PHOSPHORUS,NITROGEN AND THE GROWTH OF ALGAE IN LAKE KINNERET1

The intracellular concentrations of carbon, nitrogen, phosphorus and chlorophyll a of phytoplankton and zooplankton in Lake Kinneret were determined from 1969 to 1973. The ratios C:P, C:N, chlorophyll a:P, chlorophyll a:N of the algae showed fluctuations which could be related to the nutrient conditions that influence the annual pattern of phytoplankton development, especially in respect to the dinoflagellate bloom of Peridinium cinctum (OFM) Ehr. f. westii (Lemm.) Lef. Relatively high intracellurar P values at the start of the bloom indicated adequate availability of this nutrient and luxury consumption over a short period of time. Later, Peridinium continued to grow despite unusually high C:P ratios (> 300:1). In most years, phosphorus may have eventually limited growth, however, in 1970, the bloom censed despite comparatively high intracellular P concentrations. These observations, together with supplementary evidence from nutrient addition experiments and determinations of specific alkaline phosphatase levels, indicated that, for most of the growth phase of the bloom, Peridinium cells were not directly limited by P. The decline of the bloom usually, but not always (e.g., in 1970), was marked by very high C:P ratios. Thus, a shortage of P may often be a contributory factor to the cessation of the Peridinium bloom and may be limiting phytoplankton growth in the fall. Over the years 1969–73, possibly due to an overall drop in salinity, there appears to be a trend to lower levels of biologically bound phosphorus in Lake Kinneret, without a concomitant decrease in carbon biomass.     

Influence of metazooplankton on interactions of bacteria and phytoplankton in an oligotrophic lake

Commensalism based on organic carbon supplied by phytoplanktonand competition for mineral nutrients are important interactionsbetween bacteria and phytoplankton in oligotrophic clear-watersystems. Both interactions are influenced by zooplankton activity.To examine the relation ship between algae and bacteria in LakeLa Caldera, we studied: the correlations among phyto plankton,bacteria and phosphorus (P) dynamics; the ratio of organic carbonsupplied by algae to organic carbon demand by bacteria; andthe importance of P remineralized by metazooplankton for bothcommunities. Phytoplankton and bacteria had a similar seasonaldynamics, and there was a sig nificant and positive relationshipbetween bacterial abundance and algal biomass (P<0.01). However,the release of organic carbon from phytoplankton was usuallyhigher than the bacterioplankton carbon requirement. P availablevia zooplankton remineralization satisfied between 74 and 316%of the minimum P demands of algae and bacteria. To elucidatewhether zooplankton operate similarly on algae and bacterialgrowth or indirectly influence bacterial growth through phytoplanktonmetab olism, we performed zooplankton manipulation experiments.High zooplankton biomass in these experiments stimulated bothprimary and bacterial production, but release of organic carbonfrom phytoplankton declined. These results suggest a directstimulus of bacterial growth, so algae and bac teria can balancegrazing losses by compensatory growth. Further, the algal decreaseof the organic carbon supply for bacteria could, over time,lead to a change in the algae-bacteria interaction from competitionto commensalism. This reduction in organic carbon excretioncould affect the balance of the competitive interaction.     

Seasonal changes of C:P ratios of seston, bacteria, phytoplankton and zooplankton in a deep, mesotrophic lake

• 1 The C:P ratios of seston, bacteria, phytoplankton and zooplankton were measured twice a week in situ in mesotrophic, large and deep Lake Constance from April to December 1995. Except for zooplankton, a strong seasonality was exhibited with low C:P ratios during P‐enriched early spring conditions and high values during P‐depleted summer conditions.
• 2 Molar C:P ratios of seston varied between 180:1 and 460:1 demonstrating moderate phosphorus limitation in spring and during the clear‐water phase, and strong limitation for the rest of the season. The sestonic C:P ratio increased significantly during two decades of re‐oligotrophication of Lake Constance, reflecting an enhanced phosphorus limitation of the plankton community in summer. Molar C:P ratios of bacteria and phytoplankton varied seasonally between 50:1 and 130:1 and 180:1 and 500:1, respectively, and indicate carbon or light limitation in winter and phosphorus limitation in summer. Zooplankton had a molar C:P ratio of about 124:115 which was nearly constant throughout the seasons.
• 3 These differences in the C:P ratios of planktonic organisms have direct implications for phosphorus recycling within the food web as C:P ratios of excreta should be highly variable.

