Abundance Of Heterotrophic Nanoflagellates Research Articles

Functional structure of microbial food web in the Senegal River Estuary (West Africa): impact of metazooplankton

We studied the impact of various metazooplanktonic predators (two calanoid copepods and barnacle larvae) on microbial plankton isolated from the Senegal River Estuary. Experiments performed in microcosms were based on size-class fractionation (3, 12, 20 and 60 µm size classes) of the microbial community in order to obtain different assemblages, including bacteria, heterotrophic nanoflagellates (HNF), ciliates, pico- and nanophytoplankton. Removal of bacterial predators >3 µm increased the net growth rate of heterotrophic bacteria from 0.437 day−1 in the presence of all predators (fraction <60 µm) to 0.682 day−1 (fraction <3 µm). Removal of protozoa >12 µm caused an increased net growth rate of flagellates from 0.102 day−1 (fraction <60 µm) to 0.332 day−1 (fraction <12 µm). The same results were also observed on protozoa dynamics in the presence of Temora stylifera (0.498 day−1), suggesting a trophic control of the protozoa >12 µm by this copepod, allowing an increase in HNF growth rates through indirect effect. The highest ingestion rate over 60 h of experiment was recorded for the copepod Temora (61.1 µm3 ngC−1 h−1) and the lowest value for barnacle larvae (25.6 µm3 ngC−1 h−1). An intermediate value (47.5 µm3 ngC−1 h−1) was recorded for the copepod Acartia clausi. All these ingestion rates were mainly channeled in the size classes between <3 and 15–18 µm equivalent spherical diameter (ESD) (6 and 2435 µm3, respectively), with the highest values noted between <3 and 6–9 µm classes. In the presence of Temora, the cascading effects can explain both the decreases in mean volume and in growth rate for bacteria, due to the filtration of ciliates by the zooplanktonic predator, allowing the development of the HNF. The presence of a large size class of bacteria (among the five classes identified by flow cytometry) and a high abundance of HNF suggest an efficient heterotrophic pathway within the microbial loop of this sub-saharan estuary.

Response of East Mediterranean surface water to Saharan dust: On-board microcosm experiment and field observations

Abstract An on-board microcosm experiment was performed during the CYCLOPS May 2002 cruise to track the biogeochemical response of Eastern Mediterranean surface seawater to a gradient addition of fresh and pre-leached Saharan dust, mimicking the potential fertilization effect as opposed to the impact of adding particles alone. Response parameters examined were P-turnover time, bacterial production and abundance, chlorophyll a, other phytopigments, abundance of different pico and nanophytoplankton groups, primary production rates, abundance of heterotrophic nanoflagellates and ciliates. The addition of fresh Saharan dust (range: 0.2– 4.9 mg l - 1 ) and the subsequent nutrient release triggered an increase in phytopigments and primary production, while no response was detected for pre-leached dust particles. Most responses were linearly related to the amount of fresh dust added. Synechococcus and prymnesiophytes increased in abundance along with cellular pigment content while Prochlorococcus disappeared, heterotrophic bacteria increased production rates, and ciliates showed a small increase in cell density. A less clear response was recorded by in situ measurements following a Saharan dust storm during a cruise in the Levantine Basin in May 2001. The calculated amount of nutrients and dust particles delivered by such an event to a 15-m thick mixed surface layer is low ( ∼ 0.3 nmol P l - 1 , ∼ 9 nmol N l - 1 and 0.06 mg dust l - 1 ) , falling close to the lowest dust addition in our microcosm experiment. Even so, an enhancement of phosphate turnover time, a sharp decline of Prochlorococcus abundance, and slight increases in chlorophyll a and bacterial activity were observed in response to the dust storm. Considering the linear effect of fresh dust concentrations on the bacterial activity, primary production and pigment concentration (total and per cell), and the likely stimulation of grazing, it is not surprising that changes due to moderate strength dust storms are mostly close to detection limit of either field or remote sensing measurements.

