In situ experiments on the effects of increased sediment loads on littoral rocky shore communities in Lake Tanganyika, East Africa

1. Two in situ experiments investigated the responses to artificially increased sediment loads of the gastropod and fish communities, and of the whole benthos, of the rocky shores of Lake Tanganyika. Sediments were emptied once onto randomly selected quadrats using SCUBA, with control quadrats receiving no addition of sediment. Quadrats were monitored up to 6 months after sediment addition. 2. While the abundance of Lavigeria grandis (Mollusca; Gastropoda) was reduced greatly by the addition of sediment, low abundance and biomass of the whole gastropod community on sediment‐impacted quadrats 6 months after sediment addition suggests more general impacts of inundation in the longer term. 3. Benthic invertebrate communities inundated by sediment had significantly fewer individuals and numbers of taxa than control communities. No significant community recovery was recorded on sediment‐impacted quadrats within the timescale of the experiment. A short‐term period of high turbidity brought about by the emptying of sediments onto sediment‐impacted quadrats also caused severe decreases in benthic invertebrate abundance on controls 10 days after sediment addition. This suggests that even short‐term pulses of sediment may have significant detrimental effects on littoral communities. 4. Although significant impacts of sediment on fish community dynamics were found, and a significant reduction in the number of benthic algivorous species was recorded immediately after sediment addition, littoral fish communities showed strong resistance overall to the addition of sediments. This apparent resistance, however, may be a consequence of the small scale at which the experiment took place. Littoral fish communities were also shown to be more dynamic than indicated previously.     

Zooplankton responses to predation by larval bluegill: an enclosure experiment

SUMMARY 1. Larval fish are gape-limited predators that forage on prey of specific sizes, and thus may be expected to differentially affect members of a zooplankton community, possibly altering the size-structure or species composition.
2. I used an enclosure experiment to look at the effect of predation by larval bluegill on the dynamics of two zooplankton communities, one dominated by large-bodied individuals and the other by small-bodied individuals. Enclosures containing these zooplankton received a zero, low, medium, or high density of larval bluegill predators.
3. Increasing larval density had a negative effect on zooplankton abundance and abundance declined similarly in the large-bodied and small-bodied communities.
4. Zooplankton size-structure, as estimated by the length of the average zooplankton, increased and then decreased during the experiment, decreasing faster at higher larval fish densities. When zooplankton size-structure was estimated as the length of the average cladoceran, size-structure declined in the large-bodied but not in the small-bodied community and the greatest decline in size-structure was seen in the medium and high larval density treatments.
5. Ordination of each community using multidimensional scaling (MDS) indicated that the trajectory of change in species composition differed between the presence and absence of larval fish. In both communities, the degree of response by individual taxa depended on the density of bluegill larvae. This effect on zooplankton abundance, size-structure and community composition suggests that larval fish may make an important contribution to zooplankton dynamics in many lakes and ponds.     

pmoA-Based Analysis of Methanotrophs in a Littoral Lake Sediment Reveals a Diverse and Stable Community in a Dynamic Environment

Diversity and community structure of aerobic methane-oxidizing bacteria in the littoral sediment of Lake Constance was investigated by cloning analysis and terminal restriction fragment length polymorphism (T-RFLP) fingerprinting of the pmoA gene. Phylogenetic analysis revealed a high diversity of type I and type II methanotrophs in the oxygenated uppermost centimeter of the sediment. T-RFLP profiles indicated a high similarity between the active methanotrophic community in the oxic layer and the inactive community in an anoxic sediment layer at a 10-cm depth. There were also no major changes in community structure between littoral sediment cores sampled in summer and winter. By contrast, the fingerprint patterns showed substantial differences between the methanotrophic communities of littoral and profundal sediments.     

