Microbial Web Research Articles

Population structure and functional diversity of mycorrhizal fungi and Pseudomonas fluorescence in different growth forms of Dendrocalamus strictus

Bamboo, called as the poor man’s timber, is one of the fastest growing giant grass species. Having shallow root system and fast growth rate, the dependability of the plant on the rhizospheric microbial web cannot be denied. The study was conducted to explore the population and seasonal variations of indigenous mycorrhizal fungi as well as the functional diversity of plant growth promoting bacteria, especially fluorescent pseudomonas from the different growth forms of Dendrocalamus strictus, the most commonly present bamboo species in Indian sub-continent. In a past study, it was established that the growth forms of D. strictus which developed over time in their respective locations, were genetically varied. The present research further explores the variations in their respective rhizospheric microbes and looks for the role of plant selection phenomenon. Considerable variation in mycorrhizal structures and in the functional diversity of Pseudomonas fluorescence was registered. Nutrient deficient conditions, especially low soil phosphorous (P), maybe created a niche or condition that could promoted the occurrence of high numbers of P solubilizers which, in turn, boosted the mycorrhizal as well as bamboo growth.

From webs, loops, shunts, and pumps to microbial multitasking: Evolving concepts of marine microbial ecology, the mixoplankton paradigm, and implications for a future ocean

AbstractEmerging knowledge of mixoplankton—ubiquitous microbes that employ phototrophy and phagotrophy synergistically in one cell—reshapes our knowledge of the flow of materials and energy, with wide‐reaching impacts on marine productivity, biodiversity, and sustainability. Conceptual models of microbial interactions have evolved from food‐chains, where carbon‐fixing phytoplankton are conceived as being grazed solely by zooplankton that, in turn, support fisheries and higher trophic levels, to microbial webs, loops, and shunts, as knowledge about abundance, activity, and roles of marine microbial organisms—as well as the complexity of their interactions—has increased. In a future world, plankton that depend on a single strategy for acquiring nutrition (photo‐autotrophy or phago‐heterotrophy) may be disadvantaged with increasing temperatures and ocean acidification impacting vital rates, thermal stratification decreasing water column nutrient exchange, and anthropogenic pollution shifting amounts, forms, and proportions of nutrients. These conditions can lead to stoichiometric imbalances that may promote mixoplanktonic species with an increasing likelihood of harmful blooms. Such changes in plankton species composition alters the interconnectivity of oceanic microbes with direct consequences on biogeochemical cycling, trophic dynamics, and ecosystem services. Here, the implications of the mixoplankton paradigm relative to traditional concepts of microbial oceanography in a globally‐changing, anthropogenically‐impacted world are explored.

High salinity gradients and intermediate spatial scales shaped similar biogeographical and co-occurrence patterns of microeukaryotes in a tropical freshwater-saltwater ecosystem.

Microeukaryotes play key ecological roles in the microbial web of aquatic ecosystems. However, large knowledge gaps urgently need to be filled regarding the biogeography with associated shaping mechanisms and co-occurrence patterns of microeukaryotes under freshwater-saltwater gradients, especially true in tropical regions. Here, we investigated microeukaryotes of six mixed freshwater-saltwater regions in the Pearl River Estuary and surrounding coasts in southern China, with salinity ranging 0.1-32.0% and distances spanned up to 500 km, using molecular ecological methods. Results indicate that the biogeography of abundant and rare microeukaryotic communities was similar, both their co-occurrence patterns and biogeographical patterns were driven by deterministic and stochastic processes. The environmental factors with higher selective pressure than dispersal limitation meant that the role of deterministic process in structuring communities was more significant than that of stochastic process, and salinity played important role in structuring both microeukaryotic communities and networks. The abundant communities had stronger influence on entire microeukaryotic communities and seemed to be more sensitive to environmental changes than their rare counterparts, while rare ones had stronger interspecific relationships. Finally, the geographic scale and environmental gradients of study regions should firstly be clarified in future research on the ecological processes of microeukaryotes before conclusions are drawn.

