Lake Neusiedl, the largest steppe lake in Europe, is particularly sensitive to climate variations due to its extreme shallowness (z(max) = 1.8 m) and low ratio of catchment to lake area (3.5 : 1). Changes in water budget, salinity and turbidity have key implications for the lake's ecology and management. Here, we present a multi-proxy palaeolimnological reconstruction of the evolution of Lake Neusiedl since the end of its last complete desiccation (1865-1868), based on an undisturbed radiometrically dated core taken from the open water portion of the lake. Geochemical and biological (algal) proxies outline the succession of three major ecological stages since 1873 +/- 16 yrs, with the first major changes appearing already in the 1930s as driven by climate related hydrological variability. Subfossil diatoms proved to be reliable for tracking long-term changes in the trophic conditions of Lake Neusiedl while diatominferred lake conductivity revealed to be unreliable due to a combination of lake environmental settings and the absence of a site-specific training set. Nonetheless, multivariate statistical analyses and comparisons with limnological data confirm a great potential of subfossil diatoms for revealing past ecological changes and tipping points of shallow lakes, as long as studies rely on a multi-proxy approach. In agreement with limnological surveys, the sediment record corroborates the high vulnerability of Lake Neusiedl, both in present and past times, towards climate-driven changes in water level and salinity, and allows the prediction, by analogy with the past, of future ecological changes in a context of global warming and increasing nutrient inputs from non-point sources. (C) 2021 The Authors. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.

Zusammenfassung Die vorliegende Arbeit gibt einen Überblick über die Untersuchungen zur mikrobiologisch-hygienischen Wasserqualität des Neusiedler Sees. Am Beginn steht ein historischer Abriss über die Entwicklung eines entsprechenden Monitorings. Es folgt eine statistische Analyse mikrobiologischer Langzeitdaten (1992 bis 2013) und deren Verknüpfung mit Wasserqualitätsparametern und Wetterdaten. Die Ergebnisse wiesen die Punkte des offenen Sees und die EU-Badestellen mit ausgezeichneter Wasserqualität aus, nur 3 Perioden waren an der EU-Badestelle Rust mit „gut“ klassifiziert. Es ließen sich aber auch Hotspots der fäkalen Belastung im Neusiedler See identifizieren. Es zeigte sich, dass diese Hotspots der fäkalen Verschmutzung mit menschlichen Aktivitäten in Verbindung standen, diffuser Eintrag aus der Landwirtschaft oder Eintrag durch Wildtiere eher lokal eine Rolle spielten. Bei den Hotspots standen mehr als die Hälfte aller Verschmutzungsereignisse in zeitlichem Zusammenhang mit extremen Wetterereignissen. Neben heftigen Regenfällen waren auch Starkwindereignisse mit den Verschmutzungsereignissen korreliert. Einschwemmung von tierischem Fäzes über Kanäle oder diffus und das Überlaufen von Kläranlagen bei Starkregenereignissen sowie windbedingtem Aufwirbeln von Sedimenten und Einblasen kontaminierter Wässer aus dem Schilfgürtel durch Starkwinde stehen höchstwahrscheinlich in kausalem Zusammenhang mit diesen Beobachtungen. Abschließend werden die Erkenntnisse aus der Pilotstudie für ein nachhaltiges Gesamtkonzept zum Management des mikrobiologisch-hygienischen Zustandes des Neusiedler Sees in Vorschläge für ein zukünftiges Monitoring fäkaler Verschmutzung eingebracht.

