Boundary mixing and nutrient fluxes in Mono Lake, California

Temperature-gradient microstructure and nutrient profiling were undertaken at both an inshore and an offshore site on Mono Lake, California, to determ ine whether boundary mixing occurred and the effects on nutrient flux withi n 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 sediment-water interface. In addition, temperature profiles were m ore incrementally stepped in the pycnocline inshore than offshore. The Turn er angle indicated that double-diffusive processes may have augmented turbu lent transport in the upper 10 m, where temperatures were inversely stratif ied, but not in the main pycnocline. Within the pycnocline, a exceeded the threshold value for buoyancy flux (epsilon(thr) = 15vN(2)) in 21% of the tu rbulent layers inshore but in only 1% of the layers offshore. The coefficie nt of vertical eddy diffusivity, K-z, was two to four orders of magnitude h igher within 4 m of the bottom inshore than offshore at the same depths. Sp atially averaged values of K-z, obtained from the heat-flux method using da ta obtained from both conductivity-temperature-depth (CTD) profiles and moo red thermistor chains, were two orders of magnitude less than those obtaine d nearshore with microstructure profiling. From the differences in K-z, we inferred that most heat flux occurred due to boundary mixing at the base of the pycnocline inshore with the heat redistributed laterally by advection. Boundary mixing was initiated after winds were strong enough for the Lake number to decrease to a value of 2; thermocline compression and steepening of internal waves at the base of the pycnocline occurred, followed by packe ts of high-frequency internal waves critical for wave breaking. Calculated ammonium fluxes at the inshore site were sufficient to support daily rates of primary productivity in the deep chlorophyll maximum throughout the lake . These results indicate the vertical flux of nutrients across the nutricli ne in Mono Lake occurs over a limited area during intense mixing events ini tiated by high winds.