Diel variation in concentration,assimilation and respiration of dissolved free amino acids in relation to planktonic primary and secondary production in two eutrophic lakes

Concentration of dissolved free amino acids (DFAA) and assimilation of the 5 most abundant DFAA (glutamic acid, serine, glycine, alanine and ornithine) were measured at 3-h intervals over 27 h in two Danish, eutrophic lakes. The carbon flux of the amino acid assimilation was compared with the major routes of carbon flux, including primary production, bacterial production and zooplankton grazing. In Frederiksborg Slotssø, the mean DFAA concentration was 275 nM with distinct peaks (up to 783 nM) 3 h after sunrise. Assimilation rates of the 5 amino acids amounted on the average to 2.03 µg Cl^–1 h^–1, but high values up to 7.41 µg Cl^–1 h^–1 occurred 3 h after sunrise and at midnight. The mean turnover time of the amino acid pools was 3.2 h. In Lake Mossø, the mean DFAA concentration was 592 nM with peak of 1 161 nM at dusk. The assimilation rate averaged 0.44 µg Cl^–1 h^–1, and the mean turnover time of the amino acid pools was 39 h. In Lake Mossø, similar turnover times of glutamic acid and serine were determined from the ¹⁴C-amino acid tracer technique and Michaelis-Menten uptake kinetics, indicating that the tracer technique gave reliable values of the actual assimilation. The average respiration percentages of the assimilated amino acids were 45% in Frederiksborg Slotssø and 51% in Lake Mossø. Extracellular organic carbon (EOC) released from the phytoplankton contributed DFAA to the water. In Lake Mossø, 81% of the ambient EOC pool was <700 daltons and 9.3% of the EOC was DFAA. This corresponded to about 2.4% of the DFAA pool. Bacterial productivity, determined by means of frequency of dividing cells and ³⁵S-SO₄ dark uptake techniques gave similar results and constituted 4.5 and 3.7 µg Cl^–1 h^–1 in Frederiksborg Slotssø and Lake Mossø, respectively. The bacterial productivity suggested that DFAA were essential substrates to the bacteria, especially in Frederiksborg Slotssø. The zooplankton biomass in Frederiksborg Slotssø was six times larger than that in Lake Mossø, but cladocerans were dominant in both lakes. The zooplankton grazing probably was an important regulatory factor for the bacterial productivity.     

Incorporation of [h]leucine and [h]valine into protein of freshwater bacteria: field applications

Incorporation of leucine and valine into proteins of freshwater bacteria as a measure of bacterial production was tested in two eutrophic Danish lakes and was related to bacterial production measured by thymidine incorporation. In a depth profile (0 to 8 m) in Frederiksborg Castle Lake, incorporation of 100 nM leucine and valine gave similar rates of protein production. In terms of carbon, this production was about 50% lower than incorporation of 10 nM thymidine. In another depth profile in the same lake, incorporations of 10 nM valine and 100 nM leucine were identical, but differed from incorporations of 10 nM leucine and 100 nM valine. Bacterial carbon production calculated from incorporations of 10 nM thymidine and 10 nM leucine was similar, whereas 10 nM valine and 100 nM leucine and valine indicated an up to 2.4-fold-higher rate of carbon production. In a diel study in Lake Bagsvaerd, incorporation of 100 nM leucine and valine indicated a similar protein production, but the calculated carbon production was about 1.9-fold higher than the production based on uptake of 10 nM thymidine. Different diel changes in incorporation of the two amino acids and in incorporation of thymidine were observed. In both lakes, concentrations of naturally occurring leucine and valine were <5 nM in most samples. This means that the specific activity of a H isotope added at a concentration of 100 nM usually was diluted a maximum of 5%. Net assimilation of natural free amino acids in the lakes sustained 8 to 69% of the net bacterial carbon requirement, estimated from incorporation of leucine, valine, or thymidine. The present results indicate that incorporation of leucine and valine permits realistic measurements of bacterial production in freshwater environments.     

