Seasonal dynamics of free and attached heterotrophic nanoflagellates in an oligomesotrophic lake

1. The seasonal development of heterotrophic nanoflagellates (HNF), bacteria, rotiferans and crustacean zooplankton was studied in the epilimnion of Lake Pavin, an oligomesotrophic lake in the Massif Central of France.
2. HNF abundance varied from 0.1 to 2.5 × 10³ mL^–1. Free-living HNF reached their highest density in spring when the copepod Acanthodiaptomus denticornis dominated the metazooplankton. They were present in low numbers when rotifers and cladocerans were numerous.
3. Attached HNF, consisting of bicoecids and choanoflagellates, were fixed to large diatoms and to the colonial cyanobacterium Anabaena flos-aquae . The abundance of attached HNF was significantly correlated to bacterial abundance, which fluctuated between 1.1 and 2.7 × 10⁶ mL^–1. Highest abundance of these epiphytic protists was recorded when free-living heterotrophic nanoflagellates declined.
4. The comparison of the dynamics of heterotrophic nanoflagellates, bacteria, and the impact of zooplankton grazing suggested that prey abundance, the presence of suitable attachment sites and limited competition from the free-living forms were the main factors controlling the development of the epiphytic flagellate protists. In contrast, the low abundance of free-living forms during the period of rotiferan and cladoceran development suggests the prevalence of a top-down control by predation of the metazoopankton.     

Trophic interactions among heterotrophic microplankton,nanoplankton, and bacteria in Lake Constance

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 µm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics.Clearance rates varied between 0.2 and 122.8 nl HNF^–1 h^–1 and between 0.2 and 53.6 µl ciliate^–1 h^–1, respectively.Ingestion and clearance rates measured for HNF and ciliates are in good agreement with results obtained by other investigators from different aquatic environments and from laboratory cultures. Both the abundance of all three major microheterotrophic categories — bacteria, HNF, and ciliates — and the grazing pressure within the microbial loop show pronounced seasonal variations.     

The microbial food web structure of a hypertrophic warm-temperate shallow lake, as affected by contrasting zooplankton assemblages

The composition of zooplankton is known to affect the structure of the microbial trophic web. The zooplankton of the hypertrophic Laguna Chascomús (Argentina) is generally dominated by rotifers and cyclopoids copepods. An unusual dominance by small-cladocerans was observed after a massive winter fish kill in 2007. We hypothesized that small-cladocerans would increase the grazing pressure on heterotrophic flagellates (HF), reducing the degree of coupling between HF and picoplankton. The aim of this study was to investigate the microbial food web structure under two contrasting zooplankton assemblages. The lake was sampled every other week between 2007 and 2009. The abundances of heterotrophic bacteria (HB) and picocyanobacteria (Pcy) laid among the highest values reported for aquatic systems (>10⁸ and 10⁷ cells ml^?1, respectively). Pcy averaged 53% of total picoplanktonic biomass. When small-cladocerans dominated zooplankton HF reached the higher abundance (>10⁵ cells ml^?1) and picoplankton showed the opposite pattern, while the proportion of grazing resistant morphologies (i.e. microaggregates of Pcy) was higher. In contrast, when rotifers dominated, HF abundance decreased and picoplankton increased. Our data suggest that the degree of HF–HB coupling was affected by changes in zooplankton dominance. In contrast to our initial hypothesis, the present results suggest that large numbers of rotifers (>5,000 ind. l^?1) are more efficient than small-cladocerans at controlling HF populations.     

Relationship between bacteria, phytoplankton and heterotrophic nanoflagellates along the trophic gradient

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well as bacterial production and chlorophylla levels, were measured at five sites extending from the coastal zone toward the open Adriatic in the period from March to October 1995. The investigated areas were grouped into trophic categories according to concentrations of chlorophylla. All the biotic-para-meters increased along the trophic gradient, leading to eutrophy, but they did not increase at the same rate. The bacterial biomass: phytoplankton biomass (BB: chla) ratio decreased from about 10 in the very oligotrophic area to 0.8 at the eutrophic site. In contrast, the bacterial abundance: HNF abundance ratio (B: HNF) increased from 1000 bacteria per 1 flagellate in the oligotrophic system to 1700 bacteria flagellate⁴ in the eutrophic area. Decreasing BB: chla and increasing B: HNF ratios along the trophic gradient might reflect the different structures of the microbial food web. Relationships between bacterial abundance and production, and chla and HNF showed that bacterial abundance along the trophic gradient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacterial production and chla than in the eutrophic system, suggesting stronger control of bacterial abundance by substrate supply. On the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial abundance and production in the eutrophic system, showed that the importance of bacteriovory increased in richer systems.     