Benthic microbial abundance and activities in an intensively trawled ecosystem (Thermaikos Gulf, Aegean Sea)

Abundance of benthic bacteria, heterotrophic nanoflagellates and ciliates, extracellular enzymatic activities, bacterial C production, C mineralisation and sediment community oxygen consumption rates were measured in the Thermaikos Gulf (Northeastern Mediterranean), before (September 2001), and during intense trawling activities (October 2001 and February 2002). The biochemical composition of sedimentary organic matter has revealed that bottom trawling had an effect on the trophic state of Thermaikos Gulf. Changes on the benthic microbial food web were also recorded, during the three sampling seasons. Even though trawling-induced sediment resuspension did not alter significantly the abundance of the microbial components, with the exception of the most impacted station, it determined changes regarding their relative importance. Thus, the ratios of bacterium to nanoflagellates and ciliate to nanoflagellates abundance increased in the trawled stations, causing a sudden increase in bacterial C production, in comparison to the non-trawled station. Four months later, the effects of trawling on the microbial food web were less evident, masked possibly by the drastic decrease in the water temperature. The results of the present work suggest that bottom trawling induces alteration of the sedimentological variables and can be considered as a factor affecting the function of the microbial food web in marine coastal ecosystems. These alterations cause faster mobilisation of organic C buried in the sediment and increase nutrient concentrations and availability in the system, thus inducing an effect that could lead to coastal eutrophication.

Relative strength of resource and predation limitation of heterotrophic nanoflagellates in a low-productive sea area

The magnitude of resource and predation limitation of heterotrophic nanoflagellates (HNF) was studied in two short-term enclosure experiments performed in a low-productive sea area in the northern Baltic Sea in 2001. A cross-factorial design was used to simultaneously assess the relative importance of the two factors. Resource limitation was removed by adding bacteria, and predation limitation was eliminated by selective filtration. The first experiment was performed in June just after the spring bloom decline and the second in September at the end of the productive season. Resource limitation prevailed during both experiments, contributing to 60% of the net growth increase in June and 74% in September. Removal of predators had a significant effect only in June. Evidence for simultaneous resource and predation limitation was thus found only during the postbloom situation. The results were applied to a model on resource and predation control of HNF abundances. To evaluate seasonal differences, field data on HNF and bacteria from a whole year study were applied to the model. Except for a few occasions during spring, the model indicated prevailing resource control of HNF at two locations with slightly different productivity.

Grazing impact of different heterotrophic nanoflagellates on eukaryotic (Ostreococcus tauri ) and prokaryotic picoautotrophs (Prochlorococcus and Synechococcus)

Autotrophic picoplankton (<3 microm) composed of both prokaryotes and eukaryotes are the most abundant primary producers on Earth. In this study we examined the ingestion of the picoeukaryote Ostreococcus tauri by different marine heterotrophic nanoflagellates (HNF) with various morphologies, swimming and feeding behaviours. Cultures of specific bacterivorous nanoflagellates (Rhynchomonas nasuta, Jakoba libera, and a culture of Cafeteria sp./Monosiga sp.) and natural nanoflagellate populations were used as grazers. For comparison with Ostreococcus, we used similar-sized prokaryotes as prey, Prochlorococcus and Synechococcus. We observed large species-specific differences in terms of: use of picoautotrophs among nanoflagellates, time lag between prey addition and prey consumption (0-196 h), grazing rate (0-0.12 h(-1)), growth rate (0-0.3 h(-1)) and maximum abundance of HNF reached in experimental bottles (e.g. from 10(4) to 10(5) cells ml(-1), for a natural coastal population and a Cafeteria sp./Monosiga sp. culture feeding Ostreococcus respectively). Overall, this study shows that the nanoflagellate community composition is conclusive for picoautotrophic community structure and, vice versa, the picoautotrophic community structure favours or inhibits the growth of some nanoflagellate groups.