Incorporation of [3H]Leucine and [3H]Valine into Protein of Freshwater Bacteria: Field Applications

Incorporation of leucine and valine into proteins of freshwater bacteria as a measure of bacterial production was tested in two eutrophic Danish lakes and was related to bacterial production measured by thymidine incorporation. In a depth profile (0 to 8 m) in Frederiksborg Castle Lake, incorporation of 100 nM leucine and valine gave similar rates of protein production. In terms of carbon, this production was about 50% lower than incorporation of 10 nM thymidine. In another depth profile in the same lake, incorporations of 10 nM valine and 100 nM leucine were identical, but differed from incorporations of 10 nM leucine and 100 nM valine. Bacterial carbon production calculated from incorporations of 10 nM thymidine and 10 nM leucine was similar, whereas 10 nM valine and 100 nM leucine and valine indicated an up to 2.4-fold-higher rate of carbon production. In a diel study in Lake Bagsvaerd, incorporation of 100 nM leucine and valine indicated a similar protein production, but the calculated carbon production was about 1.9-fold higher than the production based on uptake of 10 nM thymidine. Different diel changes in incorporation of the two amino acids and in incorporation of thymidine were observed. In both lakes, concentrations of naturally occurring leucine and valine were <5 nM in most samples. This means that the specific activity of a ³H isotope added at a concentration of 100 nM usually was diluted a maximum of 5%. Net assimilation of natural free amino acids in the lakes sustained 8 to 69% of the net bacterial carbon requirement, estimated from incorporation of leucine, valine, or thymidine. The present results indicate that incorporation of leucine and valine permits realistic measurements of bacterial production in freshwater environments.     

Contribution of Bacterial Cell Wall Components to DOM in Alkaline,Hypersaline Mono Lake,California

Soda lakes are often characterized by high densities of prokaryotes and high concentrations of dissolved organic carbon. Since bacterial cell walls are less degradable than most other cell constituents, accumulation of cell wall material may occur in these lakes and contribute to the DOM pool, but composition of DOM in soda lakes has rarely been examined. Here we report concentrations of DOM components likely originating from bacterial cell walls, including D amino acids, glucosamine (GluA) and muramic acid (MurA), in depth profiles of stratified, alkaline, hypersaline Mono Lake, CA. Concentrations of cell wall components were related to total pools of dissolved free and combined amino acids (DFAA and DCAA), and bacterial density and production. In the free pool, total DFAA ranged from 50 to 3250 nM and typically increased with depth, while GluA (5 to 140 nM) and MurA (< 0.5 nM and only detected in 2005) fluctuated with depth. In the combined pool, DCAA varied between 5000 and 15000 nM and did not show clear depth-related trends. GluA ranged from 1000 to 5000 nM and tended to increase in the hypolimnion, while MurA varied between 25 and 75 nM. Free D isomers in the DFAA pool either made up < 13% (Asp and Ser) or varied from 10 to 57% (Glu and Ala). In the combined pool, D isomers of Asp, Glu, Ser and Ala made up 24-48% of these DCAA and typically showed minor changes with depth. In 2005, lysozyme activity had highest rates in the surface and correlated negatively with most D isomers among the combined amino acids. Our observations demonstrate that the pool of dissolved combined amino compounds in the lake was about 5-fold higher than in other eutrophic lakes and that a substantial portion of these amino compounds originated from bacterial cell walls.     

Are dissolved free amino acids free?

