Climate Related Impacts on a Lake - DiVA portal

Climate Related Impacts on a Lake

Thorsten Blenckner

From Physics to Biology

Dissertation for the Degree of Doctor of Philosophy in Limnology presented at UppsalaUniversity in 2001

Abstract

Blenckner, T. 2001: Climate Related Impacts on a Lake. From Physics to Biology. ActaUniversitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from theFaculty of Science and Technology 674. 37 pp. Uppsala. ISBN 91-554-5184-5

Climatic variation and change affect the dynamics of organisms and ecosystem processes.This thesis examines phytoplankton as a target variable to trace climatic impacts on LakeErken (Sweden) with special emphasis on the spring bloom.

A strong correlation between the timing of the spring bloom and the North AtlanticOscillation (NAO) illustrates the link between atmospheric pressure variations and localbiological processes. The predictive power increased by applying a recently establishedregional Scandinavian Circulation Index (SCI). Changes to an earlier timing of the springbloom and elevated water temperature were induced by the global warming trend. The climatesignal was still persistent in summer manifested by an enhanced summer phytoplanktonbiomass.

Between spring and summer, the phytoplankton was mainly controlled by phosphoruslimitation. The application of a new method to measure alkaline phosphatase activity revealedthat P-limitation varied between species and among individual cells.

Combining the above knowledge and literature data, the impact of the NAO on the timingof life history events, biomass and trophic cascade in aquatic and terrestrial ecosystems wasquantitatively tested with a meta-analysis. In all environments, pronounced effects of theNAO were apparent, indicating the generality of climate effects found in differentecosystems.

Finally, a regional climate model was applied, forcing a physical lake model from whichfuture lake conditions were simulated. The simulation revealed a one-month shorter ice coverperiod with two years out of ten being completely ice free. Internal eutrophication is one ofthe expected consequences.

In conclusion, the strong influences of global and regional climate are apparent in localphysical, chemical and biological variables and will most probably also in future affect thestructure and function of processes in lakes.

Key words: Ecosystem processes, climate, phytoplankton, NAO, SCI, ice cover, modeling.

Thorsten Blenckner, Erken Laboratory, Department of Limnology, Evolutionary BiologyCentre, Norr Malma 4200, SE – 76173 Norrtälje, Sweden. [email protected]

Thorsten Blenckner 2001

ISSN 1104-232XISBN 91-554-5184-5

Printed in Sweden by Uppsala University, Tryck & Medier, Uppsala 2001

“Climate plays an important part in determining the average numbers of a species,and periodical seasons of extreme cold or drought seem to be the most effective of all

checks. I estimated (chiefly from the greatly reduced numbers of nests in the spring)that the winter of 1854-5 destroyed four-fifths of the birds in my own grounds.”

CHARLES DARWIN – The Origin of Species

Preface

This thesis is based on the following papers, which will be referred to in the text by theirRoman numerals.

I Rengefors, K., Pettersson, K., Blenckner, T. and D. M. Anderson (2001). Species-specific alkaline phosphatase activity in freshwater spring phytoplankton: Applicationof a novel method. Journal of Plankton Research, 23, 435-443.

II Weyhenmeyer, G., Blenckner, T. and K. Pettersson (1999). Changes of the planktonspring outburst related to the North Atlantic Oscillation. Limnol. & Oceanogr., 44,1788-1792.

III Blenckner, T. and D. Chen. Comparison of the impact of regional and north-atlanticatmospheric circulation on an aquatic ecosystem. (submitted to Climatic Change).

IV Blenckner, T., Pettersson, K. and J. Padisak. Lake plankton as a tracer to discoverclimate signals. (in press by Verh. Internat. Verein. Limnol. Vol 28).

V Blenckner, T. and H. Hillebrand. North Atlantic Oscillation signatures in aquatic andterrestrial ecosystems. A meta-analysis. (in press by Global Change Biology).

VI Blenckner, T., Omstedt, A. and M. Rummukainen. A Swedish case study ofcontemporary and possible future consequences of climate change on a lakeecosystem. (submitted to Aquatic Science).

Special thanks go to the different publishers who gave the permission for reprints andpreprints (papers I, II, III and V).

Table of Contents

Introduction ....................................................................................................................................7

Aim of the study..............................................................................................................................8

Background.....................................................................................................................................9

Climatic and atmospheric indices ................................................................................................9

Climatic impacts on ecosystems.................................................................................................10

Climatic impacts on lakes ............................................................................................................................... 11

Study site .......................................................................................................................................12

Results ...........................................................................................................................................13

Phosphorus limitation (paper I).................................................................................................13

Climatic impact on the interannual variability of the spring bloom (papers II and III)............14

Winter climate impact on summer bloom (paper IV).................................................................16

NAO signatures in aquatic and terrestrial ecosystems (paper V)..............................................17

Potential impacts of future climate on a lake ecosystem (paper VI)..........................................18

General discussion .......................................................................................................................19

A conceptual frame.....................................................................................................................19

Outlook..........................................................................................................................................26

Summary in German (Deutsche Zusammenfassung) ...............................................................27

Acknowledgements.......................................................................................................................30

References .....................................................................................................................................31

Comprehensive Summary

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IntroductionThe global environmental changes that are apparent today are mainly anthropogenic due to thegrowing human population and its high activity of resource consumption (Vitousek 1994;Vitousek et al. 1997; Tilman & Lehman 2001). Nowadays, human activities are affecting thewhole earth, ranging from the smallest organism to the alteration of global biogeochemicalcycles, such as that of carbon. The most obvious ecological trends are the human-inducedchanges in biotic diversity and alterations to the structure and functioning of ecosystems(Vitousek et al. 1997). Ecosystem processes, such as productivity and nitrogen mineralizationrate, respond directly to human modification and climate (Vitousek 1994). Today, climaticchange is one of the most distinct changes discussed within the global change issues and,therefore, the impact on different ecosystems has been widely investigated using historical data,experiments and model simulations.

Scientifically, investigation of the impacts of climatic change and variation on ecosystems notonly implies collecting facts or observations. It is a process of identifying and testinggeneralizations or theories that go beyond observed facts and the need to explain those facts asinstances of a general pattern (Rigler & Peters 1995). In order to scientifically test the impacts ofclimatic change (or global change in general), several circumstances are necessary.

Climatic effects on lakes (and ecosystems in general) might only be detected when long-termdata (at least 20 years) are available, such as proxy data (sediment analyses, see for exampleSmol & Cumming 2000) or adequate and long-term monitoring. In the optimal case, long-termdata are gathered with the same sampling and analytical methods. Practically, that seldom is thecase and this has an influence on the quality of the data. Another possible problem dealing withlong-term data is that the human impact might not be constant over time. For example, in the past20-30 years or so, many lakes, in particular in Europe and the US, have been subjected to ananthropogenically-caused increase in the supply of nutrients. This supply decreased again in thelate 1980s or early 1990s, as a result of improved wastewater treatment. Also otheranthropogenically-induced changes (e.g. catchment alteration) might complicate the separation ofthe influence on such long-term data sets.

Human influences might be excluded from the data sets by different statistical methods, suchas detrending, but the transformation of the data should be kept in mind. However, if the testsshow that a target variable is influenced by climatic change, the study should be extended to awider perspective (e.g. other target variables) and by experiments and/or models. Only theinclusions of the latter can help us find general processes in ecosystems. From that perspective,global change issues, such as climatic change, provide a challenge to every scientist, becauseimpact mechanisms and effects derived from global change studies, have to be understood inorder to manage or prevent the impacts in future. Thus, the different responses of ecosystems willprovide more insights in general ecosystem processes if a careful ecological test is performed. Inmy thesis, phytoplankton was the target variable used to test the impacts of climatic change andvariability on a lake ecosystem. This organism group is well-studied and has a relatively fastresponse time, which makes it a very suitable “test-organism” in order to study the effects ofclimatic change and variability.

As Reynolds (1997) stated, phytoplankton is an assemblage of photoautotrophicmicroorganisms which live entirely, during the vegetative stages of their life cycles, in openwaters, or the pelagic zone of the sea, of lakes, ponds and rivers. This group is mainly controlledby light, nutrients, water temperature, turbulence, trophic interactions (grazing) and viruses andparasites (Harris 1986). Climate directly affects light, turbulence and water temperature and


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influences the phytoplankton via these factors. Indirect effects are, for example, grazing; ifzooplankton biomass is enhanced in warmer water this will lead to a reduction in thephytoplankton biomass.

