Top-down impact by copepods on ciliate numbers and persistence depends on copepod and ciliate species composition

Experiments with the copepods Acartia clausi, Centropages hamatusand Pseudocalanus sp. were performed to assess the species-specificeffect of these copepods on the development of monospecificalgae (Nephroselmis pyriformis) and ciliate communities (Strombidiumvestitum, Strombidium conicum, Strombidium sp. and Lohmanniellaoviformis). It was hypothesized that potentially switching copepodslike A. clausi will stabilize the algal community by switchingbetween ciliate and algal food, in contrast to copepods withstereotypic filter feeding behaviour (Pseudocalanus sp.). Intreatments with Pseudocalanus sp. and C. hamatus, all ciliatespecies were wiped out in 2 days, resulting in blooms of N.pyriformis. In treatments with A. clausi, two of the ciliatespecies were able to persist, but the combined ciliate and copepodcommunity was not able to control the algal bloom. Ciliatesbecame abundant in control treatments without copepods, butonly S. vestitum and S. conicum seemed able to establish grazingcontrol. Hence, when evaluating the role of ciliates in foodwebs, their actual numbers and species composition should betaken into account. Likewise, the species composition of copepodsmay be crucial; these experiments demonstrate that small filterfeeding copepods may have tremendous impact on ciliate numbers.     

The influence of food resources on the development, survival and reproduction of the two cyclopoid copepods: Cyclops vicinus and Mesocyclops leuckarti

Laboratory experiments were conducted to determine whether thetwo cyclopoid copepods. Cyclops vicious and Mesocyclops leuckaru.exploit the same food resources. The food requirements of juvenilesof the two cyclopoid copepods were investigated. Moreover, theimportance of algae for the predaceous adults was studied. Naupliiof both M leuckaru and C.vicinus successfully developed intocopepodites when fed the motile algae Chlamydomonas reinhardtii.Chlamydomonas sphaeroides and Cryptomonas sp. Threshold foodconcentrations for naupliar development varied between offeredalgae and between the two cyclopoid species. The food thresholdfor successful naupliar development, when reared on C.reinhardui,was lower for M.leuckarti (0.3 mg C 1^–1) than for C.vicinus(0.5 mg C l^–1) whereas a similar food threshold was foundusing Cryptornonas sp (0.3 mg C ^–1) and C.sphaeroides(<0.2 mg C 1^–1), Naupliar development time was inverselyrelated to food concentration. Food required for copepoditedevelopment differed for the two cyclopoid species. Cyclopsvicinus was able to develop to the adult stage on a pure dietof any one of the three algal species. whereas M.leuckarti requireda prey supply of the rotifer Brachionus rubens. Food composition.i.e. algal species, algal concentration and rotifer abundance,influenced copepodite survivorship of both cyclopoids and wasalways higher in the presence of B.rubens. Under similar foodconditions, mortality was higher for M.leuckarti than for Cvicinus. Mesocyclops leuckaru females were very dependent onanimal food. The predation rate of M.leuckaru was not lowerin the presence of algae. Egg production of M.leuckarti waslow on a pure algal diet and significantly higher when B rubenswas present. The results were used to discuss the life cyclestrategy and the possibility of exploitative competition ofthe two cyclopoid copepods.     

Assimilation numbers in cultures of marine phytoplankton

During exponential growth in batch culture, assimilation numbersof eleven algal species ranged from 1.6–20.8, with a meanvalue of 5.3 g C/g Chlorophyll a/hr. The highest assimilationnumber of 20.8 g C/g Chlorophyll a/hr was observed in Coccolithuspelagicus, due to the relatively low concentration of chlorophylla/cell. The assimilation number declined from exponential tostationary phase in batch cultures for ten algal species, butincreased with age in batch culture in Amphiprora paludasa (abenthic diatom). The assimilation number declined with decreasinggrowth rate in nitrate-limited chemostat cultures of Phaeodactylumtricornutum and in iron-limited chemostat cultures of Phaeodactylumtricornutum and Isochrysis galbana.     