Microbial assimilation of 3 amino acids (glutamic acid, alanine, and ornithine) was characterized in 3 lakes and 2 marine stations using the Michaelis-Menten kinetic approach. The calculated K_t + S_n concentrations were related to chemical concentration measurements of dissolved free amino acids (DFAA) to evaluate the biological and the chemical determinations of the DFAA pools. Concentrations of K_t + S_n always were larger than chemical measurements of the S_n concentrations. K_t + S_n and S_n varied from 11.5 and 9.5 nM (alanine, oligotrophic lake) to 288.7 and 89.9 nM (ornithine, marine harbor station), respectively. Subtracting S_n from the K_t + S_n concentrations, K_t was found to range from 12–897% of the chemically measured S_n concentrations. To test whether the DFAA actually were free, dissolved molecules, dissolved material in the water samples was separated into various molecular size classes by means of gel permeation chromatography. From 47–116% of the DFAA in the untreated water samples was recovered in the low molecular fraction (<700 Daltons). Variation in recoveries mainly appeared to be due to an incomplete chromatographic separation and difficulties in obtaining proper blank levels. The present observations suggest that labeled tracers can be used in the study of DFAA assimilation and that the DFAA are free, dissolved molecules. This partly conflicts with previously published reports.     

Diel Production and Microheterotrophic Utilization of Dissolved Free Amino Acids in Waters Off Southern California

Diel patterns of dissolved free amino acid (DFAA) concentration and microheterotrophic utilization were examined in the spring and fall of 1981 in euphotic waters from the base of the mixed layer off the southern California coast. The average depths of the isotherms sampled were 19.2 m for spring and 9.0 m for fall. Total DFAA levels were generally higher in the spring than in the fall, 18 to 66 nM and 14 to 20 nM, respectively. Two daily concentration maxima and minima were observed for total DFAAs as well as for most individual DFAAs. Maxima were usually measured in the mid-dark period and in the early afternoon; minima were typically observed in early morning and late afternoon. Bacterial cell numbers reached maximal values near midnight in both seasons. These increases coincided with one of the total DFAA maxima. The second total DFAA maximum occurred in early to midafternoon, during the time of maximum photosynthetic carbon production and rapid dissolved amino acid utilization. Microbial metabolism (incorporation plus respiration) of selected ³H-amino acids was 2.7 to 4.1 times greater during the daylight hours. DFAA turnover times, based on these metabolic measurements, ranged between 11 and 36 h for the amino acids tested, and rates were 1.7 to 3.7 times faster in the daylight hours than at night. DFAA distributions were related to primary production and chlorophyll a concentrations. Amino acids were estimated to represent 9 to 45% of the total phytoplankton exudate. Microheterotrophic utilization or production of total protein amino acids was estimated as 3.6 μg of C liter⁻¹ day⁻¹ in spring and 1.9 μg of C liter⁻¹ day⁻¹ in the fall. Assimilation efficiency for dissolved amino acids averaged 65% for marine microheterotrophs.     

Gradient of dissolved free amino acids and phytoplankton in a shallow bay

A 3 h survey of the concentrations of individual free amino acids, chlorophyll a and phytoplankton species biomass was conducted in the surface waters of a shallow bay. Significant coefficients of correlation were found between chlorophyll a, nanoflagellates, and DFAA concentrations. Although phytoplankton biomass variations sometimes relate to DFAA concentration patterns, consideration of the physiological activity of both phytoplankton and microheterotrophs would undoubtedly explain a more significant fraction of the DFAA variation.This work is a contribution of GREPMA (Groupe Régional d'Etudes Pélagiques en Manche-Atlantique)     

The use of antibiotics to reduce bacterioplankton uptake of phytoplankton extracellular organic carbon (EOC) in the Potomac River estuary