Die vorliegende Arbeit behandelt die Nährstoffdynamik am Neusiedler See, ein Thema, das untrennbar mit den Einträgen und Austauschprozessen der Fest- oder Schwebstoffe verbunden ist. Es werden zusammenfassend ältere Untersuchungen von Anfang der 1980er-Jahre diskutiert und neueren Erkenntnissen aus einem laufenden EU-INTERREG-Projekt (REBEN) gegenübergestellt. Der Beitrag behandelt zunächst die Schwebstoff- und Nährstofffrachten in den Neusiedler See über den größten Zubringer des Sees, die Wulka. In einem zweiten Abschnitt werden Veränderungen durch Umsetzungsprozesse beim Durchtritt der Wulka durch den Schilfgürtel-Mündungsbereich Höhe Donnerskirchen beschrieben. Die Langzeitentwicklung der Nährstoffe im offenen See bildet den dritten Abschnitt, während abschließend die horizontale Verteilung von Schweb- und Nährstoffen im Schilfgürtel und Rückschlüsse auf horizontale Strömungen und Austauschprozesse behandelt werden. Der wichtigste Befund der Datenanalysen ist der Rückgang der Phosphorfrachten der Wulka in den letzten Jahrzehnten, eine entsprechende Reaktion der trophischen Verhältnisse im See selbst sowie die Bedeutung horizontaler Austauschprozesse zwischen offenem See und Schilfgürtel für den langfristigen Erhalt einer guten Wasserqualität im Freiwasser und eines guten ökologischen Zustands des Neusiedler Sees.

This paper deals with nutrient dynamics at Lake Neusiedl, an issue that is inseparable from the inputs and exchange processes of suspended matter. Older studies from the early 1980s are summarised, discussed and compared with new findings from an ongoing EU INTERREG project (REBEN). The article first deals with the suspended matter and nutrient loads in Lake Neusiedl via the largest tributary of the lake, the river Wulka. In a second section changes due to chemical processes during the passage of the river Wulka through the reed belt at Donnerskirchen are described. The long-term development of nutrients in the open lake are discussed in the third part of the paper, which closes with the description of the horizontal distribution of suspended matter and nutrients in the reed belt and conclusions about horizontal currents and exchange processes. The most important finding of the data analysis is the decline of the phosphorus load of the river Wulka in recent decades, a corresponding reaction of the trophic conditions in the lake itself and the importance of horizontal exchange processes between the open lake and the reed belt for the preservation of a good water quality in the open water zone and a good ecological status of Lake Neusiedl.

To minimize the risk of negative consequences for public health from fecal pollution in lakes, the continuous surveillance of microbiological water quality parameters, alongside other environmental variables, is necessary at defined bathing sites. Such routine surveillance may prove insufficient to elucidate the main drivers of fecal pollution in a complex lake/watershed ecosystem, and it may be that more comprehensive monitoring activities are required. In this study, the aims were to identify the hotspots and main driving factors of fecal pollution in a large shallow Central European lake, the Neusiedler See, and to determine to what degree its current monitoring network can be considered representative spatially. A stochastic and geostatistical analysis of a huge data set of water quality data (~ 164,000 data points, representing a 22-year time-series) of standard fecal indicator bacteria (SFIB), water quality and meteorological variables sampled at 26 sampling sites was conducted. It revealed that the hotspots of fecal pollution are exclusively related to sites with elevated anthropogenic activity. Background pollution from wildlife or diffuse agricultural run-off at more remote sites was comparatively low. The analysis also showed that variability in the incidence of SFIB was driven mainly by meteorological phenomena, above all, temperature, number of sunny hours, and wind (direction and speed). Due to antagonistic effects and temporal undersampling, the influence of precipitation on SFIB variance could not be clearly determined. Geostatistical analysis did reveal that the current spatial sampling density is insufficient to cover SFIB variance over the whole lake, and that the sites are therefore in the most part representative of local phenomena. Suggestions for the future monitoring and managing of fecal pollution are offered. The applied statistical approach may also serve as a model for the study of other such areas, and in general indicate a method for dealing with similarly large and spatiotemporally heterogeneous datasets.

