolioletta gegenbauri Uljanin (Tunicata, Thaliacea)

Grazing and ingestion rates of laboratory-born Thalia democraticaaggregates and Dolioletta gegenbauri gonozooids, phorozooidsand oozooids were determined while fed Isochrysis galbana (4–5µm diameter) alone or in combination with Peridinium trochoideum(16–18 µm diameter) at concentrations of 0.15–0.70mm³ x 1^–1. Grazing rates (ml x zooid^–1 x 24 h ^–1)ranged from 10 to 355, and at zooid weights greater than 5 µgcarbon were in order oozooid > gonozooid > aggregate.Grazing rates increased exponentially with increasing zooidweight. Weight-specific grazing rates (ml x µgC^–1x 24 h^–1) were independent of the four-fold initial foodconcentration. Mean weight-specific grazing rates increasedlinearly with increasing zooid weight for the aggregates andoozooids, but gonozooid mean rates were independent of zooidweight. Aggregate and gonozooid ingestion rates (10⁶ µm³x zooid^–1 x 24 h^–1) ranged from 4 to 134 while oozooidrates ranged from 3 to 67. All ingestion rates were independentof the initial food concentration but increased linearly withincreasing zooid weight at similar rates. All mean weight-specificingestion rates (ml x µgC^–1 x 24 h^–1) wereindependent of zooid weight. The mean aggregate daily ration(µgC ingested x µg body C^–1) was 59% and themean doliolid ration was 132%. Field studies indicate that normalconcentrations of D. gegenbauri in the Georgia Bight clear theirresident water volume (1 m³) in about 4 months, but that highlyconcentrated, swarm populations which occur along thermohalinefronts clear their resident water volume in less than 1 day. ¹Current address: MacLaren Plansearch Ltd., P.O.Box 13250, sta.A.,St.John's, Nfld. A1B 4A5     

Abundance and productivity of heterotrophic nanoplankton in Georgia coastal waters

The population abundances and rates of biomass production ofheterotrophic nanoplankton (HNAN) in Georgia coastal waterswere evaluated by epifluorescence microscopy. HNAN populations(mostly non-pigmented microflagellates <10 µm in diameter)ranged from 0.3 x 10³ cells ml^–1 in shelf waters 15 kmoffshore to 6.3 x 10³ cells ml^–1 in waters 0.25 km fromthe coast. There was a strong correlation (r = 0.83) betweenHNAN and free bacterioplankton population abundances, but noapparent relation (r = 0.38) between HNAN and phototrophic nanopLankton(PNAN) abundances. HNAN biomass production in estuarine andnearshore shelf waters, as estimated from increases in HNANpopulations during laboratory incubations of natural water samples,ranged from 0.10 to 0.79 mg C m^–3 h^–3, with populationgeneration times of 9.7 to 26.5 h. There was a significant linearrelation (r = 0.95) between HNAN biomass and HNAN productivity.We calculated that HNAN may graze at least 30% to 50% of dailybacterioplankton production in Georgia coastal waters.