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Microbial planktonic communities are the basis of food webs in aquatic ecosystems since they contribute substantially to primary production and nutrient recycling. Network analyses of DNA metabarcoding data sets emerged as a powerful tool to untangle the complex ecological relationships among the key players in food webs. In this study, we evaluated co‐occurrence networks constructed from time‐series metabarcoding data sets (12 months, biweekly sampling) of protistan plankton communities in surface layers (epilimnion) and bottom waters (hypolimnion) of two temperate deep lakes, Lake Mondsee (Austria) and Lake Zurich (Switzerland). Lake Zurich plankton communities were less tightly connected, more fragmented and had a higher susceptibility to a species extinction scenario compared to Lake Mondsee communities. We interpret these results as a lower robustness of Lake Zurich protistan plankton to environmental stressors, especially stressors resulting from climate change. In all networks, the phylum Ciliophora contributed the highest number of nodes, among them several in key positions of the networks. Associations in ciliate‐specific subnetworks resembled autecological species‐specific traits that indicate adaptions to specific environmental conditions. We demonstrate the strength of co‐occurrence network analyses to deepen our understanding of plankton community dynamics in lakes and indicate biotic relationships, which resulted in new hypotheses that may guide future research in climate‐stressed ecosystems.

As one of the few remaining lakes that are freely connected with the Yangtze River, Poyang Lake exhibits large annual water level (WL) fluctuations. In this study, weekly samples were collected at the north end of Poyang Lake from September 2011 to December 2012, and we investigated the mechanism of limnological responses to fluctuations in the WL. The study covers three seasons that were associated with WL fluctuations ranging from 8 to 19 m. Spearman's rank correlations and multivariate non-metric multidimensional scaling analyses indicated that low and high WL periods differed in a number of water quality characteristics. The low WL period coincided with the non-growing season and was associated with the peak concentrations of nitrogen, the highest turbidity (Turb), and the lowest water temperature. The high WL period was mainly characterized by enhanced chlorophyll a concentration. Spearman's rank correlations revealed positive relationships between the WL and the concentrations of NO3-N and PO4-P and negative relationships between the WL and the Turb, total nitrogen, total phosphorus, NO2-N, and NH4-N concentrations. All results support the conclusion that the large WL fluctuations are the principal drivers for physicochemical variables in this floodplain lake ecosystem.

Lesser Flamingo, the flagship species of saline wetlands of Africa and India, is a specialised feeder subsisting on microscopic cyanobacteria and algae. To establish the relationship between flamingo occurrence and food algal abundance and quality, an extensive microphyte survey in more than 150 sampling trips to seven countries over a 15-years period (2001–2015) was carried out. The 44 habitat sites included the core soda lakes in eastern Africa (Bogoria, Nakuru, Elmentaita, Oloidien), where the highest numbers of flamingos were observed, and five breeding sites in eastern and southern Africa as well as in north-western India. A reference describing the diversity of microphytes was established including members of three orders of cyanobacteria and nine orders of eukaryotic algae that potentially could act as food source for Lesser Flamingos. Preferred food organisms consisted of filamentous cyanobacteria, mainly Arthrospira, as well as benthic diatoms. Further investigation on the suitability of other microphytes as alternative flamingo diet revealed the food potential of chlorophytes and euglenophytes. This paper discusses a phycological perspective in the feeding ecology of Lesser Flamingos. The survey findings can assist scientists and conservationists in evaluating the potential of wetlands to support flocks of this endangered bird.

Lake restoration is commonly assessed by changes in water transparency, nutrients and biomass of phytoplankton, while information about changes in zooplankton that is triggered by lake management is often missing. In our 19-year study we used rotifers and crustaceans to document the effect of restoration measures on zooplankton in the oxbow lake Alte Donau, a former side-arm of the Danube River which is most popular for recreation and angler (cyprinid-dominated shallow water). The record covers four management periods: the period before restoration, the restoration (including years of chemical phosphate precipitation by Riplox treatment), the re-establishment of macrophytes and the sustained ‘stable conditions’. We found the highest abundance of all zooplankton in the first Riplox-year, with decreasing zooplankton abundance in following periods associated with the decline of phytoplankton. In the long term, the main compositional change related to a shift from a cladoceran-rotifer-rich to a copepod-rotifer-rich zooplankton assemblage. Thus, the large-bodied zooplankton shifted from a community composed of mainly filter-feeding herbivorous cladocerans under eutrophic algal-turbid conditions to mainly selective-feeding omnivorous and herbivorous copepods under mesotrophic transparent-water conditions. While the carbon ratio between zoo- and phytoplankton increased significantly during the first three periods and remained high under ‘stable conditions’, the mean body size of zooplankton did not exhibit a long-term trend. Short-term increases of large-bodied zooplankton coincided with an intermittent increase of calanoid copepod abundance (Eudiaptomus gracilis) during the chemical treatment concomitant with a drastic phytoplankton biomass reduction and the occurrence of large-bodied cladocerans (Simocephalus vetulus and Sida crystallina) in some years with re-established underwater vegetation. Besides the main response of zooplankton to the ‘bottom up’ control that was triggered by the reduction of phytoplankton food supply by one order of magnitude, we studied the zooplankton response to climate change. The impact of climate warming was evident from intra-annual coincidence of the climate signal (NAODJFM) and water temperature (WT) in winter and early spring, the increase of surface water temperature (SWT) by 1.52 °C per decade in April and the prolongation of the warm period (SWT > 22 °C) by 10.5 days per decade in summer. This prolongation of the warm season seemed to support the summer development of the medusa stage of freshwater jellyfish (Craspedacusta sowerbii). During the transition from spring to summer, the progressively earlier clear-water phase followed two trends. The first period with a 33-day earlier clear-water phase per decade coincided with pronounced ecosystem changes from a high to a low eutrophic state created by chemical restoration measures. The second period with a moderate earlier progression of 7 days per decade was accompanied by a further, slight TP decrease associated with the re-establishment of macrophytes. When comparing rotifers, cladocerans, calanoid and cyclopoid copepods, the latter group benefits most from seasonal temperature increases and climate warming.

