The distribution of microplankton in the McMurdo Dry Valley Lakes,Antarctica: response to ecosystem legacy or present-day climatic controls?

Plankton abundance and biomass were investigated in five lakes of the McMurdo Dry Valleys, Antarctica: Lakes Bonney, Fryxell, Joyce, Hoare and Miers. Despite plankton communities being dominated by organisms <100 m in length, there were striking differences between the lakes, including large variations in plankton vertical distribution and differences in total plankton biomass. Bacterial biomass was highest in the anoxic monimolimnia of the meromictic lakes, reaching 191 g C l^–1 in Lake Fryxell. Photosynthetic nanoflagellates dominated phytoplankton in the five lakes studied. Highest chlorophyll a concentrations were recorded at the chemocline of Lake Fryxell (21 g chl a l^–1). Heterotrophic nanoflagellate concentrations were low, ranging from 2 cells ml^–1 in Hoare to 237 cells ml^–1 in Bonney. By Antarctic standards, ciliates were relatively successful in terms of biomass and diversity in Lakes Fryxell and Hoare. In contrast, Lake Miers possessed extremely low ciliate abundance (<0.04 cells ml^–1). On both sampling occasions, copepod nauplii were observed in Lake Joyce. This is the first recording of crustacean zooplankton within the McMurdo Dry Valley Lakes. Because the foodwebs of these lakes are structured by bottom-up forces, differences in plankton distributions could be related to the physicochemical characteristics of each lake. The effect of lake evolution (legacy) and present-day climate change on planktonic dynamics is discussed.     

Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica

1. The influence of inorganic nitrogen and phosphorus enrichment on phytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell and Vanda, which lie in the ablation valleys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were conducted during the austral summer on phytoplankton populations just beneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01) by phosphorus enrichment (2 μM) with further stimulation by simultaneous phosphorus plus NH₄⁺ (20 μM) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, although they were not statistically significant at P < 0.05. Photosynthesis in this lake was never enhanced by the addition of 20 μM NH₄⁺ alone. Simultaneous addition of phosphorus plus nitrogen stimulated photosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxell. No nutrient response occurred in Lake Vanda, where activity in nutrient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the field resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4⁺+ NO₂^?+ NO₃^?): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN : SRP ratios also imply phosphorus deficiency in Lake Vanda, although not as strong as in Lake Bonney. Particulate carbon (PC): particulate nitrogen (PN) ratios all exceed published ratios for balanced phytoplankton growth, indicative of nitrogen deficiency. 4. Vertical nutrient profiles in concert with low advective flux, indicate that new (sensu Dugdale & Goering, 1967) phytoplankton production in these lakes is supported by upward diffusion of nutrients from deep nutrient pools. This contention was tested by computing upward DIN : SRP flux ratios across horizontal planes located immediately beneath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flux ratios further corroborated nutrient bioassay results and bulk DIN : SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neither biochemical reactions nor physical processes appear to be responsible for differences in nutrient deficiency among the study lakes. The differences may instead be related to conditions which existed before or during the evolution of the lakes.     

Particulate organic and dissolved inorganic carbon stable isotopic compositions in Taylor Valley lakes,Antarctica: the effect of legacy

