Search

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 ( 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 cm2 s−1 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.

This study explores the changes in the surface water fugacity of carbon dioxide ( f CO 2 ) and biological carbon uptake in two Southern Ocean iron fertilisation experiments with different hydrographic regimes. The Southern Ocean Iron Release Experiment (SOIREE) experiment was carried out south of the Antarctic Polar Front (APF) at 61°S, 141°E in February 1999 in a stable hydrographic setting. The EisenEx experiment was conducted in a cyclonic eddy north of the APF at 48°S, 21°E in November 2000 and was characterised by a rapid succession of low to storm-force wind speeds and dynamic hydrographic conditions. The iron additions promoted algal blooms in both studies. They alleviated algal iron limitation during the 13-day SOIREE experiment and probably during the first 12 days of EisenEx. The f CO 2 in surface water decreased at a constant rate of 3.8 μatm day −1 from 4 to 5 days onwards in SOIREE. The f CO 2 reduction was 35 μatm after 13 days. The evolution of surface water f CO 2 in the iron-enriched waters (or ‘patch’) displayed a saw tooth pattern in EisenEx, in response to algal carbon uptake in calm conditions and deep mixing and horizontal dispersion during storms. The maximum f CO 2 reduction was 18–20 μatm after 12 and 21 days with lower values in between. The iron-enriched waters in EisenEx absorbed four times more atmospheric CO 2 than in SOIREE between 5 and 12 days, as a result of stronger winds. The total biological uptake of inorganic carbon across the patch was 1389 ton C (±10%) in SOIREE and 1433 ton C (±27%) in EisenEx after 12 days (1 ton=10 6 g). This similarity probably reflects the comparable size of the iron additions, as well as algal growth at a similar near-maximum growth rate in these regions. The findings imply that the different mixing regimes had less effect on the overall biological carbon uptake across the iron-enriched waters than suggested by the evolution of f CO 2 in surface water.

International audience; Grazing rates and grazing selectivity of Antarctic copepods (Rhincalanus gigas, Calanus simillimus and copepods 0.8 to 2 mm in size) were determined using incubation experiments comparing plankton community development with and without added grazers. Estimates of grazing rates were corrected for trophic cascade effects using a simple ecosystem model composed of 5 compartments: (1) nanophytoplankton, (2) nanozooplankton, (3) microphytoplankton, (4) microzooplankton (mainly protozoans) and (5) copepods. Copepod grazing rates on the different planktonic compartments were derived by solving for model parameters as constrained by microscopy enumeration of nano- and microphytoplankton and nano- and microprotozooplankton in treatments with and without added copepods. Simulations based on different parameter values and model configurations support the notion that grazing estimates were robust and between 18 and 30 % higher than uncorrected estimates. Furthermore, model analyses suggest the presence of an additional trophic cascade effect due to grazing of R. gigas on small copepods and copepod developmental stages present in incubation bottles.

