Central European Lakes Research Articles

Impact of changing flood regime on a lakeshore plant community: Long-term observations and individual-based simulation

Disturbance events shape plant communities depending on the disturbance regime as well as on the properties of the species constituting the community. We studied a lakeshore community at Lake Constance, a pre-alpine lake in Central Europe, where disturbance by flooding has a strong impact as the water level is almost not regulated by man. The lakeshore habitats are shaped by annual summer flooding during the vegetation period of plants, with seasonal course and magnitude of water-level fluctuations varying considerably between years. By a combination of field work and modelling we examined (1) whether flooding tolerance and interactions between individuals can explain zonation and species coexistence at lakeshore, (2) how strongly invasive species are affecting habitat specific species, and (3) whether changing flooding regimes due to potential climate changes will affect species composition.The study was based on a long-term monitoring record (23 years; 1988–2005) of presence/absence of six species along two transects in a species-poor community at Lake Constance and on detailed records of annual flooding. We succeeded in parameterizing a spatially explicit individual-based life-cycle model of community dynamics on a pattern-oriented basis. This gave results on species demography, dispersal, and interactions. The parameterized model could be verified along additional data recorded in 2011.The results of model parameterization showed that the habitat specialists (characteristic species for the studied lakeshore community) will be able to coexist with stable zonation over the coming decades, mainly due to a contrast between flood tolerance and strength of competitive interactions forming a variant of the competition–colonization trade-off model of coexistence. As a consequence, long-term shortening of the average flooding period, as predicted for the future, should negatively affect flood-tolerant but weakly competing habitat specialists. Here, an avoiding strategy, a shift of flood-tolerant plant species to lower parts of the lakeshore beyond transect boundary and hence beyond present model range, is to be expected.The modelling results revealed that two native but invasive species will have an increasing impact on the community, and they are predicted to endanger habitat specialists on a long run. This was verified already by the re-examination in 2011. Following the parameter values, invasive species threatening the lakeshore community are both flood-tolerant and competitive. Their expected importance for the fate of the community even exceeds that of possible changes in flood duration.The study demonstrates that an individual-based model can be developed on the basis of nonstationary, temporally and spatially changing local abundance data. Such a model goes beyond conventional matrix modelling by the inclusion of nonlinear features as a consequence of individual interactions, and such features turn out to be decisive factor for dealing with species coexistence and displacement in a plant community.

Orbital, ice-sheet, and possible solar forcing of Holocene lake-level fluctuations in west-central Europe: A reply to Magny

In his comment on Bleicher (2013) Magny presents a revised version of his score method and reaffirms that it can prove a possible influence of solar activity on Central European lake levels. He also stresses the diagnostic value of archaeological lake site settlements. This paper points at methodological problems that are not yet resolved in the revised version. Since there is great consensus that not only is the the Sun a primary forcing factor in the climate’s system, but also that its effect cannot be expected to be linear for all of the Holocene, the use of the score record is questioned. The proposed relation between archaeological lake site settlements and solar activity is shown to be doubtful.

Summed radiocarbon probability density functions cannot prove solar forcing of Central European lake-level changes

It has been argued that Central European lake levels were driven by solar activity. This interpretation rests on the comparison of a score record of radiocarbon dates with the 14C residual curve. This score record is a variant of a cumulative probability density function (CPF). In this paper it is argued that this method is not valid because the shape of the CPF is determined by the calibration curve. This hypothesis is tested by a set of null models. An alternative interpretation is given according to which only few episodes of Holocene lake-level changes were climatically driven, while most of the time non-climatic factors were dominating. On a more general level some methodological issues of CPFs are discussed and exemplified by the comparison of dendrochronological results from archaeological settlements and a CPF created using radiocarbon dates from the same settlements.

