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Abiotic heterogeneity of the littoral zone of Lake Balaton influences both horizontal and vertical distribution of macrophytes, but biotic differences could shape the nature of a community. Vertical and tem- poral (small timescale) biomass distributions of Potamogeton perfoliatus and Myriophyllum spicatum were analysed in relation to their photosynthetic capacities to understand their coexistence, general presence over the northern shore of the lake seasonal shift of dominance within the community. Our results indicated the adaptation of these macrophytes to the rapidly changing and mostly low irradiance of the Lake Balaton originated from its high turbidity: both P. perfoliatus and M. spicatum had high photosynthetic activity (20 to 50 mg O2.g drw x1 .h x1 ), low dark respiration (around 12-14% of maximal photosynthetic capacity) and high shade tolerance (Ic of plants were 29 i 18 and 26 i 18 mmol.m x2 .s x1 , re- spectively). The majority of photosynthetic parameters had no (or little) seasonal changes. On the other hand, in Lake Balaton P. perfoliatus and M. spicatum differed in vertical distribution of biomass: Myriophyllum concentrated its biomass in the upper, well-lit region of the water more strongly than the Potamogeton. Results suggest that the autogenic shift of dominance from Potamogeton to Myriophyllum in Lake Balaton can't be explained only by seasonal patterns of photophysiological changes, but supported by constitutive differences in plants architecture could led to the described phenomena.

The aim of the study was to design and put together a compact, easy-to-assemble and cost- efficient incubation system for aquatic plant photosynthesis measurements. Incubation tank consisting of glass sidewalls and mirror inner walls was constructed. The tank was split into six incubation cells and two water collecting cells. Each incubation cell was built of mirror with reflective side turned into the incubation cells to prevent the self shading of plants and minimized the variance of light intensity within the cell. The wall of each incubation cell facing the source of light was made of 3 mm glass and was covered by light absorbing film. To produce different light intensities single light source (fluorescent tubes) and light absorbing film (3M Scotchtint TM Sun Control RE50NEARL) were chosen. The sidewall of each incubation cell was covered with metal-coated neutral sun control film to produce six distinct light intensities. The different transparency of the sidewalls was achieved by lamination of the chosen film in increasing numbers of layers. The effect of the lamination on optical properties of the film was also studied. The variation of photon flux density within a cell was 3.9%. Continuous use of the system and occasional repetitive measurements of film's transparency showed that it maintained its neutral optical properties over a long period of time.

The aim of this research was to test whether NH4 + and NO3 − affect the growth, P demand, cell composition and N2 fixation of Cylindrospermopsis raciborskii under P limitation. Experiments were carried out in P-limited (200 μg l−1 PO4-P) chemostat cultures of C. raciborskii using an inflowing medium containing either 4,000 μg l−1 NH4-N, 4,000 μg l−1 NO3-N or no combined N. The results showed the cellular N:P and C:P ratios of C. raciborskii decreased towards the Redfield ratio with increasing dilution rate (D) due to the alleviation of P limitation. The cellular C:N and carotenoids:chlorophyll-a ratios also decreased with D, predominantly as a result of an increase in the chlorophyll-a and N content. The NH4 + and NO3 − supply reduced the P maintenance cell quota of C. raciborskii. Consequently, the biomass yield of the N2-grown culture was significantly lower. The maximum specific growth rate of N2-grown culture was also the lowest observed. It is suggested that these differences in growth parameters were caused by the P and energy requirement for heterocyte formation, nitrogenase synthesis and N2 fixation. N2 fixation was partially inhibited by NO3 − and completely inhibited by NH4 +. It was probably repressed through the high N content of cells at high dissolved N concentrations. These results indicate that C. raciborskii is able to grow faster and maintain a higher biomass under P limitation where a sufficient supply of NH4 + or NO3 − is maintained. Information gained about the species-specific nutrient and pigment stoichiometry of C. raciborskii could help to access the degree of nutrient limitation in water bodies.