Intensively used urban water bodies are vulnerable to eutrophication. The shallow lake Alte Donau (Vienna) can be seen as an example for the extent of anthropogenic influence. Human impacts paired with changes in environmental conditions gave way to eutrophication processes in Alte Donau. Due to the great public interest restoration concepts and subsequently management programs were established. This chapter provides a synthesis of the key aspects to evolve and implement a successful water management plan. An attempt is made to generalise our specific solutions to serve as a basis for the development of similar strategies for other urban lakes.

Summary In a combined laboratory and field study over 10 years, we investigated how two centric diatom species, Cyclotella baicalensis and Cyclotella minuta, have coexisted in the plankton of Lake Baikal in Siberia for over 100,000 years. The species differ substantially in cell size: C. baicalensis has some of the largest cells found in freshwater phytoplankton at up to 135 μm diameter, but C. minuta can be as small as 10 μm. They illustrate the advantages and disadvantages of small versus large size in a planktonic environment. Other differences included a faster division rate of C. minuta but a faster rate of biovolume increase in C. baicalensis. Timing of their life cycles and peak cell abundances differed. C. baicalensis peaked in June or July after ice break-up, but C. minuta usually peaked in autumn, because it was able to tolerate higher temperatures and survive higher in the water column during summer stratification, allowing growth to begin earlier than other species. Satellite-derived surface temperature data combined with temperature tolerances of each species revealed how variation in upwelling affected population development. Speciation may have occurred after a change in environmental conditions allowed larger cells to remain in suspension throughout the year. Long-term co-existence became possible because ecological differences reduced interspecific competition.

The cyanoprokaryote Planktothrix rubescens has been studied for over four decades in an alpine lake, Mondsee, Austria. We hypothesise that impacts of climate change can be distinguished from other environmental constraints controlling its population dynamics. During thermal stratification, P. rubescens formed a deep chlorophyll maximum in the metalimnion. Seasonal and diurnal depth distributions indicated that Planktothrix lived well below the euphotic zone, at low light levels and moderate temperatures. Photosynthetic parameters indicated a shade adapted population. The eutrophication and rehabilitation periods of Mondsee were characterised by fluctuations controlled by phosphorus, with periods below the P-threshold associated with low biovolumes and relatively stable populations. Positive net changes of Planktothrix biovolume occurred during the spring–summer transition and in autumn. During summer, the population usually declined to an annual minimum. The standardised residuals of annual biovolumes responded positively to the climate signal of the Winter North Atlantic Oscillation and the timing of the onset of stratification. An inverse relationship existed between off-set and persistence of stratification. P. rubescens only benefits from climate warming early in the year, during late spring overturn and early summer. Longer periods of summer stratification did not favour biovolume development.

In vivo delayed fluorescence (DF) and HPLC ⁄ CHEMTAX pigment analyses were used to investigate seasonal and depth distributions of phytoplankton in a deep alpine mesotrophic lake, Mondsee (Austria). Using chl a equivalents, we determined significant relationships with both approaches. Community structure derived from pigment ratios of homogenous samples was compared with microscopic estimations using biovolume conversion factors. An advantage of the HPLC ⁄CHEMTAX method was that it gave good discrimination among phytoplankton groups when based on a pigment ratio matrix derived from multiple regression analysis. When a single algal group was dominant, such as epilimnetic diatoms or hypolimnetic cyanobacteria in the deep chl maxima, HPLC ⁄CHEMTAX results were significantly correlated with microscopic estimations (diatoms: r = 0.93; cyanobacteria: r = 0.94). Changes in the composition of photosynthetically active pigments were investigated with DF and benefited from excitation spectra that considered all light-harvesting pigments, which made it possible to assess the enhancement of accessory photosynthetically active pigments relative to active chl a (chl aDF672). Changes in similarity index, based on normalized DF spectra, confirmed compositional shifts observed by microscopy. At chosen wavelengths of DF spectra, 534 and 586 nm, we generally observed a significantly inverse relationship between normalized DF intensities and temperature and light along both seasonal and depth gradients. The relative increase in photosynthetically active pigments other than chl aDF672 under low light and temperature was caused by an increasing dominance of diatoms and ⁄or phycobilin-rich cyanobacteria and Cryptophyta. DF spectra provided a more accurate picture of community pigments acclimated to light and temperature conditions than the b-carotene:chl a ratio derived from HPLC.