     

Elemental ratios and the uptake and release of nutrients by phytoplankton and bacteria in three lakes of the Canadian shield

The dynamics of carbon (C), nitrogen (N), and phosphorus (P), elemental ratios, and dark uptake/release of N and P in bacterial and phytoplankton size fractions were studied during summer 1992 in three lakes of contrasting food web structure and trophic status (L240, L110, L227). We wished to determine if phytoplankton and bacteria differed in their elemental characteristics and to evaluate whether the functional role of bacteria in nutrient cycling (i.e., as sink or source) depended on bacterial elemental characteristics. Bacterial contributions to total suspended particulate material and to fluxes of nutrients in the dark were substantial and varied for different elements. This indicated that some techniques for assaying phytoplankton physiological condition are compromised by bacterial contributions. C/N ratios were generally less variable than C/P and N/P ratios. Both elemental ratios and biomass-normalized N and P flux indicated that phytoplankton growth in each lake was predominantly P-limited, although in L227 these data reflect the dominance of N-fixing cyanobacteria, and N was likely limiting early in the sampling season. In L227, phytoplankton N/P ratio and biomass-normalized N flux were negatively correlated, indicating that flux data were likely a reasonable measure of the N status of the phytoplankton. However, for L227 phytoplankton, P-flux per unit biomass was a hyperbolic function of N/P, suggesting that the dominant L227 cyanobacteria have a limited uptake and storage capacity and that P-flux per unit biomass may not be a good gauge of the P-limitation status of phytoplankton in this situation. Examination of N-flux data in the bacterial size fraction relative to the N/P ratio of the bacteria revealed a threshold N/P ratio (22:1 N/P, by atoms), below which, bacteria took up and sequestered added N, and above which, N was released. Thus, the functional role of bacteria in N cycling in these ecosystems depended on their N/P stoichiometry.     

Uncoupling of Bacterioplankton and Phytoplankton Production in Fresh Waters Is Affected by Inorganic Nutrient Limitation

Pelagic bacterial production is often positively correlated, or coupled, with primary production through utilization of autotrophically produced dissolved organic carbon. Recent studies indicate that inorganic N or P can directly limit both bacterial and phytoplanktonic growth. Our mesocosm experiments, with whole communities from mesotrophic Calder Lake, test whether this apparent bacterial-algal coupling may be the result of independent responses to limiting inorganic nutrients. In systems without N additions, numbers of bacteria but not phytoplankton increased 2- to 2.5-fold in response to P fertilization (0 to 2.0 μmol of P per liter); this resulted in uncoupled production patterns. In systems supplemented with 10 μmol of NH₄NO₃ per liter, P addition resulted in up to threefold increases in bacteria and two- to fivefold increases in total phytoplankton biomass (close coupling). P limitation of pelagic bacteria occurred independently of phytoplankton dynamics, and regressions between bacterial abundance and phytoplankton chlorophyll a were nonsignificant in all systems without added N. We describe a useful and simple coupling index which predicts that shifts in phytoplankton and bacterioplankton growth will be unrelated (Δ bacteria/Δ phytoplankton → either + ∞ or - ∞) in systems with inorganic N/P (molar) ratios of <~40. In systems with higher N/P ratios (>40), the coupling index will approach 1.0 and close coupling between bacteria and phytoplankton is predicted to occur.     

Can bacteria outcompete phytoplankton for phosphorus? a chemostat test

Although the bacterioplankton of lakes are usually considered primarily in terms of mineralization processes, recent studies suggest that they may also strongly compete for phosphorus with the phytoplankton. In the present study, we have tested in chemostat culture, and found support for the hypotheses that (1) a freshwater bacterium (Pseudomonas paucimobilis), whose carbon source is excretion from a phosphorus-limited alga (Synedra ulna var.danica), can outcompete that alga for phosphorus (P) under widely varied P supply rates; (2) exogenously-supplied organic carbon positively influences bacterial biomass and negatively influences algal biomass; (3) the ratio of bacterial to algal phosphorus uptake in short-term³²P orthophosphate uptake experiments is an accurate predictor of their relative long-term phosphorus assimilation (i.e., growth) in mixed culture.     

