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.     

Zooplankton grazing impact in the plume of dilution of the Gironde estuary (France) prior to the spring bloom

The Bay of Biscay is a coastal area intensively exploited forfishing and is submitted to important human actions (proximityof important industrial and agricultural areas). Thus, the understandingof the materials and energy transfers in such ecosystems isof great interest. However, investigations on zooplankton (animportant component of the marine food web) are very scarcein this area. Our study concerns the grazing impact of the zooplanktonin shelf waters in the plume of dilution of the Gironde estuaryprior to the spring bloom. Samples were taken through the photiclayer in April 1993. Our results have shown that Temora longicornisand Paracalanus parvus dominated the ‘herbivorous’planktonic community. The grazing impact of the zooplanktoncommunity on the total phytoplankton stock and on the totalprimary production was low (9–14 and 17–21% day^–1,respectively) during this period, which is in accordance withmost results in coastal areas. However, due to the small sizeof the algae (     

Copepod grazing on a phytoplankton community containing the toxic dinoflagellate Dinophysis acuminata

Copepod grazing on the toxic dinofiagellate Dinophysis acuminatafrom the west coast of France (La Rochelle) was studied witha concentrated (40–70 µm) phytoplankton assemblagedominated by Leptocylindrus danicus, D.acuminata, Ceratium fususand Ceratium furca. Copepod nauplii were also present. Threeto five copepods/copepodites (Acartia clausi, Isias clavipesand Centropages typicus) were incubated together with the phytoplankton.Dinophysis acuminata was grazed upon by all copepod species.However, to some extent, I.clavipes and C.typicus avoided itas food. Dinophysis acuminata cells represented for them only5–10% of total ingested carbon during the first 24 h,and almost all individuals survived and thrived well. In contrast,A.clausi did not avoid D.acuminata. which represented 30% ofingested carbon in 1 day. Acartia clausi then had a lower survivalthan the two other copepod species. However, the survival ofA.clausi was high in control incubations, where a plankton communitywithout D.acuminata was used as food. It is concluded that theokadaic acid of D acuminata is potentially toxic to some grazers,and/or might function as an allelopathic grazer repellent.     

The importance of crustacean zooplankton in structuring rotifer and phytoplankton communities; an enclosure study

During the spring clear-water phase of 1993, an enclosure experimentwas performed in the mesotrophic Schöhsee (Plön, FRG)in order to assess the impact of crustacean zooplankton on therotifer and phytoplankton community. Among the crustacean plankton,calanoid and cyclopoid cope-pods were abundant, but Daphnia‘longispina’ reached the highest densities in thisexperiment. The colonial rotifer Conochilus unicomis was notaffected by crustacean plankton. The two most abundant species,Synchaeta peclinata and Keratella cochlearis, increased exponentiallywhen macrozooplankton had been excluded from the enclosures,but did not increase when crustaceans were present. Birth anddeath rates of K.cochlearis could be reliably determined inthis field experiment, suggesting that this rotifer specieswas mainly controlled by exploitative competition rather thanby mechanical interference or predation. Daphnia ‘longispina’generally grazed selectively on the smaller ciliates and algae,thus depriving the rotifers of their phytoplankton resources.The dominant alga, the chrysophycean Dinobryon, increased, whethercrustaceans were present or not, but appeared to be grazed uponto a certain extent despite its considerable cell size and colonialorganization.     

Effect of large- and of small-bodied zooplankton on phytoplankton in a eutrophic oxbow

Macrozooplankton and microzooplankton effects on the phytoplanktonwere measured in situ in a eutrophic lake. Indigenous phytoplanktonwere incubated for 5 days in 301 mesocosms with either the macro-and microzooplankton (complete), microzooplankton only (micro)or no zooplankton (none). Changes in phytoplankton biovolumewere investigated. Rotifer densities became significantly higherin the ‘micro’ treatment than in the ‘complete’and ‘none’ treatments. Total algal biovolume changedlittle in the ‘complete’ and ‘none’treatments, but increased significantly in the ‘micro’treatment. The results suggest that macrozooplankton (Daphniamagna) suppressed it and microzooplankton (Keratella cochlearis)enhanced it. They had opposite net effects on the phytoplankton.Suppression of microzooplankton by Daphnia probably had an indirectnegative effect on the phytoplankton.     