This thesis provides a comprehensive assessment of the response and vertical distribution of major components of the pelagic ecosystem and highlights the species interactions shaping this open ocean environment and its associated biogeochemical cycles.During the iron fertilization experiment (EisenEx), carried out in the Polar Frontal Zone of the Southern Ocean in austral spring (November), the composition, succession and temporal development of the nano-, microprotozooplankton (protozoa between 2 and 20 µm and 20 and 200 µm respectively), mesozooplankton (protozoa > 200 µm) and small metazooplankton (copepod nauplii, copepodites and adults of small species) assemblage was successfully followed for three weeks and compared with non-fertilized, surrounding water. The grazing impact of microprotozooplankton on the natural phytoplankton assemblage was also studied under controlled conditions using the dark incubation method. These experimental results combined with data on microphyto- and microprotozooplankton composition and temporal development were used to estimate microprotozoan grazing impact on primary producers, in particular the diatoms, during the fertilization experiment. The temporal development of particles produced by proto and small metazooplankton (empty diatom frustules, fecal pellets, skeletons and empty loricae) and their contribution to carbon and silica vertical fluxes during EisenEx were also investigated.Key words: Polar Frontal Zone; EisenEx; protozooplankton; small metazooplankton; iron fertilization; temporal development; vertical fluxes; grazing impact; copepod nauplii; small copepods, microphytoplanktonDistinct differences in microprotozoan assemblage inside and outside in relation with the phytoplankton bloom development were not observed. The microprotozooplankton population was dominated by dinoflagellates followed by ciliates. Dinoflagellates showed a ~ three-fold increase in abundance and biomass in the first 10 d of the experiment inside the patch, but decreased thereafter to about two-fold higher values compared to pre-fertilization values. The decline after day 10 indicates increasing grazing pressure by copepods. Dinoflagellate population was dominated by small sized (20-40 µm) athecate dinoflagellates which comprised up to 58 % of population stocks suggesting that also food limitation could be a further explanation for the strong decrease since the phytoplankton bloom was dominated by long chain-forming diatoms in the second half of the experiment which are not available for these dinoflagellates as food source. Ciliate abundances increased only slightly due to heavy grazing pressure by copepods, but biomass doubled with the majority of the increase caused by a three-fold increase in aloricate choreotrichs.Grazing impact by microprotozoa estimated for EisenEx showed that microprotozoa consume only a rather small fraction of primary production, including the diatoms (6 to 18 % of primary production grazed d-1, corresponding to 3 to 9 % of diatom daily carbon stock). Grazing rate on other phytoplankton (without diatoms) was relatively higher accounting for 9 to 28 % of other phytoplankton carbon stock d-1. Thus, microprotozooplankton grazing strongly affected nanoplankton populations, but was low on the diatom assemblage.Tintinnids, thecate dinoflagellates, foraminifera, radiolarian and acantharia comprised the bulk of larger (> 50 µm) protozoan assemblage. Inside the patch, acantharian numbers increased three-fold, but only negligibly in surrounding waters. This is of major interest, since acantharia are suggested to be responsible for the formation of barite found in sediments and which is possibly a paleoindicator of high productivity regimes. Foraminiferans also increased significantly in abundance, however the marked increase of juvenile individuals after a full moon event suggests a lunar periodicity in the reproduction cycle of some foraminiferan species rather than a reproductive response to enhanced food availability. Larger thecate dinoflagellates almost doubled in numbers and biomass, but also showed an increase outside the patch. In contrast, adult radiolaria showed no clear trend during the experiment, but juveniles increasing three-fold indicating elevated reproduction. Tintinnids decreased two-fold, whereas their carbon stocks remained more or less constant caused by a change in the relative importance of species. Empty tintinnid loricae, however, increased by a factor of two indicating that grazing pressure on this group intensified during EisenEx. In mixed layer (0 to 80 m depth), overall vertical distribution followed mainly vertical distribution of potential prey particles. The stocks of larger protozoa in the surface layers was negligible when compared to the rest of the protozoa and zooplankton. Below 80 m, biomass was dominated by foraminifera indicating important influence on biological processes occurring below the mixed layer during EisenEx.Within the small metazoans, copepod nauplii numbers stayed more or less constant, whereas biomass doubled indicating individual growth during EisenEx. Numbers of small sized copepodites and adults of cyclopoids (predominantly Oithona similis) and small calanoid copepods (predominantly Ctenocalanus citer) increased ~ three-fold suggesting either a enhanced recruitment from naupliar and copepodite stages, respectively, and/or an accumulation of numbers due to migration of individuals from below the mixed layer into the upper water column caused by a diet shift.Particles produced by proto- and metazooplankton during this study showed an increase in the course of the experiment and were correlated to the vertical distribution of their producers. Protozoan fecal pellets were significantly correlated to athecate dinoflagellates indicating that they were mainly responsible for fecal pellet production. The dominance of olive green ellipsoidal to spherical pellets discriminated during EisenEx suggest that particularly larger protozoan were also actively involved in either re-ingestion and repackaging of larger metazoan pellets. Copepods were the main source of large recognisable metazoan fecal pellets as indicated by the dominance of compact cylindrical pellets characteristic for copepod feces. Both protozoan, metazoan fecal pellets, empty diatom frustules and empty tintinnid loricae showed very high abundances as compared to radiolarian skeletons. Comparison with metazoan fecal pellet abundance indicate that contribution of protozoan fecal pellets and empty diatom frustules to total vertical fluxes could be significant, despite low sinking rates.