Ecology of phytoplankton communities dominated by Aureococcus anophagefferens: the role of viruses, nutrients, and microzooplankton grazing

We investigated the impact of viruses, nutrient loading, and microzooplankon grazing on phytoplankton communities in two New York estuaries that hosted blooms of the brown tide alga Aureococcus anophagefferens during 2000 and 2002. The absence of a bloom at one location during 2002 allowed for the fortuitous comparison of a bloom and non-bloom year at the same location as well as a comparison of two sites experiencing bloom and non-bloom conditions during the same year. During the study, blooms were found at locations with high levels of dissolved organic nitrogen and lower nitrate concentrations compared to a non-bloom location. Experimental additions of inorganic nitrogen and phosphorus yielded growth rates within the total phytoplankton community which significantly exceeded control treatments in 83% of experiments, while A. anophagefferens experienced significantly increased growth during only 20% of experimental inorganic nutrient additions. Consistent with prior research, these results suggest brown tides are not caused by eutrophication, but instead are more likely to occur when sources of labile DOM are readily available. Microzooplankton grazing rates on the total phytoplankton community during a bloom were lower than grazing rates at a non-bloom site, and grazing rates on A. anophagefferens were lower than grazing rates on the total community on some dates, suggesting that reduced grazing mortality may also promote brown tides. Mean densities of viruses during blooms (3 × 10⁸ ml⁻¹) were elevated compared to most estuarine environments and were twice the levels found at a non-bloom site. Experimental enrichment of the natural viral densities yielded a significant increase in A. anophagefferens growth rates relative to control treatments when background levels of viruses were low (<1.7 × 10⁸ ml⁻¹), suggesting that viruses may promote bloom occurrence by regenerating DOM or altering the composition of microbial communities.     

Production characteristics of bacteria and phytoplankton in the Sea of Okhotsk and Bering Sea during spring–summer

Production parameters for bacterioplankton were assessed during the spring–summer period in the western parts of the Sea of Okhotsk and the Bering Sea, as well as in northwestern Pacific Ocean. The lowest values of bacterial production were observed in early June during the spring phytoplankton bloom (0.08 mg C day^–1 m^–3), while the maximum values (up to 55 mg C day^–1 m^–3) occurred in late July?early August, 1.5 to 2 months after the bloom. The concentration of dissolved organic matter, the substrate for bacterioplankton, was assessed using satellite data. The ratio between bacterial and primary production in the surface samples varied from 0.5% at the peak of phytoplankton bloom to 180% at the peak of bacterioplankton development.     

Metabolic diversity of heterotrophic bacterioplankton over winter and spring in the coastal Arctic Ocean

Metabolic diversity of heterotrophic bacterioplankton was tracked from early winter through spring with Biolog Ecoplates under the seasonally ice covered arctic shelf in the Canadian Arctic (Franklin Bay, Beaufort Sea). Samples were taken every 6 days from December 2003 to May 2004 at the surface, the halocline where a temperature inversion occurs, and at 200 m, close to the bottom. Despite the low nutrient levels and low chlorophyll a , suggesting oligotrophy in the winter surface waters, the number of substrates used (NSU) was greater than in spring, when chlorophyll a concentrations increased. Denaturing gradient gel electrophorisis analysis also indicated that the winter and spring bacterial communities were phylogenetically distinct, with several new bands appearing in spring. In spring, the bacterial community would have access to the freshly produced organic carbon from the early phytoplankton bloom and the growth of rapidly growing specialist phenotypes would be favoured. In contrast, in winter bacterioplankton consumed more complex organic matter originated during the previous year's phytoplankton production. At the other depths we tested the NSU was similar to that for the winter surface, with no seasonal pattern. Instead, bacterioplankton metabolism seemed to be influenced by resuspension, advection, and sedimentation events that contributed organic matter that enhanced bacterial metabolism.     