Ammonium and nitrate uptake and N2-fixation of the heterocystous cyanoprokaryote Cylindrospermopsis raciborskii was examined in continuous cultures under different nitrogen concentrations and dilution rates using the 15N technique. It was found that at luxury phosphorus supply (5 mg PO4-P l−1) the biomass was similar in all cultures irrespective of the amount and portioning (continuous or pulsed) of available nitrogen forms. The added ammonium and nitrate was fully taken up by C. raciborskii and the remaining nitrogen demand was met by N2-fixation. Different ammonium concentrations (300, 750, 1500 and 3000 μg 15N l−1) added at the same dilution rate did not affect the growth of C. raciborskii. In the culture supplied with pulsed ammonium, N2-fixation was detected prior to ammonium addition only. After the ammonium pulse, the N2-fixation continued for a while then decreased and stopped. In addition, the inflowing ammonium was fully taken up by the organism. The rate of nitrogen fixation reached its original level after 8–24 hours, depending on the dilution rate. It can be suggested that the nitrogen fixation system stopped and was then activated again depending on the nitrogen content of the cells.

The cyanoprokaryote Cylindrospermopsis raciborskii bloomed many times in Lake Balaton. Some physical parameters and nitrogen forms controlling the growth of C raciborskii were investigated in batch and flow-through cultures. The optimum light intensity was 121 ± 13 μmol m -2 s -1 , while light requirement for growth was low (I k was between 22 and 26 μmol m -2 s -1 ). The optimum temperature was 30 °C, and the maximum growth rate was depending on the nitrogen source. The maximum growth rate lies between 0.84 and 0.88 d -1 when the atmospheric nitrogen (N 2 ) was the sole nitrogen source. When the NH 4 -N was the nitrogen source, the maximum growth rate reached a value of 1.6 d -1 . C. raciborskii has fixed the atmospheric nitrogen even if other nitrogen source as NO 3 -N was available. The strategy of growth may depend on: 1 - nitrogen fixation, when other algae were nitrogen limited; 2 - fast PO 4 -P uptake even if very low concentration is available; and 3 - it has low light requirement, and high temperature.

Underwater light climate, phytoplankton composition and photosynthesis, bacterioplankton production and heterotrophic nanoflagellate abundance were studied in the River Danube near Budapest in 1991. During the winter months water transparency was high, the euphotic zone deep (3 m), but the phytoplankton biomass and the primary production were low as a consequence of low water temperature and reduced global radiation. An increase in phytoplankton abundance (chlorophyll-a concentration reaching 164 μg.l -1 ) during the spring low water period in April and May was observed. This large population was lost to the summer floods. The months of July and August experienced low chlorophyll-a concentrations even though water temperature increased to 20°C. This spring peak was superseded by a second peak in September. The abundance of phototrophic picoplankton in the river was low when compared to that of lakes (10 3 -10 5 cells.ml -1 ). These low concentrations suggest that their contribution towards primary production in the river is not important. The value of the vertical attenuation coefficient (Kd) was mainly influenced by the concentration of suspended solids. In winter the Z eu / Z avg ratio approached 1.0 suggesting that the algae were not light limited this period. The ratio ranged from 0.5-0.7 during the spring and autumn algal blooms and decreased to 0.2-0.3 during the summer floods. The P/R ratio of phytoplankton in the whole water column approached 1.0 in June and July and exceeded 2.0 during the rest of the period. This result supported the hypothesis that light limitation is solely sufficient to prevent algal growth during the summer floods. The mineral nutrients (N, P) do not limit algal growth in the River Danube. Bacterial production measured by 3 H-thymidine incorporation averaged 1.19 nmol.l -1 .d -1 in 1991. The annual average net bacterial production was found to be only 10% that of primary production. Bacterial production seasonal dynamics were found to be independent of phytoplankton production. Zooplankton grazing was considered unimportant in the River Danube. Heterotrophic nanoflagellates (HNF) attained high numbers and were considered to be the main consumer of bacterioplankton and so could play a determining role in the processes of the nutrient cycling in the River Danube.

