The oceanic mixed layer is defined as the quasi-homogeneous region of the upper ocean that directly interacts with the atmosphere. The mixed layer dynamics are primarily determined by the action of turbulent mixing of the water mass due to wind stress and convection driven by buoyancy fluxes at the air-sea interface, which reveal variations on hourly through diurnal and seasonal to interannual timescales. Understanding of the processes by which the mixed layer properties vary is essential for quantitative diagnostics of the coupled ocean and atmosphere system and its effect on biogeochemical cycles and ecosystems. The interaction between the mixed layer and the underlying pycnocline determines the ventilation of the ocean interior [e.g., Luyten et al., 1983; Woods, 1985], influences the large-scale circulation and also the mesoscale variability [Strass et al., 1992], and may impact the atmosphere in remote regions where the flow returns to the mixed layer [Blanke et al., 2002]. A paradigm in marine biology that has existed since the pioneering work of Sverdrup  is that the vernal shallowing of the mixed layer sets the conditions for the phytoplankton spring bloom. Also, ice edge blooms appear in fact to be triggered by the meltwater-mediated shallowing of the mixed layer [Strass and Nöthig, 1996]. Deep mixed layers have been put forward to explain the rather low productivity of whole ocean regions such as the Antarctic Circumpolar Current [Tranter, 1982]. Climatic differences of the Southern Ocean mixed layer depth on geological timescales and their impact on the biological pump of carbon that is associated with phytoplankton primary production have even been proposed as an explanation of glacial-interglacial variations of the atmospheric carbon dioxide concentration [Francois et al., 1997]. Detailed observations of mixed layer structures and processes are, however, particularly rare in the open Southern Ocean in particular.
 Various definitions of the mixed layer depth (MLD) have been published. Basically, two types of criteria are mostly in use: difference criterions and gradient criterions, summarized and discussed by Lukas and Lindstrom , Brainerd and Gregg , Kara et al. , and Zawada et al. . The first class is based on specifying a difference in temperature or density from the surface value, whereas the second class is based on specifying a gradient in temperature or density. The different estimates of MLD can deviate by 1 order of magnitude depending on the chosen criterion [Zawada et al., 2005]. In regions where both the daily and seasonal cycles of the forcing are mainly dominated by the heat flux, the temperature-based criteria are sufficient to predict the mixed layer depth. When the wind is constant over periods longer than 1 d, the mixed layer often shows a strong diurnal cycle with nighttime convection due to cooling driving active mixing from the surface to the seasonal pycnocline, while during daytime, a shallower restratification may result from radiative heating. However, precipitation and ice melt can lead to stratifying pools of fresh water, which demands a density-based criterion.
 Kara et al.  compared different gradient and difference criteria for estimating the MLD and concluded that considerable differences in the MLD result. Therefore they introduced an optimal algorithm for isotherm layer depth and MLD calculation developed through subjective analysis of temperature and density profiles from the work by Levitus et al.  and Levitus and Boyer . One problem of estimating MLD from the available global data sets is, however, the coarse vertical resolution [Lorbacher et al., 2006] and the fact that data from a wider latitude/longitude grid cell are horizontally averaged. While the surface layer of the ocean exhibits in conceptual models a very simple structure, a well-mixed layer of variable depth at the sea surface, and a seasonal pycnocline connecting the mixed layer to the permanent pycnocline, several processes can alter this picture [Moum and Smyth, 2001]. In consequence, the surface layer can include strongly stratified layers, actively mixing layers, salinity barrier layers, fossil mixed layers, and inversions [Sprintall and Roemmich, 1999].
 In order to understand the diurnal cycle of heat storage within the surface mixed layer, Brainerd and Gregg  proposed two concepts of mixed layers, differing in the timescale over which they are mixed. They distinguished between the mixed layer, representing the zone of relatively homogeneous water formed by the history of mixing, and the actively mixing layer (AML), the zone in which mixing is occurring. To identify the mixing layer, they analyzed profiles of the kinetic energy dissipation rate (ɛ) and the length scale of turbulent overturns (LT) and compared them to the signals both in temperature and density. The authors showed that both difference and gradient criteria are capable of describing the mixed layer depth. However, neither definition gave mixing layer depths that consistently matched the turbulence measurements. They concluded that overturning or so-called Thorpe scales [Thorpe, 1977] are the most reliable measure of mixing layer depths.
 For both the ocean-atmosphere coupling and the primary production by phytoplankton, it makes a difference if the surface layer is turbulent and actively mixing or merely homogenized by previous mixing events. Sverdrup , in his fundamental theory about the development of phytoplankton blooms, defined as the criterion a critical depth, which is given by that depth at which, starting with integration at the surface, the vertically integrated photosynthetic production of phytoplankton cells that are assumed to be rapidly vertically mixed equals the integrated phytoplankton losses. Thus Sverdrup considered phytoplankton primary production in the actively mixing layer depth (AMLD). Despite that, in most applications of Sverdrup's theory to explain observed phytoplankton blooms, the MLD is taken as the control variable of light limitation instead of the AMLD.
 Here we present the results of a case study performed to determine the differences of the depths of the AML and the mixed layer (ML). While our results can contribute to a more accurate assessment of the role of light limitation for open ocean primary production and to a better understanding of variations of food supply for the higher trophic levels worldwide, they are of particular interest for the Southern Ocean. In our study, we also derive estimates of vertical diffusivity within and below the mixed layer. Estimates of vertical diffusivity are needed for calculating, for instance, the exchange of total carbon across the mixed layer base and the replenishment of phytoplankton nutrients in the mixed layer from below.
 Our study was conducted in the vicinity of the Antarctic Polar Front (APF) during the European Iron Fertilization Experiment (EIFEX) (Polarstern Cruise ANT XXI/3) [Smetacek, 2005]. The experiment was carried out in Lagrangian manner within a cyclonic eddy of a diameter of nearly 100 km. The eddy, centered at about (2°15′E, 49°15′S), was obviously shed from the APF by detachment of a northward protruding meander (Figure 1a). Since the experiment has been conducted within one particular hydrographic feature, it is ideally suited for the study of temporal changes, as advective effects are kept low.
 The paper is organized as follows. Section 2 describes the instrumentation and the data set. In section 3, we present the various methods for investigating the mixing regime within the upper 400 m from the different types of data. In order to analyze the response of the mixed layer to the forcing, we distinguish in section 4 between horizontal eddy effects and atmospheric fluxes. In section 5, we present conclusions.