The structure of winter phytoplankton in Lake Nero, Russia, a hypertrophic lake dominated by Planktothrix-like Cyanobacteria

BackgroundThe permanent dominance of Planktothrix-like сyanobacteria has been often reported for shallow eutrophic\\hypertrophic lakes in central Europe in summer\\autumn. However studies on phytoplankton growth under ice cover in nutrient-rich lakes are very scarce. Lake Nero provides a good example of the contrasting seasonal extremes in environmental conditions. Moreover, the ecosystem underwent a catastrophic transition from eutrophic to hypertrophic 2003–05, with dominance of filamentous cyanobacteria in summer\\autumn. Towards the end of the period of ice cover, there is an almost complete lack of light and oxygen but abundance in nutrients, especially ammonium nitrogen, soluble reactive phosphorus and total phosphorus in lake Nero. The aim of the present study was to describe species composition and abundance of the phytoplankton, in relation to the abiotic properties of the habitat to the end of winters 1999–2010. We were interested if Planktothrix-like сyanobacteria kept their dominant role under the ice conditions or only survived, and how did the under-ice phytoplankton community differ from year to year.ResultsSamples collected contained 172 algal taxa of sub-generic rank. Abundance of phytoplankton varied widely from very low to the bloom level. Cyanobacteria (Limnothrix, Pseudanabaena, Planktothrix) were present in all winter samples but did not always dominate. Favourable conditions included low winter temperature, thicker ice, almost complete lack of oxygen and high ammonium concentration. Flagellates belonging to Euglenophyta and Cryptophyta dominated in warmer winters, when phosphorus concentrations increased.ConclusionA full picture of algal succession in the lake may be obtained only if systematic winter observations are taken into account. Nearly anoxic conditions, severe light deficiency and high concentration of biogenic elements present a highly selective environment for phytoplankton. Hypertrophic water bodies of moderate zone covered by ice in winter and dominated by Planktothrix - like сyanobacteria in summer/autumn may follow several scenarios in the end of winter. It may be intense proliferation сyanobacteria normally dominating in summer, or the switch to the other species like the euglenoids and cryptomonads flagellates, or almost total depletion of phytoplankton.

Repeated Lake-Stream Divergence in Stickleback Life History within a Central European Lake Basin

Life history divergence between populations inhabiting ecologically distinct habitats might be a potent source of reproductive isolation, but has received little attention in the context of speciation. We here test for life history divergence between threespine stickleback inhabiting Lake Constance (Central Europe) and multiple tributary streams. Otolith analysis shows that lake fish generally reproduce at two years of age, while their conspecifics in all streams have shifted to a primarily annual life cycle. This divergence is paralleled by a striking and consistent reduction in body size and fecundity in stream fish relative to lake fish. Stomach content analysis suggests that life history divergence might reflect a genetic or plastic response to pelagic versus benthic foraging modes in the lake and the streams. Microsatellite and mitochondrial markers further reveal that life history shifts in the different streams have occurred independently following the colonization by Lake Constance stickleback, and indicate the presence of strong barriers to gene flow across at least some of the lake-stream habitat transitions. Given that body size is known to strongly influence stickleback mating behavior, these barriers might well be related to life history divergence.

Speciation leads to divergent methylmercury accumulation in sympatric whitefish

Central European lake whitefish (Coregonus spp.) colonized Swiss lakes following the last glacial retreat and have undergone rapid speciation and adaptive radiation. Up to six species have been shown to coexist in some lakes, and individual species occupy specific ecological niches and have distinct feeding and reproductive ecologies. We studied methylmercury (MeHg) accumulation in sympatric whitefish species from seven Swiss lakes to determine if ecological divergence has led to different rates of MeHg bioaccumulation. In four of seven lakes, sympatric species had distinctly different MeHg levels, which varied by up to a factor of two between species. Generally, species with greater MeHg levels were smaller in body size and planktivorous, and species with lower MeHg were larger and benthivorous. While modest disparities in trophic position between species might be expected a priori to explain the divergence in MeHg, δ15N of bulk tissue did not correlate with fish MeHg in five of seven lakes. Results of a nested ANCOVA analysis across all lakes indicated that only two factors (species, lake) explained substantial portions of the variance, with species accounting for more variance (52 %) than inter-lake differences (32 %). We suggest that differences in MeHg accumulation were likely caused by diverging metabolic traits between species, such as differences in energy partitioning between anabolism and catabolism, potentially interacting with species-specific prey resource utilization. These results indicate substantial variability in MeHg accumulation between closely related fish species, illustrating that ecological speciation in fish can lead to divergent MeHg accumulation patterns.