Aim of the studyThe main objective of this study was to investigate the effects of climatic change and variabilityon lake ecosystems, in particular phytoplankton blooms (see also Fig. 1). In order to separateclimatic effects from other effects, the influence of nutrients, especially phosphorus, on thephytoplankton bloom was analyzed (paper I). Atmospheric indices, the North Atlantic Oscillation(NAO) and the Scandinavian Circulation Index (SCI) were related to the phytoplankton springbloom in order to distinguish the impact of climatic variability and change (papers II and III).Furthermore, an analysis was made whether the influence of the winter climate variability wasstill persistent in the summer phytoplankton bloom (paper IV). In order to classify the effects ofthe NAO on the phytoplankton, the impact of the NAO on life history events, biomass andcascading effects in aquatic and terrestrial ecosystems was statistically compared (paper V).Finally, by using knowledge of the impact of the historical climate on the lake system, thepotential influence of a future climate was projected with a model approach (paper VI).

Fig. 1: Principle overview of the papers included in the thesis


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BackgroundClimatic and atmospheric indicesThe terms `climate´ and `weather´ are all to frequently misused and therefore a definition is given(M. Rummukainen, pers. comm.):Weather: The state of the atmosphere at a given time and place, described by specification ofvariables such as temperature, moisture, wind velocity and barometric pressure.Climate: The average statistics (mean, variability, even extremes) of the meteorologicalconditions, including temperature, precipitation and wind, that characteristically prevail in aparticular region.

Climate has always been changing or, more precisely, been varying, independently of the timescale of the observer, including geological time scales (Bennett 1997; Bluemle et al. 2001). Itshould be noted that we can, and should, separate between climatic change, which is the change(a trend) of the climate over a longer time period, at least 20 years, and climatic variability, whichdescribes the variation of the climate, for example year-to-year oscillations. Climatic change andclimatic variability do not exclude each other, because a long-term warming trend is still possibleand observed (i.e. from the 1960s until now) combined with year-to-year variations or quasi-cyclical 11-years sun spot cycles.

Natural changes and variability in the climatic system are due to solar forcing, volcanoeruptions, orbital variations of the earth (Milankovitch cycles) and the interaction betweengeosphere/biosphere with the atmosphere, if we can consider them to be separated (see the Gaiahypothesis by Lovelock 1979). All these natural forces are operating on different time scales,which makes it difficult to distinguish between different forces when studying the causes ofclimatic change and variability. This illustrates also the complexity of the climate system,addressing the sometimes high uncertainty levels in climate models (Allan et al. 2000).

Anthropogenically-caused changes and the debate about them are not a recent phenomenon.For example, Theophratus (4th century BC) speculated about an impact of climatic change onhumans (climatic determinism, see Stehr & Storch 1999). The understanding of past climaticchange was focused on man-made deforestation. The Scottish philosopher David Hume (1711-1776) speculated that climatic warming would be caused by human deforestation. The Swedishresearcher Arrhenius (1896; see also AMBIO vol. 26 (1), 1997) was the first to calculate howincreased concentrations of carbon dioxide in the atmosphere might affect the air temperature.These few examples from the history of science illustrate that climatic change is not just amodern issue.

Recently, the Third Assessment Report from the Intergovernmental Panel of Climate Change(IPCC 2001) pointed out that the global average surface air temperature increased over the 20th

century by about 0.6 C ( 0.2 C). As the report illustrates, the warming trend is mainlyanthropogenic caused by an increase in the emission of greenhouse gases – in contrast to the pastclimate view (see above). Moreover, the patterns of warming are not constant over time (most ofthe warming occurred between 1910 to 1945 and 1976 to 2000) associated with a high variabilityover the whole period, but consistent with the patterns predicted with global circulation models(GCMs) (IPCC 2001). The 1990s were the warmest decade in the instrumental record, since1861. Furthermore, new analyses of proxy data from the Northern Hemisphere indicate that theair temperature in the 20th century was the warmest for the past thousand years (IPCC 2001)(Jones et al. 2001), whereby the uncertainty of the proxy data is high.

However, the recent global climatic change is an average warming which is not constant overtime and space, since some regions (partly Southern Hemisphere and Antarctica) have not


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become warmer (IPCC 2001). On the other hand, the annual Swedish air temperature, forexample, increased by 0.68 C from 1861 to 1994, the largest increase being 1.4 C in spring(Moberg & Alexandersson 1997). However, these are trends over the last 1000 years (see, forexample, IPCC 2001) which, from the geological time perspective, is a very short period andtherefore climatic warming cannot in principal be tested when considering geological time scales.As Schindler (1996) pointed out, environmental researchers should focus on the effects of climateon the ecosystem to improve our understanding of it, regardless whether the change hasanthropogenic or natural causes, because the effects on the ecosystems are likely to be similar.

Climatologists also established several ways of characterizing regional-scale changes in theglobal climate. Some are based on synoptic analysis of daily weather maps, like the Lamb system(Lamb 1950). Another uses zonal indices that describe particular features of the atmosphericcirculation. The most well-known zonal index is the El-Ni o-Southern Oscillation (ENSO)located in the tropical Pacific and generating impacts in both aquatic and terrestrial environmentsover a large part of the globe (Allan et al. 1996; Jaksic 2001). In the North Atlantic, anotheratmospheric system exists, which to a substantial portion is associated with the climaticvariability over the Northern Hemisphere: the North Atlantic Oscillation, NAO (Hurrell 1995;Hurrell 1996; Hurrell & Loon 1997; Hurrell et al. 2001). The NAO refers to the meridionaloscillation in atmospheric mass with centers of action near Iceland and over the subtropicalAtlantic. The positive phase is combined with warm weather over Europe, as well as wetconditions from Iceland through Scandinavia and dry conditions over southern Europe. Thenegative phase is associated with cold weather over Europe. This atmospheric circulation pattern,which has been recognized at least since Walker and Bliss (1932), is most pronounced during thewinter period (Hurrell 1995; Hurrell 1996).

Chen and Hellström (1999) found that the NAO has an important effect on the Swedishtemperature in winter and, therefore, forces exerted on ecosystems can be expected. Aremarkable feature of the NAO is its trend towards a more positive phase over the past 30 years(Stockton & Glueck 1999). As the NAO is a natural phenomenon, it might be possible thatanthropogenic climatic change influences the NAO towards a predominating positive phase(Corti et al. 1999).

Recently, another atmospheric circulation index was established, the Scandinavian CirculationIndex (SCI) (Chen 2000). The SCI is based on monthly pressure differences on grid-point dataover Scandinavia and has been found to explain 70% of the total variance in the January airtemperature in Sweden (for further details, see Chen 2000).

In general, zonal indices are very useful for ecosystem research investigating climatic effectsbecause they integrate the different climate variables (such as air temperature, precipitation,cloud cover) and describe year-to-year variability of the regional climate. In particular, the NAOmay be seen as a proxy for regulating forces in aquatic and terrestrial ecosystems over a largepart of the Northern Hemisphere (Ottersen et al. 2001).

Climatic impacts on ecosystemsIt is reasonable to assume that climatic change and variability will affect species and theirinteractions at different trophic levels, but effects may vary regionally and with species. Theresponses to climate are dependent on the seasonal timing and magnitude of warming, as well asthe sensitivity of life-history forms present in different seasons (Chen & Folt 1996). However, the


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direct potential effects of climate on organisms can be broadly summarized into four categories(modified from Hughes 2000):

Effects on the physiology: climate variables directly affect the metabolic and growth rates oforganismsEffects on distributions: species moving up or downwards in latitude or elevation in responseto shifting climate zonesEffects on phenology: life cycle events triggered by climate, such as degree daysAdaptations: species with short generation times might undergo microevolutionary change insitu.

As a consequence, changes in species interactions (e.g. trophic interactions) associated with analtering community structure and composition have to be expected. All these interactions andeffects operate on different time scales. Responses to the recent warming trend such as upwardmovements of alpine-nival floras (Gottfried et al. 1999), earlier breeding by amphibians and birds(Forchhammer et al. 1998), northward range changes in butterflies (Parmesan et al. 1999),increased photosynthesis (Myneni et al. 1997) and changes in the community composition ofgrass (Alward et al. 1999) have been found.

Additionally, also responses caused by year-to-year variations in climate have been found formany organisms by applying zonal indices (Allan et al. 1996; Beamish et al. 1999; see also theNAO review from Ottersen et al. 2001). However, most of the responses apparent so far are thoseof individual species (Hughes 2000).

