Description: On voyages in the Iceland Basin in 2007 and 2009, we observed low (ca. 0.1nM) total dissolved iron concentrations dFe in surface waters (〈150m), which increased with depth to ca. 0.2-0.9nM. The surface water dFe was low due to low atmospheric Fe inputs combined with biological uptake, with Fe regeneration from microbial degradation of settling biogenic particles supplying dFe at depth. The organic ligand concentrations LT in the surface waters ranged between 0.4 and 0.5nM, with conditional stability constants (logK'FeL) between 22.6 and 22.7. Furthermore, LT was in excess of dFe throughout the water column, and dFe was therefore largely complexed by organic ligands (>99%). The ratio of LT/dFe was used to analyse trends in Fe speciation. Enhanced and variable LT/dFe ratios ranging between 1.6 and 5.8 were observed in surface waters; the ratio decreased with depth to a more constant LT/dFe ratio in deep waters. In the Iceland Basin and Rockall Trough, enhanced LT/dFe ratios in surface waters resulted from decreases in dFe, likely reflecting the conditions of Fe limitation of the phytoplankton community in the surface waters of the Iceland Basin and the high productivity in the Rockall Trough.Below the surface mixed layer, the observed increase in dFe resulted in a decrease of the LT/dFe ratios (1.2-2.6) with depth. This indicated that the Fe binding ligand sites became occupied and even almost saturated at enhanced dFe in the deeper waters. Furthermore, our results showed a quasi-steady state in deep waters between dissolved organic Fe ligands and dFe, reflecting a balance between Fe removal by scavenging and Fe supply by remineralisation of biogenic particles with stabilisation through ligands. © 2011 Elsevier Ltd.

Description: The Southern Ocean is an important biogeochemical region on a global scale, in which mineralising phytoplankton play a role in cycling energy, carbon and nutrients. Mineralising phytoplankton with cells 2-20. μm in diameter (nannoplankton) are poorly enumerated by traditional preservation and microscopy techniques, yet may fulfil an important role in the Southern Ocean. Here we define the spatial and temporal biogeography for these mineralising nannoplankton assessed by scanning electron microscopy in conjunction with an array of biological, physical, and chemical variables during two cruises to the Scotia Sea region of the Southern Ocean. The cruises encompassed two seasons, austral summer (January-February 2008) and austral autumn (March-April 2009).The biogeography of the three most numerous mineralising nannoplankton groups, the coccolithophore Emiliania huxleyi, the smaller (〈10μm) species of the diatom genus Fragilariopsis, and chrysophytes of the genus Tetraparma (mostly Tetraparma pelagica) were found to be related to the boundaries of the major circumpolar fronts. E. huxleyi abundances were relatively high in the northern water masses (maximum of 650cellsml -1), while T. pelagica abundances were high in the southern water masses (maximum of 1910cellsml -1). Small Fragilariopsis spp. abundances were also highest in the southern water masses (maximum of 1820cellsml -1), but this group was present throughout the Scotia Sea.Multivariate statistical analysis found that the most influential environmental variables controlling mineralising nannoplankton biogeography were sea surface temperature and silicate concentration. Estimates of biomass indicated that the Scotia Sea mineralising nannoplankton community formed a substantial part of the total phytoplankton community, particularly south of the Southern Antarctic Circumpolar Current Front (SACCF) during the austral autumn, where mineralising nannoplankton biomass reached 36 of the total phytoplankton biomass. The results that are obtained suggest that traditional microscopic surveys of large Southern Ocean phytoplankton may underestimate total biomass by excluding key mineralising nannoplankton groups. Greater appreciation of the ecological significance of mineralising nannoplankton in the Southern Ocean will improve our understanding of the relationships between environmental parameters, primary production, and the biological carbon pump in this ecosystem. © 2011 Elsevier Ltd.

Description: The Southern Ocean is largely a High Nutrient Low Chlorophyll (HNLC) region where macronutrient concentrations are high and phytoplankton productivity is low. However, there are productive 'hot spots' that sustain large phytoplankton blooms. These areas, maintained by natural iron (Fe) fertilization, are important for the Southern Ocean ecosystem and for driving carbon export. Fe addition on-deck bioassay experiments were conducted on two cruises to the Scotia Sea region of the Southern Ocean (austral spring 2006 and summer 2008) to better understand how Fe controls the microplankton (20-200μm) community structure on a seasonal basis. Light microscopy and fast-repetition rate fluorometry were used to examine changes in the species composition and physiological status of the microplankton community. Bioassays were carried out in three contrasting regions of the Scotia Sea: (1) a naturally Fe-fertilized, high chlorophyll area downstream (north and northwest) of the Islands of South Georgia (DSG); (2) a low Fe, low chlorophyll area upstream (south) of the Islands of South Georgia (USG); and (3) a naturally Fe-fertilized area north of the South Orkney Islands (SOI). Multivariate statistics were applied to the light microscopy results, showing significant differences between the initial microplankton communities for each of the bioassays. These differences were primarily spatial (between regions) and secondarily temporal (between seasons). Significant microplankton community shifts occurred in three of five bioassays, those in spring and summer USG and in summer DSG only. In summer, USG community responses increased significantly in medium (100-1000pgCcell -1) and large (>1000pgCcell -1) diatom species in response to Fe addition. Such a response was consistent with relief from in situ Fe limitation, which favours larger microplankton species with higher Fe requirements and subject to lower grazing pressures. The largest biomass increase in Fe-treated bioassay bottles was in Pseudonitzschia spp., which suggests that this genus may be a particularly important member of the microplankton community in the Scotia Sea. © 2011 Elsevier Ltd.