Impact of warming on phyto‐bacterioplankton coupling and bacterial community composition in experimental mesocosms

Global warming is assumed to alter the trophic interactions and carbon flow patterns of aquatic food webs. The impact of temperature on phyto‐bacterioplankton coupling and bacterial community composition (BCC) was the focus of the present study, in which an indoor mesocosm experiment with natural plankton communities from the western Baltic Sea was conducted. A 6°C increase in water temperature resulted, as predicted, in tighter coupling between the diatom‐dominated phytoplankton and heterotrophic bacteria, accompanied by a strong increase in carbon flow into bacterioplankton during the phytoplankton bloom phase. Suppressed bacterial development at cold in situ temperatures probably reflected lowered bacterial production and grazing by protists, as the latter were less affected by low temperatures. BCC was strongly influenced by the phytoplankton bloom stage and to a lesser extent by temperature. Under both temperature regimes, Gammaproteobacteria clearly dominated during the phytoplankton peak, with Glaciecola sp. as the single most abundant taxon. However, warming induced the appearance of additional bacterial taxa belonging to Betaproteobacteria and Bacteroidetes. Our results show that warming during an early phytoplankton bloom causes a shift towards a more heterotrophic system, with the appearance of new bacterial taxa suggesting a potential for utilization of a broader substrate spectrum.     

Pirellula and OM43 are among the dominant lineages identified in an Oregon coast diatom bloom

Although bacterioplankton and phytoplankton are generally perceived as closely linked in marine systems, specific interactions between discrete bacterioplankton and phytoplankton populations are largely unknown. However, measurements of bacterioplankton distributions during phytoplankton blooms may indicate specific microbial lineages that are responding to phytoplankton populations, and potentially controlling them by producing allelopathic compounds. Here we use a comprehensive molecular approach to identify, characterize and quantify bacterioplankton community responses to an Oregon coast diatom bloom. Total DAPI counts increased by nearly sevenfold in bloom samples, reaching 5.7 x 10(9) cells l(-1), and lineage-specific cell counts using fluorescence in situ hybridization (FISH) indicated that Bacteria accounted for approximately 89% of observed increases. Several dominant members of the bacterial community present outside the bloom (SAR11 and SAR86) did not contribute significantly to observed increases in bloom samples. Clone library and FISH data indicated that uncultured planctomycetes most closely related to Pirellula, and members of the OM43 clade of beta proteobacteria, reached 0.5 x 10(8) and 1.2 x 10(8) cells l(-1), respectively, and were among the dominant lineages in bloom samples.     

Testing the metabolic theory of ecology with marine bacteria: different temperature sensitivity of major phylogenetic groups during the spring phytoplankton bloom

Although temperature is a key driver of bacterioplankton metabolism, the effect of ocean warming on different bacterial phylogenetic groups remains unclear. Here, we conducted monthly short‐term incubations with natural coastal bacterial communities over an annual cycle to test the effect of experimental temperature on the growth rates and carrying capacities of four phylogenetic groups: SAR11, Rhodobacteraceae, Gammaproteobacteria and Bacteroidetes. SAR11 was the most abundant group year‐round as analysed by CARD‐FISH, with maximum abundances in summer, while the other taxa peaked in spring. All groups, including SAR11, showed high temperature‐sensitivity of growth rates and/or carrying capacities in spring, under phytoplankton bloom or post‐bloom conditions. In that season, Rhodobacteraceae showed the strongest temperature response in growth rates, estimated here as activation energy (E, 1.43 eV), suggesting an advantage to outcompete other groups under warmer conditions. In summer E values were in general lower than 0.65 eV, the value predicted by the Metabolic Theory of Ecology (MTE). Contrary to MTE predictions, carrying capacity tended to increase with warming for all bacterial groups. Our analysis confirms that resource availability is key when addressing the temperature response of heterotrophic bacterioplankton. We further show that even under nutrient‐sufficient conditions, warming differentially affected distinct bacterioplankton taxa.     