Centennial glacier retreat increases sedimentation and eutrophication in Subantarctic periglacial lakes: A study case of Lake Uruguay

High resolution XRF scanning documented inter-annual paleolimnological changes of a Subantarctic periglacial lake, during a process of centennial glacier retreat in King George Island, Antarctica. Two major paleoenvironmental stages were inferred from the combined analysis of elemental, molecular and isotopic biomarkers, with a boundary or transition set at about 3200 yr BP. The first stage was characterized by a relatively low allochthonous organic content, reduced productivity and nitrogen levels. Such paleoenvironmental conditions are interpreted as a terrestrial system under periglacial influence, where material influx was related to erosion process from the melt water discharge, because of the proximity to the Collins Glacier ice cap. After the major Holocene glacier advance dated at about 3500 yr BP, the ice cap retreat led to the formation of Lake Uruguay, which involved in filling processes leading to moraine deposits, proglacial meltwater channels, and lakes next to the land glacier. During the second stage, with the onset of the Current Warm Period, prior to 1900 CE the stabilization of the Zr/Rb ratio within the laminated sediments documented the origin of the lacustrine sedimentation system, with subsequent increases in the sedimentation rate and biomass content (total nitrogen and organic carbon). Time series analyses revealed that the lake displayed variability cycles related to El Niño Southern Oscillation (ENSO), as reflected by high resolution sedimentological proxies for grain size, weathering, allochthonous inputs from the watershed, increase of biomass and productivity, and changes in redox conditions, all of which displayed similar oscillation cycles from 2 to 6 yr. During this periglacial recession and associated eutrophication process, we detected a striking loss in both bacterial specific richness and diversity as inferred from preliminary selected ancient DNA analyses. Thus, the Antarctic warming scenario leading to glacier depletion appears to exert deterioration consequences on the Subantarctic microbial web.

Effects of the exotic rotifer Kellicottia bostoniensis (Rousselet, 1908) on the microbial food web components

Species introduction can alter the structure and dynamics of biological communities and, therefore, understanding their feeding behavior and the effects that an exotic species can cause in the food web configuration is pivotal. We aimed to experimentally investigate the effects of the potential invasive species Kellicottia bostoniensis on different components of the microbial food web and to evaluate if the food preferences of this species change under different conditions of resource availability and interspecific interactions. We tested the hypothesis that the presence of K. bostoniensis would have direct and indirect effects on the different components of the planktonic food web. We designed three different assays (E) using prey size fractionation, being E1 composed of bacteria (HB) and picophytoplankton (PPP), E2 composed of HB, PPP and autotrophic and heterotrophic flagellates and, E3 with the whole planktonic community. Each one composed of a control in the absence of K. bostoniensis and a treatment with the presence of this species. Results showed that K. bostoniensis caused direct effects on its main food items, the heterotrophic and autotrophic flagellates, whereas no evidence of indirect effects was observed on the base components of the microbial web food, such as heterotrophic bacteria and picophytoplankton. In addition, a negative effect of the exotic rotifer on ciliates was observed. Finally, we emphasize that the impact of K. bostoniensis on aquatic ecosystem may be quite harmful, since this specie can act as a sink of matter and energy to higher trophic levels.

Biotic Energy Flow Passing through the Microbial Trophic Web in the Pelagic Zone of Lake Ecosystems

An algorithm of a dynamic mass-balance model emulating the biotic energy flow in a planktonic community with the involvement of microbial organisms has been developed. The abiotic input parameters of the model are the latitude, mean depth of the water body, total phosphorus content, and water color. The model makes it possible to forecast the biomass and production of key plankton groups. It is shown that the food-base production for fish and their growth are determined by the degree of protozoan plankton involvement in the trophic web.