Climatic impacts on lakesClimatic changeOnly few lakes over the world are being monitored on a long-term basis. One of the bestexamples is the Experimental Lake Area (ELA) in Canada, where a reliable monitoring started inthe late 1960s (Schindler 1996). In the ELA the climate warmed by about 1.6 C (annual averageair temperature) since 1960, associated with a general tendency for lake temperature to becomewarmer (De Stasio et al. 1996; Schindler et al. 1996). The warming is also associated withshorter ice cover periods in the ELA regions (Schindler 1996) and even over the NorthernHemisphere (Magnuson et al. 2000). Additionally, the increased temperature led to changes inthe stratification pattern. In large, dimictic lakes, stratification might be expected to be strongerand shallower (De Stasio et al. 1996) or weaker (Schindler et al. 1996), depending on the color ofdissolved organic carbon and the lake size together with its morphometry (King et al. 1999).

Furthermore, a prolongation of the water renewal time has been found, which is likely to havecritical effects on eutrophication, on the retention of nitrate (Schindler 1996, and lit. therein) andincreases in the relevance of internal processes. These physical and partly chemical changesinduced by a warmer climate will affect the organisms and their interactions drastically (see alsoclimatic impacts on ecosystems) but the responses might differ between lakes. For example, inthe ELA lakes, a decline of chlorophyll was observed (Schindler et al. 1996), whereas in Castlelake, the climatic warming caused increases in both phytoplankton production and standing crop(Bryon & Goldman 1990). Chen and Folt (1996) found that fall warming could trigger restingstages and the occurrence of sexual or asexual reproduction of zooplankton species. The


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production of resting stages of different species might also be decisive for which species surviveundesirable environment periods (Hansson 1996).

Climatic variabilityNot only a general warming trend, but also year-to-year variations in the climate have a profoundeffect on the physical conditions, phytoplankton and zooplankton dynamics (Harris 1986; Catalan& Fee 1994; Adrian & Deneke 1996; Adrian et al. 1999). Year-to-year variations in climate arepartly related to the atmospheric circulation and become detectable by applying indices as climateproxy. For example, ENSO events can influence the dynamics of lakes around the Pacific Ocean(Anderson et al. 1996; Schindler et al. 1996). Around the Atlantic region, the NAO and themovements of the Gulf Stream have been documented to influence the year-to-year variations ofdifferent lake variables (George & Taylor 1995; Gerten & Adrian 2000; Straile 2000; Straile &Adrian 2000; Gerten & Adrian 2001). The year-to-year variations affected physical variables aswell as organisms at different trophic levels.

Study siteLake Erken (Fig. 2) is situated in Eastern Sweden (59º25´N, 18º15´E) at 11 m above sea levelwith a surface area of 24 km2, a maximum depth of 21 m, mean depth of 9 m and a turnover timeof 7 years (Weyhenmeyer 1999). The lake is always ice-covered in winter and the ice break-up,registered since 1954, occurs between March and the beginning of May. The lake is meso-eutrophic with an annual mean for total phosphorus of 27 µg l-1, total nitrogen of 657 µg l-1 and ayearly mean Chl a value of 5.7 µg l-1. Water samples have been taken since 1954 with moreintensive sampling both in the 1970s and from 1993 onwards. Thus, at least 20 years of data fromLake Erken are available. The latter and the fact that the lake has never undergone any obviousanthropogenic eutrophication, makes the lake very suitable for studies of climate related impacts.

Fig. 2: A map from Lake Erken, Sweden.

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Results

Phosphorus limitation (paper I)Phosphorus deficiency of phytoplankton cells can be measured with different approaches(Beardall et al. 2001). One is to analyse the alkaline phosphatase activity (APA) of a whole watersample by performing an ordinary bioassay, because algal cells synthesize the enzymephosphatase only at very low orthophosphate concentrations (see, for example, Pettersson 1980).The main limitation of these APA measurements is the difficulty to detect P limitation on theindividual species level. Therefore, we focused on the application of a new method (enzymelabeled fluorescence, ELF) to analyse alkaline phosphatase activity (APA) in a natural springphytoplankton community. By directly labelling the enzyme in the cells, the APA can be tracedunder the microscope and therefore P-limitation can be detected on a single cell level.

We could show that the method is appropriate for detection of APA activity in single markedcells. However, the values found were partly lower than those being documented with theconventional method using a whole water sample. One reason might be the considerabledestruction rate of flagellates which otherwise contribute to a substantial proportion of the APAactivity measured conventionally.

Fig. 3: Percentage of algalpopulations with ELF activity, asobserved in different phytoplanktonspecies/genera, at different datesduring the spring season (April 4 toJune 3, 1998) in Lake Erken. PanelA shows four diatom species andpanel B shows the two dinoflagellategenera with ELF activity.


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In spite of P-concentrations being below the detection limit (<5 µg P l-1), there were still someindividuals within one species present which did not show any APA, presumably due to differentindividual P-requirements, physiological status (cell cycle, etc.) or differences in enzymeactivation. Moreover, variations in APA over time were found both on the intra- and interspecificlevel (Fig. 3).

However, we could clearly state that there is a P-limitation after the spring bloom period. Thisstudy further underlines the complex nutrient situation within a spring phytoplankton communityas it cannot exactly be stated which species, or even which individual is P-limited at any exacttime. A wide range of nutrient requirements has to be taken into consideration when studyingimpact mechanisms on phytoplankton growth.

Climatic impact on the interannual variability of the spring bloom (papers II and III)Interannual variability of European climate is substantially influenced by the North AtlanticOscillation (NAO), with the most pronounced effects during winter (Hurrell 1995). We foundthat the winter NAO index was significantly correlated to the local winter air temperature and thetiming of the ice break-up in Lake Erken. The timing of the spring bloom was related to theMarch values of the NAO index and air temperature (Fig. 4), whereas a less significantcorrelation could be achieved for the biomass. This implies that the climate in March determinesthe light conditions below the ice, with the ice break-up leading to an enormous increase in lightavailability counteracted by mixing. Therefore, the interaction between the climate in March andthe timing of the ice break-up determines the timing and composition of the bloom and itsbiomass. In contrast, the duration of the bloom and the post-bloom period depends on nutrientavailability, indicated by the P-limitation of the phytoplankton (see paper I).

The large variation in the proportion of diatoms, ranging from 20 to 98 %, was dependent onthe presence of ice and significantly correlated to the winter NAO index. In 1979, a year with avery low NAO index, for example, the peak occurred below the ice and was dominated bydinoflagellates, which are favored by a long and clear (no snow) ice-cover, resulting in sufficientlight just below the ice and low turbulence.

Paper III focused on the comparison of the impact of two atmospheric circulation indices, theNAO and the SCI. For the first time, the SCI was applied in a limnological study to analyze theimpact on the ice cover period, the timing of ice break-up and the spring bloom. In comparisonwith the NAO index, it could be shown that the SCI explains the interannual variation of thetiming of ice break-up and the spring bloom with a higher explanatory power (Fig. 4). TheScandinavian climate index (SCI), consequently, is another very suitable integrative climateparameter, especially at a higher spatial resolution, when studying climate-driven responses oflake ecosystems in Scandinavia.

Additionally to the year-to-year variation in climate, an upward trend of the NAO index inMarch was observed (paper II), causing a one-month earlier phytoplankton spring bloom todaythan 45 years ago, with a probable continuation in future. In Lake Erken, the ice break-upfollowed the same trend (Fig. 5), and without any change in the onset of stratification, theisothermal spring mixing period is prolonged. This caused also a prolonged period of P-limitation, which might influence the phytoplankton succession in the direction that low P-adapted species are favored (paper I). Thus, the phytoplankton community during this period isdominated by partly mixotrophic flagellates, increasing the trophic efficiency of the ecosystem.In that way, climatic change can visibly influence the ecosystem function.


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Air temperature March

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Fig. 4: The relationship between the timing of the spring bloom and a) the airtemperature in March, b) the regional circulation March index (SCI), c) the NorthAtlantic Oscillation (NAO) March index.