Abundância de nanoflagelados heterotróficos no plâncton de reservatórios com distintos graus de trofia

The aim of this study was to investigate the influence of the trophic state on the planktonic heterotrophic nanoflagellates abundance from three reservoirs of the State of Parana: Irai (eutrophic), Rosana (mesotrophic) and Chavantes (oligotrophic). The samples were collected during dry (July) and rainy (November) periods in 2001 and were taken at the sub-surface and mixing layers of the lacustrine zone of the reservoirs. Heterotrophic nanoflagellates abundance was determined by epifluorescence microscopy, after staining with Dapi, and cells volumes were calculated aiming to estimate biomass, using a cell volume-to-carbon conversion factor. Higher values of density and biomass were observed in Irai Reservoir, the most eutrophic environment, whereas the lower ones were registred in Chavantes Reservoir, in both studied periods. In all reservoirs, a clear vertical pattern was observed, with higher values in the sub-surface layer. The results suggested that the trophic state influenced strongly the abundance patterns of planktonic heterotrophic nanoflagellates, indicating that the resources available determined the patterns. In other words, although there certainly is a predation pressure on the heterotrophic nanoflagellates community, it is not enough to prevent that the increase of resources determine a significant enlargement in these organisms abundance

Enhanced heterotrophic activity in the surface microlayer of the Mediterranean Sea

The abundance and activity of heterotrophic and autotrophic organisms were deter- mined in the surface microlayer (SML) at marine and estuarine sites in the NW Mediterranean Sea from May to July 2003. At all study sites, community respiration in the SML exceeded that in the sub- surface water (SSW) by factors between 1.7 and 28. The abundance of heterotrophic bacteria and the major groups of the autotrophic community displayed consistent, but low enrichment factors (EF) for the SML (mean 1.3). In contrast, EF for the abundance of heterotrophic nanoflagellates and the concentration of particulate organic carbon were, on average, 4 and 7, respectively. Community respiration rates in the SML were related to the concentration of total organic carbon; however, no relationship was detectable between community respiration and the abundance of the major groups of the heterotrophic and autotrophic community. Net community production, determined at 2 oligo- trophic marine sites, displayed negative values for the SML, while in the SSW autotrophic processes balanced or exceeded heterotrophic processes. Similar rates of photochemical oxygen consumption were determined for the SML and the SSW. These results let us conclude that in situ primary produc- tion in the SML is not sufficient to support biological and photochemical mineralization at the air-water interface. We suggest that vertical transport of organic matter, mainly in colloidal and particulate form, to the air-water interface is the basis of the food web, thereby supporting net heterotrophy of the SML.

Seasonal variation of phototrophic picoplankton in Lake Biwa (1994?1998)

Seasonal abundances of phototrophic picoplankton (PP) and heterotrophic nanoflagellates in Lake Biwa were studied from 1994 to 1998. Seasonal variation in cell volume and biomass of the phototrophic picoplankton were also studied. PP were counted using disposable glass microscopic plates, which gave superior accuracy to sample filtration onto membrane filters. Phycoerythrin-rich rod-shaped cyanobacteria (PEC), one of the major components of the picoplankton community, were sparse (about 104 cells ml −1) in winter and began to increase in April. Several PEC peaks were observed during the period of thermal stratification, and a rapid fall took place after October or November. In the northern basin, PEC peaked during late June and early July in 3 of the 5 years, and in late summer in the remaining years. Phycocyanin-rich rod-shaped cyanobacteria (PCC) were abundant in the southern basin and were present in smaller numbers in the eutrophic nearshore area of the northern basin; they peaked several times during the period from July to October. Seasonal variations of these two kinds of picoplankton were correlated with seasonal changes in water temperature. Phycoerythrin-rich cylinder-shaped cyanobacteria exhibited narrow peaks in July, their abundance declining as the year progressed. The density of heterotrophic nanoflagellates was greatest in early spring. Average cell volume of PEC was largest in winter, then decreased gradually to a minimum in late summer; after the fall, it recovered to the winter cell volume. This change can likely be attributed to the depletion of nitrogen in the warmer seasons.

Temporal and spatial changes in plankton respiration and biomass in the Canary Islands region: the effect of mesoscale variability

