Boundary mixing and nutrient fluxes in Mono Lake, California

摘要

Temperature-gradient microstructure and nutrient profiling were undertaken at both an inshore and an offshore site on Mono Lake, California, to detennine whether boundary mixing occurred and the effects on nutrient flux within the lake. Turbulence, as quantified by rates of dissipation of turbulent kinetic energy, was two to three orders of magnitude higher at the inshore site where the pycnocline intersected the bottom than at the same depths at an offshore station. The intense turbulence primarily occurred within 3.5 m of the sedimentwater interface. In addition, temperature profiles were more incrementally stepped in the pycnocline inshore than offshore. The Turner angle indicated that double-diffusive processes may have augmented turbulent transport in the upper 10 m, where tem- peratures were inversely stratified, but not in the main pycnocline. Within the pycnocline, epsilon exceeded the threshold value for buoyancy fiux (epsilon-thr = 15vN~2) in 21 of the turbulent layers inshore but in only l of the layers offshore. The coefficient of vertical eddy diffosivity, K, was two to four orders of magnitude higher within 4 m of the bottom inshore than offshore at the same depths. Spatially averaged values of K obtained from the heat-flux method using data obtained from both conductivity-temperature-depth (CTD) profiles and moored thermistor chains, were two orders of magnitude less than those obtained nearshore with microstructure profiling. From the differences in K, we inferred that most heat flux occurred due to boundary mixing at the base of the pycnocline inshore wit