Production rates, abundance, chlorophyll a (Chl a) concentrations and pigment composition were measured for three size classes ( 11 μm) of phytoplankton from May to December 2000 in deep, mesotrophic, alpine lake Mondsee in Austria. The study focuses on differences among phytoplankton size fractions characterised by their surface area to volume ratio ([mm2 l−1: mm3l−1]), pigment distribution patterns and photosynthetic rates. Particular attention was paid to autotrophic picophytoplankton (APP, fraction

Size related changes of phytoplankton biovolume and species composition have been analysed for forward and reverse regime shifts in a shallow, urban, seepage lake. As a consequence of changes in the hydrology, the pelagic switched from a clear water, macrophyte dominated state to a turbid stable phase with an abundant cyanobacterial population. Experimental nutrient reduction forced the system into a recovery phase. Each state change was associated with significant changes in total biovolume, species composition and size structure affecting surface to volume ratios (S/V). Chlorophyll-a content and S/V drastically increased during the early recovering phase due to small cell sized species developing. As expected, state transitions were associated with significant alterations in size structure and composition.

Allometric relationships of phytoplankton communities were studied on the basis of a five-year data-set in a deep oligotrophic alpine lake in Austria. The seasonal phytoplankton succession in Mondsee is characterised by diatoms during winter mixing and a distinct metalimnetic population of Planktothrix rubescens during stratification in summer. The variation of phytoplankton photosynthetic efficiency between seasons was assessed using in situ carbon-uptake rates (5 years data) and Fast Repetition Rate Fluorometry (FRRF) (2 years data). The light-saturated, chlorophyll-specific rate of photosynthesis (P*max), irradiance at the onset of saturation (E k) and maximum light-utilisation efficiency (α*) were determined for winter mixing and summer stratification. Fluorescence-based parameters as the functional absorption cross section of Photosystem II (σ PSII) and the photochemical quantum yield (F v/F m) were additionally analysed in 2003 and 2004 to study the underlying physiological mechanisms for the variability in photosynthetic performance. Beyond their sensitivity to changing environmental conditions like thermal stratification, phytoplankton populations differ in their photosynthetic behaviour according to their size structure. Therefore Photosynthesis vs. Irradiance (P/E)-relationships were analysed in detail within a 1-year period from size fractionated cell counts, chlorophyll-a and carbon-uptake.

Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.

Peer reviewed. ©2015. The Authors.This is an open access article under theterms of the Creative CommonsAttribution-NonCommercial-N oDerivsLicense, which permits use and distri-bution in any medium, provided theoriginal work is properly cited, the use isnon-commerc ial and no modificationsor adaptations are made., In this first worldwide synthesis of in situ and satellite-derived lake data, we find that lake summer surface water temperatures rose rapidly (global mean = 0.34°C decade 1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factors —from seasonally ice-covered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72°C decade 1) to ice-free lakes experiencing increases in air temperature and solar radiation (0.53°C decade 1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes.

The possibility of climate change has attracted considerable attention over the past decade (BURROUGHS 2001). As the evidence for changes in weather conditions have been substantiated, environmental impacts on aquatic systems have frequently been associated with climatic indices such as the North Atlantic Oscillation (HURRELL et al. 2003). This index is strongly related to the air and surface water temperatures of lakes in Central Europe (LIVINGSTONE & DOKULIL 2002). Variables are highly synchronised between lakes within the same lake district (DOKULIL & TEUBNER 2002). Oligotrophic deep alpine lakes are excellent ecosystems to amplify climatic signals. In the past, trends in lake surface temperatures and ice cover have mainly been used as indicators for climatic change. Here we analyse the long-term development and trend of hypolimnetic water temperatures (HWT) of deep alpine lakes in Austria.

SUMMARY 1. Measurements of total phosphorus (TP) concentrations since 1975 and a 50-year time series of phytoplankton biovolume and species composition from Lake Mondsee (Austria) were combined with palaeolimnological information on diatom composition and reconstructed TP-levels to describe the response of phytoplankton communities to changing nutrient conditions. 2. Four phases were identified in the long-term record. Phase I was the pre-eutrophication period characterised by TP-levels of about 6 l gL )1 and diatom dominance. Phase II began in 1966 with an increase in TP concentration followed by the invasion of Planktothrix rubescens in 1968, characterising mesotrophic conditions. Phase III, from 1976 to 1979, had the highest annual mean TP concentrations (up to 36 l gL )1 ) and phytoplankton biovolumes (3.57 mm 3 L )1 ), although reductions in external nutrient loading started in 1974. Phases II and III saw an expansion of species characteristic of higher nutrient levels as reflected in the diatom stratigraphy. Oligotrophication (phase IV) began in 1980 when annual average TP concentration, Secchi depth and algal biovolume began to decline, accompanied by increasing concentrations of soluble reactive silica. 3. The period from 1981 to 1986 was characterised by asynchronous trends. Annual mean and maximum total phytoplankton biovolume initially continued to increase after TP concentration began to decline. Reductions in phytoplankton biovolume were delayed by about 5 years. Several phytoplankton species differed in the timing of their responses to changing nutrient conditions. For example, while P. rubescens declined concomitantly with the decline in TP concentration, other species indicative of higher phosphorus concentrations, such as Tabellaria flocculosa var. asterionelloides, tended to increase further. 4. These data therefore do not support the hypotheses that a reduction in TP concentration is accompanied by (i) an immediate decline in total phytoplankton biovolume and (ii) persistence of the species composition characterising the phytoplankton community before nutrient reduction.