SummaryIn order to elucidate the main predictors of Vibrio cholerae dynamics and to estimate the risk of Vibrio cholera‐related diseases, a recently developed direct detection approach based on fluorescence in situ hybridization and solid‐phase cytometry (CARD‐FISH/SPC) was applied in comparison to cultivation for water samples from the lake Neusiedler See, Austria and three shallow alkaline lakes over a period of 20 months. Vibrio cholerae attached to crustacean zooplankton was quantified via FISH and epifluorescence microscopy. Concentrations obtained by CARD‐FISH/SPC were significantly higher than those obtained by culture in 2011, but were mostly of similar magnitude in 2012. Maximum cell numbers were 1.26 × 106 V. cholerae per L in Neusiedler See and 7.59 × 107 V. cholerae per L in the shallow alkaline lakes. Only on a few occasions during summer was the crustacean zooplankton the preferred habitat for V. cholerae. In winter, V. cholerae was not culturable but could be quantified at all sites with CARD‐FISH/SPC. Beside temperature, suspended solids, zooplankton and ammonium were the main predictors of V. cholerae abundance in Neusiedler See, while in the shallow alkaline lakes it was organic carbon, conductivity and phosphorus. Based on the obtained concentrations a first estimation of the health risk for visitors of the lake could be performed.

Tropocyclops simplex abundance, diel distribution and the impact of the planktivore fish Stolothrissa tanganicae on the T. simplex population were studied at a deep-water site in Kigoma Bay, Lake Tanganyika during the wet period (October 2008–February 2009). The top 40 m were examined using discrete depth samples, filtered through a 40-μm mesh. Ovigerous and copepodids of Tropodiaptomus simplex exhibited a very clear diel vertical migration. Ovigerous females of Tropodiaptomus were negligibly low in the 0–30 m depth during the day, while at night they stayed above 20 m. Tropodiaptomus nauplii did not show any clear diel vertical migration. The contribution of adults, copepodite and nauplii stages to the total numbers of Tropodiaptomus simplex was 8.2, 8.6 and 83.2%, respectively. Stolothrissa tanganicae stomachs were analysed to quantify the contribution of zooplankton in their diets, which revealed a dominance of Tropodiaptomus females and the electivity indices proved that T. simplex females (with or without eggs) were highly selected by S. tanganicae, which could probably be explained by size-selective feeding. Life table analysis indicated that approximately 60% of T. simplex nauplii did not develop into copepodites, a loss which could not be explained by the results of the present study. 68% of the fish gut contents were contributed by female Tropodiaptomus individuals and the contribution of shrimps and Cyclopidae increased substantially to almost 20%. S. tanganicae consumed largely ovigerous Tropodiaptomus females. However, eggs found in the guts were significantly smaller than those found in the zooplankton discrete samples, suggesting that digestion had started. The calculated electivity indices underline that ovigerous and non ovigerous females of T. simplex are being positively selected in the feeding process. It infers existence of selective predation in L. Tanganyika by S. tanganicae on females of T. simplex (with or without eggs) and as a result distinct vertical migration as a predation escape mechanism is performed.

This review summarizes winter conditions from six polymictic European shallow lakes. The lakes range from oligotrophic to hyper-eutrophic. Four of the lakes freeze regularly while ice cover is absent or rare in the two others. Ice duration and timing of ice-out are significantly influenced by climate signals in three of the lakes. Winter water temperature remains higher in non- ice-covered lakes. No long-term trend in temperature is detectable except for one lake where winter water temperature began to increase in 1986. Secchi depth in winter is equal or greater than summer values in all six lakes indicating relatively better light conditions in winter. Total phosphorus concentration in winter ranges from 10 to 130 µg L -1 , which is equal or lower than summer values and is unrelated to chlorophyll a in five of the sites. Phytoplankton species composition during winter differs largely at the six sites. The winter assemblages largely depend on the trophic level and the conditions during the previous season. Winter chlorophyll a and phytoplankton biomass are usually lower than summer values because of reduced photosynthetic rates. Bacterial production often exceeds primary production. Epipelic algal assemblages tend to proliferate during winter in both ice-covered and non-ice-covered lakes. Primary production is low during winter because of insufficient light. Zooplankton abundances and biomass critically depend on conditions during the previous season and the winter situation and are quite variable from year to year, but their values correlate with the trophic status of the lakes. As a result, winter conditions are important to understand seasonal and annual changes in shallow lakes.