Possible food chain relationships between bacterioplankton,protozoans, and cladocerans in a reservoir

Ingestion of fluorescent particles by natural protozoan assemblage was studied in the Řimov Reservoir (Southern Bohemia) from April to October, 1987. Attached and free-living bacterial abundance, proportion of active bacteria, density of suspended particles and biomass of cladocerans were also monitored. Heterotrophic nanoflagellates (HNF; 5–12.8 10²ml⁻¹) were the dominant bacterial micrograzers during the spring period and consumed 3 to 9% of the total bacteria per day. After the spring phytoplankton bloom maximum densities of suspended particles and attached bacteria (up to 28% of the total counts) were found. Development of cladocerans in May sharply decreased the proportion of attached bacteria and kept them below 5% of the total counts. All the studied components of plankton except Cladocera decreased during the clearwater phase. The most significant drop was observed in the numbers of protozoans, and they were negligible for bacterial elimination. Bacterial losses during that time apparently were due to cladoceran grazing. During the summer period, ciliates (15–142 ml⁻¹) were mostly dominant micrograzers, and protozoan community grazing increased up to 21% of bacterial standing stock per day. The proportion of active bacteria was strongly correlated with protozoan grazing (r=0.83).     

Bacterivory of metazooplankton, ciliates and flagellates in a newly flooded reservoir

Bacterial consumption by metazoan zooplankton and phagotrophic protists was measured in situ during the period of thermal stratification in the epilimnion (1 m) and metalimnion (7 m) of a newly flooded reservoir (Sep reservoir, France). The mean bacterial consumption was 2.53 x 10⁶ bacterial l^-1 h^-1 at 1m and 0.97 x 10⁶ bacteria l^-1 h^-1 at 7m. The main consumers over the whole study period were the cladocerans Daphnia longispina and Ceriodaphnia quadrangula, accounting on average for 72% of the potential total predation of bacteria at 1 m and 56% at m, especially during the months of May-June and August. Heterotrophic nanoflagellates (HNF), which accounted for 12% estimated total predation of bacteria at 1 m and 13% at 7m, only exerted a limited predation, mainly by a Monas-type cell. Ciliates, dominated in terms of abundance by Pelagohalteria viridis, accounted for 4% of total predation in the epilimnion (0.00-0.42 x 10⁶ bacteria l^-1 h^-1). In a newly flooded reservoir, metazoan zooplankton appear to be the main consumers of bacteria. Predation of ciliates and HNF by zooplanktonic crustaceans could account for the low contribution of components of the microbial loop to bacterial consumption.      

Contrasting effects of a cladoceran (Daphnia galeata) and a calanoid copepod (Eodiaptomus japonicus) on algal and microbial plankton in a Japanese lake, Lake Biwa

Macrozooplankton may affect algal and microbial plankton directly through grazing or predation and indirectly through nutrient regeneration. They may also affect potential prey positively by removing alternative predators. Here, we examined the effects of a cladoceran (Daphnia) and a calanoid copepod (Eodiaptomus) on algal and microbial plankton in a Japanese lake using in situ experiments in which we manipulated the nutrient supply and biomass of these macrozooplankton. The response of algal and microbial plankton to macrozooplankton was diverse and varied depending on the level of nutrient supply. Eodiaptomus seemed to feed mainly on large algae (>20 µm) and microzooplankton, while direct grazing by Daphnia on algae, bacteria, heterotrophic nanoflagellates (HNF), and microzooplankton (ciliates, heliozoa, and rotifers) was pronounced. Trophic linkages within these microbial plankton was also suggested; bacteria were grazed by HNF and these in turn were grazed by microzooplankton. When the nutrient supply was high, both HNF and microzooplankton were exposed to higher amounts of algae and lower bacterial abundance. Moreover, nutrient regeneration by daphnids and Eodiaptomus copepods seemed to differentially stimulate the growth of algae and bacteria. The results suggest that the relationship between macrozooplankton and microbial plankton cannot be fully understood without taking into consideration not only the feeding characteristics of the macrozooplankton, but also the food web structure, the subsidized algal resource, and nutrient regeneration from the macrozooplankton.     

