Microplankton dynamics in a perennially ice-covered Antarctic lake – Lake Hoare

1. Temporal and spatial variation in planktonic abundance, biomass and composition were determined in Lake Hoare (McMurdo Dry Valleys, Antarctica) over two summer seasons (1996–97 and 1997–98). 2. Phototrophic nanoflagellates (PNAN) dominated planktonic biomass, with a mean monthly biomass ranging between 27.3 and 40.4 μg C L^?1. The deep chlorophyll maximum was mainly composed of cryptophytes (>87% of total PNAN biomass) and varied in depth between 6 and 12 m. 3. Maximum bacterial concentration was 11.8 × 10⁵ cells mL^?1. Bacterial abundance showed relatively little temporal variation, with the exception of a drop in numbers that occurred in late November of both years studied. 4. Ciliates were the most successful heterotrophic protozoan group, with a mean monthly biomass (1.2–3.2 μg C L^?1) being typically at least double that of heterotrophic nanoflagellate (HNAN) biomass (0.1–0.7 μg C L^?1). 5. Microbial processes within this lake appear to be dominated by bottom up control. The relative importance of allochthonous inputs into the lake (from the ice‐cover and stream flow) and autochthonous recycling (by microzooplankton regeneration) are considered. 6. Results from a horizontal transect indicate that the permanence of the main sample hole may have enhanced planktonic biomass over a relatively small spatial scale.     

Light climate and phytoplankton photosynthesis in maritime Antarctic lakes

The responses of phytoplankton populations to seasonal changes in radiation flux in two Antarctic lakes with extensive winter ice-cover are described. A phytoplankton capable of photosynthesis was found throughout the year in both systems. During winter, low incident radiation combined with thick layers of snow and ice prevented in situ photosynthesis becoming detectable. The beginning of spring was marked by a reduction in snow cover which resulted in a considerable increase in surface penetrating radiation. Planktonic algae rapidly adapted to utilise these increased levels efficiently, though they still showed characteristics of strong shade adaptation.Loss of ice cover at the start of the short open water period further increased the radiation levels and a summer population developed which was much less shade adapted. Saturation and photoinhibition effects were widespread during this period as the algae proved unable to utilise high radiation levels efficiently. They were however effective at the radiation fluxes prevalent in the lower part of the rapidly circulating water columns.     

Zooplankton induced decrease in inorganic phosphorus uptake by plankton in an oligotrophic lake

Experiments conducted on samples collected from a large oligotrophic lake revealed the following: (1) excretion rates of PO _inf4 ^sup3– by single Daphnia thorata were below detection (5 pmol animal^–1 min^–1) in 20 ml of oligotrophic lake water over a period of 10 min, (2) experimental addition of D. thorata to 20 ml aliquots of lake water decreased community-wide microbial uptake of PO _inf4 ^sup3– on two occasions (as measured by ³²PO _inf4 ^sup3– incorporation), and (3) the presence of D. thorata increased uptake by organisms smaller than 1µm, and decreased uptake by large phytoplankton. The specific mechanism for these responses remains unclear, but the results imply that when phytoplankton larger than 1µm encounter cm scale patches of water recently occupied by Daphnia they may experience decreased PO _inf4 ^sup3– availability rather than elevated concentrations of PO _inf4 ^sup3– caused by excretion. We show that ³²P uptake experiments using natural plankton assemblages can be influenced by the presence or absence of large zooplankton, and that neither grazing, turbulence, nor PO _inf4 ^sup3– excretion can account for this influence.     