In perennially ice-covered lakes of Taylor Valley, Antarctica, “legacy”, a carryover of past ecosystem events, has primarily been discussed in terms of nutrient and salinity concentrations and its effect on the current ecology of the lakes. In this study, we determine how residual pools of ancient carbon affect the modern carbon abundance and character in the water columns of Lakes Fryxell, Hoare, and Bonney. We measure the stable carbon isotopic compositions and concentrations of particulate organic carbon (POC) and dissolved inorganic carbon (DIC) in the water column of these lakes over four seasons (1999–2002). These data are presented and compared with all the previously published Taylor Valley lacustrine carbon stable isotopic data. Our results show that the carbon concentrations and isotopic compositions of the upper water columns of those lakes are controlled by modern processes, while the lower water columns are controlled to varying degrees by inherited carbon pools. The water column of the west lobe of Lake Bonney is dominated by exceptionally high concentrations of DIC (55,000–75,000 μmol l⁻¹) reflecting the long period of ice-cover on this lake. The east lobe of Lake Bonney has highly enriched δ¹³C_DIC values resulting from paleo-brine evaporation effects in its bottom waters, while its high DIC concentrations provide geochemical evidence that its middle depth waters are derived from West Lake Bonney during a hydrologically connected past. Although ancient carbon is present in both Lake Hoare and Lake Fryxell, the δ¹³C_DIC values in bottom waters suggest dominance by modern primary productivity-related processes. Anaerobic methanogenesis and methanotrophy are also taking place in the lower water column of Lake Fryxell with enough methane, oxidized anaerobically, to contribute to the DIC pool. We also show how stream proximity and high flood years are only a minor influence on the carbon isotopic values of both POC and DIC. The Taylor Valley lake system is remarkably stable in both inter-lake and intra-lake carbon dynamics. Handling editor: K. Martens     

Distribution and dynamics of bacterioplankton production in a polymictic tropical lake (Lago Xolotlán,Nicaragua)

From 1988 to 1993 we assessed the variability of bacterioplankton production and biomass in Lake Xolotlán (L. Managua), Nicaragua via [3H]thymidine incorporation into DNA and cell counting. Bacterial production ranged from 3 to 8 μg C l-1 h-1, and since production was equal throughout the water column, areal production was high (≈ 600–1200 mg C m-2 d-1). Bacterial abundance in Lake Managua was extremely high, 7–30 × 109 cells l-1. Thus, specific rates of bacterial production were low. There was a strong correlation between production and number and the specific rate of bacterial production was constant. Comparable measurements of production via [3H]leucine incorporation into proteins indicated that bacteria were experiencing ‘balanced growth’. We conclude that bacterioplankton in Lake Xolotlán had reached its carrying capacity and a significant correlation between bacterial production and concentration of phaeophytin implied that dead or dying algae was the limiting substrate for bacterioplankton. The majority of bacterial number and most of bacterial production (up to 75%) were associated with particles in the >3-μm fraction, probably lysing algal cells to which bacterioplankton were ‘attached’. Grazing on bacterioplankton must be low and bacteria should be a ‘sink’ for organic matter in Lake Xolotlán. This revised version was published online in July 2006 with corrections to the Cover Date.     

The balance between photosynthesis and grazing in Antarctic mixotrophic cryptophytes during summer

SUMMARY 1. Grazing and photosynthetic contributions to the carbon balance of planktonic, mixotrophic cryptophytes in Lakes Fryxell and Hoare in the Taylor Valley, Antarctica were measured during November and December 2000.
2. The cryptophytes never became entirely photosynthetic, although carbon derived from grazing decreased in December. Individual grazing rates ranged between 5.28 and 10.08 bacteria cell⁻¹ day⁻¹ in Lake Fryxell and 0.36–11.76 bacteria cell⁻¹ day⁻¹ in Lake Hoare. Grazing rates varied temporally and with depth in the water column. In Lake Fryxell, which is a meromictic lake, highest grazing occurred just above the chemocline. Individual photosynthetic rates ranged from 0.23 to 1.35 pg C cell⁻¹ h⁻¹ in Lake Fryxell and 0.074 to 1.08 pg C cell⁻¹ h⁻¹ in Lake Hoare.
3. Carbon acquisition by the cryptophyte community gained through grazing ranged between 8 and 31% during November in Lake Fryxell, dropping to between 2 and 24% in December. In Lake Hoare grazing contributed 12–21% of the community carbon budget in November and 1–28% in December. Around 4% of the carbon acquired from grazing and photosynthesis was remineralised through respiration.
4. Mixotrophy is probably a major survival strategy for cryptophytes in the extreme lakes of the Dry Valleys, because perennial ice-cover severely limits light penetration to the water column, whereas these phytoflagellates are not normally mixotrophic in lower latitude lakes. The evidence suggests that mixotrophy may be a mechanism for supplementing the carbon budget, as well as a means of acquiring nutrients for growth.     