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Winter climate impact on summer bloom (paper IV)The winter climate strongly influenced the timing of the ice break-up and the springphytoplankton in Lake Erken (papers II and III). In a German lake, it has been found that thewinter conditions can affect also blooms in summer (Guess et al. 2000). Therefore, we checked ifthe phytoplankton biomass in summer was affected by the previous winter conditions. Indeed, astrong relationship was found between the summer biomass and the air temperature in winter, thetiming of the phytoplankton spring bloom, as well as the water temperature in May. The linkbetween the winter climate and the summer biomass in Lake Erken is hypothesized as follows:The earlier spring bloom and the warmer water temperature in May after a warm winter caused alonger time-period for mineralization, an increase in bacterial activity and a higher nutrientrelease from the surficial sediment, thereby causing an increase in both phosphorus availabilityand the summer phytoplankton biomass. Additionally, the prolongation of the mixing period dueto an earlier ice break-up without changes in the onset of the stratification leads a mineralizationperiod that was up to one month longer in the 1990s in Lake Erken. Conclusively, it seems thatthe effects of winter climate variability and change are still persistent in summer phytoplankton.

YEAR

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Fig. 5: The upward trend line of the NAO of March (NAOM) since 1950 and the trendline of the timing of the phytoplankton spring peak in Lake Erken.


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NAO signatures in aquatic and terrestrial ecosystems (paper V)Recently, many studies have focused on climatic change and variability and the impacts onecosystems. Since the 1990s, an expanded body of work has detected influences of the NAO onorganisms in lakes (Livingstone 1999; George 2000b, paper II; Gerten & Adrian 2000; Straile2000; Gerten & Adrian 2001), marine systems (Fromentin & Planque 1996; Kröncke et al. 1998;Reid et al. 1998; Beamish et al. 1999; Belgrano et al. 1999; Irigoien et al. 2000; Ottersen &Loeng 2000) and terrestrial systems (Post & Stenseth 1998; Post et al. 1999; Post & Stenseth1999; Stenseth et al. 1999; Przybylo et al. 2000; Mysterud et al. 2001). So far, the generalinfluence of the NAO on different forcing mechanisms has been summarized by Ottersen et al.(2001), but a statistical synthesis of the different studies has yet to be prepared. In our study, weapplied a meta-analysis to quantitatively analyze the influence of the winter NAO on the timingof life history events, biomass and the cascading effects of different organisms in aquatic andterrestrial ecosystems. In all environments, the timing of life history events was earlier alwaysassociated with a positive NAO (Fig. 6). But a less pronounced effect was found in higherlatitudes. In contrast, the biomass of the organisms in the primary study responded positively inaquatic systems and negatively in terrestrial systems (Fig. 6), and the response was generally lesspronounced in Eastern Europe. No significant cascading effect could be found, but a slightdeclining trend in the response to the NAO from physical to the herbivore level was apparent(Fig. 7). These results indicate that a meta-analysis is a very useful tool when seeking for generalecological patterns. Furthermore, from these results, we recommend an inclusion of non-significant results in publications in order to obtain a more objective view of the strength of NAOand climatic impacts on species in general.

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Fig. 6: Group effect size (E+) and theconfidence intervals (95 %) of the aquatic andterrestrial environment on the timing of historyevents and biomass of the organisms. Apositive group effect size implies that thecorrelation between the NAO winter index andthe target variable was positive.

Fig. 7: Group effect size (E+) and theconfidence intervals (95 %) of the effect (fromthe meta-analysis) on the aquatic environmentincluding the physical (ice cover, watertemperature), phytoplankton and zooplanktonlevel. A positive group effect size implies thatthe correlation between the NAO winter indexand the target variable was positive.


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Potential impacts of future climate on a lake ecosystem (paper VI)As projected by global climate change models, the climate will continue to warm (IPCC 2001),which has the potential for more serious climate-driven effects in lakes than today. In paper VI,we applied a regional climate model with a horizontal resolution of 44 km for Lake Erken. Theclimate simulations were used to force a physical lake model in the contemporary and possiblefuture physical lake conditions (2*CO2) and to discuss climate-driven ecological consequences ofsuch influences. First, we compared the modelled data with the observation and it appeared thatthe lake was adequately modelled. The future scenario simulations of the climate model, whichforced the lake model, resulted in an elevated water temperature and changes in the mixingregime. Furthermore, the projection for the 2*CO2 scenario resulted in a one month shorter icecover by averaging a 10-year period combined with two of ten years being totally ice-free (Fig.8). This projection would induce several ecological consequences which were discussed byapplying the previous gathered knowledge of the climate-driven effects in Lake Erken (see papersII-IV). For example, the shorter ice period will influence the timing of the spring phytoplanktonbloom and the ice-free years might change the composition from small towards large diatomspecies. The warmer water temperature and changes in the mixing regime would induce changesin the nutrient turnover. We believe that a warmer climate will be associated with an internaleutrophication in Lake Erken, because a higher water temperature and a longer growing seasonwill increase nutrient availability and primary production. We conclude that a continuation of awarmer climate in future might especially cause an internal eutrophication in lakes.

RCA scenario time slice (year)

Perio

d (d

ays)

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9

obs. mean

calc. mean

Fig. 8: Annual maximum ice cover period of the scenario run (2*CO2), the calculated averagemean from simulation year 1 to 9 (calc. mean) in comparison to the observation average from1974-1998 (obs. mean).


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General discussionThe results suggest a strong impact of climatic variability and change on the lake ecosystem, inparticular on the spring phytoplankton bloom. The climatic variability influenced the timing ofthe spring bloom, the composition and the maximum biomass, using the NAO and the SCI as aclimate proxy (papers II and III). Following the bloom period, the phytoplankton was P-limitedand revealed a pronounced variability with regard to both P-requirements within one species andamong different species (paper I). Furthermore, the winter climate variations were still persistentin the summer phytoplankton biomass (paper IV), where a warm winter was associated with anincrease in the summer phytoplankton biomass. Moreover, the meta-analysis of the NAOrelationship with the target variables illustrated that indeed the timing of life history events, thebiomass of organisms and the trophic cascade showed a pronounced NAO signature in aquaticand terrestrial ecosystems (paper V). The ongoing climatic variability and change, as shown bythe climate model, will also have a strong influence on the lake ecosystem in future and mightcause an internal eutrophication. In particular, totally ice-free years will enforce drastically theclimate-driven interannual variability in lakes (paper VI).

A conceptual frameIn the following, the results of this thesis, together with several other studies, will be integratedinto a conceptual frame of the impacts of climatic variation and change on lake ecosystemprocesses. This conceptual frame is proposed to visualize climate-driven responses of physical,chemical and biological variables and their interaction in a wider context (Fig. 9). Due to theenormous body of work that has been done in the field of climate impacts, this overview willalways remain incomplete. However, this frame will help to classify the effects found in thisthesis. The main focus will be to synthesize the different results and achieve a betterunderstanding of the processes behind the responses. Additionally, this frame could provide anoverview of missing links and hence provide possibilities for a more holistically based research.

Effect filterBasically, lakes situated in the same geographic region frequently show synchronous patterns ofinter-annual variability (Baines et al. 2000; George et al. 2000). However, the spatial extent overwhich lakes vary coherently is poorly understood (Benson et al. 2000). In general, the strongestsynchronous behavior is found in the coherence of physical variables, e. g. water temperature,compared with a less pronounced coherence of biological variables. The response of climaticvariation seems likely to be modified by lake features such as morphometry and water clarity(Fee et al. 1996). Here, also other modifying features will be added and integrated into theconceptual frame. As all these features modify the climatic effect on each individual lake, theterm effect filter is introduced. The term comprises the following components: catchmentcharacteristics, lake morphometry, lake history and the geographic position. The term isintroduced because I hypothesize first, that the effect filter components modify the climaticresponse in lakes, and second, that lakes with a similar effect filter respond similarly to climate.These effect filter components and their liability to climatic variation will be discussed below.

The catchment of most lakes is subjected to a variety of human influences. The nutrient statusof the lake, influencing the phytoplankton, is mainly determined by the surrounding catchment as


20

lakes respond in a predictable way to changes in the catchment (Vollenweider 1975). Thus, thecatchment characteristics determine the amount and timing of the supply of nutrients and othersubstances (e.g. dissolved organic carbon, DOC). The DOC concentration, released from thesurrounding catchment, influences the water clarity in lakes and, therefore, the epilimnion depth(Fee et al. 1996). Also, the timing of the supply of inorganic and organic matter is veryimportant with regard to the different catchment characteristics. In a future climate, an increase inthe extremes, especially in the intensity of precipitation and temperature, is projected (Easterlinget al. 2000), which will change the timing and amount of run-off into the lake. Also, the land-usedistribution in combination with soil type and plant composition strongly influences the releaseof matter into the lake (see, for example, Prepas et al. 2001).