Removal of settled sediments and periphyton from macrophytes by grazing invertebrates in the littoral zone of a large oligotrophic lake

• 1 The resistance and resilience of littoral zone communities to sedimentation will depend both on the extent to which sediment deposition affects productivity, and on interactions within the communities. A series of hypotheses were set up and tested to examine interactions and feedback mechanisms among deposited sediments, periphyton, macrophytes and grazers in a large oligotrophic lake subject to fluctuating sediment loadings.
• 2 Although sediments incorporated into periphyton reduced light availability to macrophytes, periphytic algae were generally the dominant light absorbing component under natural conditions. When grazers were absent, both sediments incorporated in the periphyton and periphytic algal densities increased, and both were then important in reducing light available to macrophytes.
• 3 Grazing rate and assimilation efficiency for the dominant grazer, the prosobranch gastropod Potamopyrgus antipodarum, increased with increasing sediment content under natural lake conditions to reach a maximum at 10 mg sediment cm^?2.
• 4 An increase in sediment incorporation into periphyton films resulted in an increased grazing rate and hence grooming of sediments from macrophytes.
• 5 Grazing invertebrates can play a major role in maintenance of littoral communities by continuously grooming macrophyte hosts of periphytic algae and settled sediments.

     

First record of the insect pathogenic alga Helicosporidium sp. (Chlorophyta: Trebouxiophyceae) infection in larvae and pupae of Rhizophagusgrandis Gyll. (Coleoptera, Rhizophaginae) from Turkey

The predator beetle Rhizophagus grandis Gyll. (Coleoptera, Rhizophaginae) is one of the most important biological control agents, mass-bred and used to suppress populations of an important pest: the great spruce bark beetle, Dendroctonus micans. The achlorophyllous alga Helicosporidium sp. was first discovered in the pest. Later it was also found in the predator, but only in the adults. In this study, the pathogenic alga Helicosporidium sp. was discovered in larvae and early pupae of R. grandis for the first time. The morphological characteristics of the pathogenic alga were revealed by light and electron microscopy. Infection rates of Helicosporidium sp. in the larvae and pupae of R. grandis were 23.5% and 6.25%, respectively.     

Determining effects of suspended sediment on condition of a suspension feeding bivalve (Atrina zelandica): results of a survey, a laboratory experiment and a field transplant experiment

The horse mussel Atrina zelandica (Gray) is a large, suspension feeding pinnid bivalve, common in coastal and estuarine areas of northern New Zealand. As a suspension feeder, Atrina is likely to be influenced by suspended sediment loads. We conducted a laboratory experiment to determine the effect of short-term elevations in turbidity levels, such as those commonly recorded during storms, on the physiological condition and clearance rates of Atrina. We also conducted a field survey and a 3-month transplant experiment at multiple sites along a gradient of increasing suspended sediment load in a New Zealand estuary. Laboratory clearance rates of Atrina declined above a threshold suspended sediment concentration, and Atrina physiological condition at the end of this experiment was lower in high cf. low turbidity treatments. Decreases in Atrina condition were detected after exposure to elevated levels for only 3 days. The field survey and transplant experiment provided empirical evidence of a strong, negative effect of increasing suspended sediment flux on the physiological condition of Atrina. We suggest that relationships between the physiological condition of suspension feeders and sediment settling flux could provide a link between sediment inputs, which commonly occur as a result of catchment runoff during rainfall events, and the ecological health of estuarine and shallow coastal areas. Our study also demonstrated that Atrina have a natural distribution limit controlled by suspended sediment load. Thus, there is potential for larger-scale functional and structural effects on benthic communities in estuarine and coastal areas with high rates of sedimentation.     

Anaerobic Oxalate Degradation: Widespread Natural Occurrence in Aquatic Sediments