Elemental composition (C:N:P) and growth rates of bacteria and Rhodomonas grazed by Daphnia

The elemental composition and growth rate of Rhodomonas andheterotrophic bacteria were studied in batch cultures in thepresence and absence of Daphnia and at two different levelsof phosphorus limitation. The elemental content of single cellswas measured with X-ray microanalysis. Simultaneously, dilutionexperiments were performed in order to estimate grazing losses,growth rates and dominant nutrient sources for bacteria andRhodomonas. The phosphorus:carbon (P:C) ratios of the bacteriawere generally higher in the experiment with the stronger Plimitation of the system. High P:C ratios were taken as an indicationthat bacteria were carbon limited. The presence of Daphnia resultedin a further increase in bacterial P:C ratios and increasedspecific growth rates. Thus, grazing increased the availabilityboth of inorganic nutrients and organic substrates, stimulatingthe growth of the bacteria. P:C ratios of Rhodomonas decreasedwith increasing P limitation of the system. Only at strong Plimitation did the presence of Daphnia result in increased P:Cratios of Rhodomonas compared with the control without daphnids.This study shows that the elemental content and growth rateof heterotrophic bacteria and Rhodomonas are influenced by grazingand nutrient regeneration by daphnids. The response is dynamicand depends on the level of nutrient limitation of the system. Present address: Department of Microbiology, University of BergenJahnebakken 5, NO-5020 Bergen, Norway     

Viral lysis and grazing loss of bacteria in nutrient- and carbon-manipulated brackish water enclosures

A 3 week enclosure experiment was carried out at the Gulf of Finland, the Baltic Sea. After additions of inorganic nutrients [nitrogen (N) + phosphorus (P)] and a carbon source (sucrose), we followed bacterial, viral and heterotrophic nanoflagellate (HNF) abundances, as well as bacterial production and the frequency of bacteria visibly infected with viruses. Furthermore, the decay rate of virus particles was measured three times during the enclosure experiment from the KCN-treated water samples. Bacterial mortality caused by viral lysis was estimated using the decay rates and the fraction of bacteria infected. Nutrient (N + P) additions stimulated phytoplankton growth [the chlorophyll (Chl) a concentration increased from <5 g l^-1 up to 19 g l^-1], while sucrose additions increased bacterial production (from 4-6 x 10⁷ l^-1 h^-1). The phytoplankton blooms affected bacterial production only slightly. Bacterial mortality that was explained by viruses ranged from <2% to 13% when estimated from the visibly infected cells, and from 8% to 808% when the decay rates (range 0.052-0.765 h^-1) were used. Assuming a clearance rate of 5 nl flagellate^-1 h^-1, the HNF community could graze 16-135% of total bacterial loss.      

Most probable number quantification of hypophosphite and phosphite oxidizing bacteria in natural aquatic and terrestrial environments

Concentrations of hypophosphite and phosphite oxidizing bacteria were found to be high, relative to bacterial concentrations growing on phosphate, in sediment and soil during winter and summer seasons from 12 common terrestrial and aquatic sites using a most probable number method. The percent of total culturable bacterial concentrations that could use these reduced phosphorus compounds as a sole source of phosphorus were as follows: hypophosphite, 7–100%; phosphite, 10–67%; aminoethylphosphonate, 34–270%. The average MPN/g (±SEM) was as follows: phosphate, 6.19 × 10⁶ (±2.40 × 10⁶); hypophosphite, 2.61 × 10⁶ (±1.35 × 10⁶) phosphite, 1.91 × 10⁶ (±1.02 × 10⁶); aminoethylphosphonate, 3.90 × 10⁶ (± 1.95 × 10⁶). Relatively high concentrations of reduced phosphorus oxidizing bacteria were found in both pristine sites and sites with urban and agricultural disturbance. Concentrations of reduced phosphorus oxidizing bacteria in anoxic sediments and soil were equivalent. Our data indicate that reduced phosphorus oxidizing bacteria are abundant in the environment and provide strong evidence for the importance of bacterial P oxidation in nature.     

ELEVATED ATMOSPHERIC CARBON DIOXIDE INCREASES ORGANIC CARBON FIXATION BY EMILIANIA HUXLEYI (HAPTOPHYTA), UNDER NUTRIENT‐LIMITED HIGH‐LIGHT CONDITIONS1

