Multilayer physical system and biological structure in the upper water of Lake Biwa during summer

Lake Biwa is the largest lake in Japan and supplies water for over ten million people. Observations were made over 24-hour at a station in North Basin of Lake Biwa in summer 2008 and 2009 to detect events in relation to oxygen transport. We deployed a free-fall microstructure profiler, TurboMAP-L, to measure mixing intensity and fluorescence, and basic hydrographic structure. An F-probe (Fine-scale profiler) was also deployed to measure dissolved oxygen. During the 2008 survey we also deployed TAPS (Tracor acoustic profiling system) to detect zooplankton distribution. For 2009 LISST-100X (Laser in-situ scattering and transmissiometry), was deployed only during the night to measure particle size spectra. For both the 2008 and 2009 surveys the water column showed similar structure. The thermocline was observed between 10 to 20 m depth throughout the day and formed sharp density gradient. The buoyancy frequency, N2, for the thermocline exceeded 10-3 s-2 (Fig.1). Active mixing layers were observed above the thermocline and also beneath it, where the rate of turbulent kinetic energy dissipation, ɛ, was around 10-7 W kg-1. While, mixing intensity in stratified layer was quite weak, ɛ didn’t exceed 10-8 W kg-1 (Fig.2). Eddy diffusivity coefficient, Kρ, in this stratified and weakly turbulent layer was estimated to be on the order of 10-7 m2 s-1, which is almost equivalent to a molecular diffusion level. Thus, the strongly stratified and weakly turbulent thermocline acted as a barrier to prevent oxygen transport from the surface to the lower layer. Additionally, the chlorophyll maximum and peak in zooplankton abundance occurred in the thermocline (Fig.1). We will present multilayer biological structure around a strong thermocline that indicates a complicated interaction between physical and biological process. Fig.1 Distribution of log10 of buoyancy frequency squared (gray scale) and chlorophyll (white contours; thicker line indicates higher value)

Fig.2 Distribution of log10 of turbulent kinetic energy dissipation rate