Significant concentrations of oxalate (dissolved plus particulate) were present in sediments taken from a diversity of aquatic environments, ranging from 0.1 to 0.7 mmol/liter of sediment. These included pelagic and littoral sediments from two freshwater lakes (Searsville Lake, Calif., and Lake Tahoe, Calif.), a hypersaline, meromictic, alkaline lake (Big Soda Lake, Nev.), and a South San Francisco Bay mud flat and salt marsh. The oxalate concentration of several plant species which are potential detrital inputs to these aquatic sediments ranged from 0.1 to 5.0% (wt/wt). In experiments with litter bags, the oxalate content of Myriophyllum sp. samples buried in freshwater littoral sediments decreased to 7% of the original value in 175 days. This suggests that plant detritus is a potential source of the oxalate within these sediments. [¹⁴C]oxalic acid was anaerobically degraded to ¹⁴CO₂ in all sediment types tested, with higher rates evident in littoral sediments than in the pelagic sediments of the lakes studied. The turnover time of the added [¹⁴C]oxalate was less than 1 day in Searsville Lake littoral sediments. The total sediment oxalate concentration did not vary significantly between littoral and pelagic sediments and therefore did not appear to be controlling the rate of oxalate degradation. However, depth profiles of [¹⁴C]oxalate mineralization and dissolved oxalate concentration were closely correlated in freshwater littoral sediments; both were greatest in the surface sediments (0 to 5 cm) and decreased with depth. The dissolved oxalate concentration (9.1 μmol/liter of sediment) was only 3% of the total extractable oxalate (277 μmol/liter of sediment) at the sediment surface. These results suggest that anaerobic oxalate degradation is a widespread phenomenon in aquatic sediments and may be limited by the dissolved oxalate concentration within these sediments.     

Responses of two dominant plant species to drought stress and defoliation in the Inner Mongolia Steppe of China

There has been a dramatic shift in dominance from Stipa grandis communities to S. krylovii communities in the Inner Mongolia steppe of China, in recent decades due to climate change and human activity. We examined the growth and carbohydrate allocation pattern of S. grandis and S. krylovii under controlled conditions. The experimental approach involved a drought stress treatment and a simulated defoliation (clipping) treatment of both species. Growth (above ground biomass and root biomass) and carbon allocation (concentration of leaf total phenolics and pool of total non-structural carbohydrate) variables were evaluated at the end of the experiment. Responses to drought stress differed significantly between S. grandis and S. krylovii. For S. krylovii, growth and the pool of total non-structural carbohydrate were more negatively affected by drought stress, whereas concentration of total phenolics was positively affected. Drought stress reinforced responses to defoliation, and drought stress × defoliation interaction was significant for all of the variables. There was a distinct defoliation response level for growth after drought stress between the two species. For aboveground biomass, both species responded positively to drought stress, which changed from responses equivalence to S. krylovii being superior; for root biomass, the two species responded oppositely to drought stress, which changed from S. grandis being superior to S. krylovii being superior. There was a weak and reverse defoliation response level for the carbon allocation pattern after drought stress between the two species, with S. krylovii changing from superior in defense to superior in storage. These results suggested that S. grandis utilized an avoidance strategy (investment in defense compounds) and S. krylovii utilized a tolerance strategy (investment in storage for regrowth) in response to defoliation under drought stress, supporting the idea that stress-tolerant species may become the new dominant species because of their ability to re-grow after disturbance. This provided a possible explanation for the replacement of S. grandis communities from the view point of adaptive strategy.     

Sediments influence accumulation of two macroalgal species through novel but differing interactions with nutrients and herbivory

Despite increasing concern that sediment loads from disturbed watersheds facilitate algal dominance on tropical reefs, little is known of how sediments interact with two primary drivers of algal communities, nutrients and herbivory. We examined the effects of sediment loads on the thalli of two increasingly abundant genera of macroalgae, Galaxaura and Padina, in a bay subject to terrestrial sediment influx in Mo’orea, French Polynesia. Field experiments examining (1) overall effects of ambient sediments and (2) interacting effects of sediments (ambient/removal) and herbivores (caged/uncaged) demonstrated that sediments had strong but opposite effects on both species’ biomass accumulation. Sediment removal increased accumulation of Padina boryana Thivy 50% in the initial field experiment but had no effect in the second; rather, in a novel interaction, herbivores overcompensated for increases in tissue nutrient stores that occurred with sediments loads, likely by preferential consumption of nutrient-rich meristematic tissues. Despite negative effects of sediments on biomass, Padina maintained rapid growth across treatments in both experiments. In contrast, positive growth in Galaxaura divaricata Kjellman only occurred with ambient sediment loads. In mesocosm experiments testing interactions of added nutrients and sediments on growth, Galaxaura grew at equivalent rates with sediments (collected from thalli on the reef) as with additions of nitrate and phosphate, suggesting sediments provide a nutrient subsidy. For Padina, however, the only effect was a 50% reduction in growth with sediment. Overall, retention of thallus sediments creates a positive feedback that Galaxaura appears to require to sustain net growth, while Padina merely tolerates sediments. These results indicate that sediments can modify nutrient and herbivore control of algae in ways that differ among species, with the potential for strong and unexpected effects on the abundance and composition of tropical reef macroalgae.     