Phytoplankton production and accumulation of extracellular organic carbon (EOC) was tracked during diel intervals in microcosms by inhibiting bacterioplankton assimilation of EOC with streptomycin and kanamycin. Bacterioplankton production (³H-thymidine incorporation) and metabolism (¹⁴C-glucose incorporation) were monitored in samples collected from the Potomac River estuary to determine the effect of the antibiotics. Particulate (i.e., raw water) primary production and EOC (i.e., water passing through 1.0 μm glass fiber filter) production rates were monitored to determine the impact of antibiotics on phytoplankton. In preliminary experiments, neither streptomycin nor kanamycin alone significantly inhibited bacterioplankton activity compared to controls, but when both were present secondary production and metabolism were reduced up to 90%, and remained as such for 45 h. During field evaluations using a streptomycin and kanamycin mixture (50 μM each) particulate primary production and EOC production were not statistically different in control and antibiotic treated samples indicating that the antibiotics did not negatively influence phytoplankton production rates. In the presence of antibiotics dissolved free amino acids (DFAA) and, to a lesser extent, monosaccharides were significantly elevated compared to controls. This study demonstrates that streptomycin and kanamycin are capable of inhibiting bacterioplankton metabolism and uptake of dissolved organic carbon (DOC) in the samples tested so that the contribution of EOC to the DOC pool and to bacterioplankton metabolism could be measured and assessed.     

Zooplankton induced changes in dissolved free amino acids and in production rates of freshwater bacteria

This study examined the importance of zooplankton in the flux of dissolved free amino acids (DFAA) in the water and into bacteria. DFAA release rates were followed in laboratory grazing experiments usingDaphnia galeata andEudiaptomus graciloides as grazers, andScenedesmus acutus andSynechococcus elongatus as food sources. Except for minor initial peaks, DFAAs were released continuously during the first 2 hours and made up 6–12% (in one experiment 50%) of the calculated ingestion rates. During three diel studies in lakes, effects of removal and increase of the density of zooplankton (>200m) on the pools of DFAA as well as on the bacterial production were followed. During two of the diel studies, higher DFAA pools were measured when 3–4 times the natural zooplankton density was present, and in one study a minor increase also occurred in the bacterial production, compared with results from experiments without zooplankton and with a natural zooplankton density. The increase in bacterial growth coincided with a decline in DFAA. During the third study, neither DFAA nor the bacterial production changed significantly when the zooplankton density was increased 3 times. Removal of zooplankton, however, caused a decline in both DFAA and bacterial production. Our data suggest a close relationship between occurrence of zooplankton and release of DFAA, but the factors regulating the amount of DFAA released and its effect on bacterial growth are not yet understood.     

Inorganic carbon limitation of photosynthesis in lake phytoplankton

1. Inorganic carbon availability influences species composition of phytoplankton in acidic and highly alkaline lakes, whereas the overall influence on community photosynthesis and growth is subject to debate.
2. The influence of total dissolved inorganic carbon (DIC) and free CO₂ on community photosynthesis was studied in six Danish lakes during the summer of 1995. The lakes were selected to ensure a wide range of chlorophyll a concentrations (1–120 μg l^–1), pH (5.6–9.6) and DIC concentration (0.02–2.5 m m ). Photosynthesis experiments were performed using the ¹⁴C technique in CO₂-manipulated water samples, either by changing the pH or by adding/removing CO₂.
3. Lake waters were naturally CO₂ supersaturated during most of the experimental period and inorganic carbon limitation of photosynthetic rates did not occur under ambient conditions. However, photosynthesis by phytoplankton in lakes with low and intermediate DIC concentrations was seriously restricted when CO₂ concentrations declined. Similarly, photosynthesis was limited by low CO₂ concentrations during phytoplankton blooms in the hardwater alkaline lakes.     

Seasonal changes in the dissolved free amino acid and DOC concentrations in a hypertrophic African reservoir and its inflowing rivers

