The relative importance of wind and ship waves in the littoral zone of a large lake

Surface waves and their interactions with sediments and benthic organisms are the main hydrodynamic process affecting littoral ecosystems. Here, we present a long-term data set on surface-wave parameters, which was obtained from the analysis of measurements with a pressure sensor. The data set covers a time period of a year and allows for resolving waves with heights down to less than a centimeter and frequencies up to 0.8 Hz. Wind waves and three different types of ship waves were distinguished by their spectral properties. In Lake Constance, ship-generated waves are as important as wind-generated waves and contribute about 41% of the annual mean wave energy flux to shore. In summer, during the most productive time period, ship waves dominate the wave field in terms of the energy flux to shore and also in their frequency of occurrence. Ship waves cause a diurnal and a seasonal pattern in the frequency of occurrence and in the heights of surface waves, whereas in the case of wind waves these parameters do not vary significantly with season or between nighttime and daytime. In contrast to wind waves that occur only sporadically, ship waves propagate into the littoral zone very frequently at regular time intervals. The different pattern of occurrence of ship and wind waves results in a different pattern of disturbance in the littoral ecosystem. One of the most prominent differences between the littoral and the pelagic zone is the role of surface waves. In the littoral zone, waves interact directly with the sediment surface and biota and thus cause, for example, resuspension, erosion, and transport of particles (Luettich et al. 1990; Hawley and Lee 1999; Håkanson 2005); release of nutrients and methane (Güde et al. 2000; Bussmann 2005); reallocation and stress on zoobenthos affecting zoobenthos diversity (Scheifhacken 2006); abrasion of biofilms from stones (Cattaneo 1990; Peters 2005; Francoeur and Biggs 2006) and aquatic macrophytes (Eriksson et al. 2004); and damage of reed belts (Schmieder et al. 2004). Surface waves also influence the growth and behavior of fish that cannot escape from the fluctuating currents by vertical migration (Stoll pers. comm.), the light regime via the fluctuations in water level and light attenuation by resuspended particles (Stramski et al. 1992; Erm and Soomere 2006), and the riparian plant community (Ostendorp et al. 2004; Kotowski and Pioŕkowski 2005). Nevertheless, most descriptions of surface waves are based on studies in marine environments or shelf regions (e.g., Madsen 1976; Le Blond and Mysak 1978; Donelan et al. 2005). In oceans, waves are generated by strong and frequent winds over long fetch lengths and propagate to the coast with large amplitudes. Typical wave heights vary between 0.5 m during calm sea and several meters during storm events, whereas wave periods vary between 5 and 10 s (e.g., Komen et al. 1996; CERC 2002; Brown et al. 2005). In most lakes, winds are infrequent and wind speeds low. In addition, wind forcing at the water surface often varies on small spatial scales, and the effective fetch length is restricted to a few kilometers. Hence, the wave field in most lakes is characterized by waves with small amplitudes and high frequencies and thus differs considerably from the wave field in the ocean. The ecological effect of wind-generated surface waves in lacustrine environments has been investigated mainly in the Great Lakes (Lawrence and Davidson-Arnott 1997; Meadows et al. 1997; Hawley and Lee 1999). Only a few studies specifically investigated wind waves in smaller lakes (Jin and Wang 1998; Allan and Kirk 2000). Apart from the wind, commercial and tourist ship traffic causes surface waves. Several studies investigated the properties and the importance of regular ship traffic in rivers and channels or shelf regions (e.g., Sorensen 1973; Stumbo 1999; Bauer et al. 2002) and the relevance of highspeed catamaran ferries in coastal environments (Parnell and Kofoed-Hansen 2001; Soomere 2005), but only few studies focused on ship waves in lakes (Bhowmik 1975; Maynord 2005). Because of their specific generation, ship and wind waves have considerably different properties (e.g., wave form, period, and length) and thus potentially have a different ecological effect in the littoral zone (Bauer et al. 2002; Soomere 2005; Erm and Soomere 2006). However, the hydrodynamic forcing in the littoral zone due to ship waves is often underestimated or even neglected (Bhowmik 1975; Maynord 2005). The purpose of the current study is, therefore, to fill this gap by comparing the relative importance of windand ship-generated surface waves in a large lake over a year. Materials and methods Study sites—Lake Constance is located in the southwest of Germany and borders Switzerland and Austria. It is the 1 Corresponding author ([email protected]). Acknowledgments We thank Georg Heine and his colleagues from the electronic and mechanical workshop at the University of Konstanz for technical assistance and for the development of the pressure sensors. We gratefully acknowledge the help of the technical staff at the Limnological Institute and many students during fieldwork and data analysis at the Instituts für Seenforschung (ISF) Langenargen der Landesanstalt für Umwelt, Messungen und Naturschutz (LUBW). Helpful and valuable comments of the two anonymous referees improved the manuscript. This work was supported by the German Research Foundation (DFG) within the framework of the Collaborative Research Center 454 ‘‘Littoral Zone of Lake Constance.’’ Limnol. Oceanogr., 53(1), 2008, 368–380 E 2008, by the American Society of Limnology and Oceanography, Inc.