Primary production and phytoplankton composition in relation to DOC input and bacterioplankton production in humic Lake Örträsket

1. The biomass and production of picophytoplankton, large phytoplankton and heterotrophic bacterioplankton were measured in humic Lake Örträsket, northern Sweden during four consecutive summers.
2. High flow episodes, carrying fresh dissolved organic carbon (DOC) into the lake, always stimulated heterotrophic bacterial production at the expense of primary production. Primary production never exceeded bacterial production for approximately 20 days after such an episode had replenished epilimnial DOC. We suggest that allochthonous DOC is an energy source that stimulates bacterioplankton that, because of their efficient uptake of inorganic nutrients, are then able to outcompete phytoplankton. After the exhaustion of readily available DOC, phytoplankton were able to dominate epilimnion production in Lake Örträsket.
3. Biomass production was higher when dominated by phytoplankton than by bacterioplankton, despite a similar utilization of nutrients in the epilimnion throughout the summer. We propose that different C : N : P ratios of bacterioplankton and phytoplankton permit the latter to produce more carbon (C) biomass per unit of available inorganic nutrients than bacterioplankton.     

Seasonal variation in composition and production of planktonic communities in the lower River Rhine

1. The composition and activity of phytoplankton, zooplankton and bacterioplankton in the lower River Rhine were measured in 1990 as part of an international biological inventory of the river. A seasonal study was carried out on two stations: one in the river mouth (km 1019) and one at the German/Dutch border (km 863). 2. High densities of phytoplankton (with up to 140 μg chlorophyll a 1-^?1) and occasional depletion of dissolved silicate were observed at the upstream station. Phosphate concentrations were also lowered during blooms. 3. Phytoplankton blooms, dominated by a few species of centric diatoms, declined one order of magnitude during downstream transport. During non-bloom conditions (low) algal densities were maintained during transport, or increased slightly, indicating the suitability of the river reach for algal growth. 4. Bacterial cell number and production (measured by the ³H-thymidine method) showed a broad summer maximum with activity peaks (0.5 nK < M thymidine h^?1) coincident with declining phytoplankton blooms. Winter values of bacterial production (0.02–0.05 n < Mh^?1) were substantial, probably as a result of allochthonous input of organic matter. 5. Rotifers and crustaceans made up the greater part of the zooplankton biovolume, but at the upstream station the contribution of Dreissena larvae and rhizopods was also substantial. High zooplankton biovolumes, of over 500 × 10⁶μm³1-^?1, were observed only during the phytoplankton spring bloom. 6. Quantitative relationships between the high phytoplankton production (2.1–3.4 gCm^?2 day^?1), the high bacterial substrate uptake (0.5–1gCm^?2 day^?1), and grazing were analysed for the growing season 1990. Algal grazing by metazoan herbivores was substantial only during spring, while the role of phagotrophic microplankton and cell lysis were indicated as major factors responsible for the downstream decline of phytoplankton blooms in the lower Rhine.     

Nutrient control of bacterioplankton and phytoplankton dynamics

To determine whether positive correlations between phytoplankton and bacterioplankton growth in nutrient addition experiments are due to growth coupling or growth stimulation by the same nutrients, we examined phyto- and bacterioplankton growth in a series of eleven nutrient addition (N × P) and light/dark experiments. In mesotrophic Castle Lake, the phyto- and bacterioplankton growth responses to phosphorus (P) addition were strongly correlated (r²=0.59), while only a weak correlation (r²=0.10) was observed for the nitrogen addition treatments. After normalizing the N + P treatments for the growth stimulation observed in the respective P treatments, we found a substantial stimulation of the phytoplankton (e.g., costimulation by N + P) and no stimulation of the bacterioplankton. Bacteria growth rates were similar in both light and dark incubated P treatments. In these experiments, we found clear evidence suggesting the dynamics of bacteria and phytoplankton were correlated because they are often limited by the same resource (mainly inorganic phosphorus). We found only limited evidence that bacterioplankton growth coupling to algal dynamics was occurring in these experiments. However, we did not consider several factors such as dissolved organic nutrient availability, bacterivory, availability of physical substrates, and temperature which are also thought to influence the nature of bacterial/phytoplankton interactions. Based on the results of our experiments, we conclude the biomass of the bacterio- and phytoplankton covaried because they were stimulated by the same nutrients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Differential response of high-elevation planktonic bacterial community structure and metabolism to experimental nutrient enrichment