The morphometry of lakes influences various hydrodynamic lake patterns. The depth of thelake mainly controls the length of the persistency of the NAO signature, for instance in watertemperature. In shallow lakes, the effects of winter climate on plankton are short-lived, and aresoon overtaken by the prevailing weather and by biotic interactions (Adrian et al. 1999). On thecontrary, in deep lakes, the winter climate signal can persist until late summer (Gerten & Adrian2001). The size of a lake determines also the stratification period, in particular the thermoclinedepth. In a warmer climate, the depth of the thermocline was deeper in shallow lakes (Schindleret al. 1990; Schindler 1996; Schindler et al. 1996) and King et al. (1999) observed a shallowerthermocline in larger lakes. Furthermore, the morphometry determines the retention time withinternal (long retention time) or external (short retention time) processes being dominant in theparticular lake. Changes in rainfall are particularly relevant in lakes with a short retention time(Talling 1993).

The history of the lake might also be an important factor for the magnitude of the response toclimatic variation. Lakes, that were eutrophic in the 80s and 90s – as many lakes in Europe –have now been treated and finally improved in water quality. However, the release of phosphorusstored in the sediments often delays recovery owing the internal nutrient loading (Ahlgren 1977).Thus, the history, here represented by nutrients, is still stored in the sediment of that specific lake.So, I suggest that lakes with a different history might respond differently to climatic change andvariability. A possible increase in water temperature in a warmer climate might enhance thebacterial activity as well as the decrease in the oxygen content in the hypolimnion. Therefore, aformer eutrophic lake might release nutrients from the sediment in a much higher concentrationunder those conditions than a lake that has not undergone eutrophication. In general, lakes thatare in a recovery phase from earlier eutrophication, acidification, toxic components or any otherstrong human disturbance, might respond differently to climatic variability and change due totheir different history. The geographic position of the lake is also important for the response to climate (Livingstone& Dokulil 2001; paper V). This is basically due to the fact that climatic variations are not equallypronounced worldwide but depend on the geographic position. For example, the climaticwarming of the last 200 years was strongest in northern latitudes (Giorgi et al. 2001) comparedwith other regions. A lake situated in Scandinavia might be influenced much more strongly bythe climatic warming than a lake in the tropics. Processes in the catchment are also subjected totheir altitude. Mountain lake catchments are differently affected compared with those at loweraltitudes.


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Effect FilterFilter ComponentsCatchmentMorphologyHistoryGeographical position

ClimateClimateGlobal (NAO)Regional (SCI)Local (air temperature)

Temperature, Ice Cover

Nutrients

Foodweb Components and

Interactions: e.g. Phytoplankton, Zooplankton, Fish,

Benthos

physical

chemical

biological

Outflow

Fig. 9: The conceptual frame constructed from literature findings and results from this thesis.


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Climate related effects on lake ecosystem processesAssuming that climatic signals are differently mediated by the effect filter, I will now unite thedifferent climate-driven responses found in the literature and in my results.

Climatic variation and change directly affect the physical variables. A long-term trendtowards shorter periods of ice cover due to a later freezing and an earlier break-up has beenreported for lakes around the Northern Hemisphere (Kuusisto 1987; Assel & Robertson 1995;Livingstone 1997; Magnuson et al. 2000). Similar effects have been found for Lake Erken(papers II and III). Additionally, year-to-year variability in ice break-up dates in Europe could berelated to climatic (NAO) variation (Gerten & Adrian 2000, paper II) and the NAO signal wasstill persistent in the ice characteristic in the world largest (by volume) Lake Baikal (Livingstone1999). The trend in an earlier ice-out increases the ice-free period and lake temperature in springin Canadian (Schindler et al. 1990) and European lakes (Gerten & Adrian 2000; Straile 2000,papers II and III). A further increase in climatic warming could imply that usually dimictic lakesbecome warm monomictic, i.e. circulation through the winter (Schindler 1996, paper VI). Thelength of the period for how long the climate signal can be found depends on the morphometry(see effect filter). The year-to-year winter NAO effects were still persistent in a deep dimicticlake until the following winter, whereas a shallow dimictic lake revealed an intermediateresponse, as weather conditions both in April and midsummer probably modified the strength andpersistence of the NAO signal in the hypolimnion in that lake (Gerten & Adrian 2001).Livingstone & Dokulil (2001) found that lake temperatures in Central European lakes (Austria)from autumn to spring were related to the dominance of large-scale processes over the North-Atlantic, i.e. the NAO. Also a faster temperature rise in the spring (due to earlier ice out dates)preceded the beginning of the spring stratification, which may influence the nutrient cycling(Schindler et al. 1990; Abgeti & Smol 1995). In contrast, no change in the onset of thestratification could be observed in Lake Erken after earlier ice-out dates (papers II and IV),probably due to the fact that the lake is highly wind-exposed. However, a continuation of climaticwarming might shift the onset of stratification in the near future (see discussion in paper VI).Longer periods of summer stratification, as also found in paper VI, are also predicted to causeincreased hypolimnetic anoxia, or at least lower oxygen concentrations (Magnuson et al. 1997),which can enhance the nutrient release from the sediment (Pettersson & Grust 2001).

Considering chemical variables, especially nutrients have been of main interest when studyingclimatic impact on lakes. An increase in the retention time (as mentioned above) in a warmerclimate due to higher evaporation and decreased stream outflow, will also increase the retentionof chemical constituents, in particular the nutrient cycle. Water renewal times have been shownto have a critical effect on eutrophication (Dillon 1975; Vollenweider 1975). Also, year-to-yearvariation in winter air temperature has been found to influence the nitrate concentration in LakeWindermere, UK, as warm winters lead to a reduced nitrate concentration in the water (George2000a). Furthermore, a negative relationship between the nitrate concentration in March in astream in UK and the NAO has been found (Monteith et al. 2000). The process behind this is notyet clear, but it might be linked to the length of the time the soil profile remains frozen during thewinter, which clearly shows that catchment leaching (here nitrate) is related to climaticvariability. In terms of phosphorus, interannual variations in the ice cover period (induced byENSO events) influenced the P concentration, for example in the Great Lakes (US), as a shorterice-cover period increased the resuspension of P from the sediment due to a longer mixing period(Nicholls 1998). In contrast, we suggest that the earlier timing of ice break-up in Lake Erkenprolonged the P-limitation period for phytoplankton during the mixing period, but increased the


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nutrient availability in summer due to an enhanced bacterial activity at warmer watertemperatures in combination with the prolonged mineralization period (papers II and IV). Thelonger P-limitation period will favor those species having the potential for an increased APA orutilization of mechanisms for efficient phosphate storage (paper I). In general, the nutrientturnover might be enhanced in a warmer climate (Hamilton et al. 2001), leading to an internaleutrophication, as suggested for Lake Erken (paper VI).

Biological variables and their response to climate will be discussed with emphasis onphytoplankton, zooplankton, benthic algae and fish.

Considering phytoplankton, already Lund (1950) and Talling (1971) detailed the influence ofstratification and spring rains on the timing and magnitude of spring diatom blooms. The timingof the spring bloom is mainly dependent on light availability and turbulence, two factorsinfluenced by climatic variation and change. Therefore, strong relationships between the timingand the winter climate (and also NAO and SCI) were found in European lakes (Adrian et al.1995; Müller-Navarra et al. 1997, papers II and III; Gerten & Adrian 2000). Differences in themain trigger factor depend on the morphometry of the lake (see also effect filter). For example, inLake Constance, a large and deep lake mainly without ice cover, the spring bloom only occursunder stratified conditions, depending on short-term weather conditions, because the reducedmixing increases the light availability (Gaedke et al. 1998). In shallower lakes with ice cover, themixing depth is lower due to the morphometry of the lake and the timing of the spring bloom ismainly triggered by light availability, controlled by the ice characteristics and the snow on the iceand therefore associated with the winter climate and NAO as observed for Lake Müggelsee,Germany (Gerten & Adrian 2000) and Lake Erken (paper II).

The magnitude of the bloom and its relation to climate may have different causes. The loss ofice cover or less snow on the ice might change and/or increase the algae population during winter(Pettersson 1990; Adrian et al. 1995). Thus, the nutrient availability might be lower for the actualspring bloom, leading to a reduced algae peak (Pettersson 1990; Müller-Navarra et al. 1997).However, no relationship between the NAO and the biomass was found in Lake Constance,probably due to the dampening of the biomass by grazers (Straile 2000).