The trophic role and impact of plankton ciliates in the microbial web structure of a tropical polymictic lake dominated by filamentous cyanobacteria

The recent interest in the plankton structures and dynamics in tropical and subtropical lakes has revealed important trends that set these lakes apart from temperate lakes, and one of the main differences is the enhanced importance of the microbial food web with respect to net plankton. Ciliates are a key component of subtropical and tropical microbial webs because of their role as dominant picoplankton grazers and their ability to channel picoplankton production to the uppermost trophic levels. Plankton ciliates have been found to play a crucial role in the survival of fish larvae in lakes that share several features with Lake Catemaco, a eutrophic tropical Mexican lake. Therefore, the plankton ciliate composition, abundance, and biomass of Lake Catemaco were studied to assess their role in the microbial food web. The data were obtained from surface and bottom water samples collected at eleven points during three surveys in 2011 and an additional survey in 2013, with the surveys covering the local climatic seasons. The most abundant components of the plankton ciliate assemblages were small prostomatids (<em>Urotricha </em>spp.), choreotrichs (<em>Rimostrombidium </em>spp.), cyclotrichs (<em>Mesodinium </em>and<em> Askenasia</em>), and scuticociliates (<em>Cyclidium, Cinetochilum, Pleuronema, </em>and <em>Uronema</em>). Other important ciliates in terms of abundance and/or biomass were haptorids (<em>Actinobolina, Belonophrya, Monodinium, Paradileptus, </em>and <em>Laginophrya</em>), <em>Halteria</em>, oligotrichs (<em>Limnostrombidium</em> and<em> Pelagostrombidium</em>), <em>Linostomella, Bursaridium, Cyrtolophosis,</em> and<em> Litonotus</em>. The ciliate abundance averaged 57 cells mL<sup>-1</sup> and ranged from 14 to 113 cells mL<sup>-1</sup>. The mean ciliate biomass was 71 µg C L<sup>-1</sup> and ranged from 10 to 202 µg C L<sup>-1</sup>. Differences were not detected in ciliate abundance or biomass between the sampling points or sampling depths (surface to bottom); however, significant differences were observed between seasons for both variables. Nano-sized filamentous cyanobacteria were the most abundant component of the plankton, and their abundance was assessed through epifluorescence microscopy counts. The autotrophic and heterotrophic picoplankton abundance was measured through epifluorescence, and their abundance and biomass were higher at the study site relative to other shallow freshwater ecosystems. The total ciliate biomass distribution patterns were similar to those of filamentous cyanobacteria and autotrophic or heterotrophic picoplankton, although the nanociliate biomasses peaked when the picoplankton and filamentous cyanobacteria were least abundant. The consequences of this increased importance of ciliates on the structure of the plankton at Lake Catemaco will be discussed along with the probable causes.

Effects of fish farming on microbial planktonic communities in the middle Adriatic sea

The effects of farming on planktonic microbial communities were investigated at the coastal sea bass/sea bream farm in the oligotrophic middle Adriatic Sea. Analyses of nutrients, chlorophyll a, cyanobacteria (Prochlorococcus), heterotrophic bacteria and nanoflagellates (both pigmented and heterotrophic) were carried out on samples taken during six seasonal cruises at different water depths. Farming activity increased the natural concentrations of some nutrients (ammonium, nitrite, phosphate, dissolved inorganic nitrogen) and changed their seasonal pattern. The values still remained typical for oligotrophic environ-ments demonstrating no risk for eutrophication. Enhanced nutrient supply provoked an immediate increase in abundances of both autotrophic and heterotrophic microbial groups (except pigmented nanoflagellates) and chlorophyll a. The effect of farming was more visible for the heterotrophic microbial component. Results from this study reveal a potential common pattern of microbial response to farming in the oligotrophic environments. It also suggests the importance of heterotrophic microbial web in transferring the matter and energy released from the fish farms in nutrient poor environments.

Constructing end-to-end models using ECOPATH data

The wide availability of ECOPATH data sets provides a valuable resource for the comparative analysis of marine ecosystems. We show how to derive a bottom–up transform from the top–down ECOPATH; couple this to a simple microbial web with physical forcing; and use the end-to-end model (E2E) for scenario construction. This steady state format also provides a framework and initial conditions for different dynamic simulations. This model can be applied to shelf ecosystems with a wide range of physical forcing, coupled benthic/pelagic food webs, and nutrient recycling. We illustrate the general application and the specific problems by transforming an ECOPATH model for the Northern California Current (NCC). We adapt results on the upwelling regime to provide estimates of physical fluxes and use these to show the consequences of different upwelling rates combined with variable retention mechanism for plankton, for the productivity of fish and other top predators; and for the resilience of the ecosystem. Finally we show how the effects of inter-annual to decadal variations in upwelling on fishery yields can be studied using dynamic simulations with different prey–predator relations. The general conclusion is that the nature of the physical regimes for shelf ecosystems cannot be ignored in comparing end-to-end representations of these food webs.

