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N uptake conditions at the VERTIGO sampling sites

Nitrogen uptake conditions at VERTIGO study sites. M Elskens, N Brion, F Dehairs, W Baeyens & N Savoye Pleinlaan 2, B-1050 Brussel , Belgium (melskens@vub.ac.be).

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N uptake conditions at the VERTIGO sampling sites

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  1. Nitrogen uptake conditions at VERTIGO study sites M Elskens, N Brion, F Dehairs, W Baeyens & N Savoye Pleinlaan 2, B-1050 Brussel , Belgium (melskens@vub.ac.be) • IntroductionNitrogen uptake conditions were surveyed in the photic zone at station Aloha 22°45N-158°W du-ring 2004 VERTIGO Hawaii cruise and at station K2 47°N-161°E during 2005 VERTIGO Northwest Pacific cruise using stable isotope techniques. We aimed at determining new production rates, and therefore 15N and 13C isotopically la-belled compounds were used in a series of tracer experiments conducted over a 3 week period at the VERTIGO sites. • Experimental design • At station Aloha, combined 15N (NO3, NO2, NH4, N2) and 13C tracer experiments were conducted over periods of 12 hours under constant illumination and neutral density screens to mimic in situ light attenuation. Sampling depths were 5, 25, 50 & 125m. • At station K2, 15N (NO3, NH4) and 13C tracer experiments were conducted over periods of 2, 4 and 6 hours under constant illumination and neutral density screens to mimic in situ light attenuation. Sampling depths were 5, 25, 40 & 50m. Data analysis: A model comparison approach • New production assessment • Primary production PP (13C) • f-ratio (15N): • New production: PNEW = f-ratio x PP • Integration of PP and PNEW over the euphotic layer N uptake conditions at the VERTIGO sampling sites Primary and new production variability over the sampling period Acknowledgments This research was conducted under grant GOA22/DSCH WER4 as part of the project Geconcer-teerde Onderzoek-acties supported by the Vrije Universiteit Brussel and under grant G.0021.04 as part of a Research Foundation Flanders project. The authors would like to thank Ken Buesseler for the scientific coordina-tion of the VERTIGO project Conclusions Two regimes of new production have been identified; K2 shows relatively high nitrate specific uptake rates (VNO3) but rather low new production rates and is characteristic of HNLC regions. Aloha has low VNO3, low new production and behaves typically as an oligotrophic area. At both sites there is an agreement between the estimates of new production and the POC flux samples by neutrally buoyant traps at 150m with an export ratio ranging from 1 (Aloha) to maximally 3 at K2.

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