Nutrient fluxes from upwelling and enhanced turbulence at the top of the pycnocline in Mono Lake, California

Time series measurements of temperature at 15 depths and profiles of temperature-gradient microstructure were obtained during a period with strong wind forcing and subsequent calm in Mono Lake, California. The wind forcing increased the amplitude of basin-scale internal waves and energy at all wave frequencies relative to the calm period. Rates of dissipation of turbulent kinetic energy, epsilon, were high (epsilon > 10(-6) m(2) s(-3)) at the top of the pycnocline at both an inshore and an offshore site on a day when winds reached 10 m s(-1) and on the following two days at an inshore site (epsilon > 10(-7) m(2) s(-3)). The enhanced turbulence occurred at the depth of a subsurface temperature maximum (z(TM)) and coincidentally with elevated concentrations of NH4, reduced concentrations of chlorophyll a and particulate carbon, and increased abundance of the macrozooplankter Artemia monica. The NH4 at z(TM) was more dispersed and of lower concentration inshore than offshore and indicated greater turbulent transport inshore. Over the course of 4 days, chlorophyll a concentrations increased in the upper mixed layer, and C:N and C:Chl ratios decreased. Offshore, the change in C:N ratio indicated a relaxation of moderate nutrient deficiency. We hypothesize that excretion by A. monica and turbulent transport of the NH4 from the subsurface temperature maximum led to improved physiological status of phytoplankton in the upper mixed layer.