Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx‐2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo‐nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth‐integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

EisenEx-the second in situ iron enrichment experiment in the Southern Ocean-was performed in the Atlantic sector over 3 weeks in November 2000 with the overarching goal to test the hypothesis that primary productivity in the Southern Ocean is limited by iron availability in the austral spring. Underwater irradiance, chlorophyll a (Chl a), photochemical efficiency, and primary productivity were measured inside and outside of an iron-enriched patch in order to quantify the response of phytoplankton to iron fertilization. Chl a concentration and photosynthetic rate ( 14 C uptake in simulated in situ incubations) were measured in pico-, nano-, and microphytoplankton. Photochemical efficiency was studied with fast repetition rate fluorometry and xenon-pulse amplitude modulated fluorometry. The high-nutrient low-chlorophyll waters outside the Fe-enriched patch were characterized by deep euphotic zones (63-72 m), low Chl a (48-56 mg m , respectively. As a consequence, the euphotic depth decreased to ϳ41 m. Picophytoplankton biomass hardly changed. Nano-and microphytoplankton biomass increased. In the first 2 weeks of the experiment, when the depth of the upper mixed layer was mostly Ͻ40 m, primary productivity was highly correlated with Chl a. In the third week, productivity was much lower than predicted from Chl a, probably because of a reduction in photosynthetic capacity as a consequence of increased physical variability in the upper water column. These results provide unequivocal evidence that iron supply is the central factor controlling phytoplankton primary productivity in the Southern Ocean, even if the mixing depth is Ͼ80 m.

Clones of Cryptomonas phaseolus Skuja, Cryptomonas rostratiformis (Skuja) Skuja in Huber-Pestalozi, and Cryptomonas undulata Geruais were isolated from the deep chlorophyll maximum near the oxic/anoxic bounda7y layer of the mesoeutrophic lake Schlachtensee, Germany. Different autecologzcal features of these species were studied in batch culture experiments. Cryptomonas cf: ovata Ehrenberg and Chroomonas sf. that never dominated in the deep chlorophyll layer were also isolated from Schlachtensee to study their light-dependent growth in comparison to the deep-living species. Cryptomonas undulata, C. # ovata, and C. phaseolus had a very low light compensation point (5-7 pmol.m-2.s-1), whereas the growth rate of Chroomonas sp. and C . rostratiformis was positive above 16 and 24 pmol.m-2.s-1. Cryptomonas phaseclus and Chroomonas sp. became photoinhibited above photon jlux densities of 92 and 11 6 pmol.m-2.s-1. Cryp tomonas rostratiformis, C. CJ: ovata, and C. undulata reached a maximum growth rate at a considerably higher photon jlux density ( 1 98-250 pmol.m-2.s-1). Cryptomonas phaseolus grew fastest under light-limiting conditions. Chyptomonas phaseolus and C. undulata were best able to suruive prolonged periods of darkness. Cryptomonas phaseolus, C. rostratiformis, and C. undulata did not show any uptake of fluorescent latex beads. When labeled glucose was provided in naturally occurring concentrations, carbon uptake by C. phaseolus, C. rostratiformis, and C. undulata was negligzbly small in comparison to cellular carbon content. I suggest that the adaptation to a low-light environment is an important preadaptation for the dominance ofC. phaseolus and C. undulata near the freshwater chemocline.

Dedicated to Prof. Ju¨rgen Benndorf on the occasion of his 65th birthday. AbstractChlorophyll a (chla) concentration was evaluated as a predictor of phytoplankton biomass across a broad trophic gradient of lakes (oligotrophic -highly eutrophic). First, a literature survey was conducted to collect information on the proportion of chla in phytoplankton biomass. As a result of this study (n ¼ 21) a mean value of 0.505%70.197 S.D. chla per unit wet weight of phytoplankton was calculated. Second, analyses were performed on 756 paired measurements from an unpublished database where the specific chla content of phytoplankton biomass was regressed against phytoplankton standing stocks and chla concentration. Within an interval of 0.1-50 g m À3 of phytoplankton wet weight, a substantial decrease in chla proportion from approximately 2.5% to 0.18% was found. Likewise, the proportion in phytoplankton wet weight decreased from 0.7% to 0.15% across a chla concentration interval of 0.001-0.150 g m À3 . These results had a significant impact both on chla-based biomass calculations and the subsequent comparison with phytoplankton biomasses resulting from microscopic counts. Assuming the microscopic method was a measure of the ''true'' phytoplankton standing stocks, then the precision by which phytoplankton biomass might be predicted based on chla measurements is clearly better when using variable proportions as compared to a constant conversion factor. The same holds for temporal coherence between annual records of phytoplankton biomass. The temporal fit was apparently better when relating the results of microscopic counts and biomass estimation based on variable proportions of chla in phytoplankton biomass. Nevertheless, this effect diminished as the tropic status of the lakes increased. Because of their variable specific chla content, separate taxonomic groups of phytoplankton differently affected the proportion of chla in total ARTICLE IN PRESS www.elsevier.de/limno 0075-9511/$ -see front matter (P. Kasprzak). phytoplankton wet weight. Chlorophyceae, Cryptophyceae and cyanobacteria had a high impact, while Bacillariophyceae, Dinophyceae and Chrysophyceae were of lesser importance.