Population dynamics of bacterioplankton in an oligotrophic lake

The population ecology of bacterioplankton was studied overa 3 year period in Mirror Lake, an oligotrophic lake in thenortheastern USA. Bacterial population density, biomass, andrates of biomass production in the epilimnion and hypolimnionwere examined for their relationship with several environmentalparameters. Bacterioplankton density fluctuated between 0.5and 7 x l0 bacteria ml^–1, with highest values in the anoxichypolimnion. At all depths there was a trend towards a higherdensity of bacteria from spring to midsummer, followed by adecline in late summer to early autumn. Cocci tended to dominatebacterial cell shapes from winter to midsummer, after whichrod-shaped cells became most abundant. Rod-shaped cells contributedthe most to bacterioplankton biomass at all depths and timesof year. The mean annual biovolume of all bacterioplankton was0.12 µ cell^–1. The mean annual areal bacterioplanktonbiomass was 11–12 mmol C m^–2. The percentage ofbacterial to phytoplankton biomass per volume in summertimewas 27% in the epilimnion and 11% in the hypolimnion. Averageannual and summertime bacterial production estimated using the[3H]thymidine method was similar to previous estimates of bacterialproduction measured in Mirror Lake using other methods. Theaverage ratio of bacterial to net phytoplankton production pervolume was 0.34 in the epilimnion, and between 0.65 and 1 1.depending on depth, in the hypolimnion during summer. Of severalvariables considered in regression analyses, only temperatureexplained >50% of the variance in bacterial production inboth the hypolimnion and epilimnion. Above 14°C, however,bacterial production and growth rate in the epilimnion werenot clearly related to temperature. During the period of midsummerhypolimnetic anoxia, despite colder temperatures in the hypolimnion,bacterial production was up to 10 times greater than in theepilimnion.     

Pico- and Nanoplankton in Aquatic Ecosystems in the Valley of the Lakes and Great Lakes Depression (Mongolia)

The abundance and biomass of bacterioplankton, phototrophic picoplankton, and heterotrophic nanoflagellates has been determined in lakes, rivers, and reservoirs located in the Valley of the Lakes and Great Lakes Depression (Mongolia). The species richness of the heterotrophic flagellates and their consumption of bacteria are estimated. Pico- and nanoplankton are the most abundant in shallow mineral lakes Orog and Tatsyn and in the freshwater Durgun Reservoir. Heterotrophic nanoflagellates consume 26–92% (on average 66%) of the daily bacterioplankton production. Thus, flagellates are important in the transfer of bacterial carbon to the higher levels of planktonic trophic webs. A total of 30 species and their forms of heterotrophic flagellates from 14 large taxa are identified. The highest species diversity of these protists are found in the Durgun and Taishyr reservoirs.     

Bacterioplankton in the littoral and pelagic zones of subtropical shallow lakes

We measured bacterioplankton (phylotypes detected by fluorescent in situ hybridisation, morphometric forms, abundance and production) in samples collected in summer in the littoral and pelagic zones of 10 subtropical shallow lakes of contrasting area (from 13 to 80,800 ha). Compared to the pelagic zones, the littoral zones were overall characterised by higher macrophyte dominance and lower concentrations of total phosphorus and alkalinity and higher concentrations of dissolved organic carbon (DOC) and humic substances. Similarities of bacterial production and biomass turnover and density of active phylotypes and morphotype proportions were related to similarities in a set of environmental variables (including nutrients, humic substances content, predator density and phytoplankton biomass), and some additionally to lake area. Horizontal heterogeneity in bacterioplankton variables (littoral versus pelagic) increased with lake area. Bacterioplankton biomass and production tended to be lower in the littoral zone than in the pelagic zone despite higher concentrations of DOC and humic substances. A likely explanation is higher predation on bacterioplankton in the littoral zone, although allelophatic effects exerted by macrophytes cannot be excluded. Our results indicate that organic cycling via bacterioplankton may be less efficient in the littoral zone than in the pelagic zone of shallow lakes.     