The contribution of ammonium, nitrate and N 2 to the nitrogen demand of summer phytoplankton has been estimated in two basins of Lake Balaton. The uptake rates of ammonium and nitrate were determined using 15 N methodology. N 2 -fixation rates were measured by the acetylene-reduction technique. The light dependence of uptake and N 2 -fixation was described with an exponential saturation equation. The decrease of I k values from mid-summer to autumn coincided with an increasing contribution of cyanobacteria to the algal population, suggesting a relatively lower light requirement than that of other species. The dark uptake velocity of ammonium and nitrate were 30-63 % and 1-40 % of light saturated uptake, respectively. The contribution of ammonium to the daily nitrogen supply of algae varied between 50-90%. Nitrate uptake was much less important with a maximum of 40 %. Nitrogen fixation was significant only in the western basin at the time of highest primary production and greatest N 2 -fixing cyanobacterial contribution. Nitrogen uptake experiments and loading data suggest, that nitrogen regeneration is by far the most important process in establishing and supporting the standing crop of phytoplankton in the lake during summer.

La croissance et la composition cellulaire de C. meneghiniana soumis a des concentrations limitantes en P, N et Si en culture continue a des taux de dilution differents sont discutes. Le contenu cellulaire en carbone, azote et phosphore par cellule ou par unite de volume augmente avec le taux de croissance dans toutes les conditions precitees. Le taux maximum de croissance depend du type de nutriment limitant. Ces valeurs sont tres proches de celles calculees a partir des modeles de Droop, Caperon, Goldman et Monod. Dans le Danube, les concentrations en N et P sont beaucoup plus elevees que les besoins pour la croissance et 1' assimilation de C. meneghiniana. Cela indique que ni N ni P ne sont limitants pour la croissance de C. meneghiniana en milieu naturel.

A strain of Cyclotella meneghiniana was isolated from the River Danube. The optimum irradiance and temperature for growth were determined in batch cultures. The optimum temperature for growth was 25 °C. The estimated saturated irradiance was near 125 µE m-2 s-1. The calculated Ik for growth and photosynthesis were 30 ± 3 and 44 ± 6 µE m-2 s-1 respectively. A continuous culture technique was used to test the growth and cell composition at the transition state between batch, nutrient sufficiency, and a low rate of nutrient supply. Algal response was characterized by high intracellular N and C with negative growth rate. At a low dilution rate (0.06 d-1), the continuous culture theory was not followed. This critical rate is characterized by low chlorophyll-a content per cell and may depend on the mode of nutrition.

N 2 -fixation and 15 N-labelled ammonium and nitrate uptake were measured during a bloom of C. raciborskii in two basins of Lake Balaton. In the eastern basin when the chlorophyll-a content reached 70 μg/l the N 2 -fixation was low (59 mg N m -2 day -1 ) and the N uptake was dominated by ammonium (393 mg N m -2 day -1 ). The nitrate uptake was of minor importance in both basins (43 and 55 mg N m -2 day -1 , respectively). In the western basin, the chlorophyll-a content was 160 μg/l and recirculation of regenerated forms of nitrogen could not support the demand. Although the ammonium uptake was high (357 mg N m -2 day -1 ), the fixed nitrogen (407 mg N m -2 day -1 ) exceeded it. The daily integrated carbon and nitrogen uptake ratios were 5.2 (weight/ weight) in both basins.

A pico sized Synechococcus species isolated from Lake Balaton was studied in batch and continuous cultures. This picocyanobacterium had a pH optimum at 8.5 and a temperature optimum at 28-30 °C. The I g , value for growth was 52 μEinstein m -2 s -1 , the maximum growth rate 2.27 d -1 , the half saturation constant of growth 1.2 μg PO 4 -P1 -1 and the minimal cell quota 1.74 mg Pg dry weight -1 . The dry weight of cells showed a minimum, the chlorophyll-a/biomass ratio a maximum as a function of growth rate. Above the quota of 3.4 fg P cell -1 significant amounts of non-reactive dissolved phosphorus were released.