Delimiting sub-areas in water bodies using multivariate data analysis on the example of Lake Balaton (W Hungary)

The main aim of the European Water Framework Directive (WFD, 2000) is to commit European Union Member states to the achievement of good qualitative and quantitative status for all water bodies by 2015. To achieve this, a reference state has to be determined and appropriate monitoring has to be carried out. Based on the fact that the WFD classifies Lake Balaton, the largest shallow freshwater lake in Central Europe, as one water body, and due to the lack of funds, the number of sampling locations on the lake was decreased. The aim of this study was to determine how many sub-areas with different WFD-related attributes (in this case, parameters) can be delimited in the so-called one water body of Lake Balaton, so that a number of representative sampling locations might be retained. To determine Lake Balaton's different water quality areas (i.e. sub-areas of water body) 23 parameters (inorganic compounds) were examined from 10 sampling locations for the time interval 1985–2004 using cluster- and discriminant analysis, and Wilks' lambda distribution. With cluster analysis we were able to determine two time intervals (1985–1997 and 1998–2004) with three patterns of sub-areas, two from the first and one from the latter interval. These patterns pointed to the fact that for the whole investigated time interval (1985–2004) a total of five sub-areas were present, changing in number and alignment. Then the results were verified using discriminant analysis, and the parameters which influenced the sub-areas the most were determined using Wilks' lambda distribution. The conclusion was that to be able to follow the changes in alignment of the sub-areas and to get a comprehensive picture of Lake Balaton, a minimum of five sampling locations should be retained, one in each sub-area. Based on this study the Water Authorities chose to keep five out of ten sampling locations so that the sub-areas could be described. We consider this a great success and the methodology as an example for setting up sub-areas in a water body.

Introducing sampling frequency estimation using variograms in water research with the example of nutrient loads in the Kis-Balaton Water Protection System (W Hungary)

In most of the world along with urbanization came the deterioration of surface waters. The case of Hungary's Lake Balaton (the largest shallow freshwater lake in Central Europe) and its catchment area was no exception. In the second part of the twentieth century it became clear that measures had to be taken to prevent further water quality deterioration. In order to do so, the Kis-Balaton Water Protection System (KBWPS) was established. To measure its effectiveness in filtering Lake Balaton's water input, total phosphorus and total nitrogen loads are regularly estimated. In this study the practice of temporal sampling frequency estimation is described using data series from the KBWPS in order to suggest a sampling frequency that is suitable for estimating annual nutrient loads and still produces a dataset from which environmental processes can be followed. In this case, temporal sampling frequency estimation was conducted using variogram analysis, before which trend removal was performed on four sampling sites and two daily sampled parameters for the years 1993–2007. Although the calculations were only carried out on two parameters they are applicable to other environmental parameters as well. Nutrient load estimation is used as a verification tool. The result of the variogram analysis pointed towards a three-day sampling frequency for the whole KBWPS in the case of total phosphorus and total nitrogen (if the purpose of the monitoring is nutrient estimation). This reduction in sampling frequency would make an approximately 50% cost reduction possible. The study gives an example of sampling frequency estimation which is a key question in every aspect of environmental science, so that hopefully it will be useful for scientists dealing with water protection issues.

Past and current trophic development in Lake Ammersee - Alterations in a normal range or possible signals of climate change?

In the past decades, nutrient concentrations in several pre-Alpine lakes in central Europe have increased due to human activity in the catchment area. Here, we examine whether this trend will continue in the future, through our analysis of the development of the trophic status, over a period of 25 years (1984 – 2009), of the pre-Alpine, dimictic tempered Lake Ammersee (South-Eastern Germany). The lake showed signs of an elevated eutrophic status until the 1990s. Immense management activities in the catchment area led to a reduction in the nutrient load in the tributaries. The study of dissolved oxygen, total phosphorus (TP) and chlorophyll-a revealed a substantial improvement in the trophic conditions of the lake. We identified a transition period in the development of trophic levels during the years 1996/1997. Therefore, our investigation is divided into two periods: 1984 –1996 and 1997– 2009. By comparing values during these two periods, we found that the annual mean TP concentrations of the complete water column declined from 18.2 μg l (1984 –1996) to 8.8 μg l (1997–2009). During this period the chlorophyll-a values are in relationship to other trophic variables such as TP, dissolved oxygen and sight depths (R between 0.41 and 0.56). At the same time, during the summer months, the surface temperature in the epilimnion increased (by about 0.8 K per decade), which was approximately in line with the thermal increase (about 0.6 K per decade) in the northern hemisphere (Pearson’s correlation coefficient, omitting 2006, of 0.81). In conclusion, in the case of Lake Ammersee, a decrease in nutrient input has led, most recently (1997– 2009), to a substantial improvement in the trophic state. This improvement is fundamentally a result of measures taken to correct any anthropogenic impact. Nevertheless, we show that the thermal stability of the lake reacts to climate-triggered alterations. Therefore it seems that the impact of recent climate change on trophic conditions will have to be monitored closely