Microzooplankton grazing of phytoplankton in a tropical upwelling region

We measured grazing by herbivorous zooplankton (<200 μm fraction) in coastal and slope regions of the South Brazil Bight. Using the dilution technique, we performed nine experiments during the austral summer, when nutrient-rich South Atlantic Central Water is present on the shelf, and five during winter. These experiments provide the first estimates of microzooplankton grazing in the western South Atlantic Ocean. Model II regression showed a strong relationship between phytoplankton intrinsic growth rates and grazing, with a slope of 0.64 (±0.28; 95% confidence interval) indicating that microzooplankton grazing could account for the majority of phytoplankton mortality. Both phytoplankton growth and microzooplankton grazing were higher during the summer upwelling season, compared to winter. For the two experiments that were conducted in oligotrophic slope water, grazing accounted for >80% of phytoplankton production. A comparison of incubations with and without added inorganic nutrients showed no consistent stimulation of phytoplankton growth (slope of enriched versus unenriched treatments not significantly different from 1). Estimates from microscopic counts of heterotrophic organisms >10 μm indicated that copepod nauplii comprised the largest share of the microzooplankton biomass (mean 62.4 ± 5.8% SE). Grazing estimates were not correlated with microzooplankton biomass, whether or not nauplii were included, suggesting that most of the grazing was done by nano-sized zooplankton. Electronic Supplementary Material Electronic supplementary material is available in the online version of this article at and is accessible for authorized users. Handling editor: S. Wellekens     

Experimental study of the impacts of silver carp on plankton communities of eutrophic Villerest reservoir (France)

We examined the impact of five silver carp biomass levels (0, 8, 16, 20, and 32 g m⁻³) on plankton communities and water quality of Villerest eutrophic reservoir (France). We realized the experiments using outdoor mesocosms. The presence of silver carp led to changes in zooplankton and phytoplankton assemblages. High fish biomass strongly reduced cladoceran abundance (through predation). Silver carp inefficiently grazed down particles < 20 μm. More importantly, however, the suppression of herbivorous cladocerans resulted in the increase of small size algae which were relieved from grazing and benefit from high nutrient concentrations. In contrast, in mesocosms without fish, the dominance of cladocerans (mainly Daphnia) controlled small size algae and probably also larger size algae (colonial chlorophytes, cyanobacteria). Thus, the Secchi disc transparency increased markedly. Through cascade effects, the modification of grazers communities led to changes in the utilization patterns of the added nutrients by phytoplankton communities. In high fish biomass treatments, nutrients were more efficiently accumulated into particulate fractions compared with no-fish and low-fish biomass treatments that were characterized by higher dissolved nutrients concentrations. Zooplankton was an essential source of food for silver carp. The productivity of zooplankton sustained a moderate silver carp biomass (up to 16 g m⁻³). In the presence of the highest fish biomass, the productivity of zooplankton was not large enough and silver carps fed on additional phytoplankton. Although mesocosms with high fish biomass were characterized by a slight cyanobacteria development compared with other fish mesocosms, silver carp was not effective in reducing cyanobacteria dominance. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

Top-down effects of crustacean zooplankton on pelagic microorganisms in a mesotrophic lake

