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     

Effects of Planktonic Copepods on Transparent Exopolymeric Particles (TEP) Abundance and Size Spectra

Diatoms exude considerable quantities of polymers, mainly polysaccharides,that play an important role in the process of sestonic particleaggregation in the sea. We investigated the impact of copepodson transparent exopolymeric particles (TEP) generated by thediatom Thalassiosira weissflogii. Grazing experiments with ¹⁴C-labelledalgae exudates demonstrated that copepods typical of the BalticSea were not actively filtering TEP. Control experiments showedthat ‘uptake’ of radioactivity could be ascribedto passive uptake, such as adsorption of radioactively-labelledparticles to the body surface. Furthermore, we tested the effectof copepods on TEP size spectra. The abundance and size distributionof TEP (from 1.4 to 180 µm of Equivalent Spherical Diameter)were analysed in a 4 h incubation experiment. In the presenceof copepods, the proportion of larger TEP was higher. An increasein total volume of TEP in jars containing copepods (~2 x 10⁷µm ml^–1) compared with control jars without copepods(~0.5 x 10⁷ µm³ ml^–1) was also observed. The processof aggregation of TEP demonstrated in this work, whereby copepodsincrease downward particle flux without consuming carbon, canhave far-reaching consequences for carbon fluxes along the watercolumn and for copepods feeding dynamics.     

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.     

Density and distribution of bacterioplankton and planktonic ciliates in the Bering Sea and North Pacific

The total number of planktonic bacteria in the upper mixed layerof the Bering Sea during the late spring-early summer periodranged between 1 and {small tilde}4 x 10⁶ ml^–1 (biomass10–40mg C m^–3). In the northern Pacific, along 47–52⁶N,the corresponding characteristics of the bacterioplankton densityin the upper mixed water layer were: total number 1–2x 10⁶ cells ml^–1 and biomass 15–46mg C m^–3Below the thermocline at 50–100 m, the density of bacterioplanktonrapidly decreased. At 300 m depth, it stabilized at 0.1–0.2x 106 cells ml^–1. The integrated biomass of bacterioplanktonin the open Bering Sea ranged between 1.2 and 3.6 g C m^–2(wet biomass 6–18 g m^–2) Its production per dayvaried from 2 to 23 mg C m^–3 days^–1 in the upper0–100 m. The numerical abundance of planktonic ciliatesin this layer was estimated to be from 3 to l0 x 10³ cells l^–1,and in the northern Pacific from 0.4 to 4.5 x 10³ l^–2.Their populations were dominated by naked forms of Strombidium,Strombilidium and Tontonia. In some shelf areas, up to 40% ofthe total ciliate population was represented by the symbioticciliate Mesodinium rubrum. The data on the integrated biomassof basic groups of planktonic microheterotrophs are also presented,and their importance in the trophic relationships in pelagiccommunities of subarctic seas is discussed.     

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.     

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.     

Population structure and swarm formation of the cyclopoid copepod Dioithona oculata near mangrove cays

The cyclopoid copepod Dioithona oculata forms swarms in water>30 on deep among prop roots of red mangroves (Rhizophoramangle) which fringe protected areas of two lagoonal cays, TwinCays, Belize. During 7 of 8 months surveyed by in situ observation,swarms were present but differed in size from small cylindricalswarms (5–10 cm diameter) to bands extending up to 1200m Swarms were never observed at night Swarms formed at dawnwhen light intensities reached an average value of 13.82 (logioquanta cm^–Abstract. s^–1) and dispersed at dusk atsimilar intensities Swarms observed in June formed earlier anddispersed later in the day than swarms observed in January,their swarming behavior followed seasonal changes in light intensityMean dioithonan density in swarms (10 ml^–1) was much higherthan the mean density (0 15 ml^–1) of non-swarming dioithonansaround mangrove prop roots. In open water 3–5 m away fromthe mangroves, mean dioithonan density was 7 9 x 10^–5ml¹ during the day, and 2 68 x10^–3 ml^–1 at nightSwarms were composed predominantly of adults and copepodid stagesIV and V, although younger copepodid stages could be presentNauplii were never present. The ‘average copepodid stage’for all 95 swarms sampled was 5 3, where 6 0 represents a swarmwith only adults In open water 3–5 m away from the mangroves,the youngest copepodids (stage one) dominated the dioithonanpopulation during the day. At night when swarms dispersed toopen waters, average copepodid stage was higher (3 5) comparedwith the day value (1.2) in open waters. Although densitiesin swarms were higher in June than January, average copepodidstage in June was higher (5 6) than that in January (4.9). Ahigher percentage of adults were females during June than January.Therefore higher densities did not result from increases ofsmaller stages in swarms, but perhaps changes in behavior orpopulation structure.     

