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Institute
- FG Gewässerökologie (11)
Vor dem Hintergrund der zunehmenden Anzahl an Tagebauseen sowie den Folgen der anthropogen veränderten Klimaentwicklung und infolgedessen der Möglichkeit des Entstehens oder/und des Zusammenbrechens von Meromixis in Seen besteht die Notwendigkeit, Kenntnisse über die Dichteschichtung verursachenden chemischen Prozesse sowie die in der Wassersäule auftretenden Dichtegradienten und die Dichtebeiträge gelöster Stoffe zu gewinnen.
Aus diesem Grund wurden zwei meromiktische, in der Lausitz gelegene Seen (Waldsee, Moritzteich) untersucht. Ziel war es, neben den Ursachen der Dichteschichtung die (bio)geochemischen Prozesse in der Wassersäule sowie an der Sediment-Wasser-Grenze zu untersuchen sowie Dichtegradienten und die Dichtebeiträge gelöster Stoffe im Gewässer zu quantifizieren.
Hierzu wurden die Seewasser-, Sediment-, Sinkstoff- und Porenwasserbeschaffenheit chemisch analysiert sowie tiefenorientierte Messungen der Vor-Ort-Parameter in der Wassersäule durchgeführt. Die Daten waren Basis geochemischer Berechnungen, der Bestimmung dichterelevanter chemischer Prozesse und Parameter sowie der Berechnung der Dichtegradienten in der Wassersäule und der Dichtebeiträge gelöster Stoffe.
Die Ergebnisse zeigten, dass in den untersuchten Braunkohletagebauseen die Meromixis auf der biogeochemischen Umsetzung des Eisens mit der Bildung eisenhydroxidischer und
-hydroxosulfatischer Minerale im Mixolimnion und deren Auflösung sowie der Reduktion des Eisens(III) zu Eisen(II) im Monimolimnion beruht. Dabei war die Beschaffenheit der im Mixolimnion gebildeten Eisenverbindungen pH-abhängig. Bei pH-Werten ≤3 wurden Eisenhydroxosulfate (vermutlich Schwertmannit) und bei pH-Werten von 6 bis 7 Ferrihydrit ausgefällt.
Die in den Monimolimnionsedimenten ablaufende Sulfatreduktion führte zu einem in das Sediment gerichteten Sulfat- und Eisenfluss, wobei bei einem möglichen Eisenüberschuss dieses auch ins Wasser freigesetzt und somit zu einer Erhöhung der Dichte im Monimolimnion beitragen konnte. Die im Mixolimnionsediment ablaufenden chemischen Prozesse waren vor allem vom vorliegenden geochemischen Milieu abhängig. Dabei können in Organikreichen Sedimenten sulfatreduzierende Prozesse stattfinden, wohingegen in den sauren, geringer organischen Sedimenten möglicherweise eine diagenetische Umwandlung metastabilen Schwertmannits zu stabileren Goethit vorliegt.
Der chemische Dichtegradient beruhte im Wesentlichen auf Eisen(II) und dem Karbonatsystem (HCO3-+CO2), zu geringeren Anteilen auf DOC und Calcium sowie, ausschließlich im Moritzteich, auf Sulfat. Insgesamt war der in beiden Seen durch die chemischen Stoffe verursachte Dichtegradient zwischen Mixo- und Monimolimnion mit maximal rd. 1,0 g/l gering. Saisonale Veränderungen des chemischen Dichtegradienten in beiden Seen waren marginal. Somit basierten Schwankungen des Gesamtdichtegradienten in der Wassersäule vor allem auf den Änderungen der Temperatur und weniger auf den Änderungen der Konzentrationen gelöster Stoffe.