Factors influencing the phytoplankton steady state assemblages in a drinking-water reservoir (Ömerli reservoir, Istanbul)

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°C, 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^–1d^–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.     

Dynamics and trophic roles of heterotrophic protists in the plankton of a freshwater tidal estuary

Freshwater tidal estuaries comprise the most upstream reaches of estuaries and are often characterised by the presence of dense bacterial and algal populations which provide a large food source for bacterivorous and algivorous protists. In 1996, the protistan community in the freshwater tidal reaches of the Schelde estuary was monitored to evaluate whether these high food levels are reflected in a similarly high heterotrophic protistan biomass. Protistan distribution patterns were compared to those of metazoan zooplankton to evaluate the possible role of top-down regulation of protists by metazoans. Apart from the algivorous sarcodine Asterocaelum, which reached high densities in summer, heterotrophic protistan biomass was dominated by ciliates and, second in importance, heterotrophic nanoflagellates (HNAN). HNAN abundance was low (annual average 2490 cells ml^–1) and did not display large seasonal variation. It is hypothesised that HNAN were top-down controlled by oligotrich ciliates throughout the year and by rotifers in summer. Ciliate abundance was generally relatively high (annual average 65 cells ml^–1) and peaked in winter (maximum 450 cells ml^–1). The decline of ciliate populations in summer was ascribed to grazing by rotifers, which developed dense populations in that season. In winter, ciliate populations were probably regulated `internally' by carnivorous ciliates (haptorids and Suctoria). Our observations suggest that, in this type of productive ecosystems, the microbial food web is mainly top-down controlled rather than regulated by food availability.     

Ciliates are the Dominant Grazers on Pico- and Nanoplankton in a Shallow,Naturally Highly Eutrophic Lake

Abundance and biomass of the microbial loop members [bacteria, heterotrophic nanoflagellates (HNF), and ciliates] were seasonally measured in the naturally eutrophic and shallow (2.8 mean depth) Lake Võrtsjärv, which has a large open surface area (average 270 km²) and highly turbid water (Secchi depth <1 m). Grazing rates (filter feeding rates) on 0.5-, 3-, and 6-μm-diameter particles were measured to estimate pico- and nanoplankton grazing (filter feeding) by micro- and metazooplankton. Among grazers, HNF had a low abundance (<50 cells mL^?1) and, due to their low specific filtering rates, they only grazed a minor fraction of the bacterioplankton (≤4.2% of total grazing). Ciliates were relatively abundant (≤158 cells mL^?1) and, considering their high specific feeding rates, were able to graze more than 100% of the bacterial biomass production in the open part of the lake, whereas the average daily grazing accounted for 9.3% of the bacterial standing stock. Ciliates were potentially important grazers of nanoplanktonic organisms (on average, approximately 20% of the standing stock of 3-μm-size particles was grazed daily). Metazooplankton grazed a minor part of the bacterioplankton, accounting for only 0.1% of standing stock of bacteria. Grazing on nanoplankton (3–6 μm) by metazooplankton was higher (0.4% of standing stock). The hypothesis is proposed that ciliates dominate due to a lack of top–down regulation by predators, and HNF have a low abundance due to strong grazing pressure by ciliates.     

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. Received: June 21, 2002 / Accepted: October 16, 2002 Present address: 5-2-2-18-805 Kikusui-motomachi, Shiroishi, Sapporo 003-0825, Japan Present address: School of Environmental Science, University of Shiga Prefecture, 2500 Hassaka-cho, Hikone 522-8533, Japan Tel. +81-749-28-8307; Fax +81-749-28-8463 e-mail: ban@ses.usp.ac.jp Acknowledgments We thank Dr. H. Ueda for encouraging this study and Mr. Haruna and the members of the Plankton Laboratory, Hokkaido University, for their help in sampling. We also thank two anonymous reviewers for their critical comments. Correspondence to:S. Ban     

Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

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.     

