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The Mediterranean Sea presents several biogeochemical anomalies compared to the global ocean. An unbalanced N budget, high nitrate/phosphate ratios in subsurface waters and low 15N/14N ratios in particulate and dissolved nitrogen suggest a significant occurrence of N2 fixation. This study presents, for the first time, a basin-wide overview of direct measurements of N2 fixation, with values in the North Atlantic for comparison, during late spring 2007. Very low N2 fixation rates (0.052 ± 0.031 nmols N l-1d-1) were observed in all sub-regions of the Mediterranean, in contrast to the higher values measured in the North Atlantic (0.300 ± 0.115 nmols N l-1d-1). Higher phosphorus (inorganic or organic) concentrations were not associated with higher N2 fixation rates. Low 15N/14N ratios in particulate organic nitrogen (from -2.10 to +4.11?), associated with low N2 fixation rates, suggest that other N sources, such as atmospheric inputs, fuel the Mediterranean ecosystem. Citation: Ibello, V., C. Cantoni, S. Cozzi, and G. Civitarese (2010), First basin-wide experimental results on N2 fixation in the open Mediterranean Sea, Geophys. Res. Lett., 37, L03608, doi:10.1029/2009GL041635.

L'Adriatico meridionale è un sito dominato da una dinamica ciclonica e da processi convettivi profondi che vengono innescati da forzanti atmosferici. Questa sua caratteristica lo rende un sito marino particolarmente sensibile alle variazioni meteorologiche e climatiche che possono influenzare sensibilmente il sequestro di CO2 e la produzione di carbonio organico (pompa biologica). La disponibilità di nutrienti e le modalità con cui vengono forniti alla zona eufotica giocano un ruolo chiave nel determinare una produzione biologica dominata dalla rete trofica classica e da grandi specie fitoplanctoniche, o più spostata verso il "microbial loop". Lo studio dell'abbondanza isotopica di 15N nel particellato marino (15N-PN) e nel nitrato disciolto (15N-NO3) permette di ottenere informazioni sia sull'origine del nitrato che sostiene la produzione fitoplanctonica, che sulle modalità con cui viene fornito e assimilato nella zona eufotica. Nei primi due anni di attività del progetto VECTOR sono state effettuate 4 campagne oceanografiche nell'Adriatico Meridionale lungo il transetto Bari-Dubrovnik, durante le quali sono stati raccolti i primi dati di 15N-PN e di 15N-NO3 relativi al sottobacino. I valori di abbondanza isotopica vengono presentati e discussi in relazione alle 4 diverse situazioni oceanografiche osservate rispettivamente in novembre 2006 (AM1), febbraio 2007 (AM2), aprile 2007 (AM3) e settembre 2007 (AM4), alle masse d'acqua presenti ed alle concentrazioni di azoto inorganico disciolto. L'analisi effettuata mira a: (1) evidenziare quali siano le principali fonti di azoto che sostengono la produzione primaria nei 4 periodi considerati, (2) stimare se i nutrienti vengano forniti da un'alternanza di apporti dalla zona sub-eufotica seguiti da una ri-stratificazione ("sistema chiuso") o da una diffusione continua dalla massa d'acqua sottostante ("sistema aperto").

Nitrogen uptake experiments provide important information on the nutrient cycling in the marine environment and on the growth of planktonic communities. Plankton assimilates nitrogen mainly for structural purposes and thus its uptake basically