The effects of experimentally reduced total phosphorus concentrations (TP) from 2.4 to 0.6 mmol L 21 on plankton community in a shallow lake were evaluated from the proportion between dissolved reactive P (DRP), dissolved nonreactive P (DOP) and particulate organic P (POP), and the stoichiometry among particulate organic carbon (POC), nitrogen (PON), and POP. In both triple ratios, DRP : DOP : POP and POC : PON : POP, we used POP as a key component to indicate shifts between P fractions and between nutrients in particulate organic matter. The enhanced P accumulation by the planktonic assemblage by 14% at reduced P supply was achieved by two steps, in the first year mainly at the expense of DOP and in the second year by DRP. The evidence that this increase of %POP of TP reflects the adaptive P utilization of the organisms under low P supply was substantiated by significant relationships between P fractions and changes in the community. A higher %POP of TP was associated with higher specific alkaline phosphatase activity and with tighter coupling between producers and consumers respectively— that is, driven from both the nutrient-producer and the producer-consumer interface. A lowered %DOP of TP was significantly related to smaller overall loss rates for the carbon pool of bacteria and algae, whereas %DRP of TP was correlated to the decrease of the biomass of most organism groups. Stoichiometric shifts toward a P-rich assemblage under low TP have been justified by compositional shifts from N-rich cyanobacteria to P-rich eukaryotic algae and bacteria. The shifts toward POP in both triple ratios strongly indicated that the plankton community acted as a sink for phosphorus under reduced total pool size. Ecosystems are regulated by abiotic and biotic factors. The controversial debate over which of the two is more important finally led to the concept of bottom-up and top-down control. These are not mutually exclusive but act alternatively or simultaneously. Abiotic factors such as nutrient supply establish potential biomass increases of producers, whereas food web structure determines actual biomass increase. The concept is complicated by feedback and coupling

The different use of P-resources between two sites in the deep oligotrophic Traunsee was studied by seasonal and vertical patterns of phytoplankton and nutrients from 12/1997 to 10/1998. The P-resources were evaluated from the proportion between the P-fractions, the dissolved reactive P (DRP), dissolved non-reactive P (DOP) and particulate organic P (PP) and from the stoichiometry between nutrients, the total N (TN), the total P (TP) and soluble reactive Si (SRSi). Significant differences between an inshore site impacted by industrial tailings (Ebensee Bay, EB) and an open water reference site (Viechtau, VI) were evident from vertical profiles of both the P-accumulation (%PP of TP) evaluated by DRP:DOP:PP and the distribution of phytoplankton assessed by Si-exhaustion (TN:TP:SRSi), but not from the seasonal patterns of phytoplankton composition, S:V ratios of the algal community or surface layer nutrient dynamics. Low TP and the stable stratification from May to September triggered the relative accumulation of epilimnetic P at VI as it was evident from both the higher portion of particulate P within TP (%PP of TP) and from the shift towards P-enrichment in nutrient stoichiometry of TN:TP:SRSi. The predominance of around 55-52% algal carbon over bacteria at the surface layer to 20m coincided spatially with the lowest Si content relative to N and P. The disturbances at the impacted site was summarised by: up to 11% less P accumulation by organisms at the surface, no stoichiometric shift towards TP in the epilimnion when compared with deeper layers and a reduction of the trophogenic zone to the top 10m. Reasons for this disturbance are seen in the unstable stratification, turbidity, higher TP and the metazoan dominated food chain. Both triple ratios, DRP:DOP:PP and TN:TP:SRSi, were sensitive indicators of the use of P-resources by plankton communities, while inorganic dissolved fractions (DIN:DRP:SRSi, DIN = dissolved inorganic N) provided only insufficient information on nutrient resources in Traunsee.

Long-term phytoplankton assemblages in a large shallow Chinese lake, Lake Taihu, were presented using the monthly monitoring data from October 1991 to December 1999. Earlier research results (1960, 1981 and 1988) were applied to discuss the different trophic stages of the lake. The species composition in the lake was more closely related to eutrophication level than to lake-size, shallowness, or turbidity. Each summer, a single peak of phytoplankton biovolume appeared in Meiliang Bay. The results of principal components analysis showed a distinct temporal shift in species composition between summer and winter. A clear spatial difference in phytoplankton occurred between Meiliang Bay and the lake centre. Wind speed and direction affected the horizontal distribution of phytoplankton, especially Microcystis, in the lake. Temperature, underwater light climate, nutrients and grazing by zooplankton and by fish were discussed to explain the overwhelming dominance of Microcystis. Four nutrient-phytoplankton stages were identified in the lake: an oligo-mesotrophic stage with low algal biomass until 1981, a eutrophic situation with blooms of Microcystis during 1988-1995, hypertrophic conditions with the dominance of Planctonema and total phosphorus up to 200 mg m -3 from 1996 to 1997 and the restoration period after 1997. The wax and wane of the phytoplankton assemblages were mainly controlled by temperature, wind and turbidity while long-term biomass dynamics were influenced by the level of nutrients.