The annual cycle of heterotrophic freshwater nanoflagellates: role of bottom-up versus top-down control

The population dynamics of heterotrophic nanoflagellates (HNF)were analyzed in pre-alpine Lake Constance over three consecutiveyears. A recurrent seasonal pattern led to the identificationof five seasonal phases: winter, spring, clear-water, summerand autumn. HNF biomass was lowest in winter and highest m latespring several weeks after the phytoplankton spring bloom. Theaverage biomass of HNF was 5–12% of bacterial biomassand 13–34% of ciliate biomass respectively. The largestHNF cells were recorded during the spring phase, whereas theaverage cell size was reduced to one-third during the subsequentclear-water phase. The pronounced differences in the mean cellsize were attributed mainly to varying grazing impact on HNFThroughout most of the year, HNF production was balanced bygrazing of microzooplankton, namely ciliates, within the microbialloop. During the dear-water phase, however, the grazing impactwas mainly due to rotifers and daphnids. Changing grazing impactwas primarily responsible for the observed 2-fold interannualdifference m the mean biomass of HNF Overall, top-down controlby grazing was more important in governing the population dynamicsof HNF than bottom-up control by bacterial food supply.     

Cascading predation effects of Daphnia and copepods on microbial food web components

1. We performed a mesocosm experiment to investigate the structuring and cascading effects of two predominant crustacean mesozooplankton groups on microbial food web components. The natural summer plankton community of a mesotrophic lake was exposed to density gradients of Daphnia and copepods. Regression analysis was used to reveal top–down impacts of mesozooplankton on protists and bacteria after days 9 and 15. 2. Selective grazing by copepods caused a clear trophic cascade via ciliates to nanoplankton. Medium‐sized (20–40 μm) ciliates (mainly Oligotrichida) were particularly negatively affected by copepods whereas nanociliates (mainly Prostomatida) became more abundant. Phototrophic and heterotrophic nanoflagellates increased significantly with increasing copepod biomass, which we interpret as an indirect response to reduced grazing pressure from the medium‐sized ciliates. 3. In Daphnia‐treatments, ciliates of all size classes as well as nanoflagellates were reduced directly but the overall predation effect became most strongly visible after 15 days at higher Daphnia biomass. 4. The response of bacterioplankton involved only modest changes in bacterial biomass and cell‐size distribution along the zooplankton gradients. Increasing zooplankton biomass resulted either in a reduction (with Daphnia) or in an increase (with copepods) of bacterial biovolume, activity and production. Patterns of bacterial diversity, as measured by polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE), showed no distinct grouping after 9 days, whereas a clear treatment‐coupled similarity clustering occurred after 15 days. 5. The experiment demonstrated that zooplankton‐mediated predatory interactions cascade down to the bacterial level, but also revealed that changes occurred rather slowly in this summer plankton community and were most pronounced with respect to bacterial activity and composition.     

Benthic Bacterial Production and Protozoan Predation in a Silty Freshwater Environment

The interrelation of heterotrophic bacteria with bacterivorous protists has been widely studied in pelagic environments, but data on benthic habitats, especially in freshwater systems, are still scarce. We present a seasonal study focusing on bacterivory by heterotrophic nanoflagellates (HNF) and ciliates in the silty sediment of a temperate macrophyte-dominated oxbow lake. From January 2001 to February 2002 we monitored the standing stock of bacteria and protozoa, bacterial secondary production (BSP, ³H-thymidine, and ¹⁴C-leucine incorporation), and grazing rates of HNF and ciliates on bacteria (FLB uptake) in the oxic sediment of the investigated system. BSP ranged from 470 to 4050 µg C L^–1 wet sediment h^–1. The bacterial compartment turned out to be highly dynamic, indicated by population doubling times (0.6–10.0 d), which were comparable to those in the water column of the investigated system. Yet, the control mechanisms acting upon the bacterial population led to a relative constancy of bacterial standing stock during a year. Ingestion rates of protozoan grazers were 0–20.0 bacteria HNF^–1 h^–1 and 0–97.6 bacteria ciliate^–1 h^–1. HNF and ciliates together cropped 0–14 (mean 4)% of BSP, indicating that they did not significantly contribute to benthic bacterial mortality during any period of the year. The low impact of protozoan grazing was due to the low numbers of HNF and ciliates in relation to bacteria (1.8–3.5 × 10⁴ bacteria HNF^–1, 0.9–3.1 × 10⁶ bacteria ciliate^–1). Thus, grazing by HNF and ciliates could be ruled out as a parameter regulating bacterial standing stock or production in the sediment of the investigated system, but the factors responsible for the limitation of benthic protistan densities and the fate of benthic BSP remained unclear.     