High feeding rates on large particles by Neocalanus flemingeri and N. plumchrus, and consequences for phytoplankton community structure in the subarctic Pacific Ocean

The open subarctic Pacific Ocean is a high nitrate low chlorophyll (HNLC) system characterized by low concentrations of phytoplankton, a community dominated by small cells, and iron-limited growth of, especially, the larger phytoplankton. In such systems the main energy and material flow is through the microbial web, with large copepods considered primarily to be grazers on the larger microzooplankton occupying the top of this web. Consistent with this is the recognition that much of the nutrition of the dominant copepods in this system, Neocalanus flemingeri, N. plumchrus and N. cristatus, is derived from microzooplankton. Also, these copepods consume only a small fraction of the total phytoplankton production. In this paper, we show that the contribution made by N. flemingeri and N. plumchrus to establishing and maintaining the community structure of this ecosystem should be re-evaluated. Our experiments indicate these grazers have high clearance rates on large particles, including both large phytoplankton and microzooplankton, and this selective removal contributes to establishment and maintenance of the observed foodweb structure in the Gulf of Alaska. These high feeding rates combined with large populations of these two Neocalanus species concentrated in the upper layer of the ocean, result in population-based feeding rates approximately equal to the growth rates of large phytoplankton under iron-limited conditions. We conclude that N. flemingeri and N. plumchrus populations (a) directly prevent the accumulation of large phytoplankton cells by selectively feeding on them at high rates, and (b) indirectly stimulate the accumulation of the smaller phytoplankton by consumption of their major grazers, the microzooplankton.

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.

Easy Visualization of the Protist Oxyrrhis marina Grazing on a Live Fluorescently Labelled Heterotrophic Nanoflagellate

Planktonic heterotrophic flagellates are ubiquitous eukaryotic microorganisms that play a crucial role in carbon and nutrient fluxes through pelagic food webs. Here we illustrate for the first time a grazing model of planktonic dinoflagellate, Oxyrrhis marina, on the heterotrophic nanoflagellate Goniomonas amphinema, using the DNA-binding fluorescent dye Hoechst 33342. A solution of 1 microg/mL of the fluorochrome allowed viability of the prey for at least 48 hours, provided low fluorescence quenching, and labelled the flagellate without masking the cytoplasm. After 2 hours of contact between the fluorescent prey and the predator, O. marina population had preyed on live G. amphinema at an ingestion rate of 2.2 prey Oxyrrhis (-1) h(-1). Results show that this model is a time-effective and inexpensive approach for the direct observation of heterotrophic flagellate grazing. The fact that prey remain alive while predation occurs, as well as the low rate of quenching, could be of help in studying the fate of real-time trophic interactions between protists in microbial webs.

Microbial food web contributions to bottom water hypoxia in the northern Gulf of Mexico

Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ food web on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically >15,000 km 2 since 1993. The microbial food web is also active in the Mississippi River plume, but consists of small organisms that sink slowly. This ‘recycling’ food web has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5–15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer. In this paper we present results from two studies examining the significance of this pathway. In both 2002 and 2004, we observed high production rates of appendicularians in coastal waters. Discarded gelatinous houses and fecal pellets from the appendicularian populations often provided more than 1 g m −2 d −1 of organic carbon for the establishment and maintenance of hypoxia in the northern Gulf of Mexico. This source of organic matter flux is especially important in regions far from the river plumes and during periods of low river discharge. Autotrophic elements of this food web are primarily supported by recycled inorganic nutrients originating in the Mississippi and Atchafalaya Rivers. Sources of dissolved organic matter (DOM) supporting the heterotrophic components of this microbial food web may include in situ production, the Mississippi/Atchafalaya Rivers, and Louisiana's coastal wetlands. If significant, the latter source provides a possible link between Louisiana's high rates of coastal land loss and the large hypoxic zone observed along the coast during summer. Both of the latter DOM sources are independent of phytoplankton production stimulated by inputs of riverine inorganic nutrients.