Description: Concentrations of dissolved iron (DFe) and Fe-binding ligands were determined in the tropical Northeast Atlantic Ocean (12-30°N, 21-29°W) as part of the UK-SOLAS (Surface Ocean Lower Atmosphere Study) cruise Poseidon 332 (P332) in January-February 2006. The surface water DFe concentrations varied between 0.1 and 0.4 nM with an average of 0.22 ± 0.05 nM (n = 159). The surface water concentrations of total Fe-binding ligands varied between 0.82 and 1.46 nM with an average of 1.11 ± 0.14 nM (n = 33). The concentration of uncomplexed Fe-binding ligands varied between 0.64 and 1.35 nM with an average of 0.90 ± 0.14 nM (n = 33). Thus, on average 81 of the total Fe-binding ligand concentration was uncomplexed. The average logarithmic conditional stability constant of the pool of Fe-binding ligands was 22.85 ± 0.38 with respect to Fe 3+ (n = 33). A transect (12°N, 26°W to 16°N, 25.3°W) was sailed during a small Saharan dust event and repeated a week later. Following the dust event, the concentration of DFe increased from 0.20 ± 0.026 nM (n = 125) to 0.25 ± 0.028 (n = 17) and the concentration of free Fe-binding ligands decreased from 1.15 ± 0.15 (n = 4) to 0.89 ± 0.10 (n = 4) nM. Furthermore, the logarithmic stability constants of the Fe-binding ligands south of the Cape Verde islands were distinctively lower than north of the islands. The absence of a change in the logarithmic stability constant after the dust event south of the Cape Verde islands suggests that there was no significant atmospheric input of new Fe-binding ligands during this dust event.

Description: Based on an international workshop (Gothenburg, 14–16 May 2008), this review article aims to combine interdisciplinary knowledge from coastal and open ocean research on iron biogeochemistry. The major scientific findings of the past decade are structured into sections on natural and artificial iron fertilization, iron inputs into coastal and estuarine systems, colloidal iron and organic matter, and biological processes. Potential effects of global climate change, particularly ocean acidification, on iron biogeochemistry are discussed. The findings are synthesized into recommendations for future research areas.

Description: Amidst high-nutrient low-chlorophyll waters of the south-western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia (37°W, 55°S). South Georgia blooms have a vital role in the ecosystem surrounding the island, and have been linked to one of the strongest seasonal atmospheric-carbon uptake in the open Southern Ocean. Which environmental conditions drive such remarkable productivity are still under debate, and were investigated in the current study using a multidisciplinary approach. Satellite-derived observations of surface chlorophyll a concentrations and circulation patterns were used to study the annual and inter-annual variability of phytoplankton blooms in the region. Our analysis reveals a time series of very regular blooms, controlled in space by circulation and regulated in time by surface silicate concentrations, temperature and light. The role of the fundamental, yet limiting, micronutrient iron was investigated with the coupled hydrodynamic-biogeochemical model ROMS_AGRIF-PISCES. Model results, validated against available observations, suggest a continuous supply of dissolved iron from the island's shallow shelves that is redistributed in the region by local circulation. Conversely, aeolian sources of iron have a negligible role in the main bloom area, but appear to be more important outside the influence of the South Georgia island mass effect.

Description: In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia. Multiple sources, including shallow sediments and atmospheric dust deposition, are thought to introduce iron to the region. However, the relative importance of each source is still unclear, owing in part to the scarcity of dissolved iron (dFe) measurements in the South Georgia region. In this study, we combine results from a recently published dFe data set around South Georgia with a coupled regional hydrodynamic and biogeochemical model to further investigate iron supply around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north (i.e. downstream) of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of ~0.02 nM and ~0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and very shallow shelf-sediments (depths 〈 20 m). Dust deposition exerts almost no effect on surface chlorophyll a concentrations. Other sources of iron such as run-off and glacial melt are not represented explicitly in the model, however we discuss their role in the local iron budget.

Description: In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia. Multiple sources, including shallow sediments and atmospheric dust deposition, are thought to introduce iron to the region. However, the relative importance of each source is still unclear, owing in part to the scarcity of dissolved iron (dFe) measurements in the South Georgia region. In this study, we combine results from a recently published dFe data set around South Georgia with a coupled regional hydrodynamic and biogeochemical model to further investigate iron supply around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north (i.e. downstream) of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of �0.02nM and �0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and very shallow shelf-sediments (depths 〈 20 m). Dust deposition exerts almost no effect on surface chlorophyll a concentrations. Other sources of iron such as run-off and glacial melt are not represented explicitly in the model, however we discuss their role in the local iron budget.