Local to regional scale industrial heavy metal pollution recorded in sediments of large freshwater lakes in central Europe (lakes Geneva and Lucerne) over the last centuries

This research first focuses on the spatial and temporal patterns of heavy metals from contrasting environments (highly polluted to deepwater sites) of Lake Geneva. The mercury (Hg) and lead (Pb) records from two deepwater sites show that the heavy metal variations before the industrial period are primarily linked to natural weathering input of trace elements. By opposition, the discharge of industrial treated wastewaters into Vidy Bay of Lake Geneva during the second part of the 20th century, involved the sedimentation of highly metal-contaminated sediments in the area surrounding the WWTP outlet pipe discharge. Eventually, a new Pb isotope record of sediments from Lake Lucerne identifies the long-term increasing anthropogenic lead pollution after ca. 1500, probably due to the development of metallurgical activities during the High Middle Ages. These data furthermore allows to compare the recent anthropogenic sources of water pollution from three of the largest freshwater lakes of Western Europe (lakes Geneva, Lucerne, and Constance). High increases in Pb and Hg highlight the regional impact of industrial pollution after ca. 1750–1850, and the decrease of metal pollution in the 1980s due to the effects of remediation strategies such as the implementation of wastewater treatment plants (WWTPs). However, at all the studied sites, the recent metal concentrations remain higher than pre-industrial levels. Moreover, the local scale pollution data reveal two highly contaminated sites (> 100 μg Pb/g dry weight sediment) by industrial activities, during the late-19th and early-20th centuries (Lake Lucerne) and during the second part of the 20th century (Vidy Bay of Lake Geneva). Overall, the regional scale pollution history inferred from the three large and deep perialpine lakes points out at the pollution of water systems by heavy metals during the last two centuries due to the discharge of industrial effluents.

Zooplankton community response to enhanced turbulence generated by water‐level decrease in Lake Balaton, the largest shallow lake in Central Europe

During the unusually long European drought between 2000 and 2003, the water level of the large and shallow Lake Balaton, Hungary (area = 596 km2, mean depth = 3.25 m), decreased by 28%. Although food availability for zooplankton remained unchanged, and the fish stock declined more than the water mass, the density of populations of several planktonic rotifers, cladocerans, calanoid copepodes, and veligers decreased by 60‐90% simultaneously with the water‐level decrease and regenerated only after the drought. The generally strong turbulence of the lake was intensified during the four consecutive years of low water, as verified by instrumental monitoring of the turbulence intensity and by the estimation of the turbulent kinetic‐energy dissipation rate. In our tank experiments, turbulence conditions similar to those that existed in the lake during low water were simulated, but mineral suspended material was minimized and food was regularly resupplied. Under these experimental conditions, zooplankton taxa showing the highest mortality were the same as those that were most susceptible in situ. Increased turbulence coupled with the water‐level decrease is especially unfavorable for rotifer Keratella, the cladoceran Daphnia, Bosmina, and copepodit and adult stages of the calanoid copepod Eudiaptomus gracilis in this lake.

<strong>Environmental factors influencing the distribution of EPT assemblages in streams of Lake Balaton’s catchment area, Hungary</strong>