Evidence of Large Change in Unionid Mussel Abundance from Selective Muskrat Predation,as Inferred by Shell Remains Left on Shore

Three species of unionid mussels (Pyganodon grandis, Lampsilis siliquoidea, and Toxolasma parvus) were preyed on by muskrats (Ondatra zibethicus) that left shell remains at feeding sites (middens) along the shore of Lake LaSalle (West) in western New York State, USA. Mussels became established within five years of the construction of this small artificial lake, and individuals of all three species have grown rapidly to large size. The abundance of prey in middens in 1988–1990 and a marked decline in subsequently deposited shells indicate how dramatically muskrats had exploited this food source. They consumed > 2700 individuals of P. grandis (by far the most common prey item) through 1990, yet only 20 newly deposited shells were found in 1993. At a midden where prey depletion was directly catalogued during 1988, larger specimens of P. grandis were consumed first, suggesting size selection. L. siliquoidea has been least affected by predation, and was the mussel most often found alive in the littoral zone. No live T. parvus were ever collected, and this species is known only from middens. Complete collections of shell remains from the shoreline through 1999 suggest that the mussel population may be recovering slowly. Muskrat predation may be more important in influencing mussel population structure than previously realized, and the effects can be dramatic spatially and temporally.     

Holocene subfossil chironomid stratigraphy (Diptera: Chironomidae) in the sediment of Plešné Lake (the Bohemian Forest,Czech Republic): Palaeoenvironmental implications

A faunal record of chironomid remains was analyzed in the upper 280 cm of a 543 cm long sediment core from Ple?né jezero (Ple?né Lake), the Bohemian Forest (?umava, Böhmerwald), Czech Republic. The chronology of the sediment was established by means of 5 AMS-dated plant macroremains. The resolution of individual 3-cm sediment layers is ～115 years and the analyzed upper 280 cm of the sediment core represent 10.4 cal. ka BP. As the results of DCA show, two marked changes were recorded in the otherwise relatively stable Holocene chironomid composition: (1) at the beginning of the Holocene (ca. 10.4-10.1 cal. ka BP) only oligotrophic and cold-adapted taxa (Diamesa sp., M. insignilobus-type, H. grimshawi-type) were present in the chironomid assemblages, clearly reflecting a cool climate oscillation during the Preboreal period, and (2) during an event dated in the interval 1540–1771 AD, when most taxa vanished entirely and only Zavrelimyia sp. and Procladius sp. were alternately present accompanied by Tanytarsus sp. Although, the age of this event is in agreement with the dating of the Little Ice Age, the most probable reason for the elimination of many chironomid taxa was very low sums recorded in this part of the sediment, rather than cool conditions connected with the LIA. Variations in the chironomid fauna after the Preboreal period were reflected mainly by changes in abundances of dominant taxa rather than by changes in species composition. These variations could be explained by: (1) climatic changes, namely temperature and amount of rainfall resulting in oscillations in lake level, with changes in the occurrence of macrophytes in the littoral and (2) increasingly dense afforestation which led to a considerable input of organic material into the lake and a subsequent increase in the trophic status of the lake water.     