A change of the phytoplankton composition in the pre-bloom period due to differentenvironmental conditions, e.g. different ice pattern, might also change the nutrient availabilityand ratio caused by a different uptake and storage of nutrients by different species. The lattermight also alter the composition of algae at the bloom period, because different temperatureoptima of phytoplankton have an impact on the outcome of competition (Tilman et al. 1986). Interms of climatic change, a consecutive period of 5 mild winters led to a complete change of thespring phytoplankton bloom, from a dominance of diatoms and cryptophytes to a dominance ofcyanobacteria in a German lake (Adrian et al. 1995). This illustrates that the response ofphytoplankton, and probably also other lake biota, to climatic variation might be totally differentto the response of a warming trend (climatic change) in order that ecosystem processes are non-linear. In a warmer climate with warmer winters the composition of phytoplankton might betotally different, as observed today by considering only one extreme mild winter.

Additionally to the direct (in terms of no time lag) response, also the outbreak of blooms in thesummer period can be influenced by the warmer winter period (Hallegraeff 1993; Guess et al.2000). Similar results have been found in Lake Erken (see paper IV). A warm winter affects thewater temperature and the timing of the spring bloom, resulting in an indirect increase of thesummer phytoplankton biomass, caused by an internal eutrophication. Also the overwinteringsuccess of resting stages, for example of Gloeotrichia echinulata, a dominant summer bloomingspecies in Lake Erken (Pettersson et al. 1993; Tymowski & Duthie 2000), might be strongly


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dependent. However, not only the winter conditions influence the summer biomass, also thesummer climate directly influences the water temperature in summer. For example, in CastleLake the summer primary productivity and the standing crop were enhanced due to a warmertemperature, showed by a long-term study (Bryon & Goldman 1990). In addition, longer periodsof stratification can promote a dominance of potentially toxic cyanobacteria (George & Harris1985; George et al. 1990; Hyenstrand et al. 1998).

The microcrustacean zooplankton found in lakes can tolerate quite high summer temperatures,but small increases in the winter temperature may have significant effects on their seasonaldynamics (George & Hewitt 1999). Year-to-year variations in the winter climate (by applying theNAO) strongly affected the overwintering of zooplankton species such as Eudiaptomus, causedby the strong temperature dependence on the growth rate (Lampert & Muck 1985). Also Daphniabiomass responded to the variability of ice break-up (Jassby et al. 1990), probably due to thewater temperature variability, which is strongly related to the variability of ice break-up. A strongrelationship between the onset of the clear water phase and winter climate, here the NAO, wasfound in 28 Central European lakes and 71 shallow Dutch lakes (Straile & Adrian 2000; Schefferet al. 2001). The process behind this phenomenon is probably caused by the strong temperaturedependence of the zooplankton growth rate. Straile & Adrian (2000) suggested that the clearwater phases are more or less uncoupled from the phytoplankton, because other grazers likerotifers and ciliates grazed on the phytoplankton (as shown for Lake Constance and LakeMüggelsee). Besides, this illustrates also the different responses of zooplankton species toclimatic variation and change.

In Lake Erken, no clear water phase induced by Daphnia could be observed, probably due tothe low number of Daphnia in spring (see also Nauwerck 1963). Therefore, the decline of thephytoplankton peak in spring seems to be mainly induced by nutrient limitation and possibly alsothe grazing of ciliates, which remains to be investigated. Since an earlier onset of the clear waterphase, associated with a warm winter (high NAO index), was combined with an earlier summerdecline of Daphnia (Straile 2000), this also illustrates the fact that winter climatic variation alsoaffects successional events in summer. Nevertheless, year-to-year variations in summerconditions of the lake caused by weather variations, like wind-induced mixing, can alsoinfluence, in combination with fish predation, the summer abundance of Daphnia (George2000b). Changes in the fall temperatures, as projected by the lake model (see paper VI), canswitch the reproduction of zooplankton from sexual to asexual, resulting in a lower geneticdiversity of these organisms (Chen & Folt 1996).

In general, climate-driven responses of benthic communities in lakes are, to my knowledge,rarely investigated. Therefore, the effects can only be on a speculative basis. Direct effects onbenthic algae might be an increase in the maximum rates of photosynthesis in warmer waters(Schindler et al. 1990, and lit. therein). Additionally, the fact that filamentous green algae arefavored under higher water temperature might alter the composition of the littoral algaecommunity (Schindler et al. 1990). Also, the ice may have direct effects on the benthic algae byice scour (Snoeijs & Kautsky 1989).

Benthic algal biomass could be enhanced by the higher nutrient turnover rates, as theyinfluence the transfer of nutrients between sediment and water (Havens et al. 2001). Furthermore,the order of successional events might be altered depending on the change of light availabilityinduced by the earlier ice break-up (see paper VI). However, indirectly the benthic algae areaffected by a possibly higher pelagial phytoplankton biomass could also reduce the lightavailability. This will lead to a lower biomass of benthic algae and/or change the compositiontowards low-light adapted species. Additionally, an expected higher grazing activity at higher


25

water temperature leads to greater ingestion rates and a lower mortality in the benthic grazercommunity and benthic algae biomass will eventually be suppressed (Arnell et al. 1996).

The survival and growth of fish species strongly depends on temperature (Magnuson et al.1990; De Stasio et al. 1996; Magnuson et al. 1997). Also thermal limits of different fish specieswill be altered by global warming, which will induce distribution changes for many fish species(Magnuson et al. 1990; Carpenter et al. 1992). Temperature-induced changes in the growth rateof fish, in particular predator fish, may result in cascading effects through the entire food web(Carpenter et al. 1985). For example, warmer spring temperatures may result in an earlier shiftfrom zooplanktivory to a piscivory feeding due to the enhanced predation mortality exerted bythe piscivory fish (Olson 1996). Furthermore, an increased predation rate of fish on zooplankton,in particular Daphnia, could lead to a decline of zooplankton (Mehner 2000).

In conclusion, different species as well as physical and chemical parameters react differently.This may be relevant on a species level, but all these single effects may weigh differently in thelake ecosystem as a whole.

An increase in the ice-free season, and especially the projected totally ice-free years, in LakeErken will greatly increase the number of degree-days and profoundly alter the thermal regime ofthe lake (Schindler et al. 1996). This will have a strong impact on hatching rates anddevelopment of various organisms at different trophic levels, for example on dinoflagellates(Rengefors 1998). An example from the marine system shows that long-term trends of 4 trophiclevels (from phytoplankton to predatory fish) responded in parallel to variations in the weather(Aebischer et al. 1990). The pattern behind this phenomenon is still unclear. A correspondingresponse of several trophic levels should depend on the match and mismatch of predator-prey lifecycles, such as fish and zooplankton (George & Harris 1985). Also the onset of stratificationinfluences the growth of edible algae, with the zooplankton matched or mismatched with itspreferable food (Müller-Navarra et al. 1997). In general, Petchy et al. (1999), conductingmicrocosm experiments permitting experimental control over species composition and rates ofenvironmental change, suggested that ecosystem responses are not as clear as studies of singletrophic levels indicate. Complex responses generated in entire food webs greatly complicateinferences based on single functional groups. Here, the consideration of more general ecologicalconcepts is needed in order to understand and synthesize climatic effects on lake ecosystems. Thestrength of food web interactions is characterized by many weak and few strong interactions(McCann et al. 1998). Weak links in particular act to dampen oscillations between consumersand resources (McCann et al. 1998) and presumably also environmental stressors, as climateextremes. This means that not all responses at a specific trophic level are propagated to lowertrophic levels or have significant impacts on ecosystem processes (Pace et al. 1999). A systemapproach is necessary to examine the cascading effects in response to climatic change andvariability. The magnitude of a climate-driven response of an autotroph organism does notnecessarily have to be mediated or cascaded to the heterotroph species, or vice versa. Thepotential for misleading inferences has been highlighted (Harrington et al. 1999, and lit. therein).Furthermore, the non-linearity in the response to environmental variables (including climate) ofanimal and plants should still be kept in mind (May 1986; Mysterud et al. 2001).

In relatively simple ecosystems, a strong response to climate by the top predator, i. e. keystonespecies, may have dramatic effects on the food web. In an arctic lake, a model simulation of atemperature increase of 3 C resulted in a decline of young lake trout because they were no longerable to fulfill their food supply; the higher water temperature meant that a > 8-fold higher foodconsumption was needed. But as the model projected, the food (plankton) will not increase,causing a food limitation for the lake trout, and finally that the young lake trout population will


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not survive the next winter (McDonald et al. 1996). The decline and maybe the extinction of thetop predator might significantly change the food web and will affect all species interactions,illustrating the potential effect of climatic warming on the food web structure and ecosystemfunction. However, the change in the food web would probably be not as strong as projected ifthe ecosystem were more complex, i.e. higher number of weak interactions. For example, anotherpredator fish could compensate the effects of the lake trout, resulting in a less dramatic impact forthe food web. This is an additional argument to aim at a highly diverse ecosystem (biodiversity),in terms of a high number of species interactions, potentially compensating the impacts ofclimatic effects.