Phytoplankton succession and diversity-productivity relation in German and Brazilian shallow lakes
(2012)
Phytoplankton succession linked to productivity-diversity relationship at steady-state/non steady state conditions, mixing events as predominant disturbance in sense of Connell (1978) and influence of conditional differentiation (temperature) on phytoplankton diversity were investigated. Water samples were taken two times a week in 4 German shallow lakes in Brandenbug region (Melangsee, Petersdorfer See, Wolziger See, and Langer See) from 2007 to 2010 and in 2009/2010 during cold and rainy season in a Brazilian coastal lagoon Peri. A comparison between literature data from 4 Brazilian lakes, in order to investigate the influence of temperature on diversity and dynamic of phytoplankton species was also done. The cyanobacterial species dominated all studied lakes and the mixing had a positive effect on the supremacy of this group in tropical lakes. The high biovolume of cyanobacteria contributed to the achievement of steady state conditions in sense of Sommer et al. (1993) during consecutive periods in Petersdorfer See and in Peri lagoon practically during all phases. The monodominance of Cylindrospermopsis raciborskii was observed in warm water, under no stratification regime. Oscillatoriales of group S1 (Reynolds 2002), represented by Limnothrix redekei and Pseudanabaena limnetica, typical in turbid mixed layers and with high nitrogen affinity, were typical in Petersdorfer See. In Melangsee, steady state was observed during colder periods under dominance of Bacillariophyceae and Chrysophyceae. Lower diversity coincided with steady state conditions and higher values of resource use efficiency (RUE), while higher diversity occurred mainly at transitional state during the spring in temperate lakes, supporting the non-equilibrium concept (Hutchinson 1961) at lower RUE values. Higher biovolume occurred mainly during the late summer in temperate lakes under dominance of cyanobacteria, and/or at steady state. Concerning the temporal variation of phytoplankton, the highest peak of species richness preceded the resource use efficiency, suggesting the productivity as consequence of diversity. The richness was much higher in temperate shallow lakes, which were more similar to each other in terms of seasonality, species number and biovolume. A positive correlation between richness and biovolume of temperate lakes was found and the opposite was observed in tropical lakes. The biovolume and richness in tropical lakes were more variable and in temperate lakes presented higher richness at intermediate level of biovolume (annual mean from 10 to 18 mm3/l). Low level of biodiversity allows an ecosystem to function under constant conditions, but a greater biodiversity occurred in fluctuating environments.
During open-cast lignite mining in Lusatia (NE Germany), iron sulfide minerals associated with lignite, are exposed to air and weather. The weathering products iron (Fe), sulfate and acids subsequently enter the fluvial-lacustrine system Spree and its tributaries.
The aim of this work was to identify and explain the evolving biogeochemical signatures (spatio-temporal patterns) in River Spree. These signatures are significantly influenced by biogeochemical turnover processes triggered by iron input. To identify the signatures, a regional field study was conducted from the inland delta Spreewald to the mouth. This study was followed by two laboratory experiments under controlled redox conditions to investigate the effect of Fe on the long-term binding of phosphorus and the degradation of particulate organic material in the iron-enriched sediments.
Characteristic elements introduced by lignite mining (iron, sulfur, aluminum, cobalt, nickel) and by urban areas (zinc, lead, copper, chromium) could be determined by means of a principal component analysis. Based on these findings, the impact range of the open-cast mining was determined to be at least ~ 90 km downstream of the Spreewald. Within the urban-influenced systems Fürstenwalde and Berlin, the urban signature is more dominant than the mining signature. By means of sequential extractions and XRD analyses of the sediments, iron binding forms and characteristic iron minerals have been additionally identified (e.g., Fe(III) oxyhydroxides, pyrite, vivianite).
Sediment analyses and column experiments have shown that iron increases the phosphorus sorption and decreases the phosphorus release in the sediments of three studied lakes of the fluvial-lacustrine system. However, the application of different phosphorus retention models for Lake Neuendorfer See showed that Fe cannot reduce the eutrophic state of the lakes along the river, since the water retention times are too short and the external phosphorus loads are too high. Additionally, lake sediments with different Fe contents have been incubated with particulate organic material. Higher Fe contents resulted in reduced emissions of the greenhouse gases CO₂ and CH₄, which can be attributed to a reduced degradation of organic matter by Fe. Possible reasons include sorption processes of (dissolved) organic matter onto Fe oxyhydroxides and toxic effects of iron on microorganisms. Nevertheless, Fe (and sulfate) are available as electron acceptors during the decomposition of organic matter in River Spree.
The findings of this work deepen the understanding of the impact of iron from mining on sulfur, phosphorus and carbon turnover processes in fluvial-lacustrine systems. As a consequence of the Fe input from the Lusatian area, the signature will persist for decades in River Spree.