Contrasting bacterioplankton community composition and seasonal dynamics in two neighbouring hypertrophic freshwater lakes

We characterized the bacterioplankton community and its seasonal dynamics in two neighbouring hypertrophic lakes by denaturing gradient gel electrophoresis (DGGE) analysis of short (193 bp) 16S ribosomal DNA polymerase chain reaction (PCR) products obtained with primers specific for the domain Bacteria. Lake Blankaart is turbid and has a high phytoplankton biomass and episodic cyanobacterial blooms, whereas biomanipulated Lake Visvijver is characterized by clearwater conditions and the establishment of a dense charophyte vegetation. Both lakes were dominated by bacterial groups commonly found in freshwater habitats (e.g. ACK4 cluster of Actinomycetes; ACK stands for clones isolated from the Adirondack mountain lakes) . Yet, cluster analysis and principal components analysis (PCA) revealed that taxon composition of the bacterioplankton community of the two lakes differs substantially and consistently throughout the season. During the study year (1998), the bacterioplankton community of both lakes showed a distinct seasonal pattern. Lake Blankaart showed a clear differentiation between winter, spring, summer and autumn. In Lake Visvijver, summer samples differed greatly from spring, autumn and winter samples. We hypothesize that the contrasting bacterioplankton in the two neighbouring shallow lakes is determined largely by the presence or absence of macrophytes.     

Specific activity of cell-surface acid phosphatase in different bacterioplankton morphotypes in an acidified mountain lake

Activity of extracellular acid phosphatases was measured at single-cell level in bacterioplankton groups defined by their morphology and size, in acidified mountain Lake Certovo, during the 2003 season, with a method based on use of the substrate ELF97 phosphate which provides fluorescent precipitates upon hydrolysis by phosphatases. The bacterial cell-associated precipitates were quantified by image analysis. A specific, conspicuous, apparently homogeneous morphotype of curved cells of approximately 5 microm average length, despite its low total biomass (average of 4%), contributed significantly (in average by 31%) to the total bacterioplankton phosphatase activity in Lake Certovo (ranging from 1.0 to 12.7 micromol l(-1) h(-1), using ELF97 phosphate as a substrate). Bacterial filaments (> 10 microm), although comprising in average 85% of bacterioplankton biomass, contributed to the total bacterioplankton activity only by 45%. Biomass-specific activity of extracellular (cell-surface) phosphatases of the main bacterioplankton morphotypes increased in the order filaments < cocci and rods < curved cells. The biomass-specific activity of bacterioplankton extracellular phosphatases (0-300 nmol microgC(-1) h(-1)) was generally highest in the spring and decreased gradually during summer. These changes could result from seasonal changes in the phosphorus status of the lake and from subsequent regulation of enzyme expression by bacteria.     

Mixotrophic cryptophytes and their predators in the Dry Valley lakes of Antarctica

1. The ingestion rates of planktonic, mixotrophic cryptophytes in two perennially ice-covered Antarctic lakes in the McMurdo Dry Valleys, were investigated during the summer of 1997–1998.
2. In Lake Fryxell, which is meromictic, ingestion rates increased with depth in November and were highest in a cryptophyte maximum close to the chemocline. In Lake Hoare, which is unstratified and freshwater, there was no significant difference in ingestion rates with depth. In both lakes, the highest ingestion rates occurred in early summer, decreasing in December and January. Ingestion rates varied between 0.2 bacteria cell⁻¹ h⁻¹ and 3.6 bacteria cell⁻¹ h⁻¹.
3. During November, mixotrophic cryptophytes removed up to 13% of bacterial biomass day⁻¹ and had a greater grazing impact than heterotrophic nanoflagellates (HNAN). As summer progressed, the grazing impact of cryptophytes and HNAN became similar.
4. The maximum depth of cryptophytes in Lake Fryxell was predated by a population of the ciliate Plagiocampa. Plagiocampa had an ingestion rate of 0.13–0.19 cryptophytes cell⁻¹ h⁻¹. The grazing impact on the cryptophyte community was insignificant. However, the ciliate appeared to be indulging in temporary mixotrophy, sequestering the cryptophytes for a number of weeks before digesting them.
5. It is suggested that mixotrophy is an important survival strategy in the extreme lake ecosystems of the McMurdo Dry Valleys.     