Dissolved free amino acid (DFAA) concentration and composition and dissolved organic carbon (DOC) concentration were measured over 16 months at three depths in hypertrophic Hartbeespoort Dam, South Africa and in its two perenially inflowing rivers. The range of DFAA concentrations in the reservoir and both rivers were similar with dominant DFAA consisting of serine, glycine, alanine and ornithine in all three systems. The range of DOC concentrations in the rivers was 1.5–11.1 mg l^–1, the major river (Crocodile) having about twice the DOC concentration of the Magalies River. The DFAA/DOC ratios ranged between 0.02–1.1% in the Crocodile River and 0.13–3.7% in the Magalies River. DFAA and DOC concentrations were positively correlated to the Magalies River flow, but for the Crocodile River, which received domestic and industrial effluents, DOC was inversely correlated to flow. The source of DFAA in both rivers was mainly terrestrial, in contrast to the main DOC source in the Crocodile River which was the effluents. The DFAA load of the Crocodile River ranged between 0.22 and 208 kg C d^–1.DOC (5.0–24.8mg l^–1) in Hartbeespoort Dam generally decreased with depth but DFAA (15–4800 nmol l^–1) concentration showed no clear trend. The DFAA/DOC ratios varied between 0.02 and 2.9%. DFAA concentrations were correlated (r = 0.3, n = 30, p = 0.04) with bacterial numbers at 0 and 10 m only while no significant correlations were found with bacterial production, chlorophyll a concentration and phytoplankton primary and EDOC (extracellular DOC) production at any depth. The rate of bacterial utilization of DFAA was low compared with data from other lakes. Diurnal phytoplankton production of DFAA in the euphotic zone of the whole lake was calculated to vary between 268 and 30 780 t C d^–1 indicating autochthonous DFAA sources were dominant to allochthonous DFAA sources. The autochthonous production of DFAA was > 2 × gross bacterial production of the euphotic zone indicating that although DFAA concentrations were frequently < 10 g C l^–1, the rate of DFAA production exceeded bacterial requirements.     

Immobilization and mineralization of dissolved free amino acids by stream-bed biofilms

1. Radiolabelled (¹⁴C) amino acids were used to investigate the influence of sediment size as well as dissolved free amino acid (DFAA) concentration and composition on immobilization and mineralization of DFAAs by biofilms from a first-order stream. 2. Over time (240 min), biofilms on stony substrata immobilized a DFAA mixture more effectively than those on sandy substrata, however proportional mineralization of immobilized DFAAs was higher for sandy substrata (36 v 20%). 3. Using stony substrata, the DFAA mixture was immobilized more rapidly than glycine alone at ‘near-natural’ amino acid concentrations (c. 37 μgl^?1), as well as enriched concentrations (1 and 100 mg 1-^?1). Instantaneous rates of glycine immobilization and mineralization were not saturated at glycine enrichments of up to 980 mgl^?1. 4. With both the amino acid mixture and glycine alone, proportional mineralization of the immobilized amino acids increased on enrichment to Img 1-^?1 (DFAA mixture: from 25 to 37%; glycine alone: from 50 to 54%), but then fell on further enrichment to 100mgl^?1 (DFAA mixture: 11%; glycine alone: 7%). 5. Results are discussed in terms of the potential trophic utility of immobilized DFAAs as well as the apparent roles of biotic and abiotic immobilization mechanisms. Immobilization and mineralization responses to variables investigated in this study give an insight into potential variability of carbon immobilization and retention in stream-bed sediments. This is fundamental to an understanding of how DOC may become available to higher trophic levels.     

Bacterial Production in the Recently Flooded Sep Reservoir: Diel Changes in Relation to Dissolved Carbohydrates and Combined Amino Acids

The spatial distribution of bacterial abundance and production were measured every 4 h in a recently flooded oligo-mesotrophic reservoir (the Sep Reservoir, Puy-De-Dôme, France), in relation to concentrations of dissolved carbohydrates and combined amino acids. The concentration of dissolved organic matter (DOM) components in the recently flooded Sep Reservoir were higher than those measured in other lakes of similar trophic status. Short-term variations in the bacterial production in this new reservoir appeared cyclical and endogenous to bacterial communities. These results highlight the need for the evaluation of diel changes in bacterial production, if estimation of the daily production rate of bacteria is to be done accurately for a reliable model of carbon flow through bacterioplankton and ultimately through aquatic microbial food webs. Bacterial growth, measured over time and space, did not appear exclusively governed by DOM components from phytoplankton primary production.     