Picoplankton dynamics in Davies Reef lagoon, the Great Barrier Reef, Australia

The diel variations in abundance and frequency of dividing cells(FDC) of coccoid cyanobacteria in a coral reef lagoon were investigatedin June, September and December 1989, and April 1990. Cyanobacteriaand picoplanktonic eukaryotes (<3 µm) were sampledmonthly from January to December 1990. The average abundancesof cyanobacteria and eukaryotes ranged between 1.17–10.0610⁴cells ml^–1 and 0.16–2.4110⁴ cells ml^–1, respectively,with abundances of both being higher in summer (November-April)than in winter (May-October). The ratio of cyanobacteria toeukaryotes fluctuated from 1.93 to 8.67, independent of theseasonal variation in their abundances. The instantaneous growthrate of cyanobacteria, which was estimated from the daytimeabundance increment, ranged between 0.430 and 3.144 day^–1The estimated daily specific growth rate of cyanobacteria bythe FDC method ranged between 0.231 and 0.966 day^–1. InApril, despite the high specific growth rate and low flushingconditions. cyanobacterial abundance showed a cyclic diel pattern,suggesting a strong grazing impact on their population.     

Size, composition and distribution of particles related to wind induced resuspension in a shallow tropical lagoon

The size, composition and distribution of particles in the watercolumn were surveyed in a shallow area (1 m depth) of a tropicallagoon (Cte d'Ivoire) during a sequence of wind-induced resuspension.Water samples were collected hourly near the surface duringone tidal cycle. Three characteristic periods were distinguished:a calm period with low wind speed (average 1.2 m s^–1 awindy period with wind speed >3 m^–1 s (range between4 and 6 m s^–1) inducing sediment resuspension and a relaxationperiod during the decrease of wind velocity. From the analysisof several parameters (particle size and volume, bacteria. pico-and nanophytoplankton, ciliates and detritus), sediment resuspensioncaused a regular injection of particles from the bed. The finestparticles (1.5–6 µm: chlorophytes such as Chiorellaspp., picocyanobacteria such as Synechococcus) were the firstto be affected by wind-induced turbulence, whereas large particles(6–12 µm: diatoms. cyanobacteria such as Lyngbiaspp.) were dispersed into the water column at the highest windspeed. The fate of the different seston components differedaccording to their size. Therefore, wind-induced resuspensioncould greatly influence the food web organization through thequantity, quality and size of edible particles available ata given time.     

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     

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 (     

Diel cycles in the frequency of dividing cells of freshwater picocyanobacteria

Diel changes in the frequency of dividing cells (FDC) of freshwaterpicocyanobacteria populations from a mesotrophic lake correlatedwith the day-night light cycle and did not appear to be underendogenous control. The highest FDC values (24–27%) wereobtained between noon and early afternoon. In late summer, whenpicocyanobacterial abundance was at a seasonal high (1–310⁵cells ml^–1), doubling times estimated from dilution experimentswere     

TRANSPARENT EXOPOLYMER PARTICLES FORMATION FROM CAPSULES OF ANABAENA SPIROIDES (CYANOBACTERIA) IN CULTURE1

We report the production of large numbers of transparent exopolymer particles (TEP) from polysaccharidic capsules of Anabaena spiroides Kleb. in cultures. Two biotic pathways of TEP formation were observed: (1) fragmentation of small portions of the capsules, which occurred throughout the growth phases; and (2) transformation of the whole polysaccharidic capsules into TEP, following cellular lyses in the aging culture. Photographic documentation of these processes was performed after staining small aliquots of the samples with Alcian Blue and negative staining with India ink. Concentrations of TEP were determined in distinct culture growth phases using semiquantitative Alcian Blue staining. Concentrations of TEP increased throughout the experimental time, while Alcian Blue remaining in solution decreased. Decreasing concentrations of chl a indicated cellular death, and by the end of the experiment, TEP formed by both pathways accumulate in the culture medium. These results show that virtually all dead chains of A. spiroides are transformed into TEP in the aged culture.     