Seasonal pattern of ammonium (methylamine) uptake by phytoplankton in an oligotrophic lake

Our primary objective was to determine if a relationship existed between seasonal change in phytoplankton and high affinity for (K _m) or uptake rates (V _maX) of ammonium which might explain seasonal phytoplankton succession in oligotrophic ecosystems. We measured ammonium uptake using [¹⁴C]-methylamine and estimatedK _m andV _max using Hanes Plots at 2-week intervals during 6 months of thermal stratification in Mountain lake, Virginia (37° 22 N, 80° 32 W). Community composition, nutrient levels, and other variables were determined in all uptake experiments. A second objective was to determine if ammonium was preferentially utilized over nitrate and to characterize further the ammonium transport system.V _max increased steadily from May until the end of July, each increase coinciding with major changes in the phytoplankton community. Cryptophyceans dominated in May, chlorophyceans in June and July, and cyanophyceans from the end of July to late October. With cyanophycean dominance,V _max declined until chlorophyceans reestablished dominance in late October. By contrast,K _m values increased from May to the end of July, but thereafter showed no correlation. Acetylene reduction experiments showed no nitrogen fixation during late summer and fall when blue-green algae were present. Preference for ammonium was implied also by negative nitrate reductase assays. Overall, the coincidence ofV _max andK _m values for [¹⁴C]-methylamine uptake and changing phytoplankton community structure suggests the possibility that successive algal communities may be changing as a result of specific species differences in ammonium affinity and uptake rates.     

Vertical distribution of organic constituents in an Antarctic lake: Lake Vanda

Vertical distribution of organic constituents, i.e. total organic carbon (TOC), extractable organic carbon with ethyl acetate (EOC), hydrocarbons, phytol, sterols, fatty acids and phenolic acids in Lake Vanda was studied to elucidate their features in relation to the stratification of lake water and the distribution of lake organisms. The concentrations of TOC, EOC and sterols increased with depth and attained the maximum values of 25 and 1.5 mgC l^–1 and 1.4 g l^–1 in the bottom, respectively, while those of fatty acids showed the maximum value of 61 g l^–1 at a depth of 55.4 m, along with the highest value of the ratio of unsaturated (UC₁₆, uC₁₈) to saturated (C₁₆, C₁₈) acids (8.5) and with the highest carbon preference index (35). Hydrocarbons were only found in the bottom layers (60.4 and 65.9 m) and bottom sediment. These results suggest strongly that the vertical distribution of lake organisms and their activity are quite different due to depth. In the bottom warm anoxic layers the degradation of organic materials must have occurred significantly and thus refractory organic materials should be concentrated.     

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.     

Size-fractionated phytoplankton biomass and its implications for the dynamics of an oligotrophic tropical lake

1. Size-fractionated phytoplankton biomass was examined in relation to the hydrodynamics of tropical Lake Alchichica from 1999 to 2002.
2. Alchichica is a warm monomictic lake, in which mixing takes place from late December to early March. The lake is oligotrophic (mean total chlorophyll- a concentration 4.2 ± 4.2  μ g L⁻¹) and its phytoplankton biomass is dominated (72.3 ± 16.4%) by large individuals (>2  μ m). The degree of dominance of the large size class (nano- and microplankton) over the small size class (picoplankton) throughout the year is mainly determined by the availability of silicate and the Si/N ratio in the hypolimnion prior to the mixing period.
3. This is the first record of an oligotrophic tropical lake dominated by large size fractions of phytoplankton. Because of this dominance, the fate of most primary productivity is rapid sedimentation to the bottom followed by decomposition that promotes an anoxic hypolimnion.
4. Our findings in tropical Lake Alchichica challenge the idea that oligotrophic waters are dominated by small phytoplankton, as has been well established for the oligotrophic ocean and temperate lakes.     

The seasonal dynamics of Spirogyra in a shallow,maritime Antarctic lake

Summary The filamentous chlorophyte Spirogyra forms a mat covering extensive areas of Spirogyra Lake, a small, shallow, Antarctic lake. It has an annual growth pattern, with a maximum standing crop of 400 g chlorophyll- m^-2 during the ice-free summer period. Nutrient concentrations were low and there was evidence for P-limitation. The attainment of such a high standing crop was probably dependent on the lake's high specific dilution rate. Radiation flux was very low under winter ice cover and Spirogyra died back almost completely. The lake water became hypoxic and inorganic nutrients accumulated in both the water column and overwintering algal filaments. Spore formation was not observed, but changes in the composition of filaments indicated that polysaccharides, which had accumulated in summer, were depleted over the long, ice-covered winter period.     