Lake Balaton is the largest shallow lake in Central Europe. A large part of its catchment area is a reputable and popular tourism area, where the quality of surface waters is in the center of public and scientific interest. The inflows of Lake Balaton are vulnerable, due to the strong human activity upstream. Despite their significance, the aquatic macroinvertebrate communities of these inflow streams are poorly known. In the summer of 2008, we investigated the distributional pattern of EPT (Ephemeroptera, Plecoptera, Trichoptera) assemblages, and their relationships with selected physico-chemical factors and landscape characteristics at different spatial scales. Samples were taken according to the AQEM sampling protocol at 20 stream sections with different environmental conditions. Multivariate analyses were used for evaluation. Altogether 8131 specimens (6225 Ephemeroptera, 133 Plecoptera, 1773 Trichoptera) belonging to 55 species (25 Ephemeroptera, 2 Plecoptera, 28 Trichoptera) were identified. Cloeon dipterum (L.) and Baetis spp. were the most common mayfly taxa, while caddisfly communities were dominated by limnephilids [Anabolia furcata Brauer, Potamophylax rotundipennis (Brauer)] and hydropsychids [Hydropsyche saxonica McLachlan, H. angustipennis (Curtis)]. The distribution of EPT assemblages was highly affected by pH, conductivity, dissolved oxygen, temperature and total dissolved solids. At reach scale, relationships were detected between spatial patterns of EPT assemblages and structure of riparian vegetation, type of sediment, and some hydromorphological characteristics (e.g., depth and width of stream bed, cover of aquatic vegetation, etc.). At medium and large scale, the ratio of natural and human-impacted areas proved to be the most influential factor.

Diversity and seasonal dynamics of the photoautotrophic picoplankton in Lake Balaton (Hungary)

Lake Balaton, in Hungary, is the largest shallow lake in Central Europe. This fresh water ecosystem has been well studied, including investigations of its photoautotrophic picoplankton (PPP). Previous studies revealed that picoeukaryotes could reach an extremely high abundance in winter, while picocyanobacteria are the predominant picoplankters in warmer periods, as in other lakes in the temperate zone. In addition to epifluorescence microscopy — which allows discrimination only between phycocyanin-rich picocyanobacteria, phycoerythrin-rich picocyanobacteria and picoeu - karyotic algae — we used PCR-based molecular methods to reveal the detailed genetic diversity and seasonal dynamics of the PPP in Lake Balaton for the first time. Our results show that a single inte- grated pelagic sample, taken vertically from the whole water column, may harbor a large number of picocyanobacterial genotypes including previously unidentified groups. Based on length polymor- phism analysis of the phycocyanin operon (which contains a non-coding region of variable size), the composition of picocyanobacterial communities showed significant seasonal changes and spatial variation. The relative importance of some of the operational taxonomic units (OTUs) we detected was correlated with environmental factors, such as temperature and the concentration of available nitrogen forms. The picoeukaryotic algal community of winter PPP was dominated by chlorophytes related to the group Trebouxiophyceae. The results of this study highlight the fine internal structure and dynamics of the PPP community in freshwater ecosystems — a view that is usually blurred by the lower resolution of commonly used methods, such as epifluorescence microscopy and flow cytometry.

Analysis of long-term water quality changes in the Kis-Balaton Water Protection System with time series-, cluster analysis and Wilks’ lambda distribution

Lake Balaton is the largest shallow freshwater lake in Central Europe. Its water quality is mainly affected by the supplying rivers and other water sources. The primary source is the Zala River. Its water used to be filtered by the Kis-Balaton Wetland (KBW) before entering Lake Balaton. During the nineteenth century, as a result of artificial water level modifications, the KBW disappeared and the Zala River's waters became partially unfiltered. It is for this reason that the Kis-Balaton Water Protection System (KBWPS) had to be constructed as a mitigation wetland. The aim of the study is to examine the available physical, chemical and biological parameters to get a more comprehensive picture of the processes evolving in the functioning of the KBWPS, and to make suggestions concerning the management and preservation of the system's wetland habitat. The central concept of the present study was to group the sampling points of the KBWPS and to determine which parameters had the greatest effect on the groups, and where. Multivariate data analysis was applied to the data concerning 25 chemical, biological and physical parameters for the time period 1984–2008 from 13 monitoring stations. The sampling locations were clustered then grouped. The groups were formed annually. The change of alignment of similar sampling points shows how the border between the determining groups (covering the eutrophic pond and wetland habitats) changed over the years. This change followed the transition from macrophyte vegetation to an open water area which took place as a result of the water level being kept artificially constant, and which did not therefore follow the weather conditions (rain, drought, etc.). Using Wilks’ λ distribution it was possible to determine that the parameters responsible for eutrophication were primarily responsible for forming the groups of the sampling points. The next most important factors determining the groups were the variables in close relation with the parameters characteristic of eutrophication. The inorganic chemical components affected the conformation of the groups the least. Finally, by examining the phosphorous forms and chlorophyll-a we tried to show the milestones in the history of the mitigation wetland, the KBWPS. The result of this research was that it points out changes in the KBWPS over a long time period, which had not been done previously. This research could hopefully help scientists to gain a broader perspective on processes evolving in the KBWPS. When it comes to finishing the second phase of the reservoir system, more knowledge will be available on what can be expected regarding the quality of the water entering Lake Balaton, and the conservation of the nature preserve wetland area.