The planktonic food web of the Bizerte lagoon (south-western Mediterranean) during summer: I. Spatial distribution under different anthropogenic pressures

The structure and the trophic interactions of the planktonic food web were investigated during summer 2004 in a coastal lagoon of south-western Mediterranean Sea. Biomasses of planktonic components as well as bacterial and phytoplankton production and grazing by microzooplankton were quantified at four stations (MA, MB, MJ and R) inside the lagoon. Station MA was impacted by urban discharge, station MB was influenced by industrial activity, station MJ was located in a shellfish farming sector, while station R represented the lagoon central area. Biomasses and production rates of bacteria (7–33 mg C m −3; 17.5–35 mg C m −3 d −1) and phytoplankton (80–299 mg C m −3; 34–210 mg C m −3 d −1) showed high values at station MJ, where substantial concentrations of nutrients (NO 3 − and Si(OH) 4) were found. Microphytoplankton, which dominated the total algal biomass and production (>82%), were characterized by the proliferation of several chain-forming diatoms. Microzooplankton was mainly composed of dinoflagellates ( Torodinium, Protoperidinium and Dinophysis) and aloricate ( Lohmaniellea and Strombidium) and tintinnid ( Tintinnopsis, Tintinnus, Favella and Eutintinnus) ciliates. Higher biomass of these protozoa (359 mg C m −3) was observed at station MB, where large tintinnids were encountered. Mesozooplankton mainly represented by Calanoida ( Acartia, Temora, Calanus, Eucalanus, Paracalanus and Centropages) and Cyclopoida ( Oithona) copepods, exhibited higher and lower biomasses at stations MA/MJ and MB, respectively. Bacterivory represented only 35% of bacterial production at stations MB and R, but higher fractions (65–70%) were observed at stations MA and MJ. Small heterotrophic flagellates and aloricate ciliates seemed to be the main controllers of bacteria. Pico- and nanophytoplankton represented a significant alternative carbon pool for micrograzers, which grazing represented 67–90% of pico- and nano-algal production in all stations. Microzooplankton has, however, a relatively low impact on microphytoplankton, as ≤45% of microalgal production was consumed in all stations. This implies that an important fraction of diatom production would be channelled by herbivorous meso-grazers to higher consumers at stations MA and MJ where copepods were numerous. Most of the microalgal production would, however, sink particularly at station MB where copepods were scare. These different trophic interactions suggest different food web structures between stations. A multivorous food web seemed to prevail in stations MJ and MA, whereas microbial web was dominant in the other stations.

Plankton Food Web Responses to Experimental Nutrient Additions in a Subtropical Lake

During two controlled enclosure experiments using water from a subtropical lake, the plankton food web displayed a highly variable response to combined addition of nitrogen and phosphorus. In July, the nutrients stimulated growth of Cylindrospermopsis raciborskii, and the biomass of macrozooplankton and microbial food web components did not increase. In October, the same addition of nutrients stimulated growth of small edible Lyngbya spp., and there were coincident increases in biomass of macrozooplankton and components of the microbial web. Past generalizations that cyanobacteria blooms inhibit growth of other food web components may not always hold true.

Implementation of Microbial Active Database Using Web

연구목적(Objective) The contents of microbial information are complicated because of being hierarchical, duplicated as well as their changes are continuous and various. Much of time and cost are required for maintaining, expanding the information after representing them into a relational database. The relational database is not adequate to make the information sharable via Internet. In addition, user's meta-knowledge is insufficient to use the information via Internet and users may not be able to cope with the continuous changes of microbial information through the static web interface. Therefor, this study is conducted to develop an active microbial web database for resolving those problems. 방법(Methods) To solve these problems, object-oriented database, which was able to represent the complicated contents and knowledge, was used, and intelligent agents were implemented to cope with the continuous evolution of microbial information. Also, they were incorporated into user interface to assist users in an interactive environment, who want to use information via Internet. 결과(Results) The implemented system have shown advantages of solving difficulties arising from hierarchical and duplicated microbial data. It showed the possibility to share and manage the complex microbial data via Internet. It helped the system to be implemented as the active database, information stored in the object-oriented database. 결론(Conclusion) This study indicates that a similar approach may lead to develop an expert system based on the web and active database, which is able to be evolved, and to develop intelligent interface, using the agent.