Vertical distribution of organic constituents in an Antarctic lake: Lake Fryxell

Vertical distribution of organic constituents, i.e. total organic carbon (TOC), hydrocarbons, fatty acids and hydroxy acids in water and sediment samples from Lake Fryxell (77° 35 S, 163° 15 E) of southern Victoria Land, Antarctica were studied to elucidate their features in relation to stratification of the lake waters and likely distribution of microorganisms. The TOC content of the surface water (5.0 m; just below the ice cover of 4.50 m thickness) was 1.4 mg l^–1. It increased markedly with depth and attained a maximum value of 21.7 mg C l^–1 at a depth of 17.5 m, but decreased to the bottom (13.3 mg C l^–1). The high TOC content of the anoxic bottom layers (> 15 m) is attributable to the concentration of refractory organic substances over long periods following the degradation of labile organic constituents. Hydrocarbons were not found in the water column, but the major constituent of the bottom sediment was n-C_{29 : 2} alkene. Total concentrations of fatty acids in the oxic layers ( 10 m) were highest at 10.0 m and much higher than those in the anoxic layers (> 10 m), probably reflecting the phytoplankton population. The content of branched (iso and anteiso) fatty acids and 3-hydroxy acids in the anoxic layers were much greater than those in the oxic layers which would seem to reflect the distribution of bacterial abundance. The differences of organic composition between the water column and sediments imply that sinking dead organisms were quickly degraded in the lake bottom. Also, the composition of microorganisms in the water column must be very different from that in the sediments.     

Modelling reproductive effort in sub-and maritime Antarctic mosses

A comparison is made of the reproductive effort (RE), considered as the investment in sporophyte relative to gametophyte biomass, of eight species of moss occurring at sub-and maritime Antarctic sites. Six of the species showed smaller sporophytes and game-tophytes at the climatically more extreme maritime Antaretic sites and one species showed no size difference between regions. The remaining species, although showing no regional difference, showed some evidence of a reverse pattern, with higher altitude samples having greater biomass than lower altitude samples. Spore counts indicated a measure of compensation in maritime Antarctic samples, with no significant decrease in spore output in several species despite smaller sporophyte biomass. The relationship between sporophyte (S) and gametophyte (G) biomass within samples was described by an allometric curve (S=aG ^b ) which gave a better fit than a straight line for six species. This form of model allows comparisons of patterns of RE to be made between samples with non-or partially overlapping size distributions, even when the relationship involves size-dependence. An allometric curve was not appropriate for describing samples of one species (Andreaea regularis), and insufficient data were available to identify any relationship in Polytrichum alpinum. The exponent (b) differed between species, but there were no statistically significant differences between exponents from samples of the same species. Samples of two species could further be described by the same coefficient (a), indicating that they lie on the same curve. However, samples of three species from sub-Antarctic South Georgia gave significantly higher coefficients, indicating increased RE relative to maritime Antarctic populations.     

Seasonal dynamics of the planktonic microbial community in a maritime Antarctic lake undergoing eutrophication

The abundance, biomass and community structure of phytoplankton,bacterioplankton and protozooplankton in a maritime Antarcticlake were determined at approximately monthly intervals fromDecember 1994 to February 1996 and compared with data from earlierstudies. Heywood Lake has become significantly eutrophic duringthe last three decades because of excreta from the expandingfur seal population in its catchment. Marked seasonal variationsin the abundance, composition and productivity of biota werecorrelated with the seasonality of both physical factors andnutrient levels. Protozooplankton were abundant, diverse andusually dominated by heterotrophic nanoflagellates (HNF), withHNF abundance peaking at 2.35 x 10⁷ l^–1 in summer. Highnumbers of naked amoebae were sometimes present, reaching amaximum of 4.8 x 10³ l^–1 in March. An estimated 89 speciesof protozoa were observed during the study, indicating substantiallymore diversity than is found in continental Antarctic lakes.Diversity was highest in spring and lowest in winter, when theentire water column became anoxic and the plankton were dominatedby bacteria and a few species of relatively large anaerobicflagellates. The current status of the lake is compared withdata for continental Antarctic and lower latitude lakes. Earlierstudies of biota and physical/chemical parameters in HeywoodLake are used to examine the effects of eutrophication overthree decades. Observed changes include increased microbialabundance and changes in both community structure and seasonalpatterns.     