Phytoplankton play a key role in determining the partitioning of CO₂ between the atmosphere and the ocean on seasonal, interannual, and millennial time scales. The magnitude of biological draw‐down of atmospheric CO₂ and C storage in the oceans is affected by concurrent changes in other environmental factors, like nutrient supply. Furthermore, variations in carbon‐to‐nitrogen (C:N) and carbon‐to‐phosphorus (C:P) assimilation ratios modify the oceanic CO₂ storage capacity. Here we show that increased atmospheric CO₂ concentration enhances CO₂ fixation into organic matter by a noncalcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler only under certain conditions, namely high light and nutrient limitation. Enhanced organic matter production was accompanied by marked deviations of the C:N:P ratio from the canonical stoichiometry of marine particulate matter of 106:16:1 (C:N:P) known as the Redfield ratio. Increased cell organic carbon content, C:N, and C:P were observed at high light when growth was either nitrogen or phosphorus limited. Elevated CO₂ led to further increases in the particulate C:N and C:P ratios. Enhanced CO₂ uptake by phytoplankton such as E. huxleyi, in response to elevated atmospheric CO₂, could increase carbon storage in the nitrogen‐limited regions of the oceans and thus act as a negative feedback on rising atmospheric CO₂ levels.     

Multiple environmental controls on phytoplankton growth strategies determine adaptive responses of the N : P ratio

The controls on the ‘Redfield’ N : P stoichiometry of marine phytoplankton and hence the N : P ratio of the deep ocean remain incompletely understood. Here, we use a model for phytoplankton ecophysiology and growth, based on functional traits and resource‐allocation trade‐offs, to show how environmental filtering, biotic interactions, and element cycling in a global ecosystem model determine phytoplankton biogeography, growth strategies and macromolecular composition. Emergent growth strategies capture major observed patterns in marine biomes. Using a new synthesis of experimental RNA and protein measurements to constrain per‐ribosome translation rates, we determine a spatially variable lower limit on adaptive rRNA:protein allocation and hence on the relationship between the largest cellular P and N pools. Comparison with the lowest observed phytoplankton N : P ratios and N : P export fluxes in the Southern Ocean suggests that additional contributions from phospholipid and phosphorus storage compounds play a fundamental role in determining the marine biogeochemical cycling of these elements.     

Dissolved organic carbon in a humic lake: effects on bacterial production and respiration

Allochthonous matter was the main source of carbon for pelagic bacteria in a humic lake, accounting for almost 90% of the carbon required to support observed bacterial growth. The estimated contribution from zooplankton excretion was of the same magnitude as direct phytoplankton release, both accounting for 5–7% of bacterial demands for dissolved carbon. Bacteria were an important source of carbon both for heterotrophic phytoplankton and for filter feeding zooplankton species, further stressing the role of humus DOC in overall lake productivity. The high contribution of allochthonous DOC implies a stoichiometry of dissolved nutrients with a surplus of C relative to P. The high P cell quota of bacteria suggest that under such conditions they are P-limited and act like net consumers of P. Excess C will be disposed of, and bacterial respiration rate will increase following a transition from carbon-limited bacterial growth towards mineral-nutrient-limited growth. Thus the high community respiration and frequent CO₂-supersaturation in humic lakes may be caused not only by the absolute supply of organic C, but also by the stoichiometry of the dissolved nutrient pool.     

Survival,growth and reproduction of Daphnia galeata feeding on single and mixed Pseudomonas and Rhodomonas diets

1. Bacteria can be an important resource for zooplankton production in aquatic food webs, although the degree to which bacteria sustain zooplankton growth and reproduction is not clear. We performed a growth experiment with Daphnia galeata feeding on different ratios of P‐replete Pseudomonas and Rhodomonas, ranging from a 100% bacterial to a 100% algal diet. 2. A pure bacterial diet did not support survival, growth or reproduction of D. galeata. While a 20% share of Rhodomonas in the food allowed survival of daphniids, the occurrence of offspring on a 50% algal diet indicated that the threshold for successful reproduction was between those two proportions of algal food. Increasing the proportion of the alga further increased growth and reproductive output, indicating that Rhodomonas was a higher‐quality food than Pseudomonas. 3. A subsequent labelling experiment demonstrated that D. galeata incorporated phosphorus from Pseudomonas and Rhodomonas with similar efficiency, whereas carbon was incorporated more efficiently from Pseudomonas than from Rhodomonas. 4. we hypothesise that inadequate levels of essential biochemicals in pure bacterial diets led to decreased Daphnia performance. Concentrations of fatty acids in general, and especially of polyunsaturated fatty acids, were much lower in Pseudomonas than in Rhodomonas. This difference could explain the different growth and reproduction responses, although limitation by other essential biochemicals (e.g. sterols) cannot be ruled out. 5. Hence, where they dominate, bacteria may provide a significant part of the elemental flux to species feeding higher in the food web on the short term. However, the performance of consumers may be constrained by essential biochemicals.     

