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In April 2014, dual bloom of green algae and purple bacteria occurred in a shallow, alkaline soda pan (Kiskunság National Park, Hungary). The water was only 5 cm deep, in which an upper green layer was clearly separated from a near-sediment purple one. Based on microscopy and DNA-based identification, the upper was inhabited by a dense population of the planktonic green alga, Oocystis submarina Lagerheim, while the deeper layer was formed by purple, bacteriochlorophyll-containing bacteria, predominated by Thiorhodospira and Rhodobaca. Additional bacterial taxa with a presumed capability of anoxygenic phototrophic growth belonged to the genera Loktanella and Porphyrobacter. Comparing the bacterial community of the purple layer with a former blooming event in a nearby soda pan, similar functional but different taxonomic composition was revealed. Members from many dominant bacterial groups were successfully cultivated including potentially new species, which could be the result of the application of newly designed media. [ABSTRACT FROM AUTHOR]

Winter phytoplankton communities in the shallow alkaline pans of Hungary are frequently dominated by picoeukaryotes, sometimes in particularly high abundance. In winter 2012, the ice-covered alkaline Zab-szék pan was found to be extraordinarily rich in picoeukaryotic green algae (42-82 × 10 cells ml) despite the simultaneous presence of multiple stressors (low temperature and light intensity with high pH and salinity). The maximum photosynthetic rate of the picoeukaryote community was 1.4 μg C μg chlorophyll a h at 125 μmol m s. The assimilation rates compared with the available light intensity measured on the field show that the community was considerably light-limited. Estimated areal primary production was 180 mg C m d. On the basis of the 18S rRNA gene analysis (cloning and DGGE), the community was phylogenetically heterogeneous with several previously undescribed chlorophyte lineages, which indicates the ability of picoeukaryotic communities to maintain high genetic diversity under extreme conditions. [ABSTRACT FROM AUTHOR]

Benthic algal communities can play an important role in matter and energy flux of shallow lakes. Their contribution to total primary production of lakes has been largely unexplored. The aim of this study was to estimate the primary production of the epipsammic algal communities at different water depths in Lake Balaton (Hungary) with photosynthetic measurements performed in laboratory. The photosynthesis of the benthic algae of different origin was studied at nine different irradiance levels, in three replicates. The maximum photosynthetic rate ( P) was always higher in samples from the shallow parts than those from the deeper regions of the lake. Along the west-east longitudinal axis of the lake P decreased in the southern part and increased in the middle of the lake as a consequence of differences in the chlorophyll- a concentrations. Knowing P, I, global radiation and extinction coefficient, the primary production (mg C m day) of the epipsammic algal community was calculated at different water depths. In the shallow regions at 0.5 and 1 m water depth 75-95% and 60-85% of the production was attributable to the epipsammon. The percentage contribution of epipsammon was at 2 m water depth 20-65%. In the deeper pelagic region (>3 m) more than 85% of the primary production originated from the phytoplankton. [ABSTRACT FROM AUTHOR]

