Characteristics and implications of surface gravity waves in the littoral zone of a large lake (Lake Constance) Dissertation Zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften (Dr. rer. nat.) an der Universität Konstanz Mathematisch-Naturwissenschaftliche Sektion Fachbereich Biologie vorgelegt von Hilmar Hofmann Konstanz, 2007 Tag der mündlichen Prüfung: 23.06.2008 Referent: Prof. Dr. Frank Peeters Referent: Dr. habil. Klaus Jöhnk Referent: Prof. Dr. habil. Karl-Otto Rothhaupt Bibliografische Information der Deutschen Nationalbibliothek Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar. 1. Aufl. - Göttingen : Cuvillier, 2008 Zugl.: Konstanz, Univ., Diss., 2007 978-3-86727-714-3 © CUVILLIER VERLAG, Göttingen 2008 Nonnenstieg 8, 37075 Göttingen Telefon: 0551-54724-0 Telefax: 0551-54724-21 www.cuvillier.de Alle Rechte vorbehalten. 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Auflage, 2008 Gedruckt auf säurefreiem Papier 978-3-86727-714-3 Table of contents Summary 1 Zusammenfassung 3 General introduction 7 Chapter 1: Temporal scales of water level fluctuations in lakes and 17 their ecological implications Introduction 18 Materials and methods 19 Results 20 Discussion 25 Conclusions 32 Chapter 2: The relative importance of wind and ship waves in the 35 littoral zone of a large lake Introduction 36 Materials and methods 37 Results and discussion 43 Chapter 3: Wave-induced resuspension in the littoral zone of a large 57 lake Introduction 58 Materials and methods 59 Results 66 Discussion 73 Conclusions 75 Chapter 4: Wave-induced variability of the underwater light climate 77 in the littoral zone Introduction 77 Materials and methods 78 Results and discussion 80 Conclusions 86 Chapter 5: Wave field characteristics and currents in a wave 87 mesocosm Introduction 87 Materials and methods 88 Results 93 Discussion 96 General discussion and perspectives 99 References 107 Acknowledgments 121 Curriculum vitae 123 List of publications 125 Conference and workshop presentations 127 Summary The central issue of this thesis is to provide a detailed characterization of the surface wave field in a lacustrine ecosystem including small-amplitude wind and ship waves with their specific implications on physical processes and biological conditions (e.g., sediment resuspension and light climate). Therefore, surface waves were measured during different seasons (long-term approach), wind regimes, and water levels in the littoral zone of Lake Constance. A review of water level fluctuations revealed their different temporal and spatial scales. In the case of Lake Constance, seasonal water level fluctuations are next to high- frequency surface waves an important factor, since they change the section of the shore and thus the sediment composition exposed to high-energetic surface waves. The measurement of the surface wave field showed that wind and ship waves can be distinguished by their properties. Wind waves are characterized by small wave heights and short wave periods. Their range is determined by the prevailing wind speed and the effective fetch length at the specific site. The occurrence of wind waves is rather sporadically and infrequent. Ship waves occur very frequently at regular time intervals, have higher wave periods and longer wave lengths than wind waves, and are generated by regular car and passenger ferries, passenger ships, and a newly introduced catamaran ferry. In contrast to wind waves, the occurrence of ship waves follows a diurnal and a seasonal pattern, because of the pronounced ship traffic at daytime especially during the tourist season in summer. On Lake Constance, ship waves are as important as wind waves and during summer they even dominate the wave field. Since ship waves are generated all around Lake Constance and due to their ability to travel over long distances, the results obtained at the study site Littoral Garden are representative for most of the southern and southwestern shores. According to the different temporal patterns of wave generation, wind-wave induced resuspension occurs sporadically and less frequent than ship-wave induced resuspension. The periodic and regular occurrence of ship waves, especially of passenger ships during daytime 1 2 Summary and in summer, cause a substantial increase of the suspended sediment concentration in the shallow littoral, and hence cause a diurnal cycle as well as a seasonal pattern in the suspended sediment concentration and resuspension. Ship-wave induced resuspension is known to prevent sediment consolidation and may significantly contribute to erosion. The continuous interplay between seasonal water level fluctuations and