Diese Arbeit ist eingebettet in das BMBF-geförderte Projekt NITROLIMIT II, in dem unter anderem ein Nährstoffreduktionskonzept für die Storkower Seenkette entwickelt werden soll. Ziel dieser Bachelorarbeit war es, Nährstofffrachten für Teileinzugsgebiete in der Storkower Seenkette zu ermitteln, welche als Grundlage zur Planung und Verortung von Sanierungsmaßnahmen im Einzugsgebiet dienen sollen. Die berechneten Nährstofffrachten sollten anschließend mit den Ergebnissen einer Messkampagne des Landesamtes für Umwelt, Gesundheit und Verbraucherschutz Brandenburg aus dem Jahr 2011/12 verglichen werden sowie mögliche Sanierungsmaßnahmen diskutiert werden.
Zur Bestimmung der Nährstofffrachten wurden die Gesamtphosphor- (TP) und Gesamtstickstoff-Konzentrationen (TN) aus Wasserproben von sieben Messstellen des Untersuchungsgebietes analysiert, die von Februar bis Dezember 2015 gesammelt wurden. Die TP-Analysen wurden vom Kandidaten mittels Natrium-Peroxodisulfat-Aufschluss und anschließender photometrischer Messung nach DIN EN ISO 6878 (DIN, 2004) durchgeführt. Die TN-Analyse erfolgte mittels Fließinjektionsanalyse nach DIN EN ISO 13395 (DIN, 1996). Als Basis für die Frachtberechnungen wurden die Daten der Durchflussmessungen aus zwei Studienprojekten verwendet. Für die drei Klärwerke, die Klarwasser im Untersuchungsgebiet in die Vorfluter einleiten, wurden die Ablauffrachten berechnet.
Es zeigte sich, dass besonders die Fließstrecken unterhalb der drei Kläranlagen (Rieploser Fließ (KA Storkow; 22202 EGW) und Kuppengraben I (KA Friedersdorf; 1650 EGW und KA Wolzig; 467 EGW)) stark von Nährstoffen belastet sind und direkten Einfluss auf die Biomasseproduktion der unterhalb liegenden Gewässer nehmen. Vor allem für den davon betroffenen Stahnsdorfer See wird Handlungsbedarf gesehen, um eine Verbesserung gegenüber dem gegenwärtigen, polytrophen Zustand zu erreichen.
Der Vergleich der 2015 ermittelten Daten mit den Ergebnissen der Messkampagne des LUGV aus den Jahren 2011/12 hat gezeigt, dass die im Einzugsgebiet transportierten Nährstoffmengen relativ konstant sind. Trotz der verschiedenen Niederschlags- und somit auch Durchflussmengen in den unterschiedlichen Jahren konnten Nährstofffrachten in sehr ähnlicher Größenordnung berechnet werden.
Vor dem Hintergrund der ermittelten Nährstofffrachten wurden die geplanten Sanierungsmaßnahmen im Bereich des Rieploser Fließes betrachtet. Besonderes Augenmerk lag dabei auf der geplanten Umleitung des Rieploser Fließes. Eine mögliche Variante sieht vor, das Fließ vor seiner Mündung in den Stahnsdorfer See durch ein Wehrbauwerk abzutrennen und das Wasser in ein künstliches Feuchtgebiet südlich des Sees zu leiten. Der dort befindliche ehemalige Torfstich könnte zu einem Schilfpolder umgestaltet werden und so als Retentionsfläche dienen. Diese Maßnahme könnte eine Nährstoffentlastung um ca. 30% für den Stahnsdorfer See bringen. Außerdem wurde durch diese Arbeit nachgewiesen, dass der Kuppengraben I im Vergleich zu anderen Messstellen bis zu 10fach höhere TP-Jahresmittel bzw. 4fach höhere TN-Jahresmittelwerte aufweist. Daher sollten auch für diesen Klärwerksableiter oder an den einleitenden Klärwerken Sanierungsmaßnahmen durchgeführt werden.
Many lake water bodies not attained the goal of the European Water Framework Directive (WFD) to achieve a good ecological state by 2015. This is, among other things, because the assessment and improvement of hydromorphological conditions of lakeshores has been neglected as an important component ensuring the ecological integrity of lake ecosystems. In recent years, macroinvertebrates were emphasized to be useful indicators for the assessment of lakeshore hydromorphology. Hence, in Europe macroinvertebrate-based assessment methods were developed to evaluate the hydromorphological conditions of lakeshores.