Shallow polymictic lakes are very abundant in many regions of the world, they are more numerous than deep lakes and are of great importance especially in densely populated areas even when small. Their ecology differs from stratified lakes in many ways. During the eutrophication process, lakes may change from a clear water, macrophyte dominated state to an algal dominated state, a process known as the concept of ‘alternative stable equilibria’. The usefulness of this concept as a descriptor for the eutrophication process and as the basis for rehabilitation of lakes is tested on long-term data from two contrasting case study sites using a novel multivariate bubble-plot technique. In one case the concept proved to be useful and successful while it could not be applied to a large shallow turbid lake. The equilibrium concept is then generalised using further examples. Shallow lakes which are either macrophyte dominated or light limited because of high inorganic background turbidity have ratios of total phosphorus (TP) to algal Chlorophyll-a at or below 3:1. Deep oligotrophic or algal dominated lakes in contrast exhibit ratios between 3:1 and 1:1. Alternative stable equilibria of macrophyte or algal domination can occur within the same water-body at various locations, at different times of the year or during trophic development.

This is the first study using flow cytometry to characterize the population dynamics of freshwater autotrophic picoplankton (APP) over a full seasonal cycle, the goal of which was to accu- rately quantify and qualify the natural APP populations in relation to major environmental para- meters. In particular, we wanted to test current assumptions about the seasonal succession of prokaryotic and eukaryotic picoplankton cells, including the relationship between solitary pico- cyanobacteria and microcolonies. Using flow cytometry, we were able to efficiently characterize the abundances of 4 lake APP assemblages in Lake Mondsee, including that of a solitary picocyanobac- terial population exhibiting high 'side scatter' values. Such cells were not readily enumerated by epi- fluorescence microscopy. Unlike Lakes Constance and Maggiore, we found no evidence of a spring peak in solitary picocyanobacteria — we propose that the lack of a spring peak in Lake Mondsee was due to weak stratification in March-April and relatively deep vertical mixing. Since summer declines in the abundance of solitary picocyanobacteria were associated with extended periods of reduced light availability, it is likely that such declines were in part due to low relative growth rates. Finally, we argue that the formation of microcolonies by picocyanobacteria is unlikely to be a strategy for more efficient nutrient recycling (e.g. Stockner & Antia 1986). Rather, we suggest that microcolonies reach high concentrations in surface and near-surface waters due to the production of a photosyn- thate-rich mucilage resulting from active photosynthesis during periods of severe nutrient deficiency.

533 3.5 53.0 15.3 32.0 1.7 27.5 228,758 109,375 Mondsee 481 14.2 51.0 36.0 68.3 1.7 247.0 394,444 207,906 Attersee 469 45.9 3944.6 84.2 170.6 7.0 463.5 545,131 269,050 Hallstattersee 508 8.6 557.0 64.9 125.2 0.5 646.5 132,512 68,690 Wolfgangsee 538 12.8 667.1 52.0 113.1 3.9 124.8 246,154 113,174 Traunsee 422 25.6 2302.0 89.7 191.2 1.0 1417.0 285,396 134,031

Our concept of the 'balance of TN : TP : SRSi-ratios' attempts to determine whether the seasonal succession of phytoplankton communities is influenced by or res- ponds to nutrient stoichiometry. Because the molar average of TN : TP : SRSi = 20 : 1 : 17 for 118 measurements (7 lakes, 3 inflows, all seasons) was close to the marine optimum stoichiometric ratio, we used the ratio TN : TP : SRSi = 16 : 1 : 17 as a refer- ence point for ecological stoichiometry. Triangular diagrams of TN : TP : SRSi ratios scaled in physiological proportions of 16 : 1 : 17 were used to identify seasonal fluctua- tion patterns of major phytoplankton nutrients. In lakes with 'balanced nutrient ratios' TN : TP : SRSi ratios vary evenly around the stoichiometric optimum of 16 : 1 : 17 within a year. Lakes with balanced nutrient ratios, however, were more the exception than the rule and only occurred in 2 of 7 lakes studied. Lakes with constant concentrations of one nutrient element in relation to those of the other two were common. Unbalanced nutrient ratios were associated with relatively constant concentrations of either SRSi, or TN or TP. The relative change in availability of soluble reactive silica was more af- fected by differences in lake morphometry than that of TN or TP. Ratios of TN : TP : SRSi have the advantage of synoptically presenting the relative nutrient avail- ability for both diatoms and non-siliceous algae in phytoplankton communities. Shifts within the tripartite ratio correspond to successive substitution of phytoplankton groups, e.g. from diatoms to cyanobacteria.

The ecological integrity of a lake as a whole can only beassessed through an adequate sampling strategy. Spatialheterogeneity of phytoplankton as well as vertical andhorizontal variability of physical and chemical variables wereestimated from 57 stations at four seasons differing in theirhydrological regime. Resolution of grid positions, located byGPS, was 250 m near the impact site, 500 m for the southernpart of the lake, and 1000 m in the northern part. Data areanalysed by conventional gridding methods as well as in threedimensions with a novel GIS-technique. Horizontal large scaledifferences in several variables are associated withhydrological situations. Local variability in the southern baywas due to input of industrial tailings at times. Spatialvariation of phytoplankton biomass estimated as chlorophyll-aand relevant associated environmental variables were analysedusing a graphical multimetric approach. With this technique,the directly impacted area can be evaluated relative to theremaining part of the lake. The lake is then compared with tworeference lakes, one within the same catchement, the other ina different water-shed. An index of ecological integrity wasdeveloped describing multimetric intra- and interspecific lakevariability. The final index was used to describe the statusof lake water quality relative to a `undisturbed' referencelake. Results showed that Traunsee is ecologically intactalthough its chemistry differs substantially from an`external' reference.