Lakes Dendi, Wonchi and Ziqualla are among the few remnants of undisturbed crater lakes in the central highlands of Ethiopia, and have never been investigated reliably owing to seclusion and inaccessibility. As the lakes offer a pristine environment in a beautiful landscape and are located in the vicinity of the capital city Addis Ababa, they are highly threatened by unsustainable tourism, shoreline and crater rim modifications, water abstraction and land grabbing. We provide a first limnological description to establish baseline data against which future environmental and biological changes can be monitored. The lakes are located above 2,800 m elevation with no surface outflow and generally show low concentrations of ions, displaying an equal distribution of readily soluble components like Na or K throughout the water column, but distinct oxygen depletion in greater depths linked to rising concentrations of Fe and Mn, which indicates subterranean springs. Based on nutrients, chlorophyll a, and water transparency, lakes Dendi and Wonchi are classified as oligotrophic and Ziqualla as oligo-mesotrophic. The phytoplankton community is dominated by coccal green algae, desmids and dinoflagellates in lakes Dendi and Wonchi, typical for unpolluted dilute waterbodies; whereas chlorococcales, in particular Botryococcus braunii and benthic diatoms, prevail in Ziqualla. The zooplankton fauna is depauperate, comprising a total of 11 rotifer taxa and 13 crustaceans. Copepods were the most abundant group and contributed over 60% to the total zooplankton abundance in all three lakes, followed by rotifers and cladocerans. The conservation significance of these lakes lies predominantly in their representation of dilute, nutrient–poor highland lake systems that support diverse biota assemblages like desmids and daphnids, which are highly sensitive to eutrophication.

Fish avoidance behaviour in the trawl mouth at night was investigated in the extremely shallow and turbid Lake Neusiedl in Austria. To evaluate fish reactions, a fixed frame benthic trawl with three electricity modes (without electricity, with continuous electricity and with interrupted electricity) was used and the captured fish abundances, biomasses and size structures were compared between modes. Simultaneously, the dual-frequency identification sonar (DIDSON) monitored fish abundance and size in front of and beside the trawl mouth during all tows. White bream (Blicca bjoerkna), bleak (Alburnus alburnus) and razor fish (Pelecus cultratus) were dominant fish species in the trawl catches. We did not find differences in fish abundance or biomass when the tows with different electricity modes were compared. The length frequency distribution of fish was not significantly different between modes, but trawls with the two electrified modes contained a higher proportion of fish larger than 130mm than trawls with the non-electrified mode. Additionally, the DIDSON recordings did not display any significant differences in abundance, length frequency distribution and from length–weight relationships calculated biomass between the electrical modes. Not even the two avoidance behaviour categories used as indicators of trawling error displayed significant differences between avoidance and biomass. Our results indicate that based on trawling abundance and biomass comparisons, and supported by observations by DIDSON, all electrical modes were similarly effective for all size groups of fish. The study found minimal avoidance reactions by the dominant fish species in the trawl mouth opening when a relatively small fixed frame trawl was used to sample fish in a shallow and turbid lake at night.

The aim of the present study was to distinguish areas with different chemical properties in Neusiedler See, to determine which background processes are responsible for this pattern, and to discover their spatial distribution. Uni- and multivariate data analysis was applied to the data concerning 13 mainly chemical and some biological parameters for the time period 2000-2009 from 33 sampling sites. The sampling sites were first clustered then grouped. Besides reed belt and open water areas, smaller localities, which are influenced by water inputs (the treatment plant, the river Wulka, the channels of weekend houses) were also distinguished. Using Wilks' lambda distribution it was determined that the main components (ions) have a greater effect on forming the cluster groups than those parameters which stand in close relation to biological processes. These results concurred with those obtained from the principal component analysis (PCA) conducted on the whole lake and on the groups as well. It can be stated that most of the variance in the dataset can be explained by the main components (ions). The spatial distribution of the principal component scores was visualized with isoline maps. The results of this research lead us to the view that Neusiedler See cannot be treated as one homogeneous system. This exceptional variability originates from the lake's shallow water depth, its unstable water balance, and anthropogenic activity (agriculture, tourism, sewage treatment) in the lake's vicinity. (C) 2013 Elsevier B.V. All rights reserved.