In this thesis, some of the uncertainties and missing aspects of existing macroinvertebrate-based assessment methods were addressed. The results were obtained by sampling macroinvertebrates and macrophytes at natural shores and at shores modified by marinas and beaches in three depth zones between April and November 2011 in a large lowland lake (Lake Scharmützelsee, Germany).
Firstly, I clarified that upper littoral macroinvertebrates should be used for assessing the hydromorphology of lakeshores. It was shown that the effect of lakeshore modification on macroinvertebrate diversity and community composition was most pronounced in the upper littoral and decreased to the profundal zone. Secondly, I demonstrated that a single seasonal sampling is sufficient to capture the compositional differences of macroinvertebrate communities associated with human lakeshore modification. Seasonal effects on upper littoral macroinvertebrate diversity and composition were less important than shore type in comparison with the middle littoral and profundal zone. Thirdly, upper littoral macrophyte communities were also affected by lakeshore modification and at the same time the most important variable structuring macroinvertebrate communities. Hence, the effects of different shore types on macrophytes were transferred to macroinvertebrates, but artificial substrates were also able to partly substitute macrophyte habitats as it was shown for the studied marinas. Since lakeshore modification affected macrophytes slightly differently than macroinvertebrates, macrophytes should be considered as an additional component in lakeshore assessment. Finally, secondary production as proxy to determine the effect of lakeshore modification on the functioning of macroinvertebrates was estimated. Estimation of secondary production requires the determination of biomass. Biomass was indirectly determined by using length-mass regressions established for macroinvertebrates from temperate lakes of the central European lowland. The result showed that total secondary production and secondary production of native taxa in the upper littoral was substantially lower at the studied beaches compared to natural sites. In contrast, upper littoral secondary production at marinas did not differ to natural sites, but secondary production of non-native taxa was significantly higher at marinas. No effects of lakeshore modification on secondary production were found with increasing depth. Different scenarios based on upscaling of site-specific production to whole lake ecosystem level gave evidence that the observed local impacts of lakeshore modification can translate into alterations of the functioning of macroinvertebrates at whole lake ecosystem level. In addition, it was emphasized that secondary production as a functional measure is more sensitive in detecting hydromorphological alterations than the structural measures diversity or biomass. Secondary production should therefore be included in existing lakeshore assessment methods. In order to obtain a comprehensive overview about the changes in the functioning of macroinvertebrates following lakeshore modification, it is recommended to consider not only total secondary production but also secondary production of functional groups. With these results, this thesis contributes to the mechanistic understanding of the effect of lakeshore modification on the functioning of macroinvertebrates and the consequences for the functioning of the whole lake ecosystem. The newly generated knowledge helps to optimize the development of successful lakeshore assessment tools and identification of management measures.
Ecological research on acidic lakes has historically focused on soft-waters, whereas biota in natural and man-made acidic hard-water lakes remains poorly studied. My thesis deals with the effects of multiple abiotic stressors on species and genetic biodiversity of littoral Cladocera in two types of acidic waters in Germany: hard-water mining and soft-water bog lakes. I found that these two types of acidic lakes supported highly distinct cladoceran communities, both with respect to their species diversity and composition, what led to the rejection of my null hypothesis that pH was the only abiotic factor, determining species diversity patterns. Further analyses revealed that, apart from pH, TDS-related parameters and trophy significantly affected species composition of littoral Cladocera in the studied lakes. Particularly the role of TDS and its constituents for colonization of acidic lakes was virtually neglected by previous studies, based on soft-water lakes’ data sets. My study on the genetic diversity of a generalist cladoceran Chydorus sphaericus provided evidence for genetic erosion, i.e. negative effects of abiotic stress in the acidic hard-water mining lakes, but not in the acidic soft-water bog lakes. I conclude that the observed genetic erosion in acidic mining lakes may have negative long-term consequences for population persistence even in broadly tolerant species. In general, biodiversity at both species and genetic levels was much more impaired in the acidic hard-water mining lakes, compared to the naturally acidic soft-water bog lakes. This was possibly due to both ecological (higher abiotic stress) and evolutionary (insufficient time for long-term adaptations to evolve) mechanisms. My results suggest that colonization of acidic hard- and soft-water lakes in each case should require specific physiological adaptations at both species and population levels. The practical relevance of my studies is that they provide the scientific basis for monitoring of acidic hard-water mining lakes based on littoral Cladocera. Especially genetic diversity proved a valuable indicator of water quality in acidic mining lakes, as it provided unique information on population-level responses, being complementary to species diversity assessment. Overall, my results demonstrate that inclusion of acidic hard-water lakes into the research framework as well as comparisons among communities in chemically diverse acidic lakes can result in important new insights, concerning the community organization and adaptations in acidic environments.
