Isotopic composition of particulate organic nitrogen and its relationship to nitrate assimilation in the Mediterranean Sea

The shifts in the N-15:N-14 ratio of particulate organic nitrogen (PON) suspended in the upper layer of the ocean have normally been interpreted in terms of changes in the isotopic composition of the source nitrogen used by the phytoplankton (nitrate, ammonium and dinitrogen gas) and/or the fractionation associated with the assimilation processes (N-14 is preferentially taken up by phytoplankton). The objective of the present study was to test which process, isotopic fractionation during NO3- uptake or reduced N-form consumption by phytoplankton, was more important in explaining the variations in the N-15 signature of PON in the surface layer of a geostrophic front area (northwest Alboran Sea, Mediterranean Sea). Samples of PON were collected at 6 different stations during 4 seasonal surveys. The particulate organic carbon (POC) and PON concentrations averaged over the mixed layer (ML) ranged from 142 and 23 mu g L-1 in summer to 104 and 13 mu g L-1 in autumn respectively. The C:N molar ratio of organic matter also varied seasonally from 7.2 in summer, winter and spring to 9.3 in autumn. The range of variation of the delta N-15 of PON in the ML was -0.1 parts per thousand to 6.5 parts per thousand. The lowest values of delta N-15 (lower than 1 parts per thousand) were obtained in summer and winter. However, the delta N-15 of PON in autumn and spring was higher than 3.5 parts per thousand. Consequently, the particulate matter in the ML was enriched in N-15 by about 1.5 parts per thousand in autumn and spring with respect to summer and winter. The delta N-15 of PON in the surface layer was negatively correlated with nitrate concentration (r = -0.62, n = 22, p = 0.002). The delta N-15 was also negatively correlated with the proportion of nitrate to total inorganic nitrogen (i.e. the sum of nitrate, nitrite and ammonium). Furthermore, there was a negative correlation between nitrate relative to nitrate plus ammonium assimilation rates (rNO(3)(-)) and the delta N-15 (r = -0.71, n = 20; p<0.001) for the surface samples. rNO(3)(-) was particularly low (lower than 0.2) for the surface samples collected during autumn, when primary productivity mainly depended on the consumption of ammonium coming from the recycling of organic matter. Therefore, delta N-15 of seston was a sensitive indicator of the transition from productivity based on nitrate to productivity based on regenerated ammonium.