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     

Mixotrophic nanoplankton in oligotrophic surface waters of the Sargasso Sea may employ phagotrophy to obtain major nutrients

The vertical distribution and abundance of mixotrophic nanoplanktonwas examined during two cruises to the Sargasso Sea south ofBermuda. Fluorescently labeled bacteria and cyanobacteria wereused as tracers of ingestion in experiments designed to determineabundances of mixotrophic nanoplankton. Phagotrophic nanoplanktonic(2–20 µm) algae ranged from undetectable to >100ml^–1, and were more abundant near the surface (up to 140ml^–1) than in the deeper euphotic zone. On two occasions,50% of the phototrophic nanoplankton in surface waters wereobserved with ingested fluorescent tracers. The contributionof mixotrophic algae to the total phototrophic nanoplanktonassemblage in the deep chlorophyll maximum, however, did notexceed 0.5%. It is possible that mixotrophic algae were moreabundant in the deep chlorophyll maximum, but were not phagotrophicallyactive. Two 4 day experimental incubations were subsequentlycarried out to examine the adaptive significance of phagotrophicbehavior for algae in surface waters of the Sargasso Sea. Greatermixotrophic nanoplankton abundances were observed in treatmentsthat received no nutrient inputs and were limited by the availabilityof inorganic nutrients during the experiments. A decrease inthe abundance of mixotrophic algae or a decrease in their phagotrophicactivity occurred with nutrient enrichment. Based on the experimentalresults, we suggest that phagotrophy was a mechanism by whichthese algae supplemented nutrient acquisition during periodsof low dissolved nutrient concentrations. Higher abundancesof mixotrophic nanoplankton observed in the upper 50 m of theSargasso Sea may have been due to the generally low nutrientconcentrations in these waters.     

Community structures of heterotrophic bacteria of copepod fecal pellets

The total and heterotrophic bacterial microflora of gut andfecal pellets of the copepod Temora stylifera was studied inthe coastal zone of the northwestern Mediterranean basin. Digestivetracts and feces were always found to contain bacteria. Withmean values close to 10¹¹ cells ml^–1and 10¹⁰ colony-formingunits (CFU) ml^–1, both total and heterotrophic communitiesfound in fecal pellets largely exceeded the corresponding bacterialmicroflora observed in surrounding seawater (10⁵ cells ml¹ and10³ CFU ml^–). In the same way, the frequency of dividingcells and mean cell volumes increased, respectively, from 3%and 0.12µm³ in seawater to >6% and 0.24 µm³ infecal pellets. The bacterial community structure was investigatedby carrying out 29 morphological and biochemical tests on 147isolated strains. Although Pseudomonas was always the most frequentlyencountered genus, the bacterial communities found in copepodfecal pellets clearly differ from those inhabiting seawater.Vibrio species, which were not detected in seawater microflora,were always present in fecal pellet communities. The close correspondenceobserved between the potential metabolic characteristics ofthe fecal bacterial communities isolated from a subantarcticdeposit feeder (Mytilus edulis) and from zooplankton in thisMediterranean study suggests that the fecal community differentiationis an even more general feature.     

Factors affecting the bacteria-heterotrophic nanoflagellate relationship in oligo-mesotrophic lakes

The coupling between bacteria and heterotrophic nanoflagellates(HNF) was examined in nine lakes of low productivity for evidenceof the effects of various metazooplankton (i.e. rotifers, cladoceransand copepods) on this relationship. We considered the size ofcladocerans and, in contrast to most previous across-systemstudies, the three strata of the water column (i.e. epilimnion,metalimnion and hypolimnion). Rotifers were numerically dominantin all lakes and accounted for 45–84% of total metazooplanktonabundance, while the abundance of large cladocerans was relativelylow, ranging from 0.066 to 15.2 ind. L^–1. The across-lakerelationship between bacteria and HNF was significant in thedeeper strata (meta- and hypolimnion) but not in the epilimnionand in the two groups of lakes separated on the basis of theiraverage number of large cladocerans (<5 and >5 ind. L^–1,respectively). The results confirmed the negative impacts oflarge cladocerans on HNF, but also showed that rotifers, probablythrough grazing on HNF, may be an important factor causing variationin the bacteria–HNF relationship in unproductive waters.Quadratic models best described the relationships between metazooplanktonand the ratio of bacteria to HNF. This ratio seemed to be aresult of complex interactions between several factors, includingthe zooplankton composition and abundance and the depth of thelake. Indeed, this ratio significantly decreased across lakes,with increase in depth. In addition, shallower lakes (having<5 large cladocerans L^–1 and fewer Polyarthra vulgaris)tended to have more bacteria and HNF and a higher ratio of bacteriato HNF than deeper lakes (which had >5 large cladoceransL^–1 and substantial proportions of P. vulgaris). We suggestthat the epilimnion, metalimnion and hypolimnion of lakes betaken into account when analysing the bacteria–HNF relationshipas well as the cascading effects of zooplankton on microbialcommunities.     