Net uptake of dissolved free amino acids by the giant clam, Tridacna maxima: alternative sources of energy and nitrogen?

 The role of dissolved free amino acids (DFAA) in nitrogen and energy budgets was investigated for the giant clam, Tridacna maxima, growing under field conditions at One Tree Island, at the southern end of the Great Barrier Reef, Australia. Giant clams (121.5–143.7 mm in shell length) took up neutral, acidic and basic amino acids. The rates of net uptake of DFAA did not differ between light and dark, nor for clams growing under normal or slightly enriched ammonium concentrations. Calculations based on the net uptake concentrations typical of the maximum concentrations of DFAA found in coral reef waters (∼0.1 μM ) revealed that DFAA could only contribute 0.1% and 1% of the energy and nitrogen demands of giant clams, respectively. These results suggest that DFAA does not supply significant amounts of energy or nitrogen for giant clams or their symbionts. Accepted: 7 October 1998     

Fates of dissolved free amino acids in groundwater discharged through stream bed sediments

Laboratory simulations were used to investigate the immobilization of dissolved free amino acids (DFAAs) from groundwater discharged up through cores of stream-bed sediments from a first order stream. At natural concentrations, 99% of DFAAs supplied in groundwater were immobilized, with 14–25% of this material respired and the remainder retained in the lower layers of the cores (depth = 7.5 cm). Immobilization efficiencies increased with increasing groundwater DFAA concentrations and discharge rates. Moderate enrichments (up to 1 mg l^–1) appeared to stimulate biotic immobilization of DFAAs, while abiotic processes accounted for much of the increased immobilization at higher enrichments (tested up to 100 mg l^–1). Variability in groundwater discharge rates induced no changes in the proportional contribution of biotic and abiotic immobilization processes. Thus relative contributions of DFAA concentration and groundwater discharge rate to a given DFAA load (concentration × discharge rate) influenced the degree to which immobilized DFAAs were retained (as microbial biomass or adsorbed to the biofilm) or respired. Results showed that DFAAs in groundwater discharged through the stream-bed are in a highly dynamic state of flux, suggesting that these compounds may be more significant to the transfer of organic matter to the benthic trophic structure than their normally low concentrations in groundwater would imply.     

Microbial fluxes of free monosaccharides and total carbohydrates in freshwater determined by PAD-HPLC

Abstract A new sensitive pulsed amperometric detection (PAD) method for measurements of mono- and disaccharides in nM concentrations was used in combination with high performance liquid chromatography (HPLC) to study fluxes of dissolved free and combined carbohydrates (DFCHO and DCCHO) in lake water. In a diel study concentrations of individual free saccharides typically were 5–50 nM, while total DFCHO concentrations ranged from 67 to 224 nM. No diel trends in concentration changes were obvious. At in situ light-dark conditions, dominant DFCHO were galactose, glucose, fructose and mannose/xylose. In addition to these saccharides, an increased abundance of melibiose and arabinose was measured in a parallel dark incubation. In a 118 h laboratory incubation of 1.0 μm filtered lake water, concentrations of DFCHO decreased from 194 nM (at 12 h) to a minimum of 54 nM (at 73 h). Dominant DFCHO were glucose, fructose and cellobiose. During the incubation DCCHO varied from 1.27 to 2.20 μM. Glucose, galactose and cellobiose made up 40, 30 and 10 mol-%, respectively, of the DCCHO. Fructose was degraded during hydrolysis of the DCCHO. A decline of DCCHO at 55 h was reflected in a simultaneous increase of DFCHO, but otherwise no similarities between the two saccharide pools were found. Incrased DCCHO concentrations and a high assimilation of glucose and fructose that was not reflected in a decline of their concentrations, both indicate that carbohydrates were produced during the experiment. Polysaccharides were probably excreted by the bacteria. Net assimilation of glucose and fructose sustained 14–19% (diel study) and 32% (long-term study) of the net bacterial carbon requirement.     