Carotenoids are protective pigments present in many aquatic organisms that reduce the photooxidative stress induced by short-wavelenght solar radiation, yet increase their susceptibility to predators. Arctodiaptomus spinosus, a calanoid copepod typically found in many fishless shallow soda lakes, shows large between-lake differences in pigmentation. Here, we attribute these differences to the environmental state of these ecosystems, namely, ‘dark water’ lakes with submersed vegetation and turbid ‘white’ lakes lacking macrophytes. Copepod carotenoid concentration in the turbid ‘white’ lakes was significantly (about 20-fold) higher than in the ‘dark water’ ones, although the latter systems were characterized by higher transparency. In addition, males had on a dry weight basis around three times higher carotenoid concentrations than females. Mycosporine-like amino acids (direct UV screening substances) were found in all cases, but in low concentration. The environmental conditions in these ecosystems were largely shaped by the presence/absence of submersed macrophytes Thus, in the turbid lakes, the strong wind-driven mixis allows for copepods to be brought to the surface and being exposed to solar radiation, whereas in ‘dark water’ ones, macrophytes reduce water turbulence and additionally provide shelter. Our results explain the counter-intuitive notion of strong red pigmentation in copepods from a turbid ecosystem and suggest that factors other than high UV transparency favor carotenoid accumulation in zooplankton.

Vibrio cholerae is a human pathogen and natural inhabitant of aquatic environments. Serogroups O1/O139 have been associated with epidemic cholera, while non-O1/non-O139 serogroups usually cause human disease other than classical cholera. V. cholerae non-O1/non-O139 from the Neusiedler See, a large Central European lake, have caused ear and wound infections, including one case of fatal septicaemia. Recent investigations demonstrated rapid planktonic growth of V. cholerae non-O1/non-O139 and correlation with zooplankton biomass. The aim of this study was to elucidate the interaction of autochthonous V. cholerae with two dominant crustacean zooplankton species in the lake and investigate the influence of the natural bacterial community on this interaction. An existing data set was evaluated for statistical relationships between zooplankton species and V. cholerae and co-culture experiments were performed in the laboratory. A new fluorescence in situ hybridisation protocol was applied for quantification of V. cholerae non-O1/non-O139 cells, which significantly reduced analysis time. The experiments clearly demonstrated a significant relationship of autochthonous V. cholerae non-O1/non-O139 with cladocerans by promoting growth of V. cholerae non-O1/non-O139 in the water and on the surfaces of the cladocerans. In contrast, copepods had a negative effect on the growth of V. cholerae non-O1/non-O139 via competing bacteria from their surfaces. Thus, beside other known factors, biofilm formation by V. cholerae on crustacean zooplankton appears to be zooplankton taxon specific and may be controlled by the natural bacterial community.

In the last 40 years, the shallow steppe lake, Neusiedler See, was ice covered between 0 and 97 days. The North Atlantic Oscillation (NAO) as well as the Mediterranean Oscillation affected the lake and its conditions during winter. Both climate indices correlated negatively with the duration of ice cover and the timing of ice-out. Average winter phytoplankton biomass increased from less than 0.2 (0.05–0.84) mg FM l−1 in the late 1960s/beginning of 1970s to 3.1 (1.72–5.61) mg FM l−1 in the years 2001–2004. The increase in annual winter biomass of phytoplankton was associated with a significant shift in the composition of the algal assemblage. In the winter 1997/1998, diatoms contributed between 40 and 80% to the phytoplankton biomass while in 2006/2007 cyanoprokaryotes contributed 46%. Mean chlorophyll-a concentrations during winter were significantly correlated with those of total phosphorus (Ptot). Together with cold-water species (rotifer Rhinoglena fertöensis), perennial, eurythermal ones (copepod Arctodiaptomus spinosus) contributed to the zooplankton community. High zooplankton numbers were encountered when rotifers, particularly when densities of Rhinoglena fertöensis were high (r 2 = 0.928). Zooplankton abundance and biomass varied from year to year but correlated positively with Chl-a (biomass − r 2 = 0.69; numbers − r 2 = 0.536). Winter zooplankton populations were primarily influenced by winter conditions, but in early winter also by survival of autumn populations, i.e., the more adults of Arctodiaptomus spinosus survived into winter, the higher was the zooplankton biomass in early winter. Phyto- and zooplankton dynamics in shallow lakes of the temperate region seem to critically depend on the biomass in autumn and on winter conditions, specifically on ice conditions and thus are related to climate signals such as the NAO.

