Description: The physiological condition of larval Antarctic krill was investigated during austral autumn 2004 and winter 2006 in the Lazarev Sea, to provide better understanding of a critical period of their life cycle. The condition of larvae was quantified in both seasons by determining their body length (BL), dry mass (DM), elemental- and biochemical composition, as well as stomach content analysis, and rates of metabolism and growth. Overall the larvae in autumn were in better condition under the ice than in open water, and for those under the ice there was a decrease in condition from autumn to winter. Thus growth rates of furcilia larvae in open water in autumn were similar to winter values under the ice (mean 0.008 mm/d), whereas autumn, under ice values were higher: 0.015 mm/d. Equivalent larval stages had up to 30% lower BL and 70% lower DM in winter compared to autumn, with mean oxygen consumption 44% lower (0.54 µl O2 DM/h). However, their ammonium excretion rates doubled (from 0.03-0.06 µg NH4 DM/h) so their mean O:N ratio was 46 in autumn and 15 in winter. Thus differing metabolic substrates were used between autumn and winter, suggesting a flexible overwintering strategy, as suggested for adults. The larvae were eating small copepods (Oithona spp.) and/or protozoans as well as autotrophic food under the ice. However, pelagic Chlorophyll a (Chl a) was a good predictor for growth in both seasons. The physics (current speed/ice topography) probably has a critical part to play in whether larval krill can exploit the food that may be associated with sea ice or be advected away from such suitable feeding habitat.

Description: Measurements of Fe(II) and H2O2 were carried out in the Atlantic sector of the Southern Ocean during EisenEx, an iron enrichment experiment. Iron was added on three separate occasions, approximately every 8 days, as a ferrous sulfate (FeSO4) solution. Vertical profiles of Fe(II) showed maxima consistent with the plume of the iron infusion. While H2O2 profiles revealed a corresponding minima showing the effect of oxidation of Fe(II) by H2O2, observations showed detectable Fe(II) concentrations existed for up to 8 days after an iron infusion. H2O2 concentrations increased at the depth of the chlorophyll maximum when iron concentrations returned to pre-infusion concentrations (〈80 pM) possibly due to biological production related to iron reductase activity. In this work, Fe(II) and dissolved iron were used as tracers themselves for subsequent iron infusions when no further SF6 was added. EisenEx was subject to periods of weak and strong mixing. Slow mixing after the second infusion allowed significant concentrations of Fe(II) and Fe to exist for several days. During this time, dissolved and total iron in the infusion plume behaved almost conservatively as it was trapped between a relict mixed layer and a new rain-induced mixed layer. Using dissolved iron, a value for the vertical diffusion coefficient Kz=6.7±0.7 cm**2/s was obtained for this 2-day period. During a subsequent surface survey of the iron-enriched patch, elevated levels of Fe(II) were found in surface waters presumably from Fe(II) dissolved in the rainwater that was falling at this time. Model results suggest that the reaction between uncomplexed Fe(III) and O2? was a significant source of Fe(II) during EisenEx and helped to maintain high levels of Fe(II) in the water column. This phenomenon may occur in iron enrichment experiments when two conditions are met: (i) When Fe is added to a system already saturated with regard to organic complexation and (ii) when mixing processes are slow, thereby reducing the dispersion of iron into under-saturated waters.

Description: The mixing regime of the upper 180 m of a mesoscale eddy in the vicinity of the Antarctic Polar Front at 47° S and 21° E was investigated during the R.V. Polarstern cruise ANT-XVIII/2 within the scope of the iron fertilization experiment EisenEx. On the basis of hydrographic CTD and ADCP profiles we deduced the vertical diffusivity Kz from two different parameterizations. Since these parameterizations bear the character of empirical functions, based on theoretical and idealized assumptions, they were inter alia compared with Cox-number and Thorpe-scale related diffusivities deduced from microstructure measurements, which supplied the first direct insights into turbulence of this ocean region. Values of Kz in the range of 10**-4 - 10**-3 m**2/s appear as a rather robust estimate of vertical diffusivity within the seasonal pycnocline. Values in the mixed layer above are more variable in time and reach 10**-1 m**2/s during periods of strong winds. The results confirm a close agreement between the microstructure-based eddy diffusivities and eddy diffusivities calculated after the parameterization of Pacanowski and Philander [1981, Journal of Physical Oceanography 11, 1443-1451, doi:10.1175/1520-0485(1981)011〈1443:POVMIN〉2.0.CO;2].