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.     

Effects of limiting nutrients and N:P ratios on the phytoplankton growth in a shallow hypertrophic reservoir

The purpose of this study was to evaluate the effects of limiting nutrients and the N:P ratios on the growth of phytoplankton (mainly cyanobacteria) in a shallow hypertrophic reservoir between November 2002 and December 2003. Nutrient enrichment bioassays (NEBs) were conducted, along with analyses of seasonal ambient nutrients and phytoplankton taxa, in the reservoir. The average DIN:TDP and TN:TP mass ratios in the ambient water were 90 (range: 17–187) and 34 (13–60), respectively, during the study period. The dissolved inorganic phosphorus showed seasonal variation, but less than that of inorganic nitrogen. The TN:TP ratios ranged from 13 to 46 (mean: 27 ± 6) during June–December when the cyanobacteria, Microcystis, dominated the phytoplankton composition. The NEBs showed that phytoplankton growth was mainly stimulated by the phosphorus (all of total 17 cases), rather than the nitrogen concentration (8 of 17 cases). The rapid growth rate of cyanobacteria was evident with TN:TP ratios less than 30. According to the results of the NEBs with different N concentrations (0.07, 0.7 and 3.5 mg l⁻¹), but the same N:P ratios and when the nitrogen concentration was highest, the cyanobacterial growth reached a maximum at N:P ratios <1. Overall, the response of cyanobacterial growth was a direct function of added phosphorus in the NEBs, and was greater with increased N concentrations. Thus, cyanobacterial blooms favored relatively low N:P ratios in this hypertrophic reservoir system. An erratum to this article is available at .     

氮磷添加对内蒙古温带典型草原土壤微生物群落结构的影响

选取内蒙古温带典型草原,进行连续6a氮磷添加试验,采用土壤特征微生物PLFA生物标记技术,研究6个氮添加水平N0(0 kg N hm-2a-1)、N1(56 kg N hm-2a-1)、N2(112 kg N hm-2a-1)、N3(224 kg N hm-2a-1)、N4(392 kg N hm-2a-1)、N5(560 kg N hm-2a-1)和6个磷添加水平P0(0 kg P hm-2a-1)、P1(15.5 kg P hm-2a-1)、P2(31 kg P hm-2a-1)、P3(62 kg P hm-2a-1)、P4(93 kg P hm-2a-1)、P5(124 kg P hm-2a-1)对土壤特征微生物PLFA生物标记数量和土壤微生物群落结构的影响。结果表明:(1)随氮添加量增加,土壤微生物总磷脂脂肪酸(PLFA)含量和土壤细菌PLFA生物标记数量、放线菌PLFA生物标记数量呈上升趋势,土壤G+/G-呈增加趋势;各氮添加水平对土壤真菌PLFA生物标记数量无显著差异,随氮添加量增加,土壤真菌/细菌比降低。(2)随磷添加量增加,土壤总磷脂脂肪酸(PLFA)含量、土壤细菌PLFA生物标记数量、放线菌PLFA生物标记数量、真菌PLFA生物标记数量及真菌/细菌比值呈先上升后下降趋势,均以P3水平(62 kg P hm-2a-1)处理最高,说明适宜的磷添加对内蒙古温带典型草原土壤微生物繁殖和菌落结构有显著影响。     

Inorganic Phosphorus Stimulation of Bacterioplankton Production in a Meso-Eutrophic Lake

Experiments were conducted to determine whether production of heterotrophic bacterioplankton in a small meso-eutrophic lake was influenced by the dissolved inorganic phosphorus (DIP) supply. DIP may indirectly limit bacterial production by limiting phytoplankton, which in turn may limit the carbon available to bacteria. Direct DIP limitation of bacteria occurs where the availability of DIP for bacteria is insufficient to maintain growth. This work examined direct DIP limitation of bacteria by removing phytoplankton and incubating flasks with or without added P in the dark. Bacterial production was measured via the rate of incorporation of [³H]thymidine ([³H]TdR) into DNA. Bacterial abundance was followed with epifluorescent direct counts. Rates of [³H]TdR incorporation were significantly greater in flasks with added DIP, and changes in cell abundances generally paralleled increases in [³H]TdR incorporation. Even very small additions of P (0.05 μM) were sufficient to stimulate production. DIP addition to whole lakewater also stimulated [³H]TdR incorporation relative to that in zero-addition controls, but there was not a concurrent increase in bacterial cell numbers. The stimulation of [³H]TdR incorporation after DIP addition to whole lakewater was significantly less than the stimulation due to DIP addition to 1-μm-pore-size-filtered lakewater. In this study, addition of DIP caused as much as an eightfold stimulation of [³H]TdR incorporation.     