Nutrient enrichment of high-elevation freshwater ecosystems by atmospheric deposition is increasing worldwide, and bacteria are a key conduit for the metabolism of organic matter in these oligotrophic environments. We conducted two distinct in situ microcosm experiments in a high-elevation lake (Emerald Lake, Sierra Nevada, California, USA) to evaluate responses in bacterioplankton growth, carbon utilization, and community structure to short-term enrichment by nitrate and phosphate. The first experiment, conducted just following ice-off, employed dark dilution culture to directly assess the impact of nutrients on bacterioplankton growth and consumption of terrigenous dissolved organic matter during snowmelt. The second experiment, conducted in transparent microcosms during autumn overturn, examined how bacterioplankton in unmanipulated microbial communities responded to nutrients concomitant with increasing phytoplankton-derived organic matter. In both experiments, phosphate enrichment (but not nitrate) caused significant increases in bacterioplankton growth, changed particulate organic stoichiometry, and induced shifts in bacterial community composition, including consistent declines in the relative abundance of Actinobacteria. The dark dilution culture showed a significant increase in dissolved organic carbon removal in response to phosphate enrichment. In transparent microcosms nutrient enrichment had no effect on concentrations of chlorophyll, carbon, or the fluorescence characteristics of dissolved organic matter, suggesting that bacterioplankton responses were independent of phytoplankton responses. These results demonstrate that bacterioplankton communities in unproductive high-elevation habitats can rapidly alter their taxonomic composition and metabolism in response to short-term phosphate enrichment. Our results reinforce the key role that phosphorus plays in oligotrophic lake ecosystems, clarify the nature of bacterioplankton nutrient limitation, and emphasize that evaluation of eutrophication in these habitats should incorporate heterotrophic microbial communities and processes.     

Responses of aerobic anoxygenic phototrophic bacteria to algal blooms in the East China Sea

Aerobic anoxygenic phototrophic bacteria (AAPB) are a new functional group of heterotrophic bacteria capable of phototrophy, and are suggested to be closely related with phytoplankton. However, less known is the relationship between AAPB and dominant phytoplankton populations. In this tudy, the responses of AAPB to algal blooms (ABs) in the AB-frequent-occurrence area of the East China Sea were investigated during four cruises from March to June 2005, using an advanced ‘Time-series observation-based cyanobacteria-calibrated InfraRed Epifluorescence Microscopy (TIREM)’ approach. Generally, total bacterial abundances at the bloom stations were higher than or similar to those at the non-bloom stations during the same time period. Interestingly, the responses of AAPB to ABs seemed to be more diverse and complex. AAPB abundance was higher at the stations with ABs where Thalassiosira curviseriata Takano and Skeletonema costatum (Greville) Cleve, Noctiluca scintillans (Macartney) Kofoid et Swezy, or Prorocentrum donghaiense Lu and Karenia mikimotoi Hansen co-dominated than those at the non-bloom stations during the same time period. However at the stations with a bloom of Akashiwo sanguinea Hansen, AAPB abundance only accounted for ~20% of the average abundance of AAPB at the non-bloom stations. In addition, variations of AAPB’s proportion to total bacterial abundance (AAPB%) in response to ABs basically followed AAPB abundance. Overall, our results suggest that the responses of AAPB to ABs are AB-species specific and somewhat independent of chlorophyll a concentration.     