The temporal and spatial variabilities in the abundance and respiratory activity of plankton communities (<200 μm) were studied during three seasonal cruises around Gran Canaria Island (Canary Islands), a region of high mesoscale variability. Marked seasonal changes in respiratory activity, plankton community structure, and the ratio of heterotrophic to autotrophic biomass can be largely explained by hydrographic changes at the mesoscale level. Wind/current shearing at the flanks of the island enhances plankton respiration, presumably as the consequence of an increase in turbulence. Counter-paired cyclonic and anticyclonic eddies generated downstream of the island act as a two-way biological pump, increasing plankton production by nutrient pumping into the euphotic zone and accelerating the transport of organic matter into the aphotic zone, respectively. Coastal upwelling waters invading the Canary region in the form of filaments can transport either water with low plankton respiration and large phytoplankton cells or water with high respiratory rates associated with small cells. Plankton respiration was closely related to the abundance of Synechococcus type cyanobacteria and heterotrophic nanoflagellates during the three periods, but was only correlated with chlorophyll during the most fertile season, suggesting that respiration was mainly linked to microbial food web processes. Size-fractionated studies showed that 51–67% of the respiratory activity was due to picoplankton cells (<2 μm). Respiration rates (average values: 113±18 to 187±87 mmol C m −2 d −1) matched primary production rates during the fertile period, but were up to one order of magnitude higher during the rest of the year. Substantial inputs of organic matter from the coastal upwelling would be necessary to balance the large annual heterotrophic deficit in the region of study.

Short-term response of benthic bacteria and nanoflagellates to sediment resuspension: an experimental study

We investigated benthic bacterial abundance, biomass and activities and heterotrophic nanoflagellate (HNF) abundance and biomass during sediment resuspension and after a resilience period through short-term laboratory experiments. The strong sediment resuspension increased bacterial carbon production only slightly (Tukey's test, P < 0.05) but somehow interfered with microbial growth. During sediment resuspension, after an initial increase (at 48 and 72 h) bacterial abundance and biomass decreased until the end of resuspension. Sediment resuspension also altered the relationship between HNFs and bacteria (increase in the ratios of bacterium to HNF abundance and biomass after sediment resuspension; Tukey's test, P < 0.01). After sediment resuspension, the fraction of bacterial C production removed by flagellates decreased, whereas bacterial turnover did not change significantly, suggesting that HNF pressure on bacteria decreased. These results are consistent with the significant decrease in both HNF abundance and biomass (Tukey's test, P < 0.01) and suggest that this small benthic component is less resilient to sediment resuspension than bacteria. Sediment resuspension also determined significant changes in sediment organic matter pools (proteins and carbohydrates). Sediment resuspension determined a decrease in total protein concentrations, whereas total carbohydrate concentrations did not change significantly. Sediment resuspension had significant effects on extracellular enzymatic activities, as aminopeptidase increased and β-glucosidase activity decreased. Based on total protein and carbohydrate concentrations and enzymatic activities, we observed that sediment resuspension resulted in an increase in protein turnover and a decrease in carbohydrate turnover. These results indicate that sediment resuspension plays a major role in the early diagenesis of sediment organic matter.

Seasonal and vertical distribution of planktonic bacteria and heterotrophic nanoflagellates in the middle Adriatic Sea

Temporal and spatial patterns of bacteria and heterotrophic nanoflagellates (HNF) were studied monthly from January 1997 to December 1998 in the middle Adriatic Sea. Bacterial and HNF relationships with phytoplankton biomass and temperature were analyzed to examine how the relative importance of bottom-up and top-down factors may shift over seasons and locations. For the coastal area, an inconsistent relationship between bacterial abundance and chlorophyll a and a stronger relationship between bacterial abundance and bacterial production suggest that other substrates than those of phytoplankton origin are important for bacteria. The analysis of simultaneous effects of temperature and bacterial production on bacterial abundance showed that the effect of temperature obscured the effects of bacterial production, suggesting that bacterial growth itself is highly temperature-dependent. The relationship between HNF abundance and bacterial abundance was slightly improved by the inclusion of in situ temperature, bacterial production or both parameters, as additional independent variables. About 60% of the variability in HNF abundance can be explained by bacterial abundance, bacterial production and temperature. In the open sea, tight coupling of bacterial abundance with chlorophyll a concentrations implied that phytoplankton-derived substrates have a dominant role in controlling bacterial abundance. During the colder months, bacterial abundance was high enough to support higher HNF abundance than observed, suggesting that predation exerted a minor depressing influence on bacterial abundance during that period. During the spring-summer period, HNF controlled bacterial standing stocks by direct cropping of bacterial production.