On the Occurrence of Intracellular Blue-green Algae in Cortical Cells of the Apogeotropic Roots of Macrozamia communis L. Johnson

The occurrence of species of the cyanophytes Nostoc and Anabaenain the cortex near the algal zone is reported for apogeotropicroots of Macrozamia communis L. Johnson. Algae were found tooccur both intercellularly and intracellularly in cells of theinner and outer cortex. This is the first record of intracellularalgae in the cycads and only the second report of this phenomenonin vascular plants. By examination of cells at various stagesof invasion by algae, it is interpreted that algal invasionof cortical cells and intercellular spaces is preceded by mucusapparently secreted by algal zone cells of the host, and depositedin intercellular spaces of cortical parenchyma cells nearby.Also algal penetration of cortical cells is preceded by an algalinvasion front of finely granular mucal material which bypassesmucus already deposited in intercellular spaces and may eitherlyse part of the host cell wall or enter through the plasmo-desmata,filling much of the cell cavity. Subsequently, large numbersof the algal symbionts enter the cell and may be enclosed withinhost wall material. Electron microscopic techniques are nowbeing employed to further clarify these invasion processes.     

Immuno-flow cytometric identification and enumeration of the ichthyotoxic dinoflagellate Gyrodinium aureolum Hulburt in artificially mixed algal populations

Flow cytometric identification and enumeration of Gyrodiniumaureolum Hulburt (Dinophyceae) were performed in artiticiallymixed algal populations using direct immunofluorescence. Calibrationof the flow cytometer, performed with a mixed algal populationspiked with immuno-fluorescently labelled G.aureolum cells,showed that selection of target cells after analysis on greenand orange fluorescence can be done with a recovery of 91.8%[coefficient of variation (CV) = 0.09]. Other selection methodswere less good, with 67.4% (CV = 0.16) and 58.4% (CV = 0.43)recovery based on green and red fluorescence or green fluorescenceand perpendicular light scattering. For mixed algal populationsspiked with unlabelled G.aureolum cells, the quantificationof target cells was quite good (recovery of 76.7%; CV = 0.20).The percentage of total cell loss was high (58.0–92.0%),but this was caused mostly by loss of species smaller in sizethan G.aureolum. Estimates of the relative contribution of G.aureolumin labelled samples were therefore often far too high, but detectionand quantification were not affected. The methodological underestimation(23.3%) was partly caused by gating on green and orange fluorescence(inaccuracy 8.2%).     

Omnivory in cyclopoid copepods: comparisons of algae and invertebrates as food for three, differenfly sized species

We examined the importance of algal versus invertebrate preyfor three cyclopoid copepods by comparing reproductive successfor females fed on three diets: exclusively algal food, exclusivelyinvertebrate prey and a combination of algal plus invertebrateprey. The three cyclopoid species represent a distinct gradientin body size: Tropocyclops prasinus mexicanus (small body size),Diacyclops thomasi (medium body size) and Mesocyclops edax (largebody size). Our results provide strong evidence for the potentialimportance of algae in the diet of adult cyclopoid copepods.However, the role of herbivory is dependent on species bodysize. We found that algae are the key dietary factor for reproductivesuccess of the small T.p.mexicanus. In contrast, the availabilityof invertebrate prey had a greater influence on reproductionby the two larger species. Overall, cyclopoid copepods shouldbe generally considered as having the potential to derive theirenergy from either animal or algal sources. This has importantconsequences for interpretations of the forces influencing zooplanktoncommunity diversity as well as for general food web theory.     

Application of Flow Microflorometry to the Study of Algal Cells and Isolated Chloroplasts

The applicability of flow microfluorometry (FMF) to the studyof chlorophyll-containing cells was investigated through theuse of the blue-green alga Agmenellum quadruplicatum, the greenalgae Trebouxia, Chlorella, and Euglena spp., and isolated spinachchloroplasts. When excited by laser radiation (488 nm), algalcells emitted fluorescence with intensity positively relatedto the chloro-phyll content. The chlorophyll fluorescent signalswere used further as a differential criterion in determiningrelative size based on light scattering logic and to sort mixturesof algal cells having different chlorophyll content The FMFalso was useful in estimating nucleic acid and protein contentin completely dechlorophyUized algal cells with the use of ethidiumbromide (EB) and fluoresceinisothiocyanate (FITC), respectively.     