The influence of industrial tailings on the biological integrity of the phytoplankton was assessed from annual measurements of photosynthetic rates in the alpine lake Traunsee. The mean annual integral production of 21 mmol C m-2 d-1 corresponded to the oligotrophic nature of the lake. Effects of effluents were tested by comparing photosynthesis at a station close to the industrial outlet (EB) and at a reference site with a maximum depth of 190 m (VI). Between-site optical properties (vertical attenuation coefficient, euphotic depth) were statistically significant different. The euphotic zone at the impacted station was on average 2 m shallower than at the reference site, owing to turbidity emanating from the industrial plant. The adaptation to low light intensities by the algal community at this station was evident from a high maximum light utilisation coefficient (α* at low light saturation (E K). Algae at the deep reference site were photosynthetically less efficient but adapted to high light intensities. Photosynthetic adaptation to different light climates in the euphotic zone without significant quantitative biomass alterations at the impacted site gave a clear signature of biological integrity of the phytoplankton in the oligotrophic Traunsee.

The hypothesis that both photosynthesis and pigment pattern are more reflective of size related strategies than taxonomic composition of the assemblages was tested under natural conditions in alpine lakes during fall 1999. The small size fraction ( 10 μm) were photosynthetically less efficient but high light adapted having low ratios of chlorophyll-a to \( \beta \)-carotene. In contrast to low light adapted small cells the high light adapted large-cells increased their relative proportion of photo-protective carotenoids above saturating light levels. At light limitation in deeper water layers lipophilic accessory photo-synthetic versus photo-protective pigments increased for all fractions.

Spatio-temporal distributions of plant nutrient concentrations, turbidity, chlorophyll-a and phytoplankton biomass are evaluated for a 10-year dataset from both impoundments of the New Danube flood-relief channel at Vienna, including measurements from the main River Danube. Concentrations of phosphorus primarily are influenced by groundwater transfer of water and nutrients from the main river into the two impoundments, percolating through the gravel substratum of a newly constructed island separating the Danube from the New Danube. Variations of phosphorus input in turn affect the amounts of chlorophyll-a and phytoplankton developing in the impoundments. In the Danube, turbidity depends mainly on discharge, but in the New Danube, although influenced by wind and wave action, turbidity is basically associated with direct inputs from the river when flood-gates are opened. When the channel was first filled with water, all variables had concentration gradients down its 21-km length. Over time, trophic levels have fallen considerably, from hypertrophic to oligo-mesotrophic, with a concurrent reduction of phytoplankton biomass. Species composition of algae has changed to a lesser extent.

Cyanobacterial dominance in lakes has received much attention in the past because of frequent bloom formation in lakes of higher trophic levels. In this paper, underlying mechanisms of cyanobacterial dominance are analyzed and discussed using both original and literature data from various shallow mixed and deep stratifying lakes from temperate and (sub)tropical regions. Examples include all four ecotypes of cyanobacteria sensu Mur et al. (1993), because their behavior in the water column is entirely different. Colony forming species (Microcystis) are exemplified from the large shallow Tai Hu, China. Data from a shallow urban lake, Alte Donau in Austria are used to characterize well mixed species (Cylindrospermopsis), while stratifying species (Planktothrix) are analyzed from the deep alpine lake Mondsee. Nitrogen fixing species (Aphanizomenon) are typified from a shallow river-run lake in Germany. Factors causing the dominance of one or the other group are often difficult to reveal because several interacting factors are usually involved which are not necessarily the same in different environments. Strategies for restoration, therefore, depend on both the cyanobacterial species involved and the specific causing situation. Some uncertainty about the success of correctives, however, will remain due to the stochastic nature of the events and pathways leading to cyanobacterial blooms. Truly integrated research programs are required to generate predictive models capable of quantifying key variables at appropriate spatial and temporal scales.

Phytoplankton structure and dynamics of four eutrophic lakes of Berlin were studied in 1992. The biomass change of relevant classes - Cyanophyceae, Bacillariophyceae, Cryptophyceae, and Chlorophyceae - is well represented by HPLC-analysis of photosynthetic lipophilic pigments simultaneously carried-out. Nevertheless, an estimation of algal class biovolumes or phytoplankton composition, on the basis of chromatographically recorded pigment data is restricted to a semi-quantitative consideration. Different pigment amounts in individual species of the same algal class or varying marker pigment/chlorophyll-a ratios are discussed as a reason for the misleading pigment-based quantification of phytoplankton dynamics. However, the analysis of the obtained data points to the possibility of using HPLC-aided pigment determinations in connection with microscopic cell counting for a detection of variable physiological states under field conditions; provided that mass developments of algae are dominated by a single species as common in eutrophic lakes. For time periods with dominating Planktothrix agardhii or Microcystis spec. a positive correlation was found between their content of the cyanophyte specific pigment echinenone and increasing nitrogen availability. In contrast, no relations could be detected between the changing fucoxanthin content of Aulacoseira spec. and any of the measured environmental parameters (nutrient concentrations or light climate).