Regulation of bacterial assemblages in oligotrophic plankton systems: results from experimental and empirical approaches

Bacteria are relevant members of planktonic food webs, both in terms of biomass and production share. The assessment and comprehension of the factors that control bacterial abundance and production are, thus, necessary to understand how carbon and nutrients circulate in planktonic food webs. It is commonly believed that bacterial abundance, activity and production are either determined by the available nutrient levels (‘bottom-up’ control) or by the effect of predators (‘top-down’). These factors have also been shown to regulate the internal structure (the physiological and phylogenetic structure) of the bacterioplankton black box. We present here different empirical and experimental ways in which the factors that control bacterial communities are assessed, among them, the direct comparison of the rates of bacterial growth and losses to grazing. Application of several of these methods to open ocean data suggests that bacteria are regulated by resources at the largest scales of analysis, but that this overall regulation is strongly modulated by predators in all types of systems. In the most oligotrophic environments, bacterial abundance and growth are regulated by predators, while in the richest environments it is bacterial (phylogenetic, size, activity) community composition that is most affected by protist predators, while abundance can be influenced by metazoans. Because changes in bacterial community composition require that bacteria have enough nutrient supply, the overall effect of these regulations is that bacterial growth appears to be top-down regulated in the most nutrient-poor environments and bottom-up regulated in the richer ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Autotrophic picoplankton community dynamics in a pre-alpine lake in British Columbia,Canada

Autotrophic picoplankton (APP) were studied in Chilko Lake, a large, deep ultra-oligotrophic pre-alpine lake (elevation: 1172 m) in the south central coast mountains of British Columbia. Data from 1985 (untreated) and 1990 (treated) were used to compare and contrast APP community response to a whole-lake fertilization experiment. The APP communities of Chilko Lake were dominated by the coccoid cyanobacteria Synechococcus and its colonial morph which comprised about 99% of the APP community of Chilko Lake. Chlorella-like eukaryotic picoplankters and small cyanobacteria were rare, comprising < 1 % of the APP community. In 1990 autotrophic picoplankters contributed an average of 73% to total chlorophyll, and 54% to total photosynthesis. Average APP abundance ranged from lows of 4,000–5,000 cells ml^-1 in winter and spring to highs of 50000–150000 cells ml^-1 in early August with no apparent autumnal increase. APP populations were uniformly distributed in the epilimnion, but during calm periods in August often formed a peak near the metalimnion/hypolimnion boundary. Seasonal and vertical distribution patterns of APP showed little relation to temperature or to light. When nutrients were added to the lake in 1990, APP populations doubled within 3 wk of addition and average abundance (6.16 × 10⁴ cells · ml^-1) was twice 1985 APP numbers. Bottom-up control by scarce nutrient supplies is considered the primary factor regulating community composition and abundance during the initial population growth phase (June, July) with top-down control by grazing during nutrient colimitation periods when the epilimnion is deplete of both nitrogen and phosphorus (August, September).     

The unicellular freshwater cyanobacterium Synechococcus and mixotrophic flagellates: evidence for a functional association in an oligotrophic, subalpine lake