Response of bacterial grazing rates to experimental manipulation of an Antarctic coastal nanoflagellate community

We examined changes of bacterial losses related to heterotrophic nanoflagellate (HNF) size distribution in late spring/early summer (1998/1999) using 9 grazing experiments in a coastal Antarctic area (Johnson's Dock, Livingston Island, Bransfield Sector). Water samples were subjected to size fractionation through 50 and 5 pm pore sizes to obtain a truncation of the microbial food web. In each fraction, we estimated bacterial loss rates and abundance and biomass of HNF grouped into 4 size classes (<2, 2 to 5, 5 to 10, 10 to 20 pm). We also investigated whether grazing on bacteria was mainly due to HNF, and which HNF size class had a major impact on bacteria. We expected that in the 50 pm fraction, large protists (ciliates, dinoflagellates) would prey preferentially on nanoprotists and relieve bacterial pressure from HNF. Bacterial grazing rates were estimated by disappearance of fluorescently labeled bacteria over 24 h. These showed similar values in both experimental treatments, although they were slightly higher for the 50 pm fraction. Average grazing rates were 4.8 x 10 5 ± 3.6 × 10 5 cells ml -1 d -1 in the 5 pm treatment and 6.9 x 10 5 ± 3.2 x 10 5 cells ml -1 d -1 for the 50 pm fraction. In the 5 pm fraction, HNF abundance (integrated over 24 h, HNFi) and bacterial grazing rates were significantly related. The best relationship was obtained with the smallest HNFi size classes (from <2 and 2 to 5 pm). In the 50 pm fraction, no relationships were found between bacterial loss rate and both total HNFi and any HNFi size class in terms of abundance and biomass. However, microozooplankton was negatively related to total bacteria and both HNFi abundance and biomass. The major contributor to this negative relationship was the HNFi size classes from ≤ 2 and 2 to 5 pm. Consequently, and against our expectations, large protists contributed to microbial food-web complexity by masking carbon fluxes from bacteria to HNF, and by feeding on both bacteria and nanoprotists.

Microbial communities in oligotrophic Lake Toya, Japan

The vertical distributions of planktonic bacteria, chroococcoid cyanobacteria, and heterotrophic nanoflagellates (HNF) were examined in Lake Toya, an oligotrophic lake located in Hokkaido, the northern island of Japan, at monthly intervals from May 1993 to May 1994. The abundances of bacteria, cyanobacteria, and HNF during the study period ranged from 104 to 105,102 to 104, 10 to 102 cells ml−1, respectively. The range of bacterial abundances was among the lowest previously reported from other oligotrophic lakes. The vertical distributions of both bacteria and cyanobacteria were influenced by thermal stratification: they were homogeneous throughout the water column during the circulation period (January to April) and heterogeneous during the period of stratification (June to November). During the period of stratification, the cyanobacterial abundance decreased toward the surface in the euphotic zone while the frequency of diving cells (FDC) increased, suggesting that grazing pressure was high near the surface. This hypothesis was supported by the relatively high abundance of HNF at the surface and the negative correlation between HNF and cyanobacterial abundances in the euphotic zone (r = −0.503, n = 33, P < 0.05). On the other hand, multiple regression analysis revealed that 52% of the variation in bacterial abundance in the stratified period can be explained by chlorophyll a concentration, water temperature, and HNF abundance (df = 3, 45; F = 16.2; P < 0.01), suggesting that both substrate limitation and grazing loss by HNF were important factors controlling bacterial abundance in the lake.

Seston available as a food resource for the ribbed mussel ( Geukensia demissa) in a North American, mid-Atlantic saltmarsh