113 randomly selected frustules of Cyclotella taxa, taken from the plankton of four lakes, have been investigated with respect to variation between the two thecae of each frustule and also variation within and between populations occurring in the different lakes. Multivariate statistical analyses of phenetic similarity of the 226 thecae were based on a data matrix recording the status of 11 characters drawn from the structure of the siliceous wall. From the cluster analysis, 55% of the frustules could be characterized as heterovalvar. In these cases, the phenetically dissimilar thecae of a single frustule were allocated to different clusters. Principle component analyses showed that the variation in the relative length of the marginal striae, and also the number of marginal striae on the entire valve face, are correlated with valve circumference. The arrangement of depressions on central valve and the development of the central valvar undulation appear to be unrelated to valve size. Due to the existence o...

Anthropogenic eutrophication still is a major threat to inland waters in large parts of the world although re-oligotrophication has largely progressed in several regions. Climate change now poses a new risk effectively endangering inland waters. Impacts of climate change on inland waters are already well documented and established. Direct effects are mainly through changes in temperature and associated physico-chemical alterations. Indirect effects mediated through processes such as extreme weather events in the catchment include increased nutrient loading among others. The combination of all impacts will lead to severe and significant changes in the physical structure and the biological configuration of the waters depending on future climate scenarios. Since water quality and quantity will be affected, concern is necessary. Possible adaptation and mitigation mechanisms are discussed.

◦by 2071‐2100. The associated projections for the rainfall give even more cause for concern with the reductions in some areas approaching 50% in summer. In this chapter we analyse impacts of changing weather conditions on lakes in Central Europe. Long-term data sets from a number of lakes are used to link measured variables to climate signals. Particular attention is paid to the lakes in the perialpine region which are known to be very sensitive to short-term changes in the weather (Psenner, 2003; Thompson et al., 2005). Here, the topography and the steep orography enhance the water cycle, and result in flooding, debris flows, avalanches, vertical plant migration etc. The Alps also form a barrier to the mass movement of air and are responsible for the sharp climatic divide between Atlantic, Continental and Mediterranean influences. Central Europe is a variously and vaguely defined region. Rather than a physical entity, it is more a reflection of a shared history. The results summarized here are based on the analysis of long-term climatological and limnological data from the countries shown in Fig. 20.1. These include Germany (DE), Poland (PL), the Czech Republic (CZ), Slovakia (SK), Switzerland (CH), Lichtenstein (LI), Austria (AT) and Hungary (HU). The Central European countries are geographically diverse with landforms ranging from the North-German Lowlands, through the Alps to the Hungarian plain. The pannonian plain in the eastern part is also a major climatic ‘crossing point’ and is affected by the Eastern-European continental, the WesternEuropean oceanic and the Mediterranean influence.

Phytoplankton, an assemblage of suspended, primarily autotrophic single cells and colonies, forms part of the base of the pelagic food chain in lakes. The responses of phytoplankton to anthropogenic pressures frequently provide the most visible indication of a long-term change in water quality. Several attributes related to the growth and composition of phytoplankton, such as their community structure, abundance as well as the frequency and the intensity of blooms, are included as indicators of water quality in the Water Framework Directive. The growth and seasonal succession of phytoplankton is regulated by a variety of external as well as internal factors (Reynolds et al., 1993; Reynolds, 2006). Among the most important external factors are light, temperature, and those associated with the supply of nutrients from point and diffuse sources in the catchment. The internal factors include the residence time of the lakes, the underwater light regime and the mixing characteristics of the water column. The schematic diagram (Fig. 14.1) shows some of the ways in which systematic changes in the climate can modulate these seasonal and inter-annual variations. The effects associated with the projected changes in the rainfall are likely to be most pronounced in small lakes with short residence times (see George et al., 2004 for some examples). In contrast, those connected with the projected changes in irradiance and wind mixing, are likely to be most important in deep, thermally stratified lakes.

Recent studies have highlighted the impact of the winter North Atlantic Oscillation (NAO) on water temperature, ice conditions, and spring plankton phenology in specific lakes and regions in Europe. Here, we use meta-analysis techniques to test whether 18 lakes in northern, western, and central Europe respond coherently to winter climate forcing, and to assess the persistence of the winter climate signal in physical, chemical, and biological variables during the year. A meta-analysis approach was chosen because we wished to emphasize the overall coherence pattern rather than individual lake responses. A particular strength of our approach is that time-series from each of the 18 lakes were subjected to the same robust statistical analysis covering the same 23-year period. Although the strongest overall coherence in response to the winter NAO was exhibited by lake water temperatures, a strong, coherent response was also exhibited by concentrations of soluble reactive phosphorus and soluble reactive silicate, most likely as a result of the coherent response exhibited by the spring phytoplankton bloom. Lake nitrate concentrations showed significant coherence in winter. With the exception of the cyanobacterial biomass in summer, phytoplankton biomass in all seasons was unrelated to the winter NAO. A strong coherence in the abundance of daphnids during spring can most likely be attributed to coherence in daphnid phenology. A strong coherence in the summer abundance of the cyclopoid copepods may have been related to a coherent change in their emergence from resting stages. We discuss the complex nature of the potential mechanisms that drive the observed changes.