Das Vorkommen von Cyanobakterien, Anatoxin a und Microcystin wurde in 10 Gewässern erfaßt. Die Steuerung ihrer Dynamik wurde in 3 polymiktischen Seen untersucht und Effekte von Mixis mit Sedimentresuspension in Enclosures fokusiert. Die Gewässer waren durch Oscillatoriales, entweder Planktothrix agardhii oder Limnothrix spp. dominiert und wiesen hohe Biovolumina von Nostocales im Frühsommer auf. Hohe Lichtintensität und niedrige Stickstoff-Konzentrationen förderten die Entwicklung der Nostocales. Während des Sommers nahm die Lichtintensität aufgrund des steigenden Biovolumens der Cyanobakterien ab, was zum Rückgang der Nostocales und Dominanzen von Oscillatoriales führte. Durch Sedimentresuspension wurden Nährstoffpulse erzeugt, die zur Dominanz von P. agardhii führten. Bei Stagnation der Wassersäule waren die Nährstoffkonzentrationen niedriger und Limnothrix spp. gelangte zur Dominanz. Anatoxin a konnten auf Anabaena spp. zurückgeführt werden, Microcystine auf P. agardhii. Abnehmende Lichtintensität und/oder zunehmende Stickstoff-Konzentrationen förderten die Microcystin Produktion oder das Wachstum toxischer Stämmen.
Viele biotische und abiotische Faktoren, wie die Nahrungsqualität und diverse chemische Stressoren, bestimmen die Lebensparameter von Cladoceren. Diese Arbeit konzentriert sich auf die Effekte der unterschiedlichen Futteralgen (Pseudokirchneriella subcapitata, Monoraphidium minutum und Desmodesmus armatus) auf die Lebensparameter von vier Cladoceren-Arten (Daphnia magna, D. galeata, Moina macrocopa und M. micrura) in An- und Abwesenheit von chemischem Stress, ausgelöst durch den Huminstoff HuminFeed® (HF). Um die Effekte von HF auf D. magna zu ermitteln, wurden Lebenszyklusexperimente durchgeführt. In umweltrelevanten Konzentrationen wirkte HF lebensverlängernd, jedoch zeigten sich Verminderung in der Reproduktion. Die Ergebnisse wurden im Licht der Disposable Soma Theorie diskutiert. Es konnte keine Steuerung der Energieallokation durch HF und durch die Futteralgenqualität festgestellt werden. In einem Multigenerationsexperiment wurde der Einfluss von HF auf D. magna über drei Generationen hinweg betrachtet. Dabei bewirkte HF stets eine Verminderung der Reproduktion und eine Erhöhung der Lebensspanne. Die Ergebnisse des ersten Filialgenerationsversuchs F1 haben gezeigt, dass ein signifikanter Unterschied in der Fruchtbarkeit von Versuchstieren besteht, die in derselben Umwelt aufgewachsen sind (ohne HF), aber deren Mütter (P) unterschiedlichen Behandlungen ausgesetzt waren. Die Ergebnisse des zweiten Filialgenerationsversuchs F2 haben aber aufgezeigt, dass die Umwelt, in der die Parentalgeneration aufgewachsen ist, die Fruchtbarkeit der F2 Generation nicht nachhaltig beeinflusst. Es handelt sich vermutlich um Transgenerationseffekte (epigenetische Programmierung). In einem Langzeitexperiment wurden die Auswirkungen vom HF auf die Reproduktion und die Lebensspanne von D. galeata untersucht. Es zeigte sich, dass HF erneut eine Steigerung der Lebensdauer und eine Verringerung der Reproduktion bewirkt. Die Ergebnisse wurden im Licht der Disposable Soma Theorie diskutiert. Des Weiteren wurde der Einfluss von HF auf die Lebensparameter von M. macrocopa untersucht. Durch die Zugabe von HF bei Fütterung mit D. armatus wurde teilweise eine Verkürzung der Lebensspanne und eine Erhöhung der Nachkommenzahl hervorgerufen. Bei Fütterung mit P. subcapitata zeigte sich, dass HF keine signifikanten Effekte erzielte. Bei Fütterung mit M. minutum konnte in exponierten Ansätzen eine Verlängerung