The present study documents the findings that picophytoplankton is one of the fundamental contributors to primary biomass production not only in aquatic environments with very low content of inorganic nutrients, but also in lakes with higher trophicity. The autotrophic picophytoplankton (APP) comprises bacterialsized (0.2-2μm) picocyanobacteria and eukaryotic phototrophs (mainly green algae). These organisms can provide up to 90% of the primary production of shallow soda lakes. Shallow soda pans are very characteristic of the Pannonic Ecoregion (Austria, Hungary and Serbia) and they represent a considerable part of the lakes present in this region. Between 11 May and 20 June 2009, the authors determined the total biomass of the phytoplankton (based on chlorophyll-a) in 65 soda pans (with an average depth of 17 cm) of the Pannonic Ecoregion. They also estimated the biomass and abundance of the picophytoplankton, using epifluorescence microscopy. The physico-chemical characteristics of the water, relevant for the phytoplankton, were also investigated. The chlorophyll-a (Chl-a) concentration ranged between 1 μg l-1 and 1400 μg l-1 with an average of 115 μg l-1. The abundance varied between 1000 cells ml-1 and 88 million cells ml-1. Picoplankters were represented by 1.5 μm-sized green algae and 1.0 μmsized phycocyanin-rich bacteria. Phycoerythrin-rich picocyanobacteria have not been detected. Based on the present findings, in these water bodies, the contribution of picoplankton to total phytoplankton did not show a decreasing tendency with increasing trophic state, as it was recorded earlier by other authors in different types of aquatic environments. The present results reveal that in soda lakes the picophytoplankton has a crucial contribution to the total primary production not only under conditions of nutrient deficiency, but also in lakes with higher trophicity. The total phosphorus (TP) concentration in the soda lakes studied varied between 70 μg l-1 and 59 000 μgl-1. Based on TP, four of the lakes were considered eutrophic and 61 hypertrophic. Based on phytoplankton biomass, half of the lakes were classified as oligotrophic, respectively mesotrophic, while the rest were considered eutrophic, respectively hypertrophic. [ABSTRACT FROM AUTHOR]

Large lowland rivers with sufficient hydrological storage capacity are capable of supporting primary production, but the dynamics of the advecting phytoplankton is poorly understood. Our study aimed at exploring how longitudinal versus lateral connectivity, flow dynamics versus resource availability and continuous versus discontinuous environmental gradients shaped the species composition of phytoplankton. Samples were taken from February to October 2000 along the Hungarian Tisza River (HTR) and in its main tributaries. Longitudinal and seasonal patterns were related to resources (light and nutrients) availability and flow dynamics derived from a 1D hydrodynamic model. The HTR was autotrophic during the study period, but tributary input considerably exceeded net autochtonous production. The Szamos River was the major source of both phytoplankton and nutrients in the HTR. Chryso- and euglenophytes were flushed into the main river from floodplain oxbows during high discharge. Imported algae experienced discontinuity in environmental gradients when entering the main river. The merged impact areas of two dams (IAD) that separate the two large meandering patches of the HTR disrupted the longitudinal profiles of both physico-chemical variables and attributes of algal assemblages (biomass, species composition, richness, similarity between adjacent sampling sites). Hydraulic storage along the IAD selectively favoured the recruitment of cryptophytes that, however, could not compensate for the enhanced sedimentation of diatoms in terms of biomass. Although the meandering patches presented several small-scale differences in major environmental gradients, both patches supported the growth of planktonic diatoms. Changes in algal biomass were decoupled from nutrient availability. We conclude that various measures must be applied in various lowland rivers within the same catchment to control their trophic status as a component of the ‘good ecological status’ defined in the Water Framework Directive. [ABSTRACT FROM AUTHOR]

Since the middle of 1990s the trend of Lake Balaton towards an increasingly trophic status has been reversed, but N2-fixing cyanobacteria are occasionally dominant, endangering water quality in summer. The sources of nitrogen and its uptake by growing phytoplankton were therefore studied. Experiments were carried out on samples collected from the middle of the Eastern (Siófok) and Western (Keszthely) basins between February and October 2001. Ammonium, urea and nitrate uptake and ammonium regeneration were measured in the upper 5-cm layer of sediment using the 15N-technique. Ammonium was determined by an improved microdiffusion assay. N2 fixation rates were measured by the acetylene-reduction method. Ammonium regeneration rates in the sediment were similar in the two basins. They were relatively low in winter (0.13 and 0.16 μg N cm−3 day−1 in the Eastern and Western basin, respectively), increased slowly in the spring (0.38 and 0.45 μg N cm−3 day−1) and peaked in late summer (0.82 and 1.29 μg N cm−3 day−1, respectively). Ammonium uptake was predominant in spring in the Eastern basin and in summer in the Western basin, coincident with the cyanobacterial bloom. The amount of N2 fixed was less than one third of the internal load during summer when external N loading was insignificant. Potentially, the phytoplankton N demand could be supported entirely by the internal N load via ammonium regeneration in the water column and sediment. However, the quantity of N from ammonium regeneration in the upper layer of sediment combined with that from the water column would limit the standing phytoplankton crop in spring in both basins and in late summer in the Western basin, especially when the algal biomass increases suddenly. [ABSTRACT FROM AUTHOR]