Die Primarproduktion, im Wesentlichen der grunen Pflanzen, bildet heute in nahezu allen Okosystemen die Grundlage des Lebens. Als Grund- oder Primarproduktion eines okologischen Systems, einer Gemeinschaft oder eines Teiles davon wird die mit Hilfe der Photo- und Chemosynthese der Produzenten in Form organischer Substanz gespeicherter Energie bezeichnet. Die Primarproduktion bildet daher einen ganz wesentlichen Bestandteil der allgemeinen Okologie der Gewasser. Dargestellt werden die wichtigsten Methoden zur Messung der Primarproduktion, die Abhangigkeit der Primarproduktion von Umweltfaktoren sowie Abschatzungen der Primarproduktion in Binnengewassern. Keywords: Algen; Carotinoid; Chlorophyll; Cyanobakterien; Lichtkurve; Photosynthese; Phytoplankton; Pigmente; Respiration; Sonnenstrahlung

Phytoplankton seasonal and long-term succession can be described and functionally classified by associations similar as in terrestrial vegetation studies. Such a concept has to include 'climax' into pelagic succession which in turn leads to the question to what extent steady state assemblages occur and if during periods of dynamic equilibrium can be identified. Here we explore the situation with respect to the above question for deep, nutrient poor, alpine lakes in Austria. We first track the long-term development of phytoplankton biomass, their taxonomic structure and their relation to total phosphorus and chlorophyll-a as predictors of trophic state over the past 35 years. We then analyse this data set for coherent algal associations which can be ascribed to trait separated functional groups according to Reynolds et al. (2002). A three year period of stable environmental conditions has then be extracted from the progression of trophic state indices, having similar dominating species each year. These years were finally analysed for steady state conditions according to definitions given in Sommer et al. (1993). During thermal stratification, achievement of an equilibrium could be ruled out although coexistence of several dominating species lasted for several weeks. Habitat templates were constructed from environmental variables prior to biomass peaks for two species important in summer assemblages, the dinoflagellate Ceratium hirundinella and the diatom Fragilaria crotonensis. In summary, functional groups proved to be a valid and useful concept to describe species succession of phytoplankton in deep alpine lakes while pelagic climax is much less clear and steady state conditions were never met.

The vertical distribution pattern of algal species, chlorophylls and specific carotenoids present in the dimictic pre-alpine Ammersee (Bavaria, Germany) are given for the year 2001. A detailed taxonomic list of the phytoplankton species is recorded, along with light micrographs and detailed descriptions of the flagellates. A deep chorophyll maximum, mainly built by Planktothrix rubescens, was common in this deep mesotrophic lake. The three most dominant species among 83 identified taxa alternated seasonally and reached significant biovolumes in both the epiand the metalimnion (Planktothrix rubescens > Ceratium hirundinella > unicellular centric diatoms > Asterionella formosa > Fragilaria spp. > Anabaena lemmermannii > Phacotus lenticularis and less frequent dominant was Rhodomonas minuta). We define a steady state phytoplankton assemblage in Ammersee as a stable community in terms of species composition and standing crop. The stability of species composition was measured by Bray-Curtis similarity between monthly samples and indicate the change of individual biovolumes of species from month to month. The stability of standing crop was evaluated by the net change of total biovolume for the same time intervals. Focussing on steady state phytoplankton assemblages we compared three spatially heterogeneous environments for vertical niche separation within the top 12 m: the euphotic epilimnion (2 and 5 m), the euphotic metalimnion (7 m) and the metalimnion below the euphotic zone with dim-light less than 1% (10 and 12 m). The definition of a steady state assemblage on both ends hold true only for metalimnetic layers at dim-light levels below 1% in Ammersee. At this metalimnetic layer more than 80% similarity in phytoplankton composition between successive monthly samples was reached, associated with almost zero net changes of total biovolume only. The greater the contribution of the three most dominant taxa to biovolume, the higher were the Bray-Curtis similarities at metalimnetic depths below the euphotic zone. Zooplankton biomass had very little effect on species assemblages in the metalimnion, while parameters related to stratification (Schmidt stability) as well as those of trophy (TP, Chl) correlated with species changes. The similarity values between successive monthly samples from all the euphotic layers never reached more than 60% and were usually significantly lower, even if biovolume net changes were around zero. Both the high fluctuations of the ratio of photosynthetic versus photo-protective carotenoids (PSC:PPC) and the statistical significance of correlations between the change of species and environmental-biotic parameters separate the euphotic layer of the top 7 m as a homogenous community from deeper strata. At all sampling depths within this euphotic zone the increase of sunshine duration was associated with an increase of the carotenoid ratio PSC:PPC, but no relationship was found for the deeper layers. The change of species in the euphotic layer was not significantly related to thermal stability, TP or the dominance structure of phytoplankton, but linked with the zooplankton biomass and therefore seemed to be top-down controlled. From our observations, we can conclude that only during stratification and only in the metalimnion below the euphotic zone steady state assemblages can be expected in the deep mesotrophic Ammersee.