Boundary mixing and nutrient fluxes in Mono Lake, California

Abstract

Temperature‐gradient microstructure and nutrient profiling were undertaken at both an inshore and an offshore site on Mono Lake, California, to determine 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 sediment–water 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 temperatures were inversely stratified, but not in the main pycnocline. Within the pycnocline, ϵ exceeded the threshold value for buoyancy flux ( ϵthr = 15υN2) in 21% of the turbulent layers inshore but in only 1% of the layers offshore. The coefficient of vertical eddy diffusivity, Κz, 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 Κz, 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 Κz, 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 packets 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 nutricline in Mono Lake occurs over a limited area during intense mixing events initiated by high winds.