der Lebensspanne und Erhöhung der durchschnittlichen kumulativen Nachkommenzahl festgestellt werden. Hormonähnliche Effekte der Huminstoffe wurden in Betracht gezogen. Die unterschiedliche Antwort von M. macrocopa auf Huminstoffe bei verschiedenen Futteralgen kann darauf hindeuten, dass die Energieallokation in M. macrocopa bei HF-Exposition durch die Futteralgenqualität gesteuert wird. Ebenfalls wurde bei M. micrura festgestellt, dass die Energieallokation unter dem chemischen Stressor HF durch die Futteralgenqualität gesteuert wurde. So wurde gezeigt, dass HF einen (teilweise signifikant) negativen Einfluss sowohl auf die Lebensspanne als auch auf die Fruchtbarkeit hat, wenn die Versuchstiere mit P. subcapitata gefüttert wurden. Werden M. micrura hingegen mit M. minutum gefüttert, wirkte HF teilweise lebensverkürzend und ein Trend zur Erhöhung der Nachkommenzahl war sichtbar. Diese Arbeit gibt Hinweise darauf, dass zwischen den Zooplanktonarten Unterschiede hinsichtlich der Antwort auf chemischen Stress und des Bedarfs an einzelnen Futterqualitätsfaktoren bestehen.
Nostocalean cyanobacteria differentiate heterocysts to fix dissolved nitrogen (N2) and dormant cells (akinetes) to survive harsh environmental conditions. The distribution and proliferation of Nostocales and their hibernation strategies in deep stratified lakes of different trophic states and climate zones were investigated. Studies were carried out in the meso-eutrophic Lake Kinneret (Israel) and in the mesotrophic Lake Scharmützelsee and the oligo-mesotrophic Lake Stechlinsee (Germany).
The spatio-temporal distribution of akinetes in sediments was analysed. The abundance in the sediment surface increased with water depth and differed due to basin morphometry. Light and temperature distribution revealed shallow areas of sedimentary akinete pool as a potential inoculum to contribute to the formation of a pelagic population. Hence, a small viable akinete pool was deposited in shallow sediments.
A CARD-FISH protocol was implemented to identify and quantify the akinetes in sediments. Tested Nostocales strains and akinetes in sediment samples were successfully labelled with 16S rRNA targeted probes and microscopically detected by their fluorescence signal. Akinete enumeration in field samples confirmed the suitability of the CARD-FISH approach.
Species- or genus-specific differences in the overwintering strategies of Nostocales were observed. Akinetes were the only overwintering form of Anabaena spp. On the other hand, Aphanizomenon flos-aquae overwintered as vegetative filaments in the pelagial of Lake Stechlinsee. In the other lakes studied, Aphanizomenon spp. performed an intermediate life cycle, with akinetes and a small pelagic population in winter. This suggested that Aphanizomenon spp. had another overwintering strategy contrary to the exclusively dormant strategy of Anabaena spp.
Putative strains of cylindrospermopsin (CYN)-producing Aphanizomenon ovalisporum were isolated from Lake Kinneret. Their morphologic and phylogenetic diversity, as well as the presence of CYN-encoding gene cluster and CYN production, were analysed. Four of the six strains were assigned to Anabaena bergii. Although all the isolated strains possess at least fragments of CYN gene cluster, no CYN production was detected. Non-CYN-producing Aphanizomenon ovalisporum strains and the presence of non-toxic Anabaena bergii in Lake Kinneret have not been reported before. This suggested that toxic and non-toxic Nostocales strains can coexist in lakes.
