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

Human-caused fragmentation of habitats is threatening an increasing number of animal and plant species, making an understanding of the factors influencing patch occupancy ever more important. The overall goal of the current study was to develop probabilistic models of patch occupancy for the mountain yellow-legged frog (Rana muscosa). This once-common species has declined dramatically, at least in part as a result of habitat fragmentation resulting from the introduction of predatory fish. We first describe a model of frog patch occupancy developed using semiparametric logistic regression that is based on habitat characteristics, fish presence/absence, and a spatial location term (the latter to account for spatial autocorrelation in the data). This model had several limitations including being constrained in its use to only the study area. We therefore developed a more general model that incorporated spatial autocorrelation through the use of an autocovariate term that describes the degree of isolation from neighboring frog populations (autologistic model). After accounting for spatial autocorrelation in patch occupancy, both models indicated that the probability of frog presence was strongly influenced by lake depth, elevation, fish presence/absence, substrate characteristics, and the degree of lake isolation. Based on cross-validation procedures, both models provided good fits to the data, but the autologistic model was more useful in predicting patch occupancy by frogs. We conclude by describing a possible application of this model in assessing the likelihood of persistence by frog populations.

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Changes in vertical mixing in hypersaline Mono Lake, California, are described for the onset, persistence,

The oxidation of ammonia (NH 3 ) and methane (CH 4 ) was investigated in an alkaline saline lake, Mono Lake, California (U.S.A.). Ammonia oxidation was examined in April and July 1995 by comparing dark 14 CO 2 fixation rates in the presence or absence of methyl fluoride (MeF), an inhibitor of NH 3 oxidation. Ammonia oxidizermediated dark 14 CO 2 fixation rates were similar in surface (5-7 m) and oxycline (11-15 m) waters, ranging between 70-340 and 89-186 nM d Ϫ1 , respectively, or 1-7% of primary production by phytoplankton. Ammonia oxidation rates ranged between 580-2,830 nM d Ϫ1 in surface waters and 732-1,548 nM d Ϫ1 in oxycline waters. Methane oxidation was examined using a 14 CH 4 tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and rates in oxycline and anaerobic bottom waters (0.5-37 and 7-48 nM d Ϫ1, respectively) were 10-fold higher than those in aerobic surface waters (0.04-3.8 nM d Ϫ1 ). The majority of CH 4 oxidation, in terms of integrated activity, occurred within anoxic bottom waters. Water column oxidation reduced the potential lake-atmosphere CH 4 flux by a factor of two to three. Measured oxidation rates and water column concentrations were used to estimate the biological turnover times of NH 3 and CH 4 . The NH 3 pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH 4 is cycled on 10 3 -d time scales in surface waters and 10 2 -d time scales within oxycline and bottom waters. Our data suggest an important role for NH 3 oxidation in alkaline, saline lakes since the process converts volatile NH 3 to soluble NO 2 Ϫ , thereby reducing loss via lake-atmosphere exchange and maintaining nitrogen in a form that is readily available to phytoplankton.