Ciliated protozoa of two antarctic lakes: analysis by quantitative protargol staining and examination of artificial substrates

Planktonic and artificial substrate-associated ciliates have been identified in two perennially ice-covered antarctic lakes of the McMurdo Dry Valleys. Abundances estimated by quantitative protargol staining ranged from <5 to 31690 cells · l⁻¹, levels that are comparable to those previously obtained using other methods. Nineteen ciliate taxa were identified from these lakes, with the most frequently encountered genera being Plagiocampa, Askenasia, Monodinium, Sphaerophrya and Vorticella. The taxonomic findings compare favorably with those of previous investigators; however four previously unreported genera were observed in both Lakes Fryxell and Hoare. The variability in the depth distributions of ciliates in Lake Fryxell is explained in terms of lake physicochemical properties and ciliate prey distributions, while factors related to temporal succession in the Lake Hoare assemblage remain unexplained. Local marine or temperate zone freshwater habitats are a more likely source than the surrounding dry valleys soils for present ciliate colonists in these lakes. Although the taxonomic uncertainties require further examination, our results suggest that ciliate populations in these antarctic lakes undergo significant fluctuations and are more diverse than was previously recognized. Accepted: 27 July 1998     

The microfauna of algal mats and artificial substrates in Southern Victoria Land lakes of Antarctica

The first microfaunal colonization study of continental lakes in Antarctica is reported. In addition to the general taxonomic composition of six Antarctic lakes, detailed community composition analyses of two adjacent lakes, Lake Fryxell and Lake Hoare, using polyurethane foam artificial substrates suspended in the open water and placed directly over benthic algal mats are also presented. Protozoans, rotifers, tardigrades and nematodes constituted the microfaunal community in all lakes. Higher metazoans do not occur in lakes that we studied. Protozoans were the most diverse of the four groups, and the first dinoflagellates reported from southern Victoria Land, Gymnodinium and Glenodinium, were collected from Lake Fryxell. The greatest number of protozoan taxa (55) were associated with the algal mats in the more productive Lake Fryxell. Both the physical and chemical properties and taxonomic diversity showed marked variability between all lakes that were studied, especially the adjacent lakes, Fryxell and Hoare. Ecological variables which regulate taxonomic diversity in other lakes of the world also appear to exert similar influences in remote Antarctic lakes.     

Response of an antarctic lake heterotrophic community to high dissolved oxygen

The upper waters of Lake Hoare, Antarctica, contain dissolved oxygen at about three times the normal saturation (>/=42 mg liter). The response of the heterotrophic plankton community to this high dissolved oxygen was evaluated by the criteria of CFU and d-[U-C]glucose assimilated-respired. High dissolved oxygen was not inhibitory to d-[U-C]glucose assimilation-respiration compared with normal atmospheric dissolved oxygen in Lake Hoare water. The d-[U-C]glucose was assimilated and respired optimally at 12 degrees C in Lake Hoare. The d-[U-C]glucose assimilated-respired in the upper saturated atmospheric dissolved oxygen waters of Mountain Lake, Va., was inhibited in contrast to Lake Hoare (P < 0.05). CFU formation was inhibited in both lakes. CFU represent <1% of the fluorochrome-stained direct counts in Lake Hoare. Lake Hoare planktobacteria are smaller than the planktobacteria in Mountain Lake. ATP size fractionation revealed that 39% of the ATP biomass was <0.5 mum in Lake Hoare.     

