Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence-although each with important uncertainties-lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

494Engel et al. AbstractWe studied the direct effects of CO 2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (ϳ11 m 3 each), the partial pressure of CO 2 (pCO 2 ) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (ϳ190 parts per million by volume (ppmV) CO 2 ), present (ϳ410 ppmV CO 2 ), and high (ϳ710 ppmV CO 2 ) pCO 2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO 2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO 2 -related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO 2 . This broad influence of CO 2 on the E. huxleyi bloom suggests that changes in CO 2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

[1] Volcanoes confront Earth scientists with new fundamental questions: Can airborne volcanic ash release nutrients on contact with seawater, thereby excite the marine primary productivity (MPP); and, most notably, can volcanoes through oceanic fertilization affect the global climate in a way that is so far poorly understood? Here we present results from biogeochemical experiments showing that 1) volcanic ash from subduction zone volcanoes rapidly release an array of nutrients (co-)limiting algal growth in vast oceanic areas, 2) at a speed much faster (minute-scale) than hitherto known and that marine phytoplankton from low-iron oceanic areas can swiftly, within days, utilize iron from volcanic sources. We further present satellite data possibly indicating an increase of the MPP due to the seaward deposition of volcanic particulate matter. Our study supports the hypothesis that oceanic (iron) fertilization with volcanic ash may play a vital role for the development of the global climate. Citation: Duggen, S., P. Croot, U. Schacht, and L. Hoffmann (2007), Subduction zone volcanic ash can fertilize the surface ocean and stimulate phytoplankton growth: Evidence from biogeochemical experiments and satellite data,

Abstract. Iron is a key micronutrient for phytoplankton growth in the surface ocean. Yet the significance of volcanism for the marine biogeochemical iron-cycle is poorly constrained. Recent studies, however, suggest that offshore deposition of airborne ash from volcanic eruptions is a way to inject significant amounts of bio-available iron into the surface ocean. Volcanic ash may be transported up to several tens of kilometers high into the atmosphere during largescale eruptions and fine ash may stay aloft for days to weeks, thereby reaching even the remotest and most ironstarved oceanic regions. Scientific ocean drilling demonstrates that volcanic ash layers and dispersed ash particles are frequently found in marine sediments and that therefore volcanic ash deposition and iron-injection into the oceans took place throughout much of the Earth's history. Natural evidence and the data now available from geochemical and biological experiments and satellite techniques suggest that volcanic ash is a so far underestimated source for iron in the surface ocean, possibly of similar importance as aeolian dust. Here we summarise the development of and the knowledge in this fairly young research field. The paper Correspondence to: S. Duggen (svend duggen@skoleforeningen.de) covers a wide range of chemical and biological issues and we make recommendations for future directions in these areas. The review paper may thus be helpful to improve our understanding of the role of volcanic ash for the marine biogeochemical iron-cycle, marine primary productivity and the ocean-atmosphere exchange of CO 2 and other gases relevant for climate in the Earth's history.

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms .20 mm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte-to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (.20 mm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 mm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

Ocean acidification and greenhouse warming will interactively influence competitive success of key phytoplankton groups such as diatoms, but how long-term responses to global change will affect community structure is unknown. We incubated a mixed natural diatom community from coastal New Zealand waters in a short-term (two-week) incubation experiment using a factorial matrix of warming and/or elevated p CO 2 and measured effects on community structure. We then isolated the dominant diatoms in clonal cultures and conditioned them for 1 year under the same temperature and p CO 2 conditions from which they were isolated, in order to allow for extended selection or acclimation by these abiotic environmental change factors in the absence of interspecific interactions. These conditioned isolates were then recombined into ‘artificial’ communities modelled after the original natural assemblage and allowed to compete under conditions identical to those in the short-term natural community experiment. In general, the resulting structure of both the unconditioned natural community and conditioned ‘artificial’ community experiments was similar, despite differences such as the loss of two species in the latter. p CO 2 and temperature had both individual and interactive effects on community structure, but temperature was more influential, as warming significantly reduced species richness. In this case, our short-term manipulative experiment with a mixed natural assemblage spanning weeks served as a reasonable proxy to predict the effects of global change forcing on diatom community structure after the component species were conditioned in isolation over an extended timescale. Future studies will be required to assess whether or not this is also the case for other types of algal communities from other marine regimes.

Rising atmospheric CO 2 concentrations will have profound effects on atmospheric and hydrographic processes, which will ultimately modify the supple and chemistry of trace metals in the ocean. In addition to an increase in sea surface temperatures, higher CO 2 results in a decrease in seawater pH, known as ocean acidification, with implications for inorganic trace metal chemistry. Furthermore, direct or indirect effects of ocean acidification and ocean warming on marine biota will affect trace metal biogeochemistry via alteration of biological trace metal uptake rates and metal binding to organic ligands. We still lack a holistic understanding of the impacts of decreasing seawater pH and rising temperatures on different trace metals and marine biota, which complicates projections into the future. Here, we outline how ocean acidification and ocean warming will influence the inputs and cycling of Fe and other biologically relevant trace metals globally and regionally in high and low latitudes of the future ocean; we discuss uncertainties and highlight essential future research fields. KEY WORDS: Ocean acidification · Ocean warming · Trace metals Resale or republication not permitted without written consent of the publisher Contribution to the Theme Section 'Biological responses in an anthropogenically modified ocean'OPEN PEN

Abstract. 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.Correspondence to: E. Breitbarth (ebreitbarth@chemistry.otago.ac.nz)