The importance of phytoflagellate, heterotrophic flagellate and ciliate grazing on bacteria and picophytoplankton sized prey in a coastal marine environment

The uptake of bacteria and picoplankton sized fluorescentlylabelled beads was measured off the west coast of the SouthIsland of New Zealand in winter. Phytoflagellates and heterotrophicflagellates showed similar grazing rates on 0.49 µm beads,with mean clearance rates of 1.1 and 1.8 nl ind.^–1 h^–1,respectively. Clearance rates for 1.09 µm beads were 0.9nl ind.^–1 h^–1 for heterotrophic flagellates and0.5 nl ind.^–1 h^–1 for phytoflagellates. Non-loricateciliates had clearance rates of 1.5 µl ind.^–1 h^–1for the picoplankton sized particles. The heterotrophic flagellatesshowed no significant difference between clearance rates of0.49 and 1.09 µm particles. Phytoflagellates, however,showed an apparent preference for the smaller particles. Themeasurement of significant grazing by phytoflagellate populationsin the marine environment is important and indicates that weneed to reassess our concepts of food web structure.     

Basin-scale latitudinal patterns of copepod grazing in the Atlantic Ocean

Size-fractionated copepod abundance and ingestion rates wereinvestigated along a 50°S–50°Nlatitudinal transect,during the Atlantic Meridional Transect (AMT) 4, 5 and 6 cruises(boreal spring–autumn 1997, boreal spring–summer1998). Copepod abundance was higher at high latitudes in spring,near northwest Africa, in the equatorial and Benguela upwellingsystems, and in the Subtropical Convergence, and lower in oligotrophicgyres. Gut contents were not related to phytoplankton biomassor production. Gut evacuation rate averaged 0.03 min^-1, andwas not related to latitude or body size. Conservative estimatesof copepod community total ingestion rates ranged between 3.4and 173 mg C m^-2 day^-1 for AMT4, 1.6–252 mg C m^-2 day^-1in AMT5 and 10–160 mg C m^-2 day^-1 in AMT6. Maximum valueswere always in the upwelling regions, the subtropical convergenceand high latitudes in the Northern Hemisphere during borealspring. Calculated ingestion rates translate into average dailyminimal consumption values of 2.07%, 1.89% and 2.6% of totalchlorophyll stock, or 8.02%, 14.5% and 12.9% of total primaryproduction ingested daily on AMT4, 5 and 6 respectively. Grazingimpact increases considerably if we consider ingestion of phytoplanktonlarger than 2 µm, especially under the influence of theEquatorial and North African upwellings, where copepod ingestionrepresents up to 30% of the biomass and >100% of productionby large cells.     

Response of phytoplankton and bacteria to nutrients and zooplankton: a mesocosm experiment

Although both nutrient inputs and zooplankton grazing are importantto phytoplankton and bacteria in lakes, controversy surroundsthe relative importance of grazing pressure for these two groupsof organisms. For phytoplankton, the controversy revolves aroundwhether zooplankton grazers, especially large cladocerans likeDaphnia, can effectively reduce phytoplankton populations regardlessof nutrient conditions. For bacteria, little is known aboutthe balance between possible direct and indirect effects ofboth nutrients and zooplankton grazing. However, there is evidencethat bacteria may affect phytoplankton responses to nutrientsor zooplankton grazing through direct or apparent competition.We performed a mesocosm experiment to evaluate the relativeimportance of the effects of nutrients and zooplankton grazingfor phytoplankton and bacteria, and to determine whether bacteriamediate phytoplankton responses to these factors. The factorialdesign crossed two zooplankton treatments (unsieved and sieved)with four nutrient treatments (0, 0.5, 1.0 and 2.0 µgphosphorus (P) l^–1 day^–1 together with nitrogen(N) at a N:P ratio of 20:1 by weight). Weekly sieving with 300µm mesh reduced the average size of crustacean zooplanktonin the mesocosms, decreased the numbers and biomass of Daphnia,and increased the biomass of adult copepods. Nutrient enrichmentcaused significant increases in phytoplankton chlorophyll a(4–5x), bacterial abundance and production (1.3x and 1.6x,respectively), Daphnia (3x) and total zooplankton biomass (2x).Although both total phytoplankton chlorophyll a and chlorophylla in the <35 µm size fraction were significantly lowerin unsieved mesocosms than in sieved mesocosms, sieving hadno significant effect on bacterial abundance or production.There was no statistical interaction between nutrient and zooplanktontreatments for total phytoplankton biomass or bacterial abundance,although there were marginally significant interactions forphytoplankton biomass <35 µm and bacterial production.Our results do not support the hypothesis that large cladoceransbecome less effective grazers with enrichment; rather, the differencebetween phytoplankton biomass in sieved versus unsieved zooplanktontreatments increased across the gradient of nutrient additions.Furthermore, there was no evidence that bacteria buffered phytoplanktonresponses to enrichment by either sequestering P or affectingthe growth of zooplankton.