Characterization of Arbuscular Mycorrhizal Fungus Communities of Aquilaria crassna and Tectona grandis Roots and Soils in Thailand Plantations

Aquilaria crassna Pierre ex Lec. and Tectona grandis Linn.f. are sources of resin-suffused agarwood and teak timber, respectively. This study investigated arbuscular mycorrhizal (AM) fungus community structure in roots and rhizosphere soils of A. crassna and T. grandis from plantations in Thailand to understand whether AM fungal communities present in roots and rhizosphere soils vary with host plant species and study sites. Terminal restriction fragment length polymorphism complemented with clone libraries revealed that AM fungal community composition in A. crassna and T. grandis were similar. A total of 38 distinct terminal restriction fragments (TRFs) were found, 31 of which were shared between A. crassna and T. grandis. AM fungal communities in T. grandis samples from different sites were similar, as were those in A. crassna. The estimated average minimum numbers of AM fungal taxa per sample in roots and soils of T. grandis were at least 1.89 vs. 2.55, respectively, and those of A. crassna were 2.85 vs. 2.33 respectively. The TRFs were attributed to Claroideoglomeraceae, Diversisporaceae, Gigasporaceae and Glomeraceae. The Glomeraceae were found to be common in all study sites. Specific AM taxa in roots and soils of T. grandis and A. crassna were not affected by host plant species and sample source (root vs. soil) but affected by collecting site. Future inoculum production and utilization efforts can be directed toward the identified symbiotic associates of these valuable tree species to enhance reforestation efforts.     

Salicylic Acid Alleviates the Adverse Effects of Salt Stress in Torreya grandis cv. Merrillii Seedlings by Activating Photosynthesis and Enhancing Antioxidant Systems

Background

Salt stress is a major factor limiting plant growth and productivity. Salicylic acid (SA) has been shown to ameliorate the adverse effects of environmental stress on plants. To investigate the protective role of SA in ameliorating salt stress on Torreya grandis (T. grandis) trees, a pot experiment was conducted to analyze the biomass, relative water content (RWC), chlorophyll content, net photosynthesis (Pn), gas exchange parameters, relative leakage conductivity (REC), malondialdehyde (MDA) content, and activities of superoxide dismutase (SOD) and peroxidase (POD) of T. grandis under 0.2% and 0.4% NaCl conditions with and without SA.

Methodology/Principal Findings

The exposure of T. grandis seedlings to salt conditions resulted in reduced growth rates, which were associated with decreases in RWC and Pn and increases in REC and MDA content. The foliar application of SA effectively increased the chlorophyll (chl (a+b)) content, RWC, net CO₂ assimilation rates (Pn), and proline content, enhanced the activities of SOD, CAT and POD, and minimized the increases in the REC and MDA content. These changes increased the capacity of T. grandis in acclimating to salt stress and thus increased the shoot and root dry matter. However, when the plants were under 0% and 0.2% NaCl stress, the dry mass of the shoots and roots did not differ significantly between SA-treated plants and control plants.

Conclusions

SA induced the salt tolerance and increased the biomass of T. grandis cv. by enhancing the chlorophyll content and activity of antioxidative enzymes, activating the photosynthetic process, and alleviating membrane injury. A better understanding about the effect of salt stress in T. grandis is vital, in order gain knowledge over expanding the plantations to various regions and also for the recovery of T. grandis species in the future.     

Diversity of cyanobacterial species and phylotypes in biofilms from the littoral zone of Lake Baikal

The majority of naturally occurring biofilms contain numerous microorganisms that have not yet been cultured. Additionally, there is little information available regarding the genetic structure and species diversity of these communities. Therefore, we characterised the species diversity, structure and metagenome of biofilms grown on stones and steel plates in the littoral zone of Lake Baikal (East Siberia, Russia) by applying three different approaches. First, light microscopy enabled identification of the species diversity of biofilm-forming cyanobacteria on different substrates with the dominance of Rivularia rufescens, Tolypothrix limbata, Chamaesiphon fuscus, Ch. subglobosus, and Heteroleibleinia pusilla. Additionally, scanning electron microscopy was used to show the spatial structure of biofilms. Finally, sequence analysis of 30,660 16S rRNA clones indicated a high diversity within the biofilm communities, with the majority of the microbes being closely related to Cyanobacteria (8–46% sequences), Proteobacteria (14–43%), and Bacteroidetes (10–41%). Rivularia sp., Pseudanabaena sp., and Chamaesiphon spp. were the dominant cyanobacterial phylotypes.     

