Comparison of the results of tracer experiments in lakes with predictions based on horizontal mixing models

Mixing processes playa major role in the distribution of dissolved substances and particles, including plankton, in natural waters. In lakes, vertical mixing has been studied extensively, while studies of horizontal mixing are rare. Lakes are commonly assumed to be horizontally homogeneous, and consequently horizontal mixing is frequently neglected. However, its effects can be of great importance to processes occurring at the lake boundaries and to water pollution studies, since horizontal turbulent diffusion is important in transporting substances over great distances and in diluting high concentrations caused for example by accidental pollution events. Up until now, horizontal mixing has been investigated mainly in very large water bodies ( e.g. in the ocean by OKUBO 1971 and in Lake Ontario by MURTHY 1976), or in very small lakes (e.g. by QUAY et al. 1979). With the objective of obtaining a better understanding of relative diffusion (diffusion relative to the centre of mass) in the hypolimnion of medium-sized lakes (approx. 25 km2), three tracer experiments were conducted in the course of this research. In the following section the experimental technique and the method of data reduction is described very briefly (for a detailed description see PEETERS & WOEST 1992). Subsequently, the experimental results are presented and compared with MURTHY'S (1976) Lake Ontario data and with the ocean data of OKUBO (1971) and EWART & BENDINER (1981). This is followed by a short discussion of the applicability of some models describing horizontal diffusion.