We studied the composition of the <25 μm seston size fraction as a food resource potentially available to suspension feeding ribbed mussels, Geukensia demissa, over an annual cycle in Canary Creek saltmarsh, Delaware Bay. There were significant seasonal variations in the concentration of particulate organic carbon (POC), particulate organic nitrogen (PON), and total carbohydrate, but not cellulose. The concentration of cellulose, measured by hydrolytic cellulase enzyme assay, was relatively low (seasonal range 24 to 35 μg l −1) and only comprised from 3% of total carbohydrate in May 1996 to 13% in November 1995. We used the biomass of microalgae, estimated from chlorophyll a, and abundance of free-living bacteria and heterotrophic nanoflagellates to calculate each component's equivalent carbon content. Microalgae were the most dominant carbon source (62% annually) among the four identified components (phytoplankton, bacteria, heterotrophic nanoflagellates, and cellulose) in all seasons except in August 1995 when carbon from bacteria was most abundant (55%). The annual average carbon equivalents of heterotrophic nanoflagellates and cellulose were relatively small (2 and 4%, respectively). The total concentration of POC in the seston was much greater than the carbon derived from the four identified components. The proportions that these identified components contributed to POC varied seasonally and combined only accounted for 8–24% of POC. Based on these estimates, the bulk of the POC in Canary Creek marsh was not associated with any of the four components we identified. We suggest that this uncharacterized material was some type of non-lignocellulosic, amorphous detritus of unknown utility as a food resource for ribbed mussels.

Comparing the effects of resource enrichment and grazing on a bacterioplankton community of a meso-eutrophic reservoir

We examined changes in a microbial community in reaction to shifts in predation pressure and resources in a meso-eutrophic reservoir. Manipulations consisted of size fractionation and either in situ incubation or transplantation from the dam area to the river area, which differed in P-availability. Three treatments, in which samples were incubated for 96 h in dialysis bags, were used: a bacterivore-free (<0.8 µm) treatment, an enhanced flagellate grazing (<5 µm) treatment and a control treatment (all bacterivores present). We monitored bacterial abundance, mean cell volume (MCV), bacterial production, protistan grazing, as well as bacterial community composition (BCC) using fluorescence in situ hybridisation (FISH) with oligonucleotide probes with different levels of specificity. Populations transplanted to the relatively P-rich river area showed a 3- to 6-fold increases in bacterial and heterotrophic nanoflagellates (HNF) abundance and bacterivory compared to the corresponding treatments incubated at the dam area. In the transplanted community, nearly all phylogenetic groups of bacteria distinguished showed increased growth rates, even in the top-down manipulation treatments of increased bacterivory. In contrast, at the more resource-limited dam, the top-down manipulations induced significant changes in bacterial community composition. Thus, we found that BCC changes with predation in resource-limited bacteria but that predation plays a minor role when resource limitation is relaxed.

Ecological stoichiometry in the microbial food web: a test of the light:nutrient hypothesis

The 'light:nutrient hypothesis' (LNH) states that the importance of the microbial food web relative to grazing impacts by macrozooplankton and the nature of the relationship between algae and bacteria (competition or commensalism) are affected by the balance of light and nutrients experienced by phytoplankton. We tested this hypothesis in field enclosures by manipulating irradi- ance and nutrient supply in a P-limited lake in Ontario, Canada. Shading and P-enrichment had little effect on standing biomass of small suspended particles (<1 µm) but both decreased C:P ratio in this size fraction. P-fertilization had no effect on algal biomass or bacterial abundance but shading signif- icantly lowered algal biomass and the ratio of algal biomass to bacterial abundance. Shading had no effect on heterotrophic nanoflagellates (HNF) but HNF abundance declined strongly with P-enrich- ment under both shaded and unshaded conditions, coincident with large increases in macrozoo- plankton biomass that accompanied P-fertilization. Shading and nutrient enrichment also affected resource limitation as indicated by dilution bioassays. Increased algal light limitation in the enclo- sures was associated with lower seston C:P ratio, as expected under the LNH. In addition, the C:P ratio of new seston produced in bioassay bottles was also affected by enclosure treatment; unshaded enclosures generally produced seston with moderately high C:P ratio while shaded enclosures pro- duced new seston at low C:P under low ambient light but at very high C:P ratio when algae were incubated at higher light. Our data provide support for some aspects of the LNH, including predicted impacts on the microbial loop and on resource limitations and C:P stoichiometry. However, some responses were not consistent with the LNH, indicating that the hypothesis needs to be modified to incorporate potential indirect effects.

Growth and production of free-living heterotrophic nanoflagellates in a eutrophic lake - Lake Donghu, Wuhan, China

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (-0.37-1.25 d(-1)), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5 degreesC, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from -2.25 to 35.45 mg l(-1) d(-1) with mean of 3.17 mg l(-1) d(-1). When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.