Nutrient Enrichment Coupled with Sedimentation Favors Sea Anemones over Corals

Fine sediments, which account for the majority of total fluvial sediment flux, have been suggested to degrade coral reefs on a global scale. Furthermore, sediment impacts can be exacerbated by extreme rainfall events associated with global climate change and anthropogenic nutrient enrichment. We report the findings from a series of mesocosm experiments exploring the effects of short-term sedimentation and nutrient enrichment on the interactions between the hard coral Acropora muricata, the sea anemone Mesactinia ganesis, and the green macroalga Codium edule. Mesocosms were manipulated to simulate either unimpacted reefs or reefs exposed to elevated levels of fine sediments for 10 or 14 days to simulate the effects of heavy rainfall. The first and second experiments were aimed to examine the effects of inorganic and organic sediments, respectively. The third experiment was designed to examine the interactive effects of nutrient enrichment and elevated sediment loads. Neither inorganic nor organic sediment loadings significantly affected the physiological performance of the coral, but, importantly, did reduce its ability to compete with other organisms. Photosynthetic efficiencies of both the green macroalga and the sea anemone increased in response to both sediment loadings when they were simultaneously exposed to nutrient enrichment. While organic sediment loading increased the nitrogen content of the green macroalga in the first experiment, inorganic sediment loading increased its phosphorus content in the second experiment. The coral mortality due to sea anemones attack was significantly greater upon exposure to enriched levels of organic sediments and nutrients. Our findings suggest that the combined effects of short-term sedimentation and nutrient enrichment could cause replacement of corals by sea anemones on certain coral reefs.     

Development of filamentous green algae in the benthic algal community in a littoral sand-beach zone of Lake Biwa

Temporal changes of biomass and dominant species in benthic algal communities were investigated in a littoral sand-beach zone in the north basin of Lake Biwa from December 1999 to September 2000. Chlorophyll-a amounts of benthic algal communities per unit area of the sandy sediments rapidly increased from late April to June. Increases in biomass of the benthic algal communities are considered to result from the propagation of filamentous green algae Oedogonium sp. and Spirogyra sp. The cell numbers of filamentous green algae and chlorophyll-a amounts of benthic algal communities at depths of 30 and 50cm at a station protected by a breakwater in May were significantly higher than those of a station exposed directly to wave activity. Thus, the biomass accumulation of the benthic algal communities seems to be regulated strongly by wave disturbance. The development of filamentous green algae may contribute to the increase in biomass of the benthic algal community and to the changes in seasonal patterns of biomass in the sand-beach zone of Lake Biwa. We consider that the development of the filamentous green algal community in the littoral zone of Lake Biwa is the result of eutrophication.     

Three New Species of Trichodoridae (Nematoda: Diphtherophorina) with Observations on the Vulva in Paratrichodorus

Trichodorus intermedius n. sp., Trichodorus dilatatus n. sp., and Paratrichodorus (Atlantadorus) grandis n. sp. are described from soil around native plants of Southern California. The females of the three subgenera of Paratrichodorus are further characterized on the basis of the vulva, which is pore-like in Atlantadorus, a longitudinal slit in Paratrichodorus, and a transverse slit in Nanidorus.     

The potential of agricultural headwater streams to retain soluble reactive phosphorus

Zooplankton may preferentially graze small, edible diatom species and therefore affect fossils relative to live assemblages by selective removal or increased sedimentation via egestion. Cladoceran zooplankton remains and diatom edibility were analyzed in sediment cores from Moon Lake and Coldwater Lake (North Dakota, USA) to assess changes in potential grazing pressure on algae and influence on diatom-inferred salinity (DIS) reconstructions. Sedimentary zooplankton in Moon Lake were dominated by littoral Cladocera, whereas Coldwater Lake assemblages were primarily small-bodied pelagic and littoral species. Relationships between cladocerans and environmental parameters over the past century varied by site and by species, with Chydorus brevilabris related most closely to drought at Moon and Bosmina sp. related to drought at Coldwater. A higher percentage of inedible diatoms occurred in the sediments of Moon Lake as compared to Coldwater Lake. DIS correlations with drought records improved in Moon Lake when only inedible diatom taxa were used to build a transfer function, but no improvement was seen for Coldwater Lake with this approach. These data suggest grazing pressure on diatoms differed between lakes and that zooplankton–phytoplankton interactions may affect the accuracy of drought reconstructions in the Great Plains.     

