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, ⑀ exceeded the threshold value for buoyancy flux (⑀ thr ϭ 15N2 ) in 21% of the turbulent layers inshore but in only 1% of the layers offshore. The coefficient of vertical eddy diffusivity, K z , 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 z , 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 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 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.Whether mixing events occur as frequently throughout the pycnocline as they do near lateral boundaries has implications for heat and nutrient fluxes and spatial heterogeneity of ecological processes. The importance of mixing near boundaries was inferred by Munk (1966), who determined

CO2 exchange between air and water in an Arctic Alaskan and midlatitude Swiss lake: Importance of convective mixing

“…At these wind speeds, both microscale wave breaking, convection in the water body, and wind shear contribute to aquatic turbulence. The few studies in lakes that consider high and moderate wind speeds indicate that energy dissipation rates, which represent the magnitude of turbulence in the surface waters of lakes, are comparable to conditions in oceans [MacIntyre et al, 1999]. Below wind speeds of 3 m s À1 , gas flux is independent of wind speed [Ocampo-Torres et al, 1994].…”

Effects of epilimnetic versus metalimnetic fertilization on the phytoplankton and periphyton of a mountain lake with a deep chlorophyll maxima

“…It seems likely that some source of nutrients was excluded because during the latter part of the experiment epilimnetic chlorophyll levels in the control limnocorrals were about 50% of those in the lake and the nutrient additions reversed this trend. The limnocorrals may have blocked nutrients diffusing from the epilimnetic or metalimnetic sediments (Fee 1979;Levine and Schindler 1992;MacIntyre et al 1999) and those from riverine inflows. In a lake, the temperature of the inflows will determine whether the nutrients are delivered to the epilimnion, metalimnion, or hypolimnion (Carmack et al 1979;Vincent et al 1991) and thus the impact on algal production in the different strata.…”

“…Sedimentation will move nutrients to deeper strata, whereas eddy diffusion usually moves nitrogen and phosphorus from deep, nutrient-rich strata to shallower ones. Nutrients from the sediments in contact with the epilimnion may also contribute to production in that strata (Fee 1979), andMacIntyre et al (1999) recently suggested that breaking internal waves can move nutrients from the metalimnetic-sediment interface into the metalimnion.…”

Boundary mixing in a small stratified lake

“…As a result of the internal adjustment processes within the TBBL, horizontal intrusions are generated in the TBBL and propagate from the sloping boundaries toward the lake interior. This phenomenon has been demonstrated by laboratory experiments [Ivey and Nokes, 1989;De Silva et al, 1997] and supported by field measurements [Lemckert and Imberger, 1998;MacIntyre et al, 1999]. Most importantly, the secondary circulation on a lake basin scale is generated and maintained by processes inside the TBBL, and the intrusions are not carried away from the TBBL by external processes.…”