Impact of UV radiation on the production patterns and composition of dissolved free and combined amino acids in marine phytoplankton

Using the ¹³C tracer technique in conjunction with gas chromatographic-massspectro-metric (GC-MS) techniques, we examined the patternsof synthesis and the composition of dissolved free and combinedamino acids within phytoplankton photosynthesizing in the presenceand absence of natural solar ultraviolet radiation (UVR). Atlevels that still permitted the uptake of carbon assimilationinto the cells, UVR caused a marked decline in the overall rateof carbon incorporated into amino acids and a reduction in thepool size of total cellular amino acids (TCAA). In contrast,absolute concentrations of amino acids within the intracellulardissolved free amino acid (INDFAA) pool (measured using an aminoacid analyzer) were higher in the presence of UVR. An examinationof the production patterns and composition of amino acids constitutingthe INDFAA and TCAA pools revealed a marked diminution in thesynthesis and accumulation of alanine and valine in the presenceof UVR. On the other hand, the rates of synthesis and concentrationsof glutamic acid (glutamic acid + glutamine) in the INDFAA andTCAA pools of phytoplankton were higher in samples exposed toUVR. These changes are discussed with reference to the knowneffects of UVR on nitrogen and carbon assimilation within phytoplankton.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.     

Manipulation of chemistry and phytoplankton by hydrological intervention: a whole lake experiment in the northern Netherlands

SUMMARY. 1. The small Lake Negenmad, in the nature reserve 'De Oude Venen', was isolated from the canals and lakes of the Frisian lake system. Its chemistry and phytoplankton were monitored 1 year before (1984) and 2 years after (1985–86) isolation. These characteristics were compared with those of the adjacent Lake Veertigmad which was not manipulated.
2. The hydrological intervention prevented the inflow of chloride-rich IJsselmeer water into Lake Negenmad and made its water table more dependent on evapotranspiration, precipitation and upwelling water. In comparison with the unmanipulated lake. Lake Negenmad became less saline (c. 50%), more humic (c. 50%) and total dissolved (<0.2,μm) iron concentration increased dramatically.
3. During the 2 years of damming no marked differences in the total nutrient concentrations of either lake were observed. However, the maximum phytoplankton density in the dammed lake was half that in the unmanipulated lake. The lower density may have been caused by low P availability after formation of humus-iron phosphate species.
4. Preventing the inflow of water from the Frisian lake system favoured the occurrence in Lake Negenmad of flagellated species (Chrysophyceae, Cryptophyceae) at the expense of filamentous and nuisance-causing cyanobacteria (Oscillutoria, Anabaena, Aphanizomenon spp.).
5. The implications of these results for water quality management of reservoirs in peaty areas are briefly discussed.     

A Comparative Study of Primary Production and Related Factors in Four Saline Lakes in Victoria,Australia

The purpose of the study was to compare the primary plankton productivities of lakes of different salinities and to determine the causative factors involved in their production rates. Four lakes (specific conductivity —mS cm⁻¹ at 18°C) were initially chosen: Coragulac (9), Red Rock (25), Corangamite (38), Pink (250). Sampling and production measurements were made every two to three weeks. Three lakes were dominated by specific phytoplankton blooms: Red Rock (Anabaena spiroides), Corangamite (Nodularia spumigena). Pink (Dunaliella salina). Coragulac Lake had more diverse populations. Red Rock Tarn had some of the highest production values ever recorded. Extremely high soluble phosphate and inorganic carbon concentrations were the most important causative factors. Pink Lake had very low production rates. High salinity and low nutrient concentrations were limiting factors. The other lakes were intermediate in production and nutrient levels. Zooplankton populations were also determined.