A small population of planktonic Flavobacteria with disproportionally high growth during the spring phytoplankton bloom in a prealpine lake

Bacterioplankton growth in temperate Lake Zurich (Switzerland) was studied during the spring phytoplankton bloom by in situ techniques and short-term dilution bioassays. A peak of chlorophyll a (Chl a ) concentrations was followed by a rise of bacterial cell numbers and leucine assimilation rates, of the proportions of cells incorporating 5-bromo-2-deoxyuridine (BrdU), and of community net growth rates in dilution cultures. Incorporation of BrdU was low in Betaproteobacteria (2 ± 1%), indicating that these bacteria did not incorporate the tracer. Pronounced growth of Betaproteobacteria in the enrichments was only observed after the decline of the phytoplankton bloom. An initial peak in the proportions of BrdU-positive Actinobacteria (30%) preceded a distinct rise of their cell numbers during the period of the Chl a maximum. Cytophaga–Flavobacteria (CF) changed little in numbers, but featured high proportions of BrdU-positive cells (28 ± 12%). Moreover, CF represented > 90% of all newly formed cells in dilution cultures before and during the phytoplankton bloom. One phylogenetic lineage of cultivable Flavobacteria (FLAV2) represented a small (0.5–1%) but highly active population in lake plankton. The growth rates of FLAV2 in dilution cultures doubled during the period of the Chl a maximum, indicating stimulation by phytoplankton exudates. Thus, CF, and specifically Flavobacteria , appeared to be substantially more important for carbon transfer in Lake Zurich spring bacterioplankton than was suggested by their standing stocks. The high in situ growth potential of these bacteria might have been counterbalanced by top-down control.     

Dimethylsulfoniopropionate turnover is linked to the composition and dynamics of the bacterioplankton assemblage during a microcosm phytoplankton bloom

Processing of the phytoplankton-derived organic sulfur compound dimethylsulfoniopropionate (DMSP) by bacteria was studied in seawater microcosms in the coastal Gulf of Mexico (Alabama). Modest phytoplankton blooms (peak chlorophyll a [Chl a] concentrations of approximately 2.5 microg liter(-1)) were induced in nutrient-enriched microcosms, while phytoplankton biomass remained low in unamended controls (Chl a concentrations of approximately 0.34 microg liter(-1)). Particulate DMSP concentrations reached 96 nM in the enriched microcosms but remained approximately 14 nM in the controls. Bacterial biomass production increased in parallel with the increase in particulate DMSP, and nutrient limitation bioassays in the initial water showed that enrichment with DMSP or glucose caused a similar stimulation of bacterial growth. Concomitantly, increased bacterial consumption rate constants of dissolved DMSP (up to 20 day(-1)) and dimethylsulfide (DMS) (up to 6.5 day(-1)) were observed. Nevertheless, higher DMSP S assimilation efficiencies and higher contribution of DMSP to bacterial S demand were found in the controls compared to the enriched microcosms. This indicated that marine bacterioplankton may rely more on DMSP as a source of S under oligotrophic conditions than under the senescence phase of phytoplankton blooms. Phylogenetic analysis of the bacterial assemblages in all microcosms showed that the DMSP-rich algal bloom favored the occurrence of various Roseobacter members, flavobacteria (Bacteroidetes phylum), and oligotrophic marine Gammaproteobacteria. Our observations suggest that the composition of the bacterial assemblage and the relative contribution of DMSP to the overall dissolved organic sulfur/organic matter pool control how efficiently bacteria assimilate DMSP S and thereby potentially divert it from DMS production.     

Linkages between bacterioplankton community composition, heterotrophic carbon cycling and environmental conditions in a highly dynamic coastal ecosystem

We used mesocosm experiments to study the bacterioplankton community in a highly dynamic coastal ecosystem during four contrasting periods of the seasonal cycle: winter mixing, spring phytoplankton bloom, summer stratification and autumn upwelling. A correlation approach was used in order to measure the degree of coupling between the dynamics of major bacterial groups, heterotrophic carbon cycling and environmental factors. We used catalysed reporter deposition-fluorescence in situ hybridization to follow changes in the relative abundance of the most abundant groups of bacteria (Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes). Bacterial carbon flux-related variables included bacterial standing stock, bacterial production and microbial respiration. The environmental factors included both, biotic variables such as chlorophyll-a concentration, primary production, phytoplankton extracellular release, and abiotic variables such as the concentration of dissolved inorganic and organic nutrients. Rapid shifts in the dominant bacterial groups occurred associated to environmental changes and bacterial bulk functions. An alternation between Alphaproteobacteria and Bacteroidetes was observed associated to different phytoplankton growth phases. The dominance of the group Bacteroidetes was related to high bacterial biomass and production. We found a significant, non-spurious, linkage between the relative abundances of major bacterial groups and bacterial carbon cycling. Our results suggest that bacteria belonging to these major groups could actually share a function in planktonic ecosystems.     

Drivers of interannual variability in virioplankton abundance at the coastal western Antarctic peninsula and the potential effects of climate change

An 8‐year time‐series in the Western Antarctic Peninsula (WAP) with an approximately weekly sampling frequency was used to elucidate changes in virioplankton abundance and their drivers in this climatically sensitive region. Virioplankton abundances at the coastal WAP show a pronounced seasonal cycle with interannual variability in the timing and magnitude of the summer maxima. Bacterioplankton abundance is the most influential driving factor of the virioplankton, and exhibit closely coupled dynamics. Sea ice cover and duration predetermine levels of phytoplankton stock and thus, influence virioplankton by dictating the substrates available to the bacterioplankton. However, variations in the composition of the phytoplankton community and particularly the prominence of Diatoms inferred from silicate drawdown, drive interannual differences in the magnitude of the virioplankton bloom; likely again mediated through changes in the bacterioplankton. Their findings suggest that future warming within the WAP will cause changes in sea ice that will influence viruses and their microbial hosts through changes in the timing, magnitude and composition of the phytoplankton bloom. Thus, the flow of matter and energy through the viral shunt may be decreased with consequences for the Antarctic food web and element cycling.     

Seasonal Patterns of Planktonic Production in McMurdo Sound, Antarctica

The prolonged periods of continuous darkness and light in polarregions have resulted in a unique seasonal partitioning of primaryand heterotrophic production. In McMurdo Sound for example,the biomass, size distribution and production by phytoplanktonand bacterioplankton undergo distinct seasonal cycles. The seasonalpattern of primary production appeared to be regulated by lightwhereas the three order of magnitude change in phytoplanktonbiomass during mid- to late December was largely controlledby the advection of planktonic algae from the Ross Sea intoMcMurdo Sound. The size distribution of phytoplankton was highlyseasonal; nano- and picoplankton were dominant from August throughNovember while netplankton were more abundant in December andJanuary. Seasonal variations of bacterial biomass and productionwere smaller than those of phytoplankton. During the late australwinter and spring, bacterial biomass and production exceededthose of phytoplankton. This implies that during this period,organic material from allochthonous sources sustained bacterialgrowth. During the late spring and summer, however, the patternwas reversed and autochthonous primary production was sufficientto support concurrent bacterioplankton production. The apparenttemporal disequilibrium in autochthonous bacterioplankton andphytoplankton production was maintained by the seasonal inputof allochthonous organic material into McMurdo Sound. The factsthat a) bacteria were both abundant and highly active, b) bacterivorywas common among many of the endemic protozoa and some planktonicmetazoa and c) these bacterivores consumed >95% of the bacterialproduction strongly suggest that bacteria are a crucial componentin the transfer of energy and material to metazoans in polarregions.     

Regulation of Bacterioplankton Production and Cell Volume in a Eutrophic Estuary

During three periods of 16 to 25 days, bacterioplankton production, bacterial cell volume, chlorophyll a, CO₂ assimilation, and particulate organic carbon were measured in enclosures situated in the eutrophic estuary Roskilde Fjord, Denmark. The enclosures were manipulated with respect to sediment contact and contents of inorganic nutrients, planktivorous fish, and suspension-feeding bivalves. Nutrient enrichment, the presence of suspension feeders, and sediment contact induced pronounced changes in bacterial production, as well as minor changes in bacterial cell volume; however, these effects seemed to be indirect, transmitted via phytoplankton. Bacterial production, measured as [³H]thymidine incorporation, closely followed changes in phytoplankton biomass and production, with time lags of 5 to 10 days. Good correlations of mean bacterioplankton production to chlorophyll a concentration and CO₂ assimilation suggested phytoplankton to be the dominating source of bacterial substrate, apparently independent of nutrient stress. Zooplankton >140 μm, bivalves, and sediment seemed to provide insignificant, if any, substrate for bacterioplankton, and benthic suspension feeders seemed not to act as direct competitors for dissolved organic carbon. The bacterioplankton mean cell volume, measured by image analysis, changed seasonally, with the smallest cells during the summer. Within each period, the bacterial cell volume correlated positively to growth rate and negatively to temperature.     

Relations between bacterioplankton and phytoplankton abundance in three lentic ecosystems in the Colombian Andes

We analyzed relations among phytoplankton and total bacterioplankton fractions in three lentic ecosystems (Neusa and Prado dams, and Fúquene lagoon) with different physicochemical characteristics, in the Andes of Colombia. Samplings were made in three sites of each water body during three surveys. Neusa dam (meso to oligotrophic) had the lowest bacterial concentration; Prado dam (eutrophic) had a high bacterial and algal abundance, and the Fúquene lagoon (mesotrophic) had lower concentrations of phytoplankton but a high relative concentration of bacteria, probably because of its particular conditions: high organic matter and low nutrient levels in the water. There was a negative correlation of total bacterioplankton with the phytoplankton (Pearson = -0.4479, p = 0.019, n=27) and a positive correlation between phytoplankton and heterotrophic bacteria (Pearson = 0.3866, p = 0.062, n=24) and between total bacterioplankton and DBOs (Pearson = 0.4088, p = 0.034, n=27). Apparently, total bacterioplankton and phytoplankton were not coupling, but cultivable bacteria and the phytoplankton had some degree of relationship.     

Daily and vertical dynamics of rotifers under the impact of diatom blooms in the Three Gorges Reservoir,China

The Three Gorges Dam was built in 2005 with a storage capacity of 39.3 billion m³, ranking 22nd in the world. However, since the impoundment of the reservoir, serious blooms of phytoplankton have occurred. Rotifers, having a key role in the freshwater aquatic food web, are important grazers of phytoplankton and an essential food resource to higher trophic consumers. To explore the impacts of phytoplankton blooms on the rotifer community, daily and vertical surveys of rotifers were conducted in a bay of the Three Gorges Reservoir (Xiangxi Bay). Altogether 46 rotifer species were registered, and Synchaeta tremula, Polyarthra vulgaris, and Brachionus calyciflorus were the most abundant species accounting for 36, 26, and 16% of the mean rotifer densities, respectively. Although these dominant species always prevailed in the rotifer community, their proportions changed significantly from non-bloom phase to bloom phase, e.g., the significance of S. tremula deceased from 46.8 to 33.2%, while P. vulgaris and B. calyciflorus increased from 23.9 and 13.9% to 26.2 and 16.2%, respectively. In the vertical water column, all the rotifer following phytoplankton displayed an aggregated distribution, concentrating at the upper layers (0.5–5 m), especially during the bloom phase. From the non-bloom phase to the bloom phase, rotifer densities, the dominant rotifers, Shannon–Wiener and Margalef’s diversity increased significantly, while the evenness displayed the opposite trend. Non-metric multidimensional scaling analysis (NMDS) revealed that the samples in the non-bloom phase were well separated from those in the bloom phase. This means that the outbreak of the diatom bloom in the Xiangxi Bay had significant impacts on the rotifer community. Further investigations are needed to address the impacts of the changes of rotifer community on higher trophic levels.