which can fuel 20%-100% of the nitrate uptake. Sinking particles represented <10% of total carbon fixation and -10%-50% of new production in terms of carbon and nitrogen. From these discrepancies it was suggested that (1) new production rates were overestimated because of the high level of nitrification that provided "regenerated nitrate" and (2) advection of dissolved organic carbon and nitrogen played an important role in export. The specific hydrodynamical circulation, a conveyor belt generated by upwelling at the equator and downwelling some degrees south, associated with biological in situ rcmineralization of ammonium and nitrate as well, appeared to be a very efficient system for recycling inorganic nitrogen in the euphotic layer and thus for supporting the high regenerated production levels. On the other hand, the high nitrate/silicate ratios (>1.5) observed in the upwelling waters seemed to indicate that silicate is not efficiently recycled in this specific circulation system because of its low regeneration rate as well as its sink via rapidly sedimenting diatoms cell walls; this may be also true for iron. This reinforces the idea of Si and/or Fe limitation which was put forward earlier to explain low new production levels in the equatorial Pacific. IntroductionNew production, defined as the fraction of primary production driven by the input of new nutrients (usually nitrate) into the euphotic zone [Dugdale and Goering, 1967], and export production, defined as the fraction of primary production exported as particles (carbon and nitrogen) [Eppley and Peterson, 1979], are important variables that characterize the efficiency of carbon and nitrogen cycling and particle export from the biological food web in the ocean. These fractions of photosynthetic production play a role in the transport of atmospheric carbon dioxide to the ocean interior, and their quantification is

Abstract-Experimental work during a cruise along a W-E transect in the Mediterranean Sea suggests that (1) orthophosphate concentrations in the upper photic zone show a decreasing trend from the west to the east reaching levels well below 1 nM and (2) microorganisms in the 0.6-2-m size fraction, probably Synechococcus, have, in addition to high affinity for orthophosphate, significantly higher maximum uptake rates than heterotrophic bacteria or eukaryotic algae. These specific advantages concerning orthophosphate uptake at low (Ͻ5 nM) as well as at relatively high (5-25 nM) concentrations could explain both general Synechococcus abundance in P-depleted environments and transient blooms of this species in the open ocean where episodic orthophosphate nanopulse events are likely to occur.

We developed a simple and reliable method which allows silnultaneous isotope-ratio analysis of inorganic (DIN) and organic (DON) forms of nitrogen extracted from seawater. All forms of nitrogen under analysis are converted to ammonium, by diffusion with magnesium oxide, prior to collection on glass-fiber filters appropriate for mass spectrometric assay of I5N. Oxidized DIN forms (nitrate, nitrite) are reduced to ammonium in the presence of Devarda alloy. Conversion of DON to ammonium is performed by wet oxidation using potassium persulfate and subsequent reduction of the nitrate formed. Recovery tests, both for total nitrogen and I5N content, showed that this procedure is suitable for application in DI"N-isotope dilution experiments and DON-release studies. Recovery of total nitrogen from DIN and DON was nearly complete (94 to 97%). The variability in the experimental determination of I5N abundance was < 2 % and ~4 % for DIN and DON, respectively. We used the method to balance the I5N budget in nitrate and ammonium uptake experiments conducted in an oligotrophic area (tropical North Atlantic) by including, in addition to the substrate (DIN) and biomass (PON) pool, the DON pool. However, the use of glass-fiber filters ( G F F ) for the collection of particulate matter produced a significant artifact, i.e. a large amount of small particles (<0.7 pm, PONcGF/F; prochlorophytes and/or bacteria) passed through these filters and were recovered together with the DON in a combined pool. While inclusion of this combined pool led virtually to a complete accounting for the I5N label (99%) in all samples for nitrate uptake and in those for ammonium uptake incubated for <8 h, no mass balance was achieved during ammonium uptake lasting 10 to 24 h. We suggest that the I5N that was still missing (13%) resulted mainly from bottle containment effects such as ammonium-ion adsorption and/or PON adherence onto incubation bottle walls. Transfer of I5N label to the combined pool (nitrate experiment) and to the DON and PON

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An automatic colorimetric procedure for measuring nanomolar quantities of nitrate and nitrite in seawater has been improved in sensitivity. Application of this method to in situ investigations in the North Atlantic Ocean in January 1989 and algal culture (Skeletonerna costatum) experiments in the laboratory demonstrated that one can record concentrations in the 0 to 100 nM range using classical Technicon AutoAnalyzer equipment. In this working range highest precision obtained was ± 2.87 and ± 1.24 nM for nitrate and nitrite, respectively.