NUTRITION OF TRIDACHIA CRISPATA (MORCH) (SACOGLOSSA)

Autotrophic nutrition plays an important role in adult Tridachiacrispata in the western Atlantic Ocean and results from an endosymbioticassociation between the sacoglossan and the kleptoplastids ofChlorophyta. A series of living specimens 7-80 mm in length was studied inthe laboratory. After a body length of 10 mm has been attained,the parapodial lobes of the two sides meet and fuse. Microscopicexamination of faecal strings of specimens undergoing this transitionrevealed that algal remains were produced only by juvenilesless than 13 mm long. Larger specimens defaecated only rarelyand the diminutive faecal strings contained no recognizablealgal debris. This transition probably parallels the physiologicaltransition between juvenile heterotrophic nutrition and adultautotrophic nutrition. Microscopic examination of the radulae of transitional specimensshowed no diminution of the size, number or morphology of theteeth. This indicates that in an emergency heterotrophic nutritionmay be resumed. Growth rates of Tridachia were measured in the laboratory, withand without light and the provision of algal foods. Unfed animalslost weight rather rapidly in captivity, whereas those fed uponsuitable algae (Halimeda discoidea, Caulerpa verticillata, C.racemosa and Chaetomorpha sp) did not.Caulerpa sertularioideswas suitable for food for a limited time, but proved toxic afterabout a week. Of the other species tested, Caulerpa verticillatawas the least suitable, while Halimeda, Chaetomorpha and Caulerparacemosa were the best. Ironically, C. verticillata was theonly one of the algal species tested which is known to be takenin the natural diet in Floridean waters. These experiments confirmedthat heterotrophic feeding will be resumed in Tridachia if theculture-conditions do not favour autotrophic nutrition.     

Comparison of different algal carbon estimates by use of the Droop-model for nutrient limited growth

Three different estimates of algal carbon were derived fromfield experiments in plastic enclosures and tested as measureof algal biomass in the growth model of Droop. Fresh weightderived carbon and total paniculate organic carbon correctedfor detritus were both found to behave as good estimates ofalgal biomass in the model, while total paniculate organic carbondid not. The study also provided results which suggested thatthe relation between growth rate and endogenous phosphorus differedfor two species involved. Growth was maintained at lower Q_p-ratiosin Staurastrum dominated communities compared with communitiesconsisting mainly of Anabaena flos-aquae.     

Biomass-pigment relationships in potamoplankton

During most of the growing season of 1994, pigment content,as determined by HPLC analysis of algal sample extracts, wasfollowed in the River Meuse (Belgium) potamoplankton. The concentrationof some algal pigments (chlorophylls a and b, fucoxanthin, lutein,echinenone and alloxanthin) was related to biomass estimatesof total phytoplankton and of major taxonomic components (diatoms,green algae, cyanobacteria and cryptomonads). Highly significantlinear regressions were obtained for chlorophyll a-total biomass,fucoxanthin-diatoms, lutein-green algae, chlorophyll b-greenalgae. However, no relationship was found for cyanobacteriaor cryptomonads and their specific pigments, which may be attributedto poor accuracy of biomass estimates for these non-dominantalgae. In conclusion, the good relationship found for dominantalgae and their specific pigments confirms the value of pigmentsas quantitative markers of phytoplankton, as detected in othermarine and freshwater environments.     

Species-specific algal responses to zooplankton: experimental and field observations in three nutrient-limited lakes

A series of 4-day manipulations of zooplankton biomass and nutrientavailability was performed in enclosures in three lakes to determinespecies-specific algal responses to herbivory and nutrient enrichment.Algal performance in enclosures was compared to the relationshipsbetween weekly algal growth rates and the zooplankton in situ.When in situ growth rates were significant functions of zooplanktonbiomass, the responses were generally consistent with responsesin the enclosure experiments. The importance of both nutrientsand zooplankton in mediating algal growth was demonstrated bynumerous observations: strong algal community response to enrichment,unimodal or positive responses of certain algal taxa to zooplanktonbiomass, differences in degree of nutrient limitation amongthe algal response types, lack of nutrient limitation of non-grazedalgal taxa and a preponderance of taxa with no net responseto increasing zooplankton biomass. Variation in the zooplanktoncommunity may be the largest source of variability in nutrientsupply rate during summer in stratified lakes, and causes substationalvariability in the algae. Algae responded more strongly to changesin zooplankton composition than to changes in zooplankton biomass.We conclude that, due to the close coupling of phytoplanktonand zooplankton communities in these nutrient-limited lakes,major compositional changes in the zooplankton have greatereffects on the algae than do changes in biomass of grazers alreadypresent. ¹Present address: Division of Environmental Studies, Universityof California, Davis, CA 95616, USA ²Present address: Division of Biological Sciences, Universityof California, Davis, CA 95616, USA     

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.     

