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Response of the pelagic tunicate Salpa thompsoni to an iron induced phytoplankton bloom in the Southern Ocean Frontal Zone(EIFEX)

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von Harbou, L. , Bathmann, U. , Peeken, I. , Jansen, S. and Krägefsky, S. (2004): Response of the pelagic tunicate Salpa thompsoni to an iron induced phytoplankton bloom in the Southern Ocean Frontal Zone(EIFEX) , 22. Internationale Polartagung der Deutschen Gesellschaft für Polarforschung, 18.-24.Sept.,Jena,Germany. .
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The pelagic tunicate Salpa thompsoni is an important member of Southern Ocean mesozooplankton. In the last decades, its distribution has extended southward with the declining ice coverage in certain Antarctic regions (Loeb et al 1997, Atkinson et al 2004). They especially proliferate in areas with Chlorophyll a concentrations below 1 µg L-1 (Perissinotto et al 1998) as higher particles loads seemingly harm salp feeding (Harbison et al 1986). During the European Iron Fertilisation EXperiment (EIFEX, 21.01. 25.03.2004), an eddy in the Southern Polar Frontal Zone was fertilized twice with Fe(II)SO4 to follow the build-up and fate of an iron induced bloom and the various pathways of fixed carbon. The fertilized waters (in-patch) of the eddy reached maximum chlorophyll (Chl a) concentrations of ca. 2.9 ug L-1 from surface down to 80 m and spread over ca. 500 km2 at the end of the experiment. Salp abundance and size decreased generally over time following their seasonal cycle but they disappeared earlier in fertilized waters. Gut fluorescence of salps was high indicating a strong grazing impact of salps in both areas at the beginning of the experiment. Furthermore, salps from in-patch stations showed significantly higher gut fluorescence than salps from out-patch stations with 0.69 ug and 0.28 ug Chl a Ind-1 with 3 cm body length, respectively. Microscopy and HPLC analysis revealed that the phytoplankton composition of adjacent waters down to 500 m was reflected in the salp guts and changed with the developing bloom. However, salps generally showed a higher phaeopigment to Chl a ratio than found in the water masses due to digestion. Although production rates of fecal pellets did not alter with Chl a concentrations and ranged between 0.5 to 1.0 fecal pellet Ind-1h-1 for in- and out-patch stations, the ratio of Phaeopigments to Chl a was significantly lower in guts and fecal pellets from in-patch compared to out-patch animals supposing a less effective digestion at higher food supply.In this study, we could show the impact of a changing diet on individual salps and discern effects on the salp population. These findings will help to understand the salp distribution especially in the Southern polar sea and their possible impact on the Antarctic food web and downward fluxes.

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