P-uptake kinetics of extremely P-deficient planktonic microorganisms were analyzed in shallow Lake Balaton by means of a conceptual model of P uptake. According to the model, net P uptake is a linear function of the driving force of the uptake and ceases below a substrate threshold for energetic reasons. We estimated net P-uptake and P-leakage rates from 32P-uptake data. The study period (January-May 1992) covered development, steady state growth, and collapse of a typical spring diatom bloom. 32P incorporation of 12-pm microorganisms was separated by postfiltration. On five occasions, net P-uptake rates were simultaneously obtained from chemical measurements. Chemically measured net P-uptake curves, 32P-uptake curves, and size partitioning of 32P incorporation can be recalculated from the constants of the linear force-flow model. The initial orthophosphate concentration fluctuated around 0.1 pg P liter-l. The community-average P threshold exceeded the initial P concentration by a mean factor of 1.4, that of the > 12-pm microorganisms by a mean factor of 3.9. Small-scale fluctuations of in situ P concentrations were necessary to maintain P uptake and growth of larger microorganisms. In a nonhomogeneous environment, sensitivity to the fluctuating nutrient concen- trations, efficiency of leakage reduction, and lower maintenance cell quotas may provide a competitive advantage to more complex-organisms.

1. The importance of various forms of nitrogen to the nitrogen supply of phytoplankton has been investigated in the mesotrophic eastern and eutrophic western basin of Lake Balaton. 2. Uptake rates of ammonium, urea, nitrate and carbon were measured simultaneously. The uptake rates were determined using N-15 and C-14 methodologies, and N-2-fixation was measured using the acetylene-reduction method. The light dependence of uptake was described with an exponential saturation equation and used to calculate surface-related (areal) daily uptake. 3. The contribution of ammonium, urea and nitrate to the daily nitrogen supply of phytoplankton varied between 11 and 80%, 17 and 73% and 1 and 15%, respectively. N- 2-fixation was negligible in the eastern basin and varied between 5 and 30% in the western region of the lake. The annual external nitrogen load was only 10% of that utilized by algae. 4. The predominant process supplying nitrogen to the phytoplankton in the lake is the rapid recycling of ammonium and urea in the water column, The importance of the internal nutrient loading is emphasized.

Seasonal orthophosphate uptake of natural plankton has been studied in hypertrophic and mesotrophic basins of Lake Balaton. Initial uptake of carrier-free labelled phosphoric acid followed first order kinetics in accordance with a single two-compartment model. In isotope equilibrium from 1.2 to 9.7 per cent of radioactivity was found in the filtrate. Turnover times of orthophosphate in water were some minutes in summer and two orders of magnitude longer in winter and April. Turnover time of phosphorus in microorganisms has been estimated on the basis of equilibrium isotope distribution between water and particular phases and turnover time in water. It ranged from 28 to over 1 000 min. The shortest times were found in summer, and the longest in winter. Turnover time of orthophosphate in water increased in relation to phosphate additions according to Michaelis-Menten kinetics. The values of Michaelis-constant plus the ambient orthophosphate concentration (K + Sn) suggested an overestimation of real phosphate concentrations by chemical measurements, being much lower than soluble reactive phosphorus contents in most cases, as it is well-known in many other lakes. Both K + Sn values and maximum uptake velocities (V) were significantly higher in the hypertrophic basin than in the mesotrophic one, in contrast to essentially similar values of turnover times. Maximum uptake velocity per unit chlorophyll-a P opt:V ratio showed extreme to moderate phosphorus deficiency for Lake Balaton plankton, which seemed to decrease somewhat in the course of eutrophication.

The process of eutrophication has threatened the viability for recreation of the largest lake in Middle Europe, Lake Balaton. For studying and simulating the system a deterministic model (a system of differential equations) has been constructed. The four basins of the lake are described by the same model and the models of basins are concatenated by hydrological throughflow. The one-basin model with its ten state variables describes the main mass transport processes in the pelagic zone of the lake and in the sediment, as the lake is shallow. The model can simulate seasonal changes and it is able to forecast the effect of some measures on water quality in the short term. The simulation runs show that the measures can be effective when the phosphorus-removing water-dam systems involved in tertiary treatment of sewage water work at a high rate.