A series of single-factor in situ experiments was conductedin a mesotrophic lake in Brandenburg, North Germany, to studythe predatory impact of Eudiaptomus graciloides (adults, copepodites,nauplii), cyclopoid copepods (adult Diacyclops bicuspidatus,Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina,Daphnia cucullata) on the microbial community (bacteria, autotrophicpicoplankton, flagellates, ciliates). All zooplankton speciestested reduced the ciliate community significantly and ingestionrates were always higher for ciliates in the 20–55 µmsize category as compared to smaller ciliates (10–20 µm).Adult E.graciloides, which exhibited the highest predatory impacton ciliates, differed from cyclopoids and daphnids by theirability to decimate ciliates to very low abundances. Ingestionrates of ciliates by the crustacean zooplankton followed thesequence E.graciloides > daphnids = cyclopoids = copepodites.While top-down control was evident for ciliates, top-down effectsdown to the autotrophic picoplankton and flagellates were mostlyrestricted to Daphnia-dominated treatments. Top-down effectswere never strong enough to produce negative bacterial growthrates. For all zooplankton tested, clearance rates for ciliatesexceeded those for phytoplankton. Besides the potential of thecrustacean zooplankton to influence the structure of ciliatecommunities, ciliates may contribute to the energy demands ofcopepods and daphnids, especially when phytoplankton resourcesare limited.     

Comparison of carbon-specific growth rates and rates of cellular increase of phytoplankton in large limnetic enclosures

Potential carbon-specific growth rates of phytoplankton wereestimated from a series of measurements of photosynthetic radio-carbonuptake over 4- and 24-h exposure periods in the light fieldsof three large limnetic enclosures (‘Lund Tubes’),each providing different limnological and trophic conditions.Photosynthetic behaviour and short-term, chlorophyll-specificcarbon-fixation rates conformed to well-established criteriabut, over 24 h, the net retention represented 23–82% ofthe carbon fixed during the daylight hours. Potential mean growthrates (k'_p, of the photo-autotrophic community were calculatedas the net exponential rates of daily carbon-accumulation relativeto derived, instantaneous estimates of the cell carbon-content.Apparent actual community growth rates (k'_D were calculatedas the sum of the exponential rates of change of each of themajor species present, corrected for probable rates of in situgrazing and sinking, and expressed relative to the fractionof total biomass for which they accounted. The correspondingvalues were only occasionally similar, k'_p generally exceedingK'_D by a factor of between 1 and 30 or 1 and 14, depending uponthe carbon:chlorophyll ratio used. The ratio, K'_p/K'_D was foundto vary inversely both to k'_D and to k_n, the net rate of changein phytoplankton biomass, suggesting that measured carbon fixationrates merely represent a capacity for cellular increase which,owing to other likely limitations upon growth, is seldom realized.Apparent rates of loss of whole cells do not account for theloss of carbon; that the ‘unaccounted’ loss rates(K'_p–K'_D varied in direct proportion to K'_p (i.e., losseswere least when chlorophyll-specific photosynthetic productivitywas itself limited) is best explained by physiological voidingof excess carbon (for instance, by respiration, photorespiration,excretion) prior to the formation of new cells.     

Size-fractionated biomass, photosynthesis and dark CO2 fixation in a tropical oceanic environment

This study examines the spatial distribution and size structureof phytoplankton biomass and productivity in relation to thevertical structrure of the Andaman Sea (northeastern IndianOcean). This region was characterized by low concentrationsof nutrients and high levels of insolation. Nitrogen availabilityappeared to control overall productivity with nitrate-based‘new’ production accounting for 8–24% of thetotal primary production. Euphotic column chlorophyll (chl a)averaged 52.5 mg m^–2 of which a major portion was locatedas a subsurface chl a maximum (SCM) at     

Feeding efficiency of the larval ctenophore Mnemiopsis leidyi A. Agassiz (Ctenophora, Lobata)

Larval stages of the ctenophore Mnemiopsis leidyi rely on metazoanprey, such as Acartia tonsa nauplii and copepodites, to supporthigh growth rates. However, M. leidyi larvae <0.5 mm (totallength) had low retention efficiencies (REs) (proportion ofencountered prey actually ingested), 5.78 ± 2.6% (mean± SE), of nauplii and were often damaged by their encounters.REs of nauplii rapidly increased, 38.94 ± 3.73%, as larvaegrew to a size of     

Phytoplankton and zooplankton response to ultraviolet radiation in a high-altitude Andean lake: short- versus long-term effects

Exclusion experiments on global UV (A and B) radiation and globalUVB were performed in 460 I mesocosms with plankton communitiesfrom the oligotrophic Andean lake Laguna Negra (33°35'S–70°04'W;2700 m a.s.l.). The experiments were run for 30 days duringthe summers of 1991–1992 and 1992–1993, and for48 days in 1993–1994. When UVB radiation was allowed toenter into the mesocosms (full sun), the population of Ankyrajudayi (Chlorophyta) reached the highest density, suggestingthat this species can endure high levels of UV radiation. Concurrently,an increase in chlorophyll a concentration was observed in thistreatment. The cladoceran Chydorus sphaericus and the rotiferLepadella ovallts were strongly inhibited by UVB. Conversely,UVB radiation had no effect on the survival of the differentlife stages of the calanoid copepod Boeckela gractlipes, suggestinga species-specific difference in the sensitivity to solar UVBradiation. Moreover, no reduction in the number of copepod eggsper female and the number of nauplii produced was observed.Apparently, herbivory does not strongly affect phytoplanktonabundance. Moreover, the phytoplankton species composition changedin the different treatments over the time. Fragilaria construensand Fragilaria crotonensis were dominant in those mesocosmswhere UVB was excluded. Populations fluc tuated depending ontheir life cycles and the period of time they were exposed toUVB radiation. It is important to define the time scale of exclusionexperiments, because different conclusions about the influenceof UVB irradiance result from short-, medium- or long-term exposures.     

Phytoplankton development and turbulent mixing in Lake Kinneret (1992-1996)

We have utilized data from a recently developed three-dimensionalvelocity fluctuation meter to compute the dissipation of turbulentkinetic energies (TKE) and the intensity of turbulent mixingin horizontal and vertical planes in the pelagic, epilimnicwater of Lake Kinneret, Israel. These characteristics of wind-inducedturbulent movement have been monitored from January 1992 throughDecember 1996. The turbulence parameters were strongly correlatedto wind energy inputs, calculated daily as 5 day cumulativeinputs. There have been dramatic changes in the annual and seasonaldevelopment of phytoplankton, together with unusually high levelsof primary production in this lake since 1994. We observed differentpatterns of vertical and horizontal turbulent movement and ofTKE dissipation rates during the years when ‘unusual’phytoplankton development occurred (1994–1996) comparedto ‘normal’ years (1992, 1993). The first appearanceof the filamentous cyanobacterium Aphanizomenon in this lakein August–September 1994 coincided with a period of markedlylower rates of TKE dispersion and a shift from vertical to horizontaldominance of the turbulent eddy spins. The absence of a regularwinter-spring bloom of the dinoflagellate, Peridinium, in 1996occurred when dissipation rates of TKE were extremely high,while record high amounts of dinoflagellates (1994, 1995) appearedwhen dissipation rates were very low. Correlations were shownbetween phytoplankton parameters (chlorophyll, primary productionand the ratio of primary production to chlorophyll) and boththe dissipation rate of TKE and the intensity of water turbulentmixing in the vertical plane. We suggest that the changes inthe ‘turbulence climate’ of Lake Kinneret were animportant factor in determining shifts in phytoplankton successionand the population composition of the algal assemblage.     

Phytoplankton blooms in a fluctuating environment: the roles of plankton response time scales and grazing

Our new view of planktonic ecosystems states that the picoplanktoncan outcompete the netplankton for nutrients, but are held toa relatively constant biomass because of the short responsetime scales of their protist grazers. The long response timeof the mesozooplankton grazing the netphytoplankton allows theselarger phytoplankton to respond to environmental fluctuationswith large changes in biomass. There is some ambiguity in theliterature, however, over the relative importance of ‘responsetime scales’ versus grazing in controlling the phytoplanktonbiomass. To address this issue, a simple model including explicitresponse time scales and grazing was formulated. The model wasused to explore the influence of these two dynamics in controllingthe response of phytoplankton to sudden changes in the carryingcapacity of the environment. It was applied to the IronEx IIdata to explore the implications of the two types of control.The model supports the hypothesis that the short response timescale of the protists limits the picoplankton biomass. However,it also shows that the zooplankton grazing rate (here representedby a clearance rate a) has a much stronger effect in determiningthe phytoplankton biomass than the response time scale.     

Microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): II. Ciliates and dinoflagellates

The composition and ecological role of ciliates and dinoflagellates were investigated at one station in Kongsfjorden, Svalbard, during six consecutive field campaigns between March and December 2006. Total ciliate and dinoflagellate abundance mirrored the seasonal progression of phytoplankton, peaking with 5.8 × 10⁴ cells l⁻¹ in April at an average chlorophyll a concentration of 10 μg l⁻¹. Dinoflagellates were more abundant than ciliates, dominated by small athecates. Among ciliates, aloricate oligotrichs dominated the assemblage. A large fraction (>60%) of ciliates and dinoflagellates contained chloroplasts in spring and summer. The biomass of the purely heterotrophic fraction of the ciliate and dinoflagellate community (protozooplankton) was with 14 μg C l⁻¹ highest in conjunction with the phytoplankton spring bloom in April. Growth experiments revealed similar specific growth rates for heterotrophic ciliates and dinoflagellates (<0–0.8 d⁻¹). Food availability may have controlled the protozooplankton assemblage in winter, while copepods may have exerted a strong control during the post-bloom period. Calculations of the potential grazing rates of the protozooplankton indicated its ability to control or heavily impact the phytoplankton stocks at most times. The results show that ciliates and dinoflagellates were an important component of the pelagic food web in Kongsfjorden and need to be taken into account when discussing the fate of phytoplankton and biogeochemical cycling in Arctic marine ecosystems.     

Predaceous feeding habits of Limnocalanus macrurus

Limnocalanus macrurus, a large, glacial-relict copepod, hasbeen assumed an omnivore or a herbivore; predaceous habits ofthe species are unknown. The predaceous feeding habits of Limnocalanusfrom Lake Michigan were studied in the laboratory using naturalprey. Predation rates were highest on copepod nauplii. Copepoditesof Diaptomus spp. and Cyclops spp. were preyed upon at lowerrates. Limnocalanus preyed selectively upon nauplii <300µm. Small cyclopoid copepodites (<–750 µm)were also selected over large copepodites. Experiments usingtwo prey types showed that nauplii were selected over all copepodites,and that no selectivity existed for either diaptomid or cyclopoidcopepodites. Predaceous feeding habits began in the fourth copepoditestage of Limnocalanus. Predaceous feeding rates of Limnocalanuschanged seasonally being highest in late spring and autumn andlowest in summer and early winter. Since Limnocalanus also feedson net-phytoplankton, predation rate changes may be relatedto changes in the relative abundance of large phytoplanktonand naupliar prey in nature. Limnocalanus predation may be animportant factor in structuring the zooplankton community. Present address: Great Lakes Research Division, University Michigan,Ann Arbor, MI 48109, USA     

Diurnal Phototropism in Leaves of Lavatera cretica L. under Conditions of Simulated Solar-Tracking