Anthropogenic eutrophication shapes the past and present taxonomic composition of hybridizing Daphnia in unproductive lakes

It has been proposed that anthropogenic eutrophication of lakes facilitated the establishment of populations of the cladoceran Daphnia galeata into the originally oligotrophic lakes north of the European Alps in the 1960s. This hypothesis lacks the support of studies on unproductive lakes, in which the past eutrophication is assumed to have never been on the level necessary for D. galeata to reach high abundances and to establish permanently. In order to investigate if such species shifts also happened in unproductive systems, we studied the past and present taxonomic composition of three ultra‐oligotrophic Swiss lakes that were only marginally affected by eutrophication using molecular methods on diapausing eggs sampled from sediment cores. D. galeata temporarily established in unproductive lakes, but its colonization success seemed to depend on the general trophic state of the lake and the magnitude of eutrophication. In two of the studied lakes, D. galeata could establish a significant population size, whereas it was not successful in the most unproductive lake with the weakest eutrophication. Even in unproductive lakes, eutrophication led to partly irreversible species changes, providing evidence that this anthropogenic disturbance is responsible for species shifts in many pre‐alpine lakes in Central Europe.

Interaction of Vibrio cholerae non-O1/non-O139 with Copepods, Cladocerans and Competing Bacteria in the Large Alkaline Lake Neusiedler See, Austria

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00248-010-9764-9) contains supplementary material, which is available to authorized users.

Effects of a half a millennium winter on a deep lake – a shape of things to come?

AbstractAnalyses of the effects of extreme climate periods have been used as a tool to predict ecosystem functioning and processes in a warmer world. The winter half‐year 2006/2007 (w06/07) has been extremely warm and was estimated to be a half‐a‐millennium event in central Europe. Here we analyse the consequences of w06/07 for the temperatures, mixing dynamics, phenologies and population developments of algae and daphnids (thereafter w06/07 limnology) in a deep central European lake and investigate to what extent analysis of w06/07 limnology can really be used as a predictive tool regarding future warming. Different approaches were used to put the observations during w06/07 into context: (1) a comparison of w06/07 limnology with long‐term data, (2) a comparison of w06/07 limnology with that of the preceding year, and (3) modelling of temperature and mixing dynamics using numerical experiments. These analyses revealed that w06/07 limnology in Lake Constance was indeed very special as the lake did not mix below 60 m depth throughout winter. Because of this, anomalies of variables associated strongly with mixing behaviour, e.g., Schmidt stability and a measure for phosphorus upward mixing during winter exceeded several standard deviations the long‐term mean of these variables. However, our modelling results suggest that this extreme hydrodynamical behaviour was only partially due to w06/07 meteorology per se, but depended also strongly on the large difference in air temperature to the previous cold winter which resulted in complete mixing and considerable cooling of the water column. Furthermore, modelling results demonstrated that with respect to absolute water temperatures, the model ‘w06/07’ most likely underestimates the increase in water temperature in a warmer world as one warm winter is not sufficient to rise water temperatures in a deep lake up to those expected under a future climate.