The effect of small zooplankton on the microbial loop and edible algae during a cyanobacterial bloom

SUMMARY 1. We studied the effect of the small crustacean zooplankton on heterotrophic micro‐organisms and edible phytoplankton in a eutrophic lake during a cyanobacterial bloom.2. Small (15 L) enclosures were filled with natural or screened (100 μm) lake water and incubated for 5 days in the lake. Screening removed crustacean zooplankton but the initial density of rotifers and phytoplankton remained the same in control and removal treatments. Changes in the abundance and biomass of bacteria, autotrophic picoplankton (APP), heterotrophic nanoflagellates (HNF) and ciliates were measured daily.3. The crustacean zooplankton, dominated by the small cladoceran Chydorus sphaericus, did not affect cyanobacteria, the main phytoplankton group during the experiment.4. The removal of the crustacean zooplankton induced a higher abundance of ciliates and reduced that of the HNF, indicating the importance of ciliates in controlling HNF in this system.

Top‐down effects on pelagic heterotrophic nanoflagellates (HNF) in a large river (River Rhine): do losses to the benthos play a role?

1. The abundance, production and control of pelagic heterotrophic nanoflagellates (HNF) in the Lower River Rhine (Germany) were investigated. Field samples (live‐counting technique) were taken at least every 2 weeks at Cologne (km 685) over a period of 20 months. In addition, Lagrangian sampling was carried out 160 km downstream of Cologne (Kalkar–Grieth, km 845) over a period of 12 months. Potential HNF growth rates and loss rates caused by planktonic predators were estimated in the laboratory (size fractionation experiments) and compared with the changes in HNF‐density in a water parcel flowing downstream.2. Mean abundance (±SD) ranged from 7 ± 6 to 4890 ± 560 individuals mL–1 and was positively correlated with discharge. Heterotrophic nanoflagellates abundance increased up to 30‐fold during flood events, whereas there were only minor changes in the taxonomic composition.3. HNF growth rate ranged from 0.16 ± 0.12 to 1.98 ± 0.10 day–1. Between 0 and 95% (mean: 32%) of the gross production was lost to planktonic predators; a larger portion between 0 and 195% (mean: 64%) of the HNF gross production was lost by other means.4. There may be an important role for benthic predators in the control of pelagic HNF. First, production of HNF was high and grazing losses to planktonic predators low at times when HNF abundance was low. Secondly, high in situ loss rates (not explicable by planktonic predators), which were positively related to temperature, indicate the importance of biotic interactions. Thirdly, the dependence of HNF abundance on discharge indicated a decrease grazing intensity with rising water levels (increase in water volume/colonised river bed ratio).5. The impact of discharge on planktonic HNF mediated by the grazing impact of benthic predators was modelled, showing a good fit with the field data.

Water discharge-regulated bacteria/heterotrophic nanoflagellate (HNF) interactions in the water column of the river Rhine.

Heterotrophic nanoflagellates (HNF) make up a large fraction of the zooplankton biomass of rivers. Their abundance can be strongly affected by water discharge, but the consequences of this highly dynamic factor for their main prey, the bacteria, is still unknown. The focus of this study was on bacterial/HNF interactions in the Lower River Rhine (Germany) with respect to the discharge-dependent dynamics. The bacterial and HNF abundances and biomasses were determined over the course of 17 months. The potential consumption of bacteria by HNF was calculated based on the biomass data and on data on the HNF production. The mean bacterial abundance in the Rhine at Cologne ranged from 0.3 x 10(6) to 3.5 x 10(6) cells mL(-1), with lowest abundances in winter and highest in late spring. No significant changes in abundance during the downstream passage were found. Neither could a significant correlation be found between bacterial and HNF abundance. The ratio of bacterial to HNF abundance showed high variations which lay between 166 and 19,055 and was negatively dependent on water discharge. Monthly routine calculations on the potential bacterial consumption by HNF revealed a clearance of between 2 and 82% of the bacterial standing stock d(-1). The values increased greatly with water discharge and could exceed 100% d(-1) at times of high water flow. The presented data suggests a change in the top-down control of the planktonic bacteria due to the water discharge: The importance of benthic predation at low water flow (high contact probability to benthic predators) gives way to an increased importance in predation by planktonic HNF at high water flow.