The biological availability of dissolved organic carbon (DOC) was experimentally studied in water samples collected at the mouth of the River Zala and in the western and eastern basin of Lake Balaton (Hungary) in four seasons. The water samples were filter-sterilized and inoculated with the in situ bacterial population. The concentration of fulvic and humic acids were analyzed at the beginning of the experiment. The bacterioplankton biomass and DOC concentration were measured on day 0 and 28. The decrease in the DOC concentration and the ratio of the bacterial C/initial DOC concentration showed the microbial utilization of DOC. No seasonal changes in DOC availability were found at any of the sampling stations. The DOC bioavailability was higher in the river than in the lake water. A strong positive correlation was found between the bioavailability and the humic properties of DOC. The observed changes in the organic matter composition of Lake Balaton support the view that much of the not readily utilizable ‘refractory’ DOM resides in the non-humic pool in standing waters. [ABSTRACT FROM AUTHOR]

The aim of the present study was to assess the effect of ultraviolet radiation (UVR, 280–400 nm) on phytoplankton primary production in Lake Balaton, a shallow lake with the largest surface area in Central Europe possessing unique optical properties. The experiments were performed in situ in summer 1999 in the western basin with generally higher primary production rates and light attenuation coefficient due to higher algal biomass and in the eastern basin of the lake. Primary production was measured using 14C-technique, samples collected on-site were incubated at different depths for 4 h centred around noon. Underwater photosynthetically active radiation (PAR, 400–700) was measured prior to incubation. The incubation was carried out in three treatments using cut-off filters to exclude various portions of the UVR spectrum. Surface photoinhibition reached 72–78% in the samples exposed to PAR and UVR as compared to the control treatment from which UVR was excluded, however, areal primary production showed only an 8–14% decrease in the presence of UVR. Although the filters applied absorb ca. 15% PAR as well, comparison of the treatments pointed out that it was UVAR (320–400 nm), and not UVBR (280–320 nm) that predominantly inhibited photosynthesis. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is quantitatively the most significant pool of organic matter in lakes. Within DOM, the pool of dissolved organic carbon (DOC) is dominated (50–80%) by humic substances (HS). The aim of the present study was to determine the changes in quality of DOM along the longitudinal axis of Lake Balaton from the mouth of the Zala River (main inflow) to the eastern part (single outflow) of the lake. The composition of organic fractions and the contribution of HS to the total organic carbon (TOC) pool were determined. Different analytical methods (organic carbon analysis, size fractionation, colour and fluorescence measurements) were used. Our results demonstrated that in the total organic carbon pool of the lake, DOC was the dominant fraction (82–96%). The humic substances made up 75% of DOC at the mouth of the Zala River, and decreased to 55% toward the eastern part of the lake, while the contribution of fulvic acids (FA) to the HS pool increased from 75 to 99%. Along the longitudinal axis of the lake not only did the contribution of humic substances to the total DOC pool decrease, but also the Pt-colour and the fluorescence intensity, while the proportion of the higher molecular weight fractions increased, suggesting that the organic substances became more refractory. [ABSTRACT FROM AUTHOR]

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 &sup15;N and &sup14;C methodologies, and N[sub 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[sub 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. [ABSTRACT FROM AUTHOR]