All in all, the magnitude of the climate-driven response in the lake ecosystem depends not onlyon the abiotic constraints (see effect filter) but also on the structure of the biotic interactions in theentire food web.

OutlookFuture research should include additional trophic levels in order to improve the understanding ofclimatic effects on the entire food web and its interaction. Therefore, national monitoringprogrammes have to be extended in terms of including a larger number of organism groupscombined with a reasonable sampling frequency. The costs of these monitoring programs couldbe kept low by a thorough and intensive sampling of only a few case study lakes. On a widerinternational scale, available national databases should be combined. A statistical analysis ofthese data might increase the chances to find general response patterns of different ecosystemcomponents to climatic variation and change. The occurrence of climatic change is not only athreat to the environment but also an ongoing “earth experiment”. Thus, there is a great potentialand challenge to improve the understanding of processes and driving forces between and withindifferent components of “Gaia”. From that more holistic point of view, climatic change should beseen as an intellectual and social challenge, from which environmental scientists take theopportunity to derive guidelines for political actions in order to compensate anthropogenicinfluences.

Furthermore, water quality models validated through historical databases and combined withregional climate models could project future changes in different regions of the world, whichagain provides an improved basis for political decisions on water management.

Today, the design of management practices is mostly based on historical climate, (i.e.Vollenweider Model). These management practices have to be re-designed according to thesenew climate conditions, leading to alterations in lake processes in future, such as an internaleutrophication (paper VI).

Only a combination of basic and applied science on one hand, and a clear communication withdecision makers on the other, might fulfill the needs of water for a growing human population infuture. As the climatic and water cycles act on a global scale, scientific and political actions alsoneed to react correspondingly. This is especially true, since water has been one of the mostmistreated and ignored natural resources in history.


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Summary in German (Deutsche Zusammenfassung)In der Diskussion über eine globale Klimaerwärmung wird im allgemeinen vor allem dieErwärmung der nördlichen Hemisphäre in den letzten 100 Jahren betrachtet. Im Zusammenhangmit dieser Erwärmung tritt eine zunehmende Variabilität hin zu extremen Bedingungen auf wiebeispielsweise extrem kalten Wintern oder extrem intensiven Regenfällen. Inwieweit dieseEntwicklung anthropogen beeinflusst ist, wird immer noch untersucht. Jedoch können dieVeränderungen als solche wesentliche Auskunft darüber geben, inwieweit eine Anpassung derÖkosysteme an die veränderten klimatischen Bedingungen erfolgt. Hierbei ist es von äußersterWichtigkeit, zwischen der Zunahme von extremen Schwankungen und der generell zubeobachtenden allgemeinen Erwärmung zu unterscheiden.

Natürliche Veränderungen des Klimas geschehen in Abhängigkeit von der Sonnenaktivität,Vulkanausstößen, dem Stand der Erde zur Sonne oder aufgrund von Interaktionen zwischen Bio-und Geosphäre (wenn man diese nicht als Einheit betrachtet wie es die Gaia Hypothesebeschreibt). Alle diese natürlichen Phänomene agieren auf sehr unterschiedlichen Zeitskalen, waseine Ursachenanalyse für Klimaveränderungen erschwert.

Das Internationale Gremium für Klimaveränderung (IPCC) beschreibt in seiner neuestenStudie (2001) eine globale Klimaerwärmung von 0,6 ºC, die gemäß ihrer Auffassunganthropogen verursacht wird. Ferner heißt es, dass die 90ger Jahre die wärmsten sind, die seit derEinführung des Thermometers gemessen wurden.

Um den Einfluss der Klimaveränderung auf Ökosysteme festzustellen, kann man sichsogenannter atmosphärischer Zirkulationen bedienen, deren Schwankungen in Indexwertenbeschrieben wird. Der bekannteste ist das weltweite El-Ni o Phänomen mit weitreichendenKonsequenzen für die Westküsten sämtlicher Kontinente.

Ein ähnliches Phänomen ist die atmosphärische Zirkulation über dem Nord-Atlantik, die alsNord-Atlantische Zirkulation (NAO) beschrieben wird. Sie wird als Luftdruckdifferenz zwischenIsland und Portugal (Island-Tief und Azoren-Hoch) gemessen und ist bekannt fûr ihren Einflussauf die jährlichen Klimaschwankungen der nördlichen Hemisphäre. Ein positiver NAO Indexwertbedeutet einen warmen Winter in Europa, der niederschlagsreich im Norden und trockener imSüden ist.

Kürzlich wurde ein weiterer, regional begrenzter sogenannter SkandinavischerZirkulationsindex (SCI) entwickelt, der, ähnlich wie die NAO, auf Luftdruckdifferenzen basiert,jedoch in diesem Fall lediglich über Skandinavien.

Insgesamt repräsentieren alle diese atmosphärischen Indizes eine sehr wesentlicheKomponente in der Klimaforschung, da jeder Index verschiedene Klimafaktoren (Temperatur,Niederschlagsmenge, Wolkendecke) in einem übergreifenden Indexwert vereinigt.

Ziel dieser Arbeit war es, die Auswirkungen sowohl von extremen Klimaschwankungen als auchvon der genellen Klimaerwärmung auf das Ökosystem See zu untersuchen. Das pflanzlichePlankton (Phytoplankton) wurde insbesondere als Testorganismus ausgewählt, da es hinreichenderforscht und durch seine kurze Generationszeit sehr geeignet ist, Veränderungen im Seeaufzuzeigen.


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Im einzelnen wurde ein Einfluss des Klimas und teilweise auch der Nährstoffkonzentration auffolgende Prozesse untersucht:

- der Einfluss des Winterklimas (NAO, SCI) auf das Frühjahrsplankton- der Einfluss der Nährstoffkonzentration auf das Frühjahrsplankton- der Einfluss des Winterklimas (NAO) auf das Sommerplankton- der Einfluss der NAO allgemein auf aquatische und terrestische Ökosysteme- der Einfluss eines modellierten Zukunftsklimas auf den See

Ein möglicher Einfluss des Klimas auf das Phytoplankton kann zum Teil modifiziert oder nichterkenntlich sein, wenn andere Faktoren stärker kontrollierend wirken. Zu diesen gehörenNährstoffkonzentrationen, insbesondere Phosphat, die nachgewiesenermaßen im Frühjahr dasWachstum des Phytoplanktons begrenzen können. Deshalb wurde die Nährstofflimitationwährend und nach der Algenfrühjahrsblüte mit einer neuen Methode analysiert, die es erlaubt,Phosphatlimitation sogar innerhalb von Zellen im Mikroskop sichtbar zu machen. Somit kannermittelt werden, welche Arten und sogar welche Individuen einer Art phosphatlimitiert sind.Vergleichend wurde noch eine konventionelle Methode angewandt, die nur die Limitation imumgebenden Wasser zellunspezifisch aufzeigt.

Beide Methoden ergaben ein sehr komplexes Bild. Am Ende der Frühjahrsblüte und einige Zeitdanach waren tatsächlich einige Arten phosphatlimitiert, jedoch zeigten einzelne Individuenselbst bei extrem niedrigen Phosphatkonzentrationen keinerlei Limitation. Die Nährstoff-erfordernisse von Arten und Individuen des Phytoplanktons erscheinen somit komplexer alsbisher angenommen, dennoch sind Nährstoffkonzentrationen ein kontrollierender Faktor amEnde der Frühjahrsblüte.

Der Beginn der Frühjahrsblüte jedoch wird weitreichend von lokalen Klimafaktoren, vor allemLicht, initiiert, wie zahlreiche frühere Untersuchungen an verschiedenen Seen ergaben. Somitwurde in dieser Arbeit statistisch ermittelt, inwieweit sich über diese lokalen Bedingungen einemögliche Verbindung zu großräumigen Klimaprozessen (NAO, SCI) herstellen lässt, und wiediese möglicherweise das Frühjahrsplankton beeinflussen.