Sediment Enzyme Activities and Microbial Community Diversity in an Oligotrophic Drinking Water Reservoir,Eastern China

Drinking water reservoir plays a vital role in the security of urban water supply, yet little is known about microbial community diversity harbored in the sediment of this oligotrophic freshwater environmental ecosystem. In the present study, integrating community level physiological profiles (CLPPs), nested polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) and clone sequence technologies, we examined the sediment urease and protease activities, bacterial community functional diversity, genetic diversity of bacterial and fungal communities in sediments from six sampling sites of Zhou cun drinking water reservoir, eastern China. The results showed that sediment urease activity was markedly distinct along the sites, ranged from 2.48 to 11.81 mg NH₃-N/(g·24h). The highest average well color development (AWCD) was found in site C, indicating the highest metabolic activity of heterotrophic bacterial community. Principal component analysis (PCA) revealed tremendous differences in the functional (metabolic) diversity patterns of the sediment bacterial communities from different sites. Meanwhile, DGGE fingerprints also indicated spatial changes of genetic diversity of sediment bacterial and fungal communities. The sequence BLAST analysis of all the sediment samples found that Comamonas sp. was the dominant bacterial species harbored in site A. Alternaria alternate, Allomyces macrogynus and Rhizophydium sp. were most commonly detected fungal species in sediments of the Zhou cun drinking water reservoir. The results from this work provide new insights about the heterogeneity of sediment microbial community metabolic activity and genetic diversity in the oligotrophic drinking water reservoir.     

Application of Molecular Biological Techniques to a Seasonal Study of Ammonia Oxidation in a Eutrophic Freshwater Lake

The autotrophic ammonia-oxidizing bacteria in a eutrophic freshwater lake were studied over a 12-month period. Numbers of ammonia oxidisers in the lakewater were small throughout the year, and tangential-flow concentration was required to obtain meaningful estimates of most probable numbers. Sediments from littoral and profundal sites supported comparatively large populations of these bacteria, and the nitrification potential was high, particularly in summer samples from the littoral sediment surface. In enrichment cultures, lakewater samples nitrified at low (0.67 mM) ammonium concentrations only whereas sediment samples exhibited nitrification at high (12.5 mM) ammonium concentrations also. Enrichments at low ammonium concentration did not nitrify when inoculated into high-ammonium medium, but the converse was not true. This suggests that the water column contains a population of ammonia oxidizers that is sensitive to high ammonium concentrations. The observation of nitrification at high ammonium concentration by isolates from some winter lakewater samples, identified as nitrosospiras by 16S rRNA probing, is consistent with the hypothesis that sediment ammonia oxidizers enter the water column at overturn. With only one exception, nested PCR amplification enabled the detection of Nitrosospira 16S rDNA in all samples, but Nitrosomonas (N. europaea-eutropha lineage) 16S rDNA was never obtained. However, the latter were part of the sediment and water column communities, because their 16S rRNA could be detected by specific oligonucleotide probing of enrichment cultures. Furthermore, a specific PCR amplification regime for the Nitrosomonas europaea ammonia monooxygenase gene (amoA) yielded positive results when applied directly to sediment and lakewater samples. Patterns of Nitrosospira and Nitrosomonas detection by 16S rRNA oligonucleotide probing of sediment enrichment cultures were complex, but lakewater enrichments at low ammonium concentration were positive for nitrosomonads and not nitrosospiras. Analysis of enrichment cultures has therefore provided evidence for the existence of subpopulations within the lake ammonia-oxidizing community distinguishable on the basis of ammonium tolerance and possibly showing a seasonal distribution between the sediment and water column.