1. We applied Reynolds's approach to the study of phytoplankton ecology through functional associations of species to identify possible algal species, which associate with the freshwater cyanobacterium Synechococcus. Previously an association among Synechococcus spp. and small‐celled chlorophytes (association Z) has been recognised by evaluating phytoplankton associations according to functional criteria. 2. Biomass data for phytoplankton and picocyanobacteria from Lago Maggiore spanning more years were organised in a matrix and a cluster analysis was performed. The results showed four groups separated at a linkage distance of 0.20. Mixotrophic species which clustered with Synechococcus spp. were Ceratium hirundinella, Chrysochromulina parva, Cryptomonas erosa, Cryptomonas ovata, Dinobryon bavaricum, Dinobryon sociale, Rhodomonas minuta and Uroglena americana. The redundancy analysis (RDA) consolidated the association of Synechococcus with C. hirundinella and R. minuta showing greater probability of occurrence than random aggregates of species. 3. The association Synechococcus spp. –C. hirundinella also appeared from temporal variation of their biomass. In early summer both these species increased at the same time; later, the peak of C. hirundinella (potential predator) coincided with a minimum of Synechococcus spp. suggesting a possible predator‐prey interaction. This implied that phytoplankton assemblages which form a functional group cannot only have similar adaptations and requirements but can also exhibit trophic interactions. 4. We propose to enlarge the association Z and create an association Z_MX (where _MX stands for mixotrophs) which would include Synechococcus spp. and C. hirundinella as the most representative of the mixotrophic species found in the oligotrophic Lago Maggiore.     

Long-term phytoplankton changes in an artificially divided,top-down manipulated humic lake

Lake Große Fuchskuhle (Brandenburg, Germany) is a naturally acidic bog lake that was artificially divided into four basins by large plastic curtains for biomanipulation experiments in 1990. Different numbers of perch were added to each compartment beginning in the spring of 1993. The species composition and abundance of phytoplankton, pH, nutrient concentrations, dissolved organic carbon (DOC) and chlorophyll a content were analyzed at regular intervals during 1991 and 1998. The division of the lake resulted in divergent developments in the physical and chemical environment of the compartments. This study compared the phytoplankton assemblages of the Northeast- (NE) and Southwest- (SW) basins which differed strongly in chemistry during the investigation period. Divergent developments in phytoplankton species composition in both basins can be explained by changes in physical and chemical conditions (bottom-up effects). Increased pH values and DOC concentrations probably favoured mass developments of the dinoflagellate Gymnodinium uberrimum since 1993, while increased nutrients (dissolved inorganic carbon, total nitrogen and especially total phosphorus) as well as further changes in pH and DOC led to the dominance of the raphidophyte Gonyostomum semen in 1998. This bloom was characterized by extreme biomasses of up to 143 mg l^–1 wet weight, corresponding with high chlorophyll a concentrations of up to 413 g l^–1 at the same time. In contrast, no significant relationship between experimental manipulations by piscivorous fish stocking (top-down effects) and phytoplankton biomass were observed.     

The temporal and spatial distribution of planktonic and benthic protozoan communities in a small productive lake

An investigation into the spatial and temporal distribution of protozoa in Esthwaite Water, a small eutrophic lake in the English Lake District, was carried out from mid-June until late September, 1977, during the period of summer stratification. Quantitative analyses of planktonic protozoan populations were performed on water samples collected at 1 m intervals throughout the water column and individual depth-time distribution profiles were constructed for the major ciliate species. Population density and species succession of the benthic protozoa was also studied.     

Size spectra and abundance of planktonic ciliates within various habitats in a macrophyte-dominated lake (Eastern Poland)

Body size, community structure, abundance and biomass of ciliates were compared in various stands of macrophytes in a macrophyte-abundant shallow lake in Eastern Poland. Samples were collected in belts of Phragmites, Typha, Ceratophyllum, Elodea, Stratiotes and Chara. Additionally, protozooplankton was collected from the open water zone surrounding the vegetation belts. Differences in numbers of ciliate taxa between micro-sites were statistically significant. The highest numbers were found in Chara and Ceratophyllum stands, lower numbers in Stratiotes and Elodea stands and the lowest in the open water, Phragmites and Typha areas. Ciliate biomass was, like density, significantly higher in submerged macrophytes than in emergent macrophytes and open water zones. Based on differences in macrophyte structure, two groups of habitats with similar patterns of size-related ciliate distribution were distinguished. The first group consisted of two vegetated zones of sparse stem structure (Phragmites and Typha) and the open water zone, the second group comprised submerged macrophyte species, which were more dense and complex. Generally, the abundance of ciliates correlated positively with total suspension solid (TSS) and total organic carbon (TOC) concentrations. In the Chara and Ceratophyllum stands, relations between ciliate numbers, TSS and TOC were stronger.