Feeding experiments were carried out with fry of silver carp fed on cultured algae, Scenedesmus ellipsoideus, Anacystis nidulans, Anabaena flos-aquae , and on phytoplankton collected in Lake Balaton. Analyses of the intestinal contents of silver carp showed that blue-green algae and diatoms were ingested and digested to the greatest extent. Colonies of blue-greens and big forms of greens were ingested, but poorly digested. Fish were not able to ingest algae smaller than 10 μm in size. Silver carp fed on blue-greens consumed 11–13% of their body weight (wet wt) in a 40 mg l −1 algal suspension. Filtration rate, at the same food density, ranged between 2.6 and 3.3 l g −1 (body weight) 4 h −1 . Algae labeled with 14 C passed through the alimentary canals in an average time of 3.2 h. Feeding of silver carp was most intense in the afternoon and lowest at night. Fecal algae ( Scenedesmus ) did not increase either the biomass or the primary production at a fecal concentration of 1 mg l −1 . At a fecal concentration of 3 mg l −1 , however, there was a slight increase in both the biomass and the primary produciton. At 10 mg l −1 fecal concentration the biomass and primary production of living cells almost doubled. Fecal algae ( Anabaena ) increased the primary production at 10 mg l −1 , but at 50 mg l −1 the biomass was three times and the primary production four times higher as compared with the control.

Adsorption of phosphate by sediments could be described by the Langmuir isotherm. The adsorption capacity of the sediments increased with their NaOH-extractable iron content, and the energy of adsorption was proportional to their calcite content. Increasing phosphate saturation of the sediments could be demonstrated near the inlet of the Zala River into the Kis-Balaton reservoir, as a result of high external loading. Phosphate saturation of the sediments remained similar in the hypertrophic and mesotrophic basins of Lake Balaton. This was explained by the continuous renewal of calcite, the main phosphate sorbent in this calcareous lake. A comparison of the interstitial SRP concentration and the adsorption/desorption concentration limit, derived from the Langmuir isotherm, indicated that sorption is of major importance in regulation of the interstitial phosphate concentration. In Lake Balaton desorption occurs whenever the sediments are resuspended; adsorption takes place only in the sediments. In the two reservoirs, adsorption and biotic uptake are equally important in the removal of phosphorus from the water, depending on the actual loading and primary production.

The aim of the investigations was to identify the main process controlling the rather low phosphate concentration in the water of Lake Balaton. Three processes were taken into consideration: a) phosphate uptake by algae, b) coprecipitation with biogenic lime, and c) adsorption on suspended sediment particles. A plexiglass box open at the bottom was placed in the shallow water with its sides extending into the sediment. The water in the box was stirred. When phosphate alone was added to the water in the box, its concentration decreased very rapidly. On the other hand, if NaOCl was also added in order to kill algae and bacteria, the phosphate concentration remained constant. This proves the priority of life processes in phosphate removal. In the next experiment phosphate and EDTA-Na2 were added simultaneously to the water in the box. The complexing agent prevented biogenic lime formation but did not influence the rapid phosphorus removal. This fact indicates that the main process involved in phosphate control is direct uptake by algae. The same conclusion was obtained in laboratory experiments where labelled phosphate was added to the bottles containing Lake Balaton water. It was demonstrated by the photo-oxidation technique that the phosphate removed from the water was incorporated into the organic particulate matter.

Enclosures isolating 17 m3 water with the underlying sediments in the mesotrophic area of Lake Balaton were used to simulate the main processes taking place in its hypertrophic area. In an enclosure with both P and N added the ~production and the biomass of phytoplankton increased immediately. Algae rapidly took up the added nutrients, transporting a third of the loadings from the water to the sediments and supplying a large amount of organic substrate for denitrification. As a result of N loss from the system, Aphanizomenonflos-aquae formed a second bloom. In an enclosure with only N added the production and the biomass of algae remained similar to those in the open water. NO,- accumulated in the water for a long time. In an enclosure with only P added the algal production and biomass showed a moderate increase, followed by an outburst of A. flos-aquae. POd3- accumulated in the water during the first part of the experiment and was used up during the second.

Phytoplankton maximum growth rate and the saturation light intensity, Is, can be estimated from vertical profiles of primary production by non-linear least-squares estimation. Solution through the normal equations leads to formulae which are relatively simple and easy to implement. The computation of confidence contours is demonstrated, and the results are contrasted to the confidence limits on the parameters individually. These can be quite misleading due to model non-linearity and correlation between parameter estimation. The procedure has been applied to primary production data from Lake Balaton, a shallow lake in Hungary. The growth rate-temperature relation is analysed by separating the parameters into two groups characteristic for “warm” and “cold” water phytoplankton, respectively. A bell-shaped curve is found for “cold” water communities, with an optimum at about 7–9°C, whereas the “warm” water phytoplankton exhibits a strong exponential dependency in the temperature range of interest (up to 25°C). Is also appears to be related to temperature except for the “cold” water group, where Is is essentially constant. However, a roughly linear relation with considerably less scatter is obtained when Is is plotted directly versus day-averaged solar radiation. This apparent fast adaptation is attributed to the extremely short turnover time in Lake Balaton. Maximum growth rates of 10–20 d−1 have been found for temperatures between 20 and 25°C. These results and a critical appraisal of available literature suggest that the common notion of maximum growth rates being in the order of 1–3 d−1 needs revision, at least for lakes with relatively high summer temperatures.

Sediment phosporus was studied by a combination of the fractionation procedure of Hieltjes & Lijklema and the isotopic dilution technique in a mesotrophic (Tihany) and a hypertrophic (Keszthely) basin of Lake Balaton. In the calcareous sediments the largest part of phosphorus was bound to calcium. Iron-bound and residual P showed higher concentrations at Keszthely than at Tihany. There was little loosely adsorbed P at both locations. Vertical differences in P fractions of the sediments were more pronounced at Keszthely than at Tihany. Exchangeability of the fractions decreased in the following sequence: iron-bound > loosely adsorbed > calcium-bound > residual. Phosphorus, particularly in the calcium-bound fraction, was more mobile at Keszthely, and its exchangeability decreased rapidly downwards at both stations. Four times more potentially mobile phosphorus has been accumulated by the upper sediment layer at Keszthely than at Tihany.

After in vitro incubation of rat epididymal fat pads with radioactive palmitic acid, the distribution of the label in the different lipid classes and in different triglycerides was determined by silica gel and silver nitrate-silica gel thin-layer chromatography (TLC).The radioactivity of the diglycerides was approximately half of the triglycerides. This ratio did not change with alteration in the time of incubation. It remained unaltered even after a subsequent 10-min incubation in a nonradioactive medium.When the fat pads were incubated, first with(14)C-, then with(3)H-labeled palmitic acid, the(3)H/(14)C ratio was slightly lower in diglycerides than in triglycerides.The fully saturated molecules contained 38% of the radioactivity of triglycerides. Addition of oleic acid or norepinephrine to the labeled palmitic acid-containing medium decreased this value. Subsequent incubation with these compounds did not alter the distribution of radioactivity.

Rat adipose tissue pieces were incubated with acetate-2-14C and glucose-14C(U), respectively, and liver slices with acetate-2-14C. The labeled tissues were then reincubated in inactive medium, and the changes of radioactivity in the different lipid classes were determined. In all three experiments a significant amount of radioactivity was incorporated in the diglycerides. During 1 hr of reincubation in inactive medium the radioactivity of diglycerides decreased from 35 to 26% of the total lipid activity in the adipose tissue labeled with acetate. In the adipose tissue labeled with glucose radioactivity fell from 25 to 19%. In liver slices 11% of the labeled acetate was incorporated in the diglycerides, and during the 2 hr of reincubation this value fell to its half. The radioactivity of the uniformly labeled glucose was distributed equally in the fatty acids and the glycerol. The distribution of radioactive glycerol between diglycerides and triglycerides was similar to that of the labeled fatty acids. Triglyceride synthesis seems to always be accompanied by the formation of diglycerides with a lastint turnover time.

Measurements of the iodine value made over a period of three years demonstrate a regular yearly cycle in the composition of the fat of crustacean plankton in Lake Balaton. The melting point of the lipid of the planktonic copepods remained during the whole year somewhat lower than the water temperature. The proportions of the C20–C22 poly-unsaturated acids in the planktonic crustaceans increased with decreasing temperature and in some species exceeded the values characteristic of marine animals. In fresh-water fish, kept at room temperature and fed on freshly collected fresh-water plankton, the lipids formed in winter resembled marine fish oil. In the crustaceans raised on algae containing no fatty acids longer than C18, the C20-C22 acids always appeared as major components of the copepods.