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

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

Role of bacteria as food for zooplankton in a eutrophic tropical pond (Ivory Coast)

The aim of the study was to determine whether bacteria could be a substantial source of carbon for zooplankton and whether the grazing pressure of these metazoan filter-feeders could influence the fate of bacterial production. Eight grazing experiments using natural bacteria labelled with³H thymidine were conducted in a tropical pond (Ivory Coast) during various phases of biological colonization (rotifer-dominated and copepod-dominated phases of the colonization). Higher grazing and clearance rates were observed with rotifers (Brachionus plicatilis andHexarthra intermedia), while very low values were obtained when the cyclopoid copepodApocyclops panamensis was dominant. Less than 1% of the bacterial production was harvested when copepods were dominant, whileB. plicatilis consumed up to 36% of this production. However, this consumption of bacteria appeared to contribute only to an insignificant proportion of the daily carbon intake (e.g. 0.9 to 7.1% of body carbon for rotifers). The low contribution of bacteria in the nutrition of zooplankton is discussed in terms of their cell size and their relative abundance in the total amount of seston available.     

Complex interactions and different possible pathways among functional components of the aquatic microbial world in Farasan Archipelago,Southern Red Sea,Saudi Arabia

This work aims to outline the dynamics of trophic links between the three main microbial components (bacteria, nanoflagellates, and ciliates) of the Farasan Archipelago in order to establish a baseline for future research in this area. The Farasan Archipelago lies along the southwestern coast of the Saudi Arabia, southern Red Sea between 16°20′–17°10′N and 41°30′–42°30′E and had been declared as marine and terrestrial reserve by the year 1996. Three different sites were chosen for this study, with each site visited bimonthly for 18 months from September 2016 to February 2018. Bacteria, nanoflagellates and ciliates were enumerated in order to explore the complex interactions between the main microbial categories in sea waters of the Farasan Archipelago. High abundances were recorded during the present study for bacteria (8.7 × 10⁶ bacteria ml⁻¹), nanoflagellates (3.7 × 10⁴ TNAN ml⁻¹) and ciliates (40.4 ciliates ml⁻¹). The paper discusses the various potential pathways controlling the complex interactions between these microbial groups in this part of the southern Red Sea. It is concluded that a linear trophic chain consisting of bacteria; heterotrophic nanoflagellates; filter feeding ciliates is a major route by which the production of bacteria is transferred to the higher consuming levels, thereby confirming the high importance of t bottom-up control (food supply), alongside top-down control (predation) in regulating bacterial abundances in the Farasan Archipelago. During the present investigation, each nanoflagellate ingested between 11 and 87 bacteria in one hour, while each ciliate consumed between 20 and 185 nanoflagellates every hour. These calculated grazing rates of protistan eukaryotes confirmed the role of heterotrophic nanoflagellates as the main consumers of bacteria, and the role of ciliates as the major control for the heterotrophic nanoflagellate population dynamics, and thus the top predators within the microbial plankton assemblage in the Farasan Archipelago.     

Changes in cell volume of bacteria and heterotrophic nanoflagellates in a hypereutrophic pond

Changes in cell volume of planktonic bacteria and heterotrophic nanoflagellates (HNF) were examined in a hypereutrophic pond from April to October, 1997. There were marked changes in the abundance of bacteria, HNF and ciliates and in protistan bacterivory during this period. The cell volume of free-living bacteria (0.121 ± 0.031 m³, mean ± SD) was large relative to that reported in the literature. The cell volumes of HNF was 71.1 ± 24.8 m³. Both cell volumes did not follow a seasonal trend. The dominant size class of bacteria was seasonally variable, whereas density of filamentous bacteria was relatively high between August and September. Biomass of filamentous bacteria accounted for up to 33.6% of total bacterial biomass. A correlation analysis for cell volume of bacteria and HNF, density of filamentous bacteria and some microbial variates was performed. The positive correlations detected (p<0.05) were between density of bacteria and cell volume of HNF, and between density of filamentous bacteria and cell volume of HNF.