Ecological studies on the decomposition rate of fish carcasses by benthic organisms in the littoral zone of Lake Constance, Germany

Using field experiments, we investigated the effects of fish carcasses (so called 'food falls') on benthic fauna. We simulated food falls using freshly killed fish during two different seasons (spring and summer) in the littoral zone of a large, pre-alpine meso-oligotrophic lake in central Europe (Lake Constance, Germany). This study provides evidence that input in the form of fish carcasses may play an important role in nutrient dynamics within this ecosystem. The benthic communities in the vicitinity and underneath the food fall were strongly influenced by the food fall. The results show that this supply of organic matter has a significant influence on the relationships within the communities, which are clearly dominated by bacteria, followed by copepod nauplii, cyclopoid copepods, chironomids, and ostracods. Total decomposition was obtained between days 80 and 68 of the experiments. The food fall had a positively but not significant effect on meiofaunal assemblages in these experiments. Furthermore, the results showed a negative relationship between bacteria and meiofauna abundance, indicating grazing by the meiobenthos on the bacterial community. These findings support other studies that found that the meiofauna exerted a grazing pressure on the microbial community, where this process was important in the decomposition of carcasses. Moreover, this study shows the potential importance of fish carcasses in Lake Constance, where food falls may generate high abundances and diversity of the benthic fauna and support high bacterial activities in this littoral ecosystem.

Morlet wavelet and autocorrelation analysis of long-term data series of the Kis-Balaton water protection system (KBWPS)

Lake Balaton is the largest shallow freshwater lake in Central Europe. Its water quality is mainly affected by the supplying rivers and other water sources. The primary source is the Zala River. The water of this river used to be filtered by the Kis-Balaton wetland (KBW) before entering Lake Balaton at Keszthely Bay. In the course of the nineteenth century, water level modifications at KBW shrank and it lost its full capacity to function as a filter for the Zala's waters, so the nutrient load of Lake Balaton greatly increased, and its water quality started to deteriorate. This is the reason an artificial extended wetland called the Kis-Balaton water protection system (KBWPS) had to be constructed–to retain the inorganic nutrients at the mouth of the Zala River and stop the further degradation of Balaton's water quality. In this study, Morlet wavelet spectrum (periodicity) analysis and autocorrelation (memory) analysis were used with weekly chemical, biological and physical data for the KBWPS for the period 1993–2007. We compared the periodicity and the memory of the parameters in the two different habitats of the KBWPS and the Zala River. We also analyzed the tracking capability of the periodical seasons of the two phases (the complete Phase I, and the incomplete Phase II) and the river. The results showed that the incomplete Phase II is unable to conserve the periodicity and sustain long memory because of the shading provided by the macrophyte cover and the extra inflows. In conclusion, we were able to point out the dependence of the system's processes on temperature and climate conditions with methods that have not yet been used for this kind of purpose.

How to cope with a superior enemy? Plant defence strategies in response to annual herbivore outbreaks

Summary 1. The perfoliate pondweed Potamogeton perfoliatus L. constitutes large monospecific macrophyte patches in many Central European lakes. Correlative evidence from the field suggests that P. perfoliatus is under an increasing grazing pressure during its short vegetation period from May to September due to seasonal outbreaks of the aquatic moth Acentria ephemerella Denis & Schiffermüller. We used a mesocosm experiment to determine the influence of A. ephemerella herbivory on P. perfoliatus shoot development and resting bud production and to study the defence strategies of this macrophyte. 2. Herbivory resulted in a reduction of the P. perfoliatus vegetation period by more than 2 months thereby reducing the average resting bud size and the overall resting bud biomass sevenfold. This suggests that besides its severe immediate effects, herbivory affects P. perfoliatus growth and dynamics also during the subsequent season. 3. As a response to herbivory P. perfoliatus translocated nutrients (phosphorus (P) and nitrogen (N)) from leaves towards buds. Acentria ephemerella larvae had a high P content, implying P limitation of larval growth especially within the herbivory treatment. This suggests that at least the P translocation from leaves towards resting buds may be viewed as an anti‐predator strategy rather than as a nutrient conservation strategy. 4. Acentria ephemerella herbivory changed the allocation strategy of P. perfoliatus in the size versus number of resting buds: only the number, but not the size of resting buds was reduced under shoot grazing by Acentria ephemerella. This change in the number versus size trade‐off might allow the plant to produce a minimum resting bud size necessary for successful sprouting in the next spring. 5. Synthesis. Overall, our results suggest an escape syndrome (after Agrawal & Fishbein, Ecology, 87 (2006) S132) as a defence strategy against herbivory for P. perfoliatus, consisting of a shortening of the growth period, a translocation of nutrients and a change in allocation strategy. The increased plant senescence that was accompanied by the shortening of the growth period has further implications for the usage of macrophyte patches as a habitat for invertebrates and fishes and for the structure of littoral food webs.