Tatsächlich ergab die Analyse der Langzeitdaten einen deutlichen Zusammenhang zwischensowohl dem Zeitpunkt des Auftretens der Algenblüte, ihrer Artenzusammensetzung sowie ihrertotalen Biomasse mit beiden atmosphärischen Indizes NAO und SCI. Das bedeutet, dass diegroßräumige Zirkulation über dem Nordatlantik lokal die drei wesentlichen Kriterien derFrühjahrsblüte (wann, wer, wieviel) kontrolliert. Verständlicherweise ist der Zusammenhang mitdem lokal kleinräumigeren SCI noch stärker. Da diese Zirkulationen von Jahr zu Jahr sehrunterschiedliche Werte (Luftdruckdifferenzen) annehmen können, unterliegt der See somitebenfalls diesen Schwankungen.

Zusätzlich konnte zudem ein Effekt der generellen Klimaerwärmung über der nördlichenHemisphäre festgestellt werden, der sich in einem langfristig ansteigenden Durchschnittswert derNAO darstellt. So tritt das Maximum der Frühjahrsblüte heute 30 Tage (1 Monat) früher auf alsin den 70ger Jahren. Eine entsprechende Verschiebung lässt sich auch für den Zeitpunkt desAufbrechen des Eises feststellen. Da sich interessanterweise der Beginn der Schichtung des Sees(Trennung in eine warme obere und kühle untere Wasserschicht) während des Sommers zeitlichnicht verändert hat, verlängert sich die Durchmischungsphase im Frühjahr um diesen einenMonat. Eine Verlängerung der phosphatlimitierenden Phase für die Algen ist nur eine


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Konsequenz, die verdeutlicht, wie die Klimaerwärmung die Prozesse im See verändern undbeeinflussen kann.

Doch ist nicht nur ein deutlicher Einfluss auf die Frühjahrsprozesse im See feststellbar. DieseArbeit ergab, dass sogar noch die Biomasse der Algen im Sommer von den Klimabedingungendes vorgehenden Winters signifikant beeinflusst wird. Man kann vermuten, dass die frühereFrühjahrsblüte zusammen mit den wärmeren Wassertemperaturen in der verlängertenDurchmischungsphase zu einer höheren bakteriellen Aktivität und einer stärkeren Freisetzung derNährstoffe aus den oberen Sedimentschichten führt. Die resultierende höhere Nährstoff-konzentration ermöglicht eine höhere Algenbiomasse im Sommer und verdeutlicht, inwiefernVeränderungen des Winterklimas sogar längerfristige Prozesse beeinflussen können.

Um die gefundenen Zusammenhänge vergleichend einzuordnen und nach allgemeingültigenBeziehungen zwischen der NAO und biologischen Prozessen zu suchen, wurden Studien vonaquatischen und terrestischen Ökosystemen ausgewertet und die Effekte der NAO auf denZeitpunkt von Lebenszyklen, Biomasse und trophische Ebenen quantitativ mit einer Meta-Analyse (einem übergeordneten, statistischen Verfahren) verglichen. Sie ergab, dass in allengetesteten Ökosystemen die Effekte von Klimaveränderungen gleich gewichtig waren, d.h. einwarmer Winter (positiver NAO Index) führte dazu, dass zu einem vergleichbar früherenZeitpunkt Algenblüten auftraten, Eier von Amphibien und Vögeln gelegt wurden und Geburtenvon Schafen, Hirschen und Elchen auftraten.

In bezug auf die Biomasse waren die Effekte interessanterweise abhängig vom Ökosystemtyp(aquatisch oder terrestrisch). In aquatischen Ökosystemen führte ein warmer Winter zu einerhöheren Biomasse, während die Biomasse in terrestischen Systemen abnahm. Dieser generelleVergleich macht deutlich, dass die gefundenen Effekte im See Erken auch auf andereÖkosysteme übertragbar und somit generellen Ursprungs sind.

Basierend auf dieser Grundlage wurde ein regionales Klimamodel benutzt, um eine möglicheVeränderung des Sees Erken in naher Zukunft (ca. 2050-2060) zu simulieren. Dazu wurden dieKlimasimulationsdaten in ein physikalisches Seemodell integriert, um Wassertemperatur,Eisdecke und Schichtung des Sees vorhersagen zu können. Auf der Basis dieser Ergebnissewurden dann mögliche ökologische Konsequenzen diskutiert. Geprüft wurde das Modell-verfahren anhand der bereits vorhandenen Daten über den See Erken aus den letzten 30 Jahren.Sowohl das Klima- als auch das Seemodell ergaben mit den historischen Testdaten adäquateSimulationsergebnisse und waren somit geeignet für eine realitätsnahe Zukunftssimulation.

Die Zukunftvorhersagen ergaben vor allem eine erheblich höhere Wassertemperatur allgemeinund eine Intensivierung der Schichtung des Sees während der Sommermonate. Zudem wurdenkürzere Eisbedeckungszeiten modelliert, sowie zwei gänzlich eisfreie Jahre von zehn.

In Anbetracht dieser Simulationsdaten ist von einer erheblichen Veränderung sämtlicherÖkosystemprozesse auszugehen. Es ist anzunehmen, dass ein weiterer Rückgang der Eis-bedeckung die Zusammensetzung der Algen gravierend verändert. Besonders großeDiatomeenarten würden von den verlängerten Durchmischungsphasen profitieren und vermehrtBestandteil der Frühjahrsalgenblüte sein. Zudem ist sehr wahrscheinlich, dass die Nährstoffe,bedingt durch die höhere Wassertemperatur, die veränderten Schichtungsverhältnisse und dieresultierende erhöhte Bakterienaktivität deutlich zunehmen. Man spricht dabei von einer internenEutrophierung, von der in einem wärmeren Klima ausgegangen werden muss. Damit verbundenist das Risiko von einem gehäuften Auftreten sogenannter giftiger Algenblüten während des


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Sommers, da die Blaualgen (Cyanobakterien) bei wärmeren Wassertemperaturen in sehr stabilenSchichtungen ihren Konkurrenten gegenüber begünstigt sind.

Fest steht, dass zukünftige Beschlüsse und Strategien für ein nachhaltiges Management vonWasserreserven sowohl die generelle Klimaerwärmung als auch zunehmend extreme Klima-schwankungen auf jeden Fall miteinbeziehen müssen. Dabei sollte man nie den Bezug zumgesamten Ökosystem verlieren.

AcknowledgementsI am very grateful to my advisor Kurt Pettersson who offered me a PhD student position in a verychallenging European Union project called REFLECT. He supported all my ideas, travelling, hasalways been positive and is a very good badminton player.

I would also like to thank my second supervisor Gesa Weyhenmeyer, who introduced me withher great enthusiasm into the field. Special thanks also go to Helmut Hillebrand, who taught methe clues in fancy statistics and the writing of scientific papers. Also our general ecologicaldiscussions encouraged me enormously.

I furthermore profited from very interesting discussions within the REFLECT project team,which certainly enlarged my view. Additionally, I really appreciated the very open-mindedSWECLIM group at the SMHI and would especially like to thank Markku Rummukainen andAnders Omstedt for their great support and provision of climate and lake model data.I am of course also very thankful to the staff of the Department of Limnology, in particular LarsTranvik, who has always been open-minded and very helpful. Lots of thanks also to the staff ofthe Erken Laboratory for all the practical and mental help in a nice atmosphere, especially to Uffefor the great music and to Anna for great fun. Special thanks also to Paul van Reeuwijk, MalvaAhlkrona and my Finnish friend Antti Lindfors.

Also warm thanks go to my earlier supervisors Wayne Wurtsbaugh, Pål Brettum and HansWuthe and especially also to Heide and Günther Engels who sponsored me with loads of booksand chocolate to survive my study.

I am very grateful to Erik and Anders for introducing me into a happy Swedish life and fortheir support through a great friendship. Special thanks also to my great friends in Germany,Hartmut and Axel and not to forget Metallica and AC/DC for their great songs, which kept meawake (“Stiff Upper Lip”).

Enormous thanks of course to my parents Hannelore and Horst Blenckner for all the love,encouragement and support of my ideas. The same for my parents-in-law, Frauke and BerndReuss, for the initiation of my critical thinking in philosophy and holistic perception. Finally, mygreatest appreciation and love goes to my wife Steph for her endless belief, love and enthusiasticsupport.

This manuscript profited from comments by Kurt Pettersson, Helmut Hillebrand, GesaWeyhenmeyer, Stephanie Blenckner and Maria Kahlert. Nigel Rollison edited the language.Economical support for research and travels were provided by grants from the European Union,Malméns stiftelse, NorFA, and ASLO.


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