Weak Coupling between Heterotrophic Nanoflagellates and Bacteria in a Eutrophic Freshwater Environment

In a study on the dynamics and trophic role of the heterotrophic nanoflagellate (HNAN) assemblage in the microbial food web of a eutrophic oxbow lake abundances, biomass, and production rates of HNAN and their potential prey organisms, namely heterotrophic bacteria and autotrophic picoplankton, were monitored for a period of 2 years. No coupling between HNAN abundance and biomass and the abundance and biomass of their picoplanktonic prey was observed for the investigation period. The ratio of heterotrophic bacterial to HNAN abundance ranged from 2.2 x 103 to 8.6 x 103 (mean: 4.2 x 103 +/- 1.8 x 103). HNAN carbon consumption could account for only 10% to 40% of bacterial secondary production. The lack of coupling between HNAN and their potential prey and the low HNAN abundance relative to bacterial abundance suggested (a) that HNAN grazing was an insignificant factor in the regulation of bacterial abundance and (b) that HNAN abundance was regulated by predation rather than by prey abundance. This hypothesis was supported by the fact that HNAN growth rates were high (in the range of 0.45 d-1 to 1.00 d-1 during spring and summer, yearly mean: 0.52 d-1), and only weakly correlated with prey abundance and biomass. The results indicated strong top-down control of HNAN and consequently a weak coupling of HNAN and picoplankton in the investigated eutrophic freshwater environment.     

Occurrence of mixotrophic flagellates in relation to bacterioplankton production, light regime and availability of inorganic nutrients in unproductive lakes with differing humic contents

1. Field data from five unproductive Swedish lakes were used to investigate the occurrence of mixotrophic flagellates in relation to bacterioplankton, autotrophic phytoplankton, heterotrophic flagellates and abiotic environmental factors. Three different sources of data were used: (i) a 3‐year study (1995–97) of the humic Lake Örträsket, (ii) seasonal measurements from five lakes with widely varying dissolved organic carbon (DOC) concentrations, and (iii) whole lake enrichment experiments with inorganic nutrients and organic carbon. 2. Mixotrophic flagellates usually dominated over autotrophic phytoplankton in Lake Örträsket in early summer, when both bacterial production and light levels were high. Comparative data from the five lakes demonstrated that the ratio between the biomasses of mixotrophic flagellates and autotrophic phytoplankton (the M/A‐ratio) was positively correlated to bacterioplankton production, but not to the light regime. Whole lake carbon addition (white sugar) increased bacterial biomass, and production, reduced the biomass of autotrophs by a factor of 16, and increased the M/A‐ratio from 0.03 to 3.4. Collectively, the results indicate that the dominance of mixotrophs among phytoplankton was positively related to bacterioplankton production. 3. Whole lake fertilisation with nitrogen (N) and phosphorus (P) demonstrated that the obligate autotrophic phytoplankton was limited by N. N‐addition increased the biomass of the autotrophic phytoplankton but had no effect on mixotrophic flagellates or bacteria, and the M/A‐ratio decreased from 1.2 to 0.6 after N‐enrichment. Therefore, we suggest that bacteria under natural conditions, by utilising allochthonous DOC as an energy and carbon source, are able to outcompete autotrophs for available inorganic nutrients. Consequently, mixotrophic flagellates can become the dominant phytoplankters when phagotrophy permits them to use nutrients stored in bacterial biomass. 4. In Lake Örträsket, the biomass of mixotrophs was usually higher than the biomass of heterotrophs during the summer. This dominance could not be explained by higher grazing rates among the mixotrophs. Instead, ratios between mixotrophic and heterotrophic biomass (the M/H‐ratio) were positively related to light availability. Therefore, we suggest that photosynthesis can enable mixotrophic flagellates to outcompete heterotrophic flagellates.     

Bacterial secondary production and it relation with primary production in the Embalse del Río III Reservoir,Argentina

During an annual period the bacterial biomass (epifluorescence) and secondary production (methyl-³H-thymidine incorporation), as well as biomass (Chlorophyll) and algal primary production (incorporation of NaH¹⁴CO₃) were studied in the Embalse del Río III Reservoir, Argentina. The relations between these variables and their responses to seasonal changes in water temperature were analyzed. A close relationship in seasonal patterns of algal primary production and bacterial secondary production was observed, with the estimated rates of bacterial production similar to that obtained by other authors in eutrophic lakes. Bacterial production was 17 to 46% of the primary production, thus, at 60% assimilation efficiency, the bacterioplankton would consume 28 to 77 % of the total fixed carbon. Seasonal trends in algal primary production and bacterial secondary production were mainly affected by temperature.     