Consumer-dependent responses of lake ecosystems to nutrient loading

The nutrient loading concept proposes that algal biomass, waterclarity and the processes of lake eutrophication are a functionof nutrient loading. We hypothesized that grazers play an importantrole in determining the impacts of nutrient loading on algalbiomass and water clarity, and the overall eutrophication process.To test how the contrasting grazer communities modify the fateof nutrients, we added nutrients (nitrate and phosphate) ata known loading rate to four large enclosures, but in two ofthe four enclosures large cladoceran grazers (Daphnia >1mm mean length) were allowed to develop by removing the planktivorousfish. In the remaining two enclosures, the development of largeDaphnia was prevented by adding planktivorous fish. The concentrationsof epilimnetic total phosphorus (TP) increased at a similarrate in all four enclosures. However, the daily accumulationof added phosphate into the participate or planktonic forms,especially into plankton <20 µm, was three times fasterwhen large Daphnia were absent than when large Daphnia wereabundant. In the enclosures with large Daphnia, added phosphatewas accumulated in the dissolved pool instead. At a constantnutrient loading, algal biomass (chlorophyll a) increased fourtimes faster in the enclosures without large Daphnia than inthose with large Daphnia. Similarly, Secchi depth declined from3.5 to <1 m when Daphnia were absent, but did not declinewhen Daphnia were common. Our results demonstrate that the samenutrient loading and the resultant increase in epilimnetic TPdo not produce the same trophic conditions, as indicated byalgal biomass and water clarity, if the grazers of the majorassimilators of nutrients (the fraction of plankton edible toDaphnia) are different. We suggest that stratified lake ecosystemshaving functionally dominant large grazer communities may beless prone to eutrophication than those lacking large grazers.Consistent with the nutrient loading concept, epilimnetic concentrationsof phosphorus increase proportionately with increased loadingof phosphorus, but the trophic conditions of ecosystems indicatedby algal biomass and water clarity do not follow the same patternsunder contrasting conditions of grazer communities. We suggestthat models predicting algal biomass from loading rates shouldaccount for the role of grazers.     

Composition and distribution of the pelagic and sympagic algal assemblages in the Laptev Sea during autumnal freeze-up

The phytoplankton and ice algal assemblages in the SiberianLaptev Sea during the autumnal freeze-up period of 1995 aredescribed. The spatial distribution of algal taxa (diatoms,dinoflagellates, chrysophytes, chlorophytes) in the newly formedice and waters at the surface and at 5 m depth differed considerablybetween regions. This was also true for algal biomass measuredby in situ fluorescence, chlorophyll (Chl) a and taxon-specificcarbon content. Highest in situ fluorescence and Chl a concentrations(ranging from 0.1 to 3.2 µg l^–1) occurred in surfacewaters with maxima in Buor Khaya Bay east of Lena Delta. Thealgal standing stock on the shelf consisted mainly of diatoms,dinoflagellates, chrysophytes and chlorophytes with a totalabundance (excluding unidentified flagellates <10 µm)in surface waters of 351–33 660 cells l^–1. Highestalgal abundance occurred close to the Lena Delta. Phytoplanktonbiomass (phytoplankton carbon; PPC) ranged from 0.1 to 5.3 µgC l^–1 in surface waters and from 0.3 to 2.1 µg Cl^–1 at 5 m depth, and followed the distribution patternof abundances. However, the distribution of Chl a differed considerablyfrom the distribution pattern shown by PPC. The algal assemblagein the sea ice, which could not be quantified due to high sedimentload, was dominated by diatom species, accompanied by dinoflagellates.Thus, already during the early stage of autumnal freeze-up,incorporation processes, selective enrichment and subsequentgrowth lead to differences between surface water and sea icealgal assemblages.     

