HR: 09:15h
AN: H51E-05    [Abstracts]
TI: Contribution of Diazotrophs to the Nitrogen Pool Along 10$\deg$N in the Subtropical Atlantic
AU: * Voss, M
EM: voss@io-warnemuende.de
AF: Baltic Sea Research Institute, Seestrasse 15, Warnemuende, 18119 Germany
AU: Mills, M M
EM: mmills@ifm-geomar.de
AF: Leibniz Institut fuer Meereswissenschaften, Duesternbrooker Weg 20, Kiel, 24105 Germany
AU: Wallace, D W
EM: dwallace@ifm-geomar.de
AF: Leibniz Institut fuer Meereswissenschaften, Duesternbrooker Weg 20, Kiel, 24105 Germany
AB: The Meteor 55 cruise from October 12th to November 17th, 2002 was carried out along 10$\deg$N across the subtropical Atlantic from 56$\deg$W to 19$\deg$W. Here we present a data set from approximately 40 stations that were sampled with a CTD system from surface to 500m depth. Nutrient and particulate organic matter concentrations as well as isotope measurements from POM, NO${3}$, and size fractionated zooplankton were used to estimate the diazotrophic contribution to the particulate nitrogen pools along the transect. An evaluation of the percentage of nitrogen originating from diazotrophs was performed by means of isotope signatures of the three zooplankton size fractions. Significantly higher N-contributions from nitrogen fixation were found at the western end of the transect from 56$\deg$ to 30$\deg$W with 19 to 61% (mean 48%) and only 11-42% (mean 30%) between 25$\deg$ and 19$\deg$W. The diazotrophic contribution to the zooplankton was correlated significantly (n=13, p$<$0.005, r$^{2}$=0.56) with the contribution calculated to the PON, suggesting a direct link between the diazotrophs and the food web. As expected the 25 - 27 potential density surfaces shallowed from west to east with the 26 density surface found below 100m depth in the west and at 50m in the east. Similarly the DIN excess followed this slope and had a significant maximum around these density surfaces implying remineralization of N-rich organic matter produced by diazotrophs. The isolines of the $\delta^{15}$N-NO$_{3}$ signature followed the DIN excess across the Atlantic with a minimum between 100 and 200m. The isotopic data confirm previous findings which find evidence of more intense nitrogen fixation in the western Atlantic compared to the east. However, direct nitrogen fixation rate measurements along the cruise track showed highest rates off Africa. This activity is not mirrored in the natural abundance data. One reason may be the pronounced contribution of upwelled nitrate and the fractionation of the nutrients off Africa, where productivity is high.
DE: 9325 Atlantic Ocean
DE: 4805 Biogeochemical cycles (1615)
DE: 4845 Nutrients and nutrient cycling
DE: 4855 Plankton
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting