Use of Metabolic Inhibitors to Estimate Protozooplankton Grazing and Bacterial Production in a Monomictic Eutrophic Lake with an Anaerobic Hypolimnion

Inhibitors of eucaryotes (cycloheximide and amphotericin B) and procaryotes (penicillin and chloramphenicol) were used to estimate bacterivory and bacterial production in a eutrophic lake. Bacterial production appeared to be slightly greater than protozoan grazing in the aerobic waters of Lake Oglethorpe. Use of penicillin and cycloheximide yielded inconsistent results in anaerobic water and in aerobic water when bacterial production was low. Production measured by inhibiting eucaryotes with cycloheximide did not always agree with [³H]thymidine estimates or differential filtration methods. Laboratory experiments showed that several common freshwater protozoans continued to swim and ingest bacterium-size latex beads in the presence of the eucaryote inhibitor. Penicillin also affected grazing rates of some ciliates. We recommend that caution and a corroborating method be used when estimating ecologically important parameters with specific inhibitors.     

Structure, synthesis and orientation of microfibrils. V. On the recovery of Oocystis solitaria from microtubule inhibitor treatment

Depending on the type of the inhibitor and its concentration one can experimentally induce two forms of aberrant microfibril orientations in O. solitaria cell walls through microtubule inhibitor application. The first form, designated "Intermediate", is characterized by the presence of cortical microtubules together with a spiral arrangement of microfibrils. The second form, designated "Parrallel", shows a wall with bundles of parallel oriented microfibrils without cortical microtubules. Taking colchicine as an example for a microtubule-inhibitor the "Parallel" form may be obtained with 10mM and the "Intermediate" with 5 to 1 mM solutions. Some microtubule-inhibitors such as methylbenzimidazole-2yl-carbamate (MBC) produce the "intermediate" form only. The recovery of normal microfibril orientation after inhibitor treatment is dependent on three factors: a) the developmental stage--young autospores just beginning to synthesize a wall are absolutely necessary; b) the application of inhibitors with the lowest effective concentration for c) the shortest possible time. Minimal concentrations for obtaining a "Full" effect range from 10 mM for colchicine to 1 micrometer for amiprophosmethyl (APM) with incubation periods from 3 to 9 hours. The return to the normal microfibril orientation has been achieved in all cases except after podophyllotoxin treatment. Since APM has been claimed to act selectively on tubulin synthesis in Chlamydomonas it was decided to compare the effects of this compound with cycloheximide (10 microgram/ml) on the recovery of microfibril orientation after colchicine treatment. In both cases no orientation recovery is possible although in the case of cycloheximide, synthesis of cellulose is drastically inhibited. This cycloheximide inhibition is fully reversible. During cycloheximide, but not APM, inhibition cortical microtubules return; however, due to the inhibition of cellulose synthesis itself, they cannot exert their orienting influence.     

Fish-induced changes in zooplankton grazing on phytoplankton and bacterioplankton: a long-term study in shallow hypertrophic Lake Sobygaard

The impact of fish-mediated changes on the structure and grazingof zooplankton on phytoplankton and bacterioplankton was studiedin Lake Søbygaard during the period 1984–92 bymeans of in vitro grazing experiments (¹⁴C-labelled phytoplankton,³H-labelled bacterioplankton) and model predictions. Measuredzooplankton clearance rates ranged from 0–25 ml l^–1h^–1 on phytoplankton to 0–33 ml l^–1 h^–1on bacterioplankton.The highest rates were found during thesummer when Daphnia spp. were dominant. As the phytoplanktonbiomass was substantially greater than that of bacterioplanktonthroughout the study period, ingestion of phytoplankton was26-fold greater than that of bacterioplankton. Multiple regressionanalysis of the experimental data revealed that Daphnia spp.,Bosmina longirostris and Cyclops vicinus, which were the dominantzooplankton, all contributed significantly to the variationin ingestion of phytoplankton, while only Daphnia spp. contributedsignificantly to that of bacterioplankton. Using estimated meanvalues for clearance and ingestion rates for different zooplankters,we calculated zooplankton grazing on phytoplankton and bacterioplanktonon the basis of monitoring data of lake plankton obtained duringa 9 year study period. Summer mean grazing ranged from 2 to4% of phytoplankton production and 2% of bacterioplankton productionto maxima of 53 and 88%, respectively. The grazing percentagedecreased with increasing density of planktivorous fish caughtin August each year using gill nets and shore-line electrofishing.The changes along a gradient of planktivorous fish abundanceseemed highest for bacterioplankton. Accordingly, the percentagecontribution of bacterioplankton to the total ingestion of thetwo carbon sources decreased from a summer mean value of 8%in Daphnia-dominated communities at lower fish density to 0.7–1.1%at high fish density, when cyclopoid copepods or Bosmina androtifers dominated. Likewise, the percentage of phytoplanktonproduction channelled through the bacteria varied, it beinghighest (5–8%) at high fish densities. It is argued thatthe negative impact of zooplankton grazing on bacterioplanktonin shallow lakes is highest at intermediate phosphorus levels,under which conditions Daphnia dominate the zooplankton community.     

Marine bacterial community structure resilience to changes in protist predation under phytoplankton bloom conditions

To test whether protist grazing selectively affects the composition of aquatic bacterial communities, we combined high-throughput sequencing to determine bacterial community composition with analyses of grazing rates, protist and bacterial abundances and bacterial cell sizes and physiological states in a mesocosm experiment in which nutrients were added to stimulate a phytoplankton bloom. A large variability was observed in the abundances of bacteria (from 0.7 to 2.4 × 10⁶ cells per ml), heterotrophic nanoflagellates (from 0.063 to 2.7 × 10⁴ cells per ml) and ciliates (from 100 to 3000 cells per l) during the experiment (∼3-, 45- and 30-fold, respectively), as well as in bulk grazing rates (from 1 to 13 × 10⁶ bacteria per ml per day) and bacterial production (from 3 to 379 μg per C l per day) (1 and 2 orders of magnitude, respectively). However, these strong changes in predation pressure did not induce comparable responses in bacterial community composition, indicating that bacterial community structure was resilient to changes in protist predation pressure. Overall, our results indicate that peaks in protist predation (at least those associated with phytoplankton blooms) do not necessarily trigger substantial changes in the composition of coastal marine bacterioplankton communities.     

Spatial distribution of bacterioplankton production across the Weddell-Scotia Confluence during early austral summer 1988–1989

Summary We applied two methods to measure bacterio-plankton production, the [³H]-thymidine (TTI) and the [³H]-leucine (LEU) incorporation into cold trichloro-acetic acid precipitate. Both methods gave similar results of the distribution of production in time and space (r ²=0.82, n=66). Using empirically determined conversion factors the TTI gave production values from 21 to 125 mg Cm^–2 day^–1, which are within the range reported earlier from the Southern Ocean. Highest production rates were associated with the open water in the Confluence area (59°S–60°S) and with the Scotia Sea front. Low production rates were recorded from the ice covered areas in the Weddell Sea and in the open Scotia Sea waters. Good correlation on an areal basis was found between bacterioplankton production and other measures of heterotrophy, including ETS (r²=0.93, n=9) and NH₄(r²=0.50, n=21). Good correlation was also found between bacterioplankton and phytoplankton production (r²=0.63, n=19). Bacterioplankton production seems to be driven by products from photosynthesis and heterotrophic processes, most likely grazing, which are tightly coupled to autotrophy. Quantitatively, bacterioplankton production was on an average 11 % of net primary production, which is clearly a lower value than the 30% based on a review from temperate freshwater and marine ecosystems, but is comparable with values reported from the spring period in subarctic ecosystems. In comparison with the measurements of ETS, bacterioplankton contribution to community respiration was also lower than predicted from results from temperate ecosystems. We concluded from these results and the results obtained from microcosm experiments (Bjørnsen and Kuparinen 1991b) that the flux of organic matter to eucaryote heterotrophs via bacterioplankton during spring and early summer periods in the Southern Ocean is of considerable, but not of equivalent importance as in temperate waters.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Mechanisms of corticotropin action in rat adrenal cells. I. The effects of inhibitors of protein synthesis and of microfilament formation on corticosterone synthesis.

The contributions of protein synthesis and formation of microtubules and microfilaments to corticotropin-stimulated steroidogenesis in rat adrenal cell suspensions has been assessed by use of a series of inhibitors to each function. Five inhibitors of protein synthesis (cycloheximide, puromycin, blastocidin S, anisomycin, and trichodermin) each exhibited time-dependent inhibition of corticotropin-stimulated steroidogenesis. For the first 30 min, steroidogenesis was more extensively inhibited than protein synthesis, after which the effectiveness of the inhibitors diminished on steroidogenesis but not on protein synthesis. The reversal effect was not observed at high levels of inhibitors. One inhibitor of microfilament formation (cytochalasin B) and four inhibitors of microtubule formation (colchicine, podophyllotoxin, vinblastine sulfate and griseofulvin) inhibited steroidogenesis without inhibiting protein synthesis and without any reversal effect with prolonged incubation. The actions of all ten inhibitors were shown to be fully reversible. Cell superfusion of adrenal cells showed that the decay of steroidogenesis upon addition of all the protein synthesis inhibitors was similar to decay upon removal of corticotropin from the medium (t1/2 = 4--6 min). Recoveries from inhibition upon removal of the inhibitors were similar to each other and comparable to initial corticotropin stimulation of the cells (lag of 3--5 min, t1/2=7--9 min). Similar kinetics of inhibition and recovery were observed for vinblastine sulfate while a direct inhibition of cytochrome P-450scc by aminoglutethimide was complete within 1 min and was rapidly reversed. Injection of each inhibitor (all classes) into hypophysectomized rats inhibited the elevation of plasma corticosterone by corticotropin. The extent of cholesterol combination with cytochrome P-450scc in adrenal mitochondria isolated from these rats was also decreased by all of the inhibitors. Decreases in plasma corticosterone correlated directly with decreases in cholesterol combination with cytochrome P-450scc (r=0.94). It is concluded that protein synthesis and steroidogenesis must be intimately coupled probably due to the requirement of a labile protein for cholesterol transport to cytochrome P-450scc. An involvement of microtubules and microfilaments in this process is clearly indicated.     

THE MELANOCYTE MODEL : Colchicine-like Effects of Other Antimitotic Agents

The effect of various agents that cause metaphase arrest in dividing cells was studied on the rapid reversible darkening of frog skin under the influence of melanocyte-stimulating hormone (MSH). Darkening is due to dispersion of melanin granules in melanocytes and is thought to be accompanied by a gel-to-sol cytoplasmic transformation. After subsequent washing the skin lightens, with aggregation of melanin granules and cytoplasmic gelation. As previously shown with colchicine, preincubation of frog skin with vinblastine, vincristine, or colcemid produced an increase in darkening induced by MSH, as compared to control skins, and a dosage-dependent inhibition of subsequent lightening. Preincubation with each drug, without subsequent MSH, produced a gradual, irreversible, dosage-dependent darkening over several hours. On a molar basis, the relative strength of the various agents was vinblastine > vincristine > colcemid > colchicine; vinblastine was about 100 times stronger than colchicine. Preincubation of frog skin with griseofulvin, followed by washing, had no subsequent effects on darkening or lightening. However, effects similar to those of the Colchicum and Vinca alkaloids were seen if griseofulvin was kept in the ambient media. These effects were rapidly reversible on removal of the drug from the media. These findings support the melanocyte model originally proposed for the action of colchicine, and emphasize certain facts that models of melanin granule movement will have to accommodate.     

Inadequacy of the Eucaryote Inhibitor Cycloheximide in Studies of Protozoan Grazing on Bacteria at the Freshwater-Sediment Interface

Four guilds from a lake sediment-water interface microbial community were isolated and tested for sensitivity to cycloheximide (0.1 to 200 mg liter⁻¹). Field experiments were conducted to compare the inhibition, dilution, and filtration methods for determining grazing rates. Cycloheximide inhibited anaerobic bacteria at 50 mg liter⁻¹, and inhibition of bacterial growth was observed in the grazing experiments. The results show that the assumption of selective inhibition of heterotrophic eucaryotes was violated and preclude the use of cycloheximide in grazing experiments.     

Combined effects of nutrients and grazers on bacterioplankton and phytoplankton abundance in an Antarctic lake with even food-chain links

The goal of this study was to address the top-down and bottom-up controls on different microbial web components (bacterioplankton, picophytoplankton, and >3 μm phytoplankton) in an Antarctic lake. Two experiments using a size fractionation approach and nutrient addition were conducted at microcosm scale (2.5 l). The variation in net growth rates (k′) of bacterioplankton and phytoplankton size fractions was analyzed after 5 days. The results determined significant differences; whereas bacterioplankton and large phytoplankton showed an increase in their k′ when their predators were removed, the picophytoplankton showed a decrease. All the studied plankton components presented the highest k′ when nutrients were added. It is suggested that, in this lake, both the top-down and bottom-up regulations account for the regulation of bacterioplankton and large phytoplankton. As for picophytoplankton, the bottom-up control was evident and grazing did not pose a negative impact and rather, had a positive effect probably due to liberation of nutrients.     

The Seasonal Dynamics and Trophic Relations of the Plankton Components in Lake Peipsi (Peipus)

The seasonal dynamics of the biomass and production of phyto-, zoo- and bacterioplankton was investigated during the vegetation periods (from May to November) in 1985 and 1986 in the pelagial of the large eutrophic lake Peipsi (Estonia). The average values of productions per vegetation period for the investigation years were as follows: phytoplanktion − 203.5 gC · m⁻²; bacterioplankton − 37.9 gC · m⁻²; filter-feeding zooplankton − 20.6 gC · m⁻² and predatory zooplankton − 1.5 gC · m⁻². The herbivorous zooplankton production constituted 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain — filtrators are feeding mostly on living algae and the detrital food chain seems of little importance. The dominance of large forms (Melosira sp., Aphanothece saxicola), in the phytoplankton during the major part of the vegetation period is assumed to be a result of high grazing pressure on small algae. Zooplankton grazing was investigated in situ in a specially constructed twin bathometer. Experimental measurements revealed, that zooplanktion presence in the experimental vessel actually stimulated the phytoplankton growth in many cases — the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), excreted by the concentrated zooplankton in the grazing chamber, which led to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11%. Grazing on bacteria increased, when grazing on phytoplankton was somehow disturbed.     

Photo- and heterotrophic pico- and nanoplankton in the Mississippi River plume: distribution and grazing activity

The abundance of pico- and nanophytoplankton, bacteria and heterotrophicnanoflagellates, and grazing rates on phototrophic pico- andnanoplankton and bacterioplankton were assessed along a salinitygradient (0.2–34.4) in the Mississippi River plume inMay 2000. Grazing rates were established by serial dilutionexperiments, and analysis by flow cytometry allowed differentiationof grazing rates for different phytoplankton subpopulations(eukaryotes, Synechococcus spp., Prochlorococcus spp.). Grazingrates on phytoplankton tended to increase along the salinitygradient and often approached or exceeded 1 day^-1. Phytoplanktonnet growth rates (growth – grazing) were mostly negative,except for positive values for eukaryotic nanoplankton in thelow-salinity, high-chlorophyll region. Grazing pressure on bacteriawas moderate (     

Control of marine bacterioplankton populations: Measurement and significance of grazing

A variety of methods have been used to estimate the degree of control exercised upon marine bacterioplankton by grazing organisms. These include filtration or dilution of samples to reduce grazers, the use of specific inhibitors to prevent growth or grazing, and the use of artificial particles or radio-labelled bacteria as tracers for the natural bacterioplankton. Each of these techniques has drawbacks which may lead to under- or overestimates of grazing. In addition, they tell us little about which organisms are doing the grazing or the degree to which viruses or lytic bacteria compete with grazers for bacterial production. Because measurements of grazing and bacterioplankton growth rates are uncertain, exact comparisons are not presently possible. Thus measurements of bacterial and bacterivore abundance, concentrated on comparisons between seasons, on diel cycles and on spatial variations, have been used to evaluate mechanisms controlling bacterial populations. These give an idea of the degree of coupling between bacterial growth and bacterivore activity and of the time scales over which growth and grazing balance. Combined with laboratory studies of grazing, they currently provide the best insight into what controls populations of bacteria in the sea.     

Top-down control of natural phyto- and bacterioplankton prey communities by Daphnia magna and by the natural zooplankton community of the hypertrophic Lake Blankaart

Biomanipulation, through the reduction of fish abundance resulting in an increase of large filter feeders and a stronger top-down control on algae, is commonly used as a lake restoration tool in eutrophic lakes. However, cyanobacteria, often found in eutrophic ponds, can influence the grazing capacity of filter feeding zooplankton. We performed grazing experiments in hypertrophic Lake Blankaart during two consecutive summers (1998, with and 1999, without cyanobacteria) to elucidate the influence of cyanobacteria on the grazing pressure of zooplankton communities. We compared the grazing pressure of the natural macrozooplankton community (mainly small to medium-sized cladocerans and copepods) with that of large Daphnia magna on the natural bacterioplankton and phytoplankton prey communities. Our results showed that in the absence of cyanobacteria, Daphnia magna grazing pressure on bacteria was higher compared to the grazing pressure of the natural zooplankton community. However, Daphnia grazing rates on phytoplankton were not significantly different compared to the grazing rates of the natural zooplankton community. When cyanobacteria were abundant, grazing pressure of Daphnia magnaseemed to be inhibited, and the grazing pressure on bacteria and phytoplankton was similar to that of the natural macrozooplankton community. Our results suggest that biomanipulation may not always result in a more effective top-down control of the algal biomass.     

Effects of Nutrients on Specific Growth Rate of Bacterioplankton in Oligotrophic Lake Water Cultures

The effects of organic and inorganic nutrient additions on the specific growth rates of bacterioplankton in oligotrophic lake water cultures were investigated. Lake water was first passed through 0.8-μm-pore-size filters (prescreening) to remove bacterivores and to minimize confounding effects of algae. Specific growth rates were calculated from changes in both bacterial cell numbers and biovolumes over 36 h. Gross specific growth rates in unmanipulated control samples were estimated through separate measurements of grazing losses by use of penicillin. The addition of mixed organic substrates alone to prescreened water did not significantly increase bacterioplankton specific growth rates. The addition of inorganic phosphorus alone significantly increased one or both specific growth rates in three of four experiments, and one experiment showed a secondary stimulation by organic substrates. The stimulatory effects of phosphorus addition were greatest concurrently with the highest alkaline phosphatase activity in the lake water. Because bacteria have been shown to dominate inorganic phosphorus uptake in other P-deficient systems, the demonstration that phosphorus, rather than organic carbon, can limit bacterioplankton growth suggests direct competition between phytoplankton and bacterioplankton for inorganic phosphorus.     

Assessing Phytoplankton and Bacterioplankton Production During Early Spring in Lake Erken, Sweden

The spring development of both phytoplankton and bacterioplankton was investigated between 18 April and 7 May 1983 in mesotrophic Lake Erken, Sweden. By using the lake as a batch culture, our aim was to estimate, via different methods, the production of phytoplankton and bacterioplankton in the lake and to compare these production estimates with the actual increase in phytoplankton and bacterioplankton biomass. The average water temperature was 3.5°C. Of the phytoplankton biomass, >90% was the diatom Stephanodiscus hantzchii var. pusillus, by the peak of the bloom. The ¹⁴C and O₂ methods of estimating primary production gave equivalent results (r = 0.999) with a photosynthetic quotient of 1.63. The theoretical photosynthetic quotient predicted from the C/NO₃ N assimilation ratio was 1.57. The total integrated incorporation of [¹⁴C]bicarbonate into particulate material (>1 μm) was similar to the increase in phytoplankton carbon determined from cell counts. Bacterioplankton increased from 0.5 × 10⁹ to 1.52 × 10⁹ cells liter⁻¹ (~0.5 μg of C liter⁻¹ day⁻¹). Estimates of bacterioplankton production from rates of [³H]thymidine incorporation were ca. 1.2 to 1.7 μg of C liter⁻¹ day⁻¹. Bacterial respiration, measured by a high-precision Winkler technique, was estimated as 4.8 μg of C liter⁻¹ day⁻¹, indicating a bacterial growth yield of 25%. The bulk of the bacterioplankton production was accounted for by algal extracellular products. Gross bacterioplankton production (production plus respiration) was 20% of gross primary production, per square meter of surface area. We found no indication that bacterioplankton production was underestimated by the [³H]thymidine incorporation method.     

Bacterivory and herbivory: Key roles of phagotrophic protists in pelagic food webs

Research on microbial loop organisms, heterotrophic bacteria and phagotrophic protists, has been stimulated in large measure by Pomeroy's seminal paper published in BioScience in 1974. We now know that a significant fate of bacterioplankton production is grazing by < 20-µm-sized flagellates. By selectively grazing larger, more rapidly growing and dividing cells in the bacterioplankton assemblage, bacterivores may be directly cropping bacterial production rather than simply the standing stock of bacterial cells. Protistan herbivory, however, is likely to be a more significant pathway of carbon flow in pelagic food webs than is bacterivory. Herbivores include both < 20-µm flagellates as well as > 20-µm ciliates and heterotrophic dinoflagellates in the microzooplankton. Protists can grow as fast as, or faster than their phytoplankton prey. Phototrophic cells grazed by protists range from bacterial-sized prochlorophytes to large diatom chains (which are preyed upon by extracellularly-feeding dinoflagellates). Recent estimates of microzooplankton herbivory in various parts of the sea suggest that protists routinely consume from 25 to 100% of daily phytoplankton production, even in diatom-dominated upwelling blooms. Phagotrophic protists should be viewed as a dominant biotic control of both bacteria and of phytoplankton in the sea.     

Effect of metabolic inhibitors on formed novocaine and neutral red segregation zones in frog erythrocytes

Effects of some metabolic inhibitors, as well as of biologically active compounds (diakarb, ethidium bromide and a phenanthridine alkaloid sanguinarine) on the formed novocaine and neutral red segregation zones were studied. The volume of granules diminished under the influence of a glycolytic inhibitor iodoacetate, uncouplers of oxidative phosphorylation (2,4-dinitrophenol and carbonyl cyanide trifluoromethoxyphenylhydrozone), and respiratory inhibitors (antimycin A and rotenone), as well as under the influence of cycloheximide - an inhibitor of protein synthesis. Diakarb, ethidium bromide or sanguinarine also provoked a regression of the segregation zones. It has been found that all these compounds are inhibitors of ATPase activity of the isolated segregation zones. A possible mechanism of volume decreasing in segregation zones under the influence of both the metabolic inhibitors and diakarb, ethidium bromide and sanguinarine is discussed.     

Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north-western Ontario, Canada.
2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet-A (UVA) + ultraviolet-B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.
3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the 'base phytoplankton' (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near-surface phytoplankton. On the other hand, production of near-surface bacterioplankton was reduced following a 4-h UVR exposure but had increased after a 48-h exposure to both UVA and UVB compared with the PAR only treatment.
4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.     

Microbial food web in a large shallow lake (Lake Balaton, Hungary)

Seasonal variations of phyto-, bacterio- and colourless flagellate plankton were followed across a year in the large shallow Lake Balaton (Hungary). Yearly average chlorophyll-a concentration was 11 µg 1^–1, while the corresponding values of bacterioplankton and heterotrophic nanoflagellate (HNF) plankton biomass (fresh weight) were 0.24 mg 1^–1 and 0.35 mg 1^–1, respectively. About half of planktonic primary production was channelled through bacterioplankton on the yearly basis. However, there was no significant correlation between phytoplankton biomass and bacterial abundance. Bacterial specific growth rates were in the range of 0.009 and 0.09 h^–1, and ended to follow the seasonal changes in water temperature. In some periods of the year, predator-prey relationships between the HNF and bacterial abundance were obvious. The estimated HNF grazing on bacteria varied between 3% and 227% of the daily bacterial production. On an annual basis, 87% of bacterial cell production was grazed by HNF plankton.     

Experimental evidence of the grazing impact of <Emphasis Type="Italic">Boeckella poppei</Emphasis> on phytoplankton in a maritime Antarctic lake

The zooplankton biomass of Lake Boeckella (Hope Bay, Antarctic Peninsula) is strongly dominated by the calanoid copepod Boeckella poppei Mrázek. This work analyses the grazing impact of this copepod on the two dominant fractions of phytoplankton, pico- and nanoplankton, and on the bacterioplankton. By means of in-situ experiments using microcosms, the following hypotheses were tested: (a) the early stages of the copepod mainly graze on phytoplankton; (b) the pre-adult and adult stages graze on phytoplankton and benthic algae. Copepods were separated into two groups of maturity: early stages, and pre-adult and adult stages. The following treatments were performed: (1) only nano- and picoplankton, (2) nano- and picoplankton+periphyton, and (3) only periphyton, for each one of the two copepod maturity groups, and (4) control (without copepods). The variation in nano-phytoplankton density was analysed after 2 and 4 days. The results determined a significant grazing on the nano-phytoplankton fraction in all microcosms containing copepods of both maturity groups (P<0.01). The effect on the nano-phytoplankton was greater when the copepods did not have another source of food (P<0.01). No significant differences between the maturity groups were observed (P>0.05). We also conclude that the copepods use the periphyton as an alternative source of food, which was corroborated by the analysis of gut content. In general, no significant differences among treatments were recorded for the pico-sized fraction (pico-phytoplankton and bacterioplankton), which would suggest that no direct grazing on this fraction exists.