Diurnal laminar reorientation was followed in solar-trackingleaves of Lavatera cretica L. under simulated conditions. Asimulated ‘sun’ was moved over the lamina in a 180?arc in the vertical plane of the mid-vein, at an angular velocityof 15? h^–1 in a regime of 12-h photoperiods. In one groupof plants the petioles of the experimental leaves were arrangedto face ‘sunrise’, while in the other they werearranged to face ‘sunset’. At ‘sunrise’,the laminae in both groups, which were inclined towards theanticipated direction of ‘sunrise’, changed theirelevation towards the rising ‘sun’, resulting inprogressive reduction in the angle of incidence (AI) of lighton the laminar surface (AI= differential between laminar and‘solar’ elevation). As a result, laminar and ‘solar’elevation converged, and laminar reorientation gradually ceased,until the ‘solar’ elevation had passed the normalto the laminar surface (AI=0?). laminar reorientation was thenresumed, but its direction was reversed to follow the directionof ‘solar’ reorientation. During most of the remaining‘day’, laminar elevation (LE) trailed that of the‘sun’ by an average of 11?-14?. Laminar reorientationthen anticipated ‘sunset’ by starting to slow down60 to 90min in advance. During the 12-h dark period, the laminareoriented towards the anticipated direction of the subsequent‘sunrise’. The time-course of nocturnal reorientationwas qualitatively different in the two groups of experimentalplants. The time-course of diurnal phototropism under naturaland simulated conditions is analysed and compared and differencesand similarities between them are discussed. Key words: Diurnal phototropism, solar-tracking, vectorial excitation     

Copepod grazing impact on the trophic structure of the microbial assemblage of the San Pedro Channel, California

In August 2002 and March 2003 the trophic structure of the microbialassemblage from the San Pedro Channel, California was studiedfollowing the experimental alteration of the number of copepods.Changes in the abundance/biomass of microorganisms <80 µmduring 3-day incubations were monitored in (i) the absence ofmetazoa >80 µm, (ii) the presence of natural abundancesof metazoa and (iii) the presence of an elevated number of copepods.Prokaryotes and small-sized eukaryotes (<4 µm) dominatedplankton biomass during both experimental months. Diatoms numericallydominated the 10–80 µm plankton in August 2002,but ciliate and heterotrophic dinoflagellate biomass generallyexceeded diatom biomass on both dates. Ingestion of protozooplankton(predominantly ciliates) contributed substantially to copepoddaily carbon rations. The adult copepod assemblage removed 4.6and 36% per day of the microzooplankton standing stocks (10–80µm size fraction) in August and March, respectively. Elevatedcopepod grazing pressure on protozooplankton resulted in increasedbiomass of nanoplankton (<5 µm) presumably via a trophiccascade. Accordingly, the copepod–protozoan trophic linkappears to be a key factor structuring the planktonic microbialassemblage in the San Pedro Channel. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.     

Cladoceran and rotifer grazing on bacteria and phytoplankton in two shallow eutrophic lakes: in situ measurement with fluorescent microspheres

Metazooplankton grazing on bacteria and on the phytoplanktonof various sizes was estimated in shallow eutrophic lakes Kaiavereand Võrtsjärv (Estonia) by in situ feeding experimentswith fluorescent microspheres (diameters 0.5 µm for bacteriaand 3, 6 and 24 µm for phytoplankton). Zooplankton communitycomposition, abundance and food density were important factorsdetermining grazing rates in these lakes. Cladocerans and rotifersfiltering rates (FR) and ingestion rates (IR) on bacteria andphytoplankton were several times higher in Lake Kaiavere wherebacterivorous rotifers and Daphnia contributed more to zooplanktonassemblage. While cladocerans were generally the main phytoplanktonconsumers, both lakes differed with respect to the groups ofbacterivores. Based on consumption of fluorescent microspheres,the metazooplankton grazing rates were relatively low and hadlow impact on production and standing stock of bacteria andingestible phytoplankton (<30 µm). On average, 0.5and 0.1% of standing stock of bacteria and 2.6 and 1.0% of standingstock of ingestible phytoplankton was grazed daily by metazooplanktonin lakes Kaiavere and Võrtsjärv, respectively. Thatcorresponded to daily grazing of 4.1% of the bacterial productionand 0.43% of the total primary production (PP) by metazooplanktonin Lake Kaiavere compared with 4.3 and 0.06% in Lake Võrtsjärv,respectively. The results suggest that the majority of consumptionof the bacterial and phytoplankton PP is most likely channelledthrough the microbial loop.