Impact of diazotrophy on N stable isotope signatures of nitrate and particulate organic nitrogen: case studies in the north-eastern tropical Atlantic Ocean.
During two independent cruises in the north-eastern tropical Atlantic Ocean, we applied two different approaches to investigate the impact of diazotrophy on nitrogen stable isotope signatures in nitrate and particulate organic nitrogen (PON) of the food-web constituents. The first approach, used during the Poseidon cruise 348 in the Mauritanian upwelling, investigated the long-term influence of diazotrophy on the natural abundance of δ(15)N-NO(-)(3) and PON. The second approach, adopted during the Cape Verde field cruise, applied stable isotope tracer addition experiments. These served to determine the instantaneous transfer of diazotrophic N to the higher trophic level. Both approaches showed that N(2) fixation was compatible with the pattern and the magnitude of the isotopic depletion of dissolved NO(-)(3) during the Mauritanian upwelling cruise, as well as PON in zooplankton and phytoplankton during the Cape Verde cruises. An N-budget using (15)N incorporation rates and diazotrophic N(2) fixation rates showed that 6 % of the daily N(2) fixation was potentially taken up by the mesozooplankton community. Direct grazing accounted for 56 % of gross mesozooplanktonic N incorporation, while 46 % occurred due to channelling through the microbial loop.
Wannicke
Nicola
N
Department of Biological Oceanography, Leibniz Institute for Baltic Sea Research, Seestrasse 15, Rostock, Germany. nicola.wannicke@io-warnemuende.de
Liskow
Iris
I
Voss
Maren
M
D000818:Animals; D001267:Atlantic Ocean; D055726:Cabo Verde; D033342:Copepoda / Q000254:growth & development / Q000378:metabolism; D000458:Cyanobacteria / Q000254:growth & development / Q000378:metabolism; D017753:Ecosystem; D020387:Food Chain; D009566:Nitrates / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D009586:Nitrogen Fixation / Q000502:physiology*; D009587:Nitrogen Isotopes / Q000378:metabolism*; D009930:Organic Chemicals / Q000378:metabolism*; D010839:Phytoplankton / Q000254:growth & development / Q000378:metabolism; D015048:Zooplankton / Q000254:growth & development / Q000378:metabolism
D009566:Nitrates; D009587:Nitrogen Isotopes; D009930:Organic Chemicals; D009584:Nitrogen