Seasonal changes in the abundance and composition of marine heterotrophic bacterial communities in an Antarctic coastal area

During a 1-year period, systematic observations of the Antarctic coastal marine bacterioplankton were recorded. Three field stations were sampled weekly in 1989 in Terre Adélie area. The survey included physicochemical (temperature and particulate organic matter) and bacteriological (total and heterotrophic counts, estimation of bacterial production) measurements. The bacterial community structure was investigated by carrying out 27 morphological and biochemical tests on 254 strains isolated during each season. Gram-negative non-fermentative rods were always dominant. However, an obvious difference exists between the communities inhabiting ice-free and ice-covered seawater. The potential metabolic abilities which were relatively significant in the summer community were severely reduced in the winter community. A general increase in bacterial biomass and production was observed in surface water after sea ice formation. The results suggest a close coupling between heterotrophic bacterioplankton and the input of allochthonous organic carbon, for example from the overlying sea-ice communities or from nearby penguin rookeries.     

Purple Sulfur Bacteria Control the Growth of Aerobic Heterotrophic Bacterioplankton in a Meromictic Salt Lake

In meromictic Mahoney Lake, British Columbia, Canada, the heterotrophic bacterial production in the mixolimnion exceeded concomitant primary production by a factor of 7. Bacterial growth rates were correlated neither to primary production nor to the amount of chlorophyll a. Both results indicate an uncoupling of bacteria and phytoplankton. In the chemocline of the lake, an extremely dense population of the purple sulfur bacterium Amoebobacter purpureus is present year round. We investigated whether anoxygenic phototrophs are significant for the growth of aerobic bacterioplankton in the overlaying water. Bacterial growth rates in the mixolimnion were limited by inorganic phosphorus or nitrogen most of the time, and the biomass of heterotrophic bacteria did not increase until, in autumn, 86% of the cells of A. purpureus appeared in the mixolimnion because of their reduced buoyant density. The increase in heterotrophic bacterial biomass, soluble phosphorus concentrations below the detection limit, and an extraordinarily high activity of alkaline phosphatase in the mixolimnion indicate a rapid liberation of organically bound phosphorus from A. purpureus cells accompanied by a simultaneous incorporation into heterotrophic bacterioplankton. High concentrations of allochthonously derived dissolved organic carbon (mean, 60 mg of C(middot)liter(sup-1)) were measured in the lake water. In Mahoney Lake, liberation of phosphorus from upwelling purple sulfur bacteria and degradation of allochthonous dissolved organic carbon as an additional carbon source render heterotrophic bacterial production largely independent of the photosynthesis of phytoplankton. A recycling of inorganic nutrients via phototrophic bacteria also appears to be relevant in other lakes with anoxic bottom waters.     

Biomass, production and heterotrophic activity of bacterioplankton in the Great Astrolabe Reef lagoon (Fiji)

 Biomass, production and heterotrophic activity of bacterioplankton were determined for two weeks in the Great Astrolabe Reef lagoon, Fiji. Bacterial and Bacterial activities were distributed homogeneously throughout the water column (20 to 40 m deep) and varied little from site to site inside the lagoon. Bacterioplankton biomass and production also varied little over a diel period with coefficients of variation of 9 and 22%, respectively. On average, over the whole study, bacterial abundance was 0.77×10⁹ cells l^-1 and bacterial production averaged 0.36 μg-at. C l^-1 d^-1. Bacterial abundance and production were greater in the lagoon than in oceanic waters. Attachment to particles seems to provide an advantage for bacterioplankton growth because specific growth rates for attached bacterioplankton were, on average, significantly greater than that of the free community. Growth efficiency, determined by correlating the net increase of bacterial biomass and the net decrease of dissolved organic carbon (DOC) in dilution cultures, was very low (average 6.6%). Using carbon growth efficiency and bacterial production rates, heterotrophic activity was estimated to average 5.4 μg-at. C l^-1 d^-1. The turn-over rate of DOC (average 114 μg-at. C l^-1) due to bacterial consumption was estimated to be 0.048 d^-1 during the period of study. Accepted: 25 July 1998