Calibration and validation of a one-dimensional complex marine biogeochemical flux model in different areas of the northern Adriatic shelf

Abstract. In this paper we show results from numerical simulations carried out with a complex biogeochemical fluxes model coupled with a one-dimensional high-resolution hydrodynamical model and implemented at three different locations of the northern Adriatic shelf. One location is directly affected by the Po River influence, one has more open-sea characteristics and one is located in the Gulf of Trieste with an intermediate behavior; emphasis is put on the comparison with observations and on the functioning of the northern Adriatic ecosystem in the three areas. The work has been performed in a climatological context and has to be considered as preliminary to the development of three-dimensional numerical simulations. Biogeochemical model parameterizations have been ameliorated with a detailed description of bacterial substrate utilization associated with the quality of the dissolved organic matter (DOM), in order to improve the models capability in capturing the observed DOM dynamics in the basin. The coupled model has been calibrated and validated at the three locations by means of climatological data sets. Results show satisfactory model behavior in simulating local seasonal dynamics in the limit of the available boundary conditions and the one-dimensional implementation. Comparisons with available measurements of primary and bacterial production and bacterial abundances have been performed in all locations. Model simulated rates and bacterial dynamics are in the same order of magnitude of observations and show a qualitatively correct time evolution. The importance of temperature as a factor controlling bacteria efficiency is investigated with sensitivity experiments on the model parameterizations. The different model behavior and pelagic ecosystem structure developed by the model at the three locations can be attributed to the local hydrodynamical features and interactions with external inputs of nutrients. The onset of the winter/spring bloom in the climatological simulations is primarily driven by local stratification conditions. During summer the major carbon-transfer pathway developed by the model is the microbial web at all the sites, indicating that a large fraction of organic matter is processed through bacteria during productive periods, as suggested by field observations. The site directly influenced by riverine inputs differs from the others, showing a more alternate shifting among trophic pathways. Applying the conceptual scheme proposed by Legendre and Rassoulzadegan (Ophelia, 41, 153-172, 1995), it can be recognized as a herbivorous spring phase tightly followed by a microbial loop development, a summer microbial web phase and a multivorous trophic web pattern during autumn with a subsequent recovery of microbial processes. Results are discussed in terms of regime shifting from transient to stable water column conditions.Key words. Oceanography: general (continental shelf processes; numerical modelling) – Oceanography: biological and chemical (biochemistry and food chains)

Response of a complex ecosystem model of the northern Adriatic Sea to a regional climate change scenario

This paper investigates the impact of a regional climate change scenario on an ecosys- tem model of the northern Adriatic Sea. The study was performed by applying a biogeochemical bio- mass-based ecosystem model (ERSEM: European Regional Seas Ecosystem Model) coupled with a 1-dimensional version of a commonly used hydrodynamical model (POM: Princeton Ocean Model). The model response was studied by performing 2 simulations: one with atmospheric forcing functions for present-day climate conditions ('Control', 1970-1999), and one for the future climate after a dou- bling of the CO2 concentration ('Scenario', 2060-2089). Time-series of meteorological forcing func- tions were obtained from a greenhouse gas experiment with the high-resolution version of the ECHAM4 atmospheric general circulation model with a horizontal resolution of about 100 km. Pres- ent-day simulation results were compared to the available historical observations and showed a sat- isfactory agreement with the main seasonal cycles of hydrodynamical variables and nutrients in the limits of the 1-dimensional representation. Under the Scenario conditions, the model predicted an overall enhancement of the water-column stratification on an annual basis, with stronger intensifica- tion during the summer periods. The diffusion of oxygen and nutrients between surface and bottom layers is reduced, and the transfer of organic matter through the food web is shifted towards the smaller components of the microbial web. The model gives indications of an increase in the specific phytoplankton uptake rates of inorganic carbon in the Scenario, but the photosynthesized carbon ends up in the DOC pool because of the reduced supply of nutrients. The mean annual concentration of DOC is thus higher in the Scenario, with possible negative consequences on the water quality. Additional experiments show that the response of primary producers is not directly linked to the increase in ambient temperature. The structure of the food web modulates the interaction of phyto- plankton with the abiotic conditions. The results give indications of a possible non-linear response of the biogeochemical cycles to the climate change warming scenario and confirm the feasibility of using the downscaling technique to couple large-scale climate models with comprehensive regional ecosystem models.