In recent years Cylindrospermopsis raciborskii (Woloszynska) Seenayya et Subba Raju a potentially toxic freshwater cyanobacterium of the order Nostocales initially classified as a tropical species, has invaded the temperate region and has established in lakes in northern Germany. The formation of dormant cells (akinetes) is an important part of the life cycle which enables Nostocales species to survive harsh conditions like the winter in the temperate zone. In this thesis, my aim is to throw more light on the life cycle of the invasive cyanobacterium Cylindrospermopsis raciborskii and also to compare it to native Nostocales species of the genus Aphanizomenon using derived field and laboratory data. This would help in the understanding of the invasion of C. raciborskii and spread and how it can be contained. Abiotic factors influencing the development and growth of photoautotrophic planktonic organisms in nature are examined and their variation with the years is discussed. A comprehensive look at C. raciborskii’s akinetes in the sediment and the filaments and akinetes in the water column is done. The timing and temperature condition of akinete germination, recruitment, formation in the pelagic and importation into the sediment is determined. An assessment of the role of abiotic factors on the dynamics and abundance of filaments is done with a statistical model used to isolate which abiotic factors are most influential on the population growth of C. raciborskii. Akinetes distribution on the bed of the lake and the effect of loss processes on their abundance are looked into. Cylindrospermopsis is compared to native species Aphanizomenon spp in terms of the seasonal dynamics and the germination condition of their akinetes. The main findings in this thesis were that the seasonal dynamics of C. raciborskii had a similar trend but different timing of each event in the four years of the study. The variabilities were due to the different timing of the abiotic conditions. But in all temperature was identified as the most important factor influencing the timing of most events with germination of akinetes occurring at 13°C which is lower than so far reported in literature Subsequent increment in temperature to at least 23°C was found to be essential for the development of high populations of C. raciborskii in the water column. With a drop of temperature in autumn the filaments are stimulated in to producing akinetes which settle out into the sediment and remain there until the next spring when favourable conditions force them to germinate. They might also be consumed before germination by non identified grazer or losses due to microbial infections are discussed. Overall, the native Nostocales species of the genus Aphanizomenon have a selective advantage over the invaded species C. raciborskii under low temperatures and temperature fluctuations in spring as observed from the field and laboratory studies.
In order to lower the phytoplankton biomass in lakes it is theoretically most effective to reduce the nutrient that is actually limiting. However, it is widely assumed that the abundance and N₂-fixation rate of N₂-fixing cyanobacteria (Nostocales) would increase in response to reduced N loading, and thereby render efforts to improve water quality by N reduction ineffective. Nostocales N₂-fixation has a huge energy demand and consequently the light intensity may affect the response of Nostocales biovolume and N₂-fixation to varying N additions. This led to the following aims, for which three different sets of experiments were conducted.
(i) Determination of the seasonal dynamic of N- and P-limitation for four lakes of differing lake types in the German lowlands and testing the power of four N:P ratios to predict the limiting nutrient: Biweekly experiments were performed in 4 lakes differing in lake. For the shallow lakes there was a trend from P limitation in spring to N or light limitation in summer and autumn, while the deep lake remained predominantly P limited. The ability of in-lake N:P ratios to predict the relative strength of N vs. P limitation was tested using linear regression. All four N:P ratios had significant positive relationships with the response ratio to N and P additions, but the TN:TP and DIN:TP ratios performed best. Nitrogen limitation was predictable, frequent and persistent, suggesting that nitrogen reduction could play a role in water quality management.
(ii) Determination of the response of Nostocales biovolume and N₂-fixation to varying N additions: To achieve this, an experiment with water from an N limited lake (LAN) was conducted. While the Nostocales biovolume did not respond to varying N additions, the N₂-fixation increased in low N microcosms. To quantify the extent to which Nostocales compensated for the varying N addition rate, we calculated a compensation rate (CR). By the end of the experiment a CR of 36 % was reached. However, at biovolumes typical for summer in LAN the CR would be much lower. Therefore, in shallow polymictic lakes like LAN, reduced N loading may lower both in-lake N concentrations and biovolumes of non-fixing phytoplankton without significantly impacting Nostocales biovolume.
(iii) Determination of the effect of the light intensity on the response of Nostocales biovolume and N₂-fixation to varying N additions: To accomplish this aim another microcosm experiment with water from LAN was conducted. While at low and intermediate light intensities the reduction of N addition had no effect on Nostocales biovolume, at high light intensities it led to an increase. The N₂-fixation increased at all light intensities when N addition got reduced. This positive response to a reduction in N addition increased with light, showing that Nostocales may take advantage of being able to fix N₂ mainly at high light intensities.