Species composition, abundance, and spatial distribution of rotifer and crustacean zooplankton were studied in Lake Ziway from late April to early July 2004. A total of 49 rotifer species was recorded, with Anuraeopsis fissa, Brachionus angularis, Filinia novaezealandiae, and Trichocerea ruttneri being numerically dominant. Variation in abundance was extremely high, ranging from 2 to 1000+ individuals per litre. There was no significant difference in the distribution of rotifer species between inshore and offshore regions. Crustacean species richness was low, with only five cladoceran and three copepod species occurring in the open water. Moina micrura and Diaphanosoma excisum dominated the cladoceran community, whereas Thermocyclops decipiens was the dominant copepod. Although numerically dominant (75%), rotifers accounted for less than 30% of mean total zooplankton biomass. Peak abundance of crustaceans was observed in May and June, following the onset of the rainy season and increased phytoplankton production. Variation in the spatial distribution of crustacean species was neither observed horizontally between inshore and offshore areas nor vertically in the highly turbid and wind exposed deeper part of the lake. On the other hand, Moina micrura varied significantly in size between inshore and offshore areas. Adult M. micrura dominated offshore, whereas juveniles were more abundant inshore, suggesting a predominantly littoral selective predation on large and adult crustaceans by fish.

Moderately saline soda lakes harbor extremely abundant and fast growing bacterial communities. An interesting phenomenon of an explosive bacterial growth in shallow soda lakes in Eastern Austria after dilution with rainwater, concomitantly with a significant decrease in temperature was observed in a former study. In the present study, we tried to identify the factors being responsible for this enhanced bacterial growth in laboratory batch cultures. Three experiments were performed with water taken from two different lakes at different seasons. Natural soda lake water was diluted with distilled water, artificial lake water, sterile filtered soda lake water, and grazer-free water to test (1) for the influence of compatible solutes released to the environment and reduced salt stress after osmotic down-shock, (2) for the influence of nutrients, which may be washed in from the dry areas of the lake bottom after rainfall and (3) for the decrease of grazing pressure due to dilution. The potential influence of (4) viruses was indirectly deduced. The response of the bacterial community to the manipulations was measured by changes in bacterial numbers, the incorporation of (3)H-leucine and the concomitant determination of the amount of (3)H-leucine uptaking bacteria by microautoradiography. The influence of the environmental factors enhancing bacterial growth after a simulated rainfall event showed variations between the lakes and over the seasons. The addition of nutrients was, in all experiments, the main factor triggering bacterial growth. The decrease in grazing pressure and viral lysis after dilution was of significant importance in two of three experiments. In the experiment with the highest salinity, we could show that either compatible solutes released after osmotic down-shock and used as a source of nutrients for the soda lake bacterial populations or reduced salt stress were most probably responsible for the observed marked enhancement of bacterial growth.

Vibrio cholerae non-O1/non-O139 strains have caused several cases of ear, wound, and blood infections, including one lethal case of septicemia in Austria, during recent years. All of these cases had a history of local recreational activities in the large eastern Austrian lake Neusiedler See. Thus, a monitoring program was started to investigate the prevalence of V. cholerae strains in the lake over several years. Genetic analyses of isolated strains revealed the presence of a variety of pathogenic genes, but in no case did we detect the cholera toxin gene or the toxin-coregulated pilus gene, both of which are prerequisites for the pathogen to be able to cause cholera. In addition, experiments were performed to elucidate the preferred ecological niche of this pathogen. As size filtration experiments indicated and laboratory microcosms showed, endemic V. cholerae could rapidly grow in a free-living state in natural lake water at growth rates similar to those of the bulk natural bacterial population. Temperature and the quality of dissolved organic carbon had a highly significant influence on V. cholerae growth. Specific growth rates, growth yield, and enzyme activity decreased markedly with increasing concentrations of high-molecular-weight substances, indicating that the humic substances originating from the extensive reed belt in the lake can inhibit V. cholerae growth.