Measures of food quality as demographic predictors in freshwater copepods

This study examines the ability of a number of short-term measuresof algal food quality to predict longer term demographic parametersfor two species of freshwater calanoid copepods, Diaptomus minuiusand Epischura lacustris. Food quality of two species of algaethat are usually considered highly edible (Cryptomonas erosavar. reflexa and Chlamydomonas reinhardtii) are compared withrespect to: (i) the biochemical constituents of the algae (totalN, total C and protein); (ii) short-term foraging responsesby D.minutus to either algal species (e.g. clearance, ingestionand assimilation rates); (iii) longer term demographic responses(e.g. survivorship and reproduction) by both D.minutus and E.lacustrisfed a diet of either algal species. Demographic responses ofthe two copepod species indicate that C.erosa is a higher qualityfood. In fact, survival and reproduction of both copepod speciesfed C.reinhardtii were not different from starved treatments.Cryptomonas erosa treatments also had greater C, N and protein.However, D.minutus ingested five times more C.reinhardtii thanC.erosa, indicative of ‘compensatory feeding’ inthe presence of poor-quality food. Based upon these higher ingestionrates, individuals fed C.reinhardai actually ingested greateramounts of C, N and protein. Hence, ingestion rates taken aloneor coupled with biochemical parameters are not reliable predictorsof consumer demographic response. Assimilation rate, which waspositive for C.erosa and zero for C.reinhardtii, was the singlebest short-term predictor of food quality.     

ULTRASTRUCTURAL AND ECOLOGICAL ASPECTS OF THE DEVELOPMENT OF CHLOROPLAST RETENTION IN THE SACOGLOSSAN GASTROPOD ELYSIA TIMIDA

Elysia timida is a common and endemic inhabitant of shallowand very well lit waters in the Mediterranean. This sacoglossanslug retains functional symbiotic chloroplasts derived fromits algal food, Acetabularia acetabulum, although the chloroplastsare not transmitted in the spawn. After hatching and until day12, Elysia timida juveniles do not retain these chloroplastsin the digestive gland. However, newly hatched juveniles retainchloroplasts from Cladophora dalmatica. Development varies seasonallybetween direct (December to April) and lecithotrophic (October,November and May), and this variation may be an adaptation toseasonal calcification of the algal food Acetabularia acetabulum. (Received 12 February 1991; accepted 15 July 1992)     

Function of cAMP as a Mat-Forming Factor in the Cyanobacterium Spirulina platensis

A dense suspension of Spirulina platensis trichomes aggregatedrapidly and formed a diskshaped algal mat when cAMP was added.Cyclic AMP significantly stimulated algal mat formation at concentrationsas low as 10^–7 M. Stimulation of the mat formation wasmost rapid at about 10^–5M cAMP, but higher concentrationswere not increasingly effective. Other nucleotides such as cGMP,ATP and AMP showed no stimulatory effect on algal mat formation.CCCP, an inhibitor of ATP synthesis, was found to suppress thecAMP-stimulated algal mat formation. Cyclic AMP also stimulatedrespiration and gliding movement of this cyanobacterium. (Received September 2, 1991; Accepted October 15, 1991)     

Studies on chlorophyllase of Chlorella protothecoides II. Formation of atypical chlorophyllide a

When chlorophyll a was incubated with a preparation of chlorophyllaseextracted with Triton X-100 from methanol-acetone powder ofChlorella protothecoides, the substrate was changed to chlorophyllidea, and subsequently to an atypical form of chlorophyllide a.The formation of an atypical form of chlorophyllide was notdetected in the reaction with chlorophyll b as substrate, norin the reaction with another preparation of chlorophyllase extractedfrom the algal cells. (Received August 18, 1969; )     

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     

Effect of cycloheximide on the process of "glucose-bleaching" in Chlorella protothecoides

The process of bleaching of Chlorella protothecoides inducedby the addition of glucose was strongly inhibited by cycloheximide,an inhibitor of protein synthesis, whereas it was suppressedonly weakly by chloramphenicol, puromycin and ethionine. Whencycloheximide was added simultaneously with glucose at the beginningof die bleaching experiment, no bleaching of algal cells occurredduring the subsequent incubation. When it was added after glucose,the bleaching of algal cells proceeded for a period of timeas actively as in the control, then gradually ceased. Cycloheximidewas found to suppress the uptake of glucose by algal cells,and to severely inhibit the assimilation of glucose into lipidswhen added at the beginning of the bleaching experiment. Theseinhibitory effects of cycloheximide are discussed in relationto the induction of "glucose-bleaching" in algal cells. (Received December 16, 1968; )