Trophic interactions between zooplankton and the microbial community in contrasting food webs: the epilimnion and deep chlorophyll maximum of a mesotrophic lake

Trophic interactions between the mesozooplankton and the microbial web (bacteria, protozoa) were studied in 2 in situ grazing experiments in the epilimnion and the deep chlorophyll maximum (DCM) of a mesotrophic lake. We combined the Landry-Hassett dilution technique, whereby growth and grazing rates can be determined simultaneously, with the presence or absence of mesozooplankton and additional nutrients. The epilimnion was cladoceran-dominated, and had relatively high ciliate, and low heterotrophic nanoflagellate (HNF) abundance, while the DCM was rotifer-dominated, with fewer ciliates, but higher HNF abundance. Temperature differed by 13°C (22°C in the epilimnion; 9°C in the DCM). The community differences between the epilimnion and DCM and the experimental manipulations had their greatest effects on ciliates, lesser effects on HNF, and essentially no net effect on bacteria. Bacteria growth rates and grazing losses did not differ between the epilimnion and DCM, despite the large differences in chlorophyll a and abiotic parame- ters, and were independent of nutrient addition or mesozooplankton removal. In contrast, the exper- imental manipulations produced relatively moderate changes in HNF growth rates and grazing losses in both the epilimnion and DCM. Although HNF densities were 3 times higher in the DCM than the epilimnion, epilimnetic HNF growth rates were approximately twice the rate found in the DCM. This was consistent with both higher temperature and a release from predation through indi- rect predatory effects on alternative HNF predators like ciliates. In the DCM, crustacean zooplank- ton were less abundant and therefore had smaller direct and indirect effects on the HNF, while the metazoan microzooplankton (mainly rotifers) were more important. Ciliate growth rates were lower in the epilimnion than in the DCM, but only in the epilimnion were ciliate growth rates enhanced by the addition of nutrients. The cladoceran-dominated community of the epilimnion was able to reduce ciliate growth rates to negative values, while in the rotifer-dominated DCM, ciliate growth rates were always positive along a predator density gradient. These results demonstrate that while ciliates, and to a lesser extent heterotrophic flagellates, respond quickly to changes in predator community com- position, there are so many other direct and indirect pressures on bacteria that changes in the meso- zooplankton community composition have no net effect on their adundance.

Temporal variability of viruses, bacteria, phytoplankton and zooplankton in the western English Channel off Plymouth

The temporal distribution of autotrophic and heterotrophic components of the planktonic community was studied from samples collected weekly at station L4, located to the south of Plymouth, UK, from October 1992 to January 1994. Phytoplankton succession followed the typical pattern of temperate waters, the development of a summer Gyrodinium aureolumbloom being the most prominent feature. Bacterial numbers were significantly correlated with temperature during autumn and winter, whereas resource availability and predation, including viruses, appear to be the most important controlling factors in spring and summer. High mesozooplankton densities, mainly copepods, were observed throughout most of the study associated with a series of diatom blooms, and also during autumn when low phytoplankton biomass was measured. This data set was analysed in order to build up conceptual trophodynamic models whereby the role of biological communities on the cycling of organic matter could be inferred. The results obtained in this study provide empirical evidence supporting the existence of a sucession of trophic organization patterns in a coastal temperate environment. Classical models (herbivorous or microbial webs) appeared episodically whereas transition models (multivorous web) dominated throughout most of the seasonal cycle.