Aerobic anoxygenic phototrophs (AAPs) are a group of photoheterotrophic bacteria common in natural waters. Here, AAP abundance and contribution to total bacterial abundance and biomass were investigated to test whether the trophic status of a lake or content of coloured dissolved organic matter (CDOM) play a role in determining AAP distribution and abundance in shallow inland lakes, with special focus on hypertrophic and polyhumic waters. Twenty-six different shallow lakes in Hungary were monitored. AAP abundance and biomass were determined by epifluorescence microscopy. The lakes exhibit a broad range of CDOM (2–7000 mg Pt L−1) and phytoplankton biomass (2–1200 μg L−1 chlorophyll a concentration). Very high AAP abundance (up to 3 × 107 cells mL−1) was observed in polyhumic and hypertrophic shallow lakes. AAP abundance was influenced by phytoplankton biomass and CDOM content, and these effects were interrelated. As determined, 40 μg L−1 chlorophyll a and 52 mg Pt L−1 CDOM are threshold levels above which these effects have a synergistic relationship. Hence, the observed high AAP abundance in some soda pans is a consequence of combined hypertrophy and high CDOM content. AAP contribution was influenced by total suspended solids (TSS) content: the success of AAP cells could be explained by high TSS levels, which might be explained by the decrease of their selective grazing control. [ABSTRACT FROM AUTHOR]

Occurrence of the smallest phototrophic microorganisms (photoautotrophic picoplankton, APP) in Lake Balaton was discovered in the early 1980s. This triggered a series of systematic studies on APP and resulted in the setting of a unique long-term picoplankton dataset. In this review, we intend to summarize the obtained results and to give a new insight on APP ecology and diversity in Lake Balaton. According to the results, APP dynamics depends on trophic state, temperature, nutrient, and light availability, as well as grazing pressure. APP abundance in Lake Balaton decreased to a low level (1-2 × 10 5  cells mL -1 ) as a consequence of decreasing nutrient supply (oligotrophication) during the past more than two decades, and followed a characteristic seasonal dynamics with higher abundance values from spring to autumn than in winter. Concomitantly, however, the APP contribution to both phytoplankton biomass and primary production increased (up to 70% and 40-50%, respectively) during oligotrophication. Regarding annual pattern, picocyanobacteria are dominant from spring to autumn, while in winter, picoeukaryotes are the most abundant, most likely due to the different light and temperature optima of these groups. Within picocyanobacteria, single cells and microcolonies were both observed with mid-summer dominance of the latter which correlated well with the density of cladocerans. Community-level chromatic adaptation (i.e., dominance of phycoerythrin- or phycocyanin-rich forms) of planktonic picocyanobacteria was also found as a function of underwater light quality. Sequence analysis studies of APP in Lake Balaton revealed that both picocyanobacteria and picoeukaryotes represent a diverse and dynamic community consisting several freshwater genotypes (picocyanobacteria: Synechococcus, Cyanobium; picoeukaryotes: Choricystis, Stichococcus, Mychonastes, Nannochloris, and Nannochloropsis)., (© 2020. The Author(s).)

Lake Balaton is the largest European shallow lake, which underwent cultural eutrophication in the '70-80s. Therefore, strict pollution control measures were introduced and the water quality has become meso-eutrophic since the millennium. Due to the touristic significance and change in trophic levels of the lake, numerous ecological studies were carried out, but none of them was focused on both benthic and planktonic microbial communities at the same time. In our study, an attempt was made to reveal the spatial bacterial heterogeneity of the Lake Balaton and Zala River by 16S rDNA terminal restriction fragment length polymorphism fingerprinting and Illumina amplicon sequencing methods in the summer of 2017. According to the molecular biology results, mostly well-known freshwater microorganisms, adapted to nutrient-poor conditions were found in the pelagic water column. The LD12 subclade member Fonsibacter ubiquis, the cyanobacterial Synechococcus sp. and unknown Verrucomicrobia species were abundant in the less nutrient-dense basins, while the hgcI clade members showed various distribution. In the estuary and in the nutrient-dense western part of the lake, some eutrophic conditions preferring cyanobacteria (filamentous Anabaena and Aphanizomenon species) were also detectable. The benthic microbial community showed higher diversity, according to the observed appearance of microorganisms adapted to the deeper, less aerated layers (e.g. members of Desulfobacteraceae, Nitrosomonadaceae).