surface waves under the condition of decreasing mean water levels can further support sediment erosion. Simultaneous, high-frequent measurements of surface wave field and underwater light climate revealed amplitudes and temporal scales of the fluctuations in the light intensity, which are caused by the process of wave focusing, the change in the surface elevation, and resuspension. Wave focusing is induced by wind-generated ripple waves. It causes fluctuations of the underwater light intensity with large amplitudes and high frequencies that are far above the pure effect of surface elevation. On the other hand, wave-induced resuspension dramatically reduces the underwater light intensity in the littoral zone due to the shading effect of suspended particles, and thus limits the availability of light for the growth of phytoplankton and biofilms. Measurements of the surface wave field and the related currents were also performed in a wave mesocosm, which is used for various biological investigations focusing on implications of waves on biota in the shallow littoral. The comparison of a typical wave in the mesocosm with waves in the field revealed the limitations as well as the advantages of mesocosm experiments compared to field experiments. Since the wave heights and periods and also the magnitude of other wave-related parameters in the wave mesocosm are much smaller than in the field, the implications on biota determined during biological experiments may underestimate the actual impact of wave motion in the field. In conclusion, this thesis is the first long-term investigation that gives a detailed characterization of the surface wave field including small-amplitude waves with a high temporal resolution in a lacustrine environment. In Lake Constance, wind as well as ship waves contribute to the overall wave field, whereas ship waves are as important as wind waves on an annual scale and even dominate the wave field during summer. The specific occurrence and properties of wind and ship waves cause different temporal patterns and have various implications on sediment resuspension and the underwater light climate, which in turn affect biotic processes in the littoral ecosystem. This thesis demonstrates the strong linkages between physical processes and biological conditions, which supports the need of integrated research. Zusammenfassung Zentrales Thema dieser Dissertation ist eine detaillierte Charakterisierung des Oberflächenwellenfeldes in einem limnischen Ökosystem, bei der sowohl Wind- als auch Schiffswellen und ihre jeweiligen Auswirkungen auf physikalische Prozesse und biologische Bedingungen berücksichtigt werden (z. B. die Resuspension von Sedimenten und das Unterwasser-Lichtklima). Dazu wurden Oberflächenwellen während unterschiedlicher Jahreszeiten (langfristige Messungen), Windregime und Wasserständen im Litoral des Bodensees gemessen. Die umfassende Darstellung von Wasserspiegelschwankungen zeigte deren unterschiedliche zeitliche und räumliche Skalen auf. Im Fall des Bodensee bilden saisonale Wasserspiegelschwankungen neben Oberflächenwellen einen wichtigen Faktor, da sie den Abschnitt des Ufers und somit die Zusammensetzung der Sedimente verändern, die den energiereichen Oberflächenwellen ausgesetzt werden. Die Messung des Wellenfeldes im Bodensee ergab, dass Wind- und Schiffwellen anhand ihrer Eigenschaften unterschieden werden können. Windwellen haben geringe Wellenhöhen und kurze Wellenperioden, die von der vorherrschenden Windgeschwindigkeit und der Wirklänge des Windes am jeweiligen Standort bestimmt werden. Sie treten relativ sporadisch und selten auf. Schiffswellen kommen dagegen sehr häufig und in regelmäßigen Zeitabständen vor, und besitzen größere Wellenperioden und Wellenlängen als Windwellen. Sie werden von Auto- und Passagierfähren, Passagierschiffen und einer neu eingeführten Katamaranfähre erzeugt. Im Gegensatz zu Windwellen, kann man bei Schiffswellen ein ausgeprägtes diurnales und saisonales Muster beobachten, das durch das fast ausschließliche Auftreten des Schiffsverkehrs am Tag und während der touristischen Kursschifffahrt im Sommer erzeugt wird. Im Bodensee sind Schiffswellen im Jahresmittel ebenso bedeutend wie Windwellen, und während des Sommers dominieren sie sogar das Wellenfeld. Das ubiquitäre Auftreten von Schiffswellen im Bodensee und ihre Eigenschaft sich über lange Strecken 3 4 Zusammenfassung auszubreiten, machen die Ergebnisse der Untersuchungen im Litoralgarten auf die meisten der südlich und südwestlich gelegenen Ufer des Bodensees übertragbar. Das zeitlich unterschiedliche Auftreten von Wind- und Schiffswellen spiegelt sich in der Resuspension