Impact of tephra on algae-bloom and carbon storage in the ocean

The fertilization potential of volcanic ash (particles of tephra <2 mm) on soil is well documented; however, its influence on the marine environment is widely unknown. Though, it has been suggested that volcanic ash in contact with water might release micro-nutrients, such as surface bound iron. Especially at conditions with HNLC (high-nutrient-low-chlorophyll), micro-nutrients may be limiting for algae growth/marine primary production (Duggen et al. 2010). The effect of addition of iron has been investigated, in terms of storage and drawdown of CO2 (Hamme et al. 2010), however, the effects are uncertain and debated in the literature. In the present work the fertilizing capacity and efficiency of volcanic ash collected from the Grimsvötn, Iceland, eruption in May 2011, is examined. The ash was sifted through different sieves (0-63, 63-90, 90-212, 212-600, and 600-1000 μm), decomposed in nitric acid at 250 ⁰C using an Milestone Ultraclave® (microwave-technique), and analysed on ICP-OES (Perkin Elmer Optima 5300 DV) to check if there were differences in elemental composition between the fractions. The size-fraction analysis showed that the one containing the most nutrients totally, was the smaller portion <63 μm. pH, salinity, and redox-conditions do affect speciation and remobilisation of trace elements. Thus in order to estimate the speciation of particulate trace metals most likely limiting algae growth in marine habitats (Co, Cu, Fe, Mn, Si, P, and Zn), tephra with particle size <2 mm was subjected to a six steps sequential chemical extraction procedure. The order of magnitude of each element that is reversible bound was rather small (approximately 4% Mn, 4% Si, 9% Zn, 10% P, 0,2% Fe, 8% Cu, and 3% Co), compared with the irreversibly bound portions. This however does not necessarily mean that the nutrient released does not create any effect with respect to algal growth. This is examined during an algae cultivation experiment. Green-algae (Tetraselmis suecica) were grown “in vitro” in the laboratory in saltwater added f/2 medium, at 23 ⁰C and a light intensity of 200 μmol photons m-2s-1. An experimental set up is in progress where cell growth is measured in 330 mL tubes filled with algae containing seawater added different amounts of ash (0,8 g, 1,6 g, and 8 g per tube) at different size fractions (<63 μm and <2mm). Cell growth is measured in terms of dry weight, cell count, and turbidity. References Duggen, S., Olgun, N., Croot, P., Hoffmann, L., Diete, H., Demelle, P. & Teschner, C. (2010). The role of airborne volcanic ash for the surface ocean biobeochemical iron sycle: a review. Biogeosciences, 7: 827-844. Hamme, R. C., Webley, P. W., Crawford, W. R., Whiteney, F. A., DeGrandpre, M. D., Emerson, S. R., Eriksen, C. C., Sabine, C. L., Batten, S. D., Coogan, L. A., et al. (2010). Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific. Geophysical research letters, 37 (L19604): 5.