Protist abundance and carbon concentration during a Phaeocystis-dominated bloom at an Antarctic coastal site

Summary Changes in the concentrations of bacteria, phytoplankton, protozoa, dissolved organic carbon (DOC), particulate organic carbon (POC), particulate carbohydrate (PCHO) and particulate organic nitrogen (PON) were followed throughout the summer at an Antarctic coastal site. The colonial prymnesiophyte Phaeocystis pouchetii was the first major phytoplankton species to bloom, reaching concentrations of 6 × 10⁷ cells · 1^–1 and remained numerically dominant for most of the summer. During the P. pouchetii bloom the concentration of most other autotrophs did not increase. Microheterotroph abundance peaked during or immediately after the Phaeocystis bloom. Their peak coincided with very high concentrations of organic carbon, particularly DOC which exceeded 100 mg · 1^–1, and low bacterial abundance. Maximum bacterial abundance was reached after the decline in microheterotroph numbers. Bacterial utilization of carbon substrates and microheterotroph grazing of bacteria and uptake of DOC may form an important link to higher trophic levels during Antarctic Phaeocystis blooms.     

Planctomycetes in Lakes: Poor or Strong Competitors for Phosphorus?

Experiments were conducted with water samples from two perialpine lakes with differing eutrophication status in order to examine the effects of inorganic-nutrient amendments (nitrogen as NO₃⁻ or NH₄⁺ and phosphorus as PO₄³⁻) on the dynamics, structure, and composition of Planctomycetes and to test the hypothesis that the community structure of Planctomycetes members and that of the other bacteria (without Planctomycetes, here referred to as bacteria-wP, the most represented groups within the community) would be similarly impacted by nutrient additions. Initial samples were characterized by high total nitrogen-to-total phosphorus ratios (range, 39 to 55), suggesting P rather than N was the limiting nutrient for microbial communities. Consistent with this, P additions stimulated phytoplankton growth and affected the community structure of bacteria-wP but, surprisingly, not that of Planctomycetes. N additions did not significantly affect the community structures of bacteria-wP and Planctomycetes or the Planctomycetes phylotype composition. The estimated generation time of Planctomycetes was 123 h. These findings could suggest that the generally well-accepted statement that bacteria (as a whole) are superior to phytoplankton in the ability to obtain phosphorus under P limitation might actually not hold for Planctomycetes. Planctomycetes might be poor competitors for P that do not respond quickly to the nutrient supply, which may help explain why their abundance is low in aquatic systems. The alternative view that Planctomycetes could be strong competitors for P (storing it) is also discussed. Our findings highlight the need for further studies examining Planctomycetes-phosphorus relationships in aquatic ecosystems.     

CARBON,NITROGEN, AND PHOSPHORUS AND THE EUTROPHICATION OF FRESHWATER LAKES1

The question of nutrients responsible for eutrophication of freshwater lakes is reviewed, and recent additions to the literature on nutrient limitation are discussed. The paper by Lange is criticized on several grounds, including the facts that utilization of HCO₃^? by phytoplankton and the invasion of lake waters by atmospheric CO₂ are ignored as sources of photosynthetic carbon. The phosphorus and nitrogen concentrations used in Lange's experiments are far higher than values published by others for Lakes Erie and Ontario. Preliminary results of fertilizing a small oligotrophic lake with nitrogen and phosphorus are described. The standing crop of phytoplankton increased by 30–50 ×, while the P:N:C ratio in seston did not change from ratios found in unfertilized lakes. Other experiments done in water columns isolated with polyethylene film showed that addition of carbon did not increase the phytoplankton standing crop. Since the fertilized lake was initially lower in total CO₂ than any other recorded in the literature, it is concluded that carbon is unlikely to limit the standing crop of phytoplankton in almost any situation. Measurements of invasion of atmospheric gases to the fertilized lake by the Rn²²² technique were compared with phytoplankton production measurements, revealing that atmospheric invasion of CO₂ is sufficient to support the high phytoplankton standing crop in the epilimnion of the lake. Possible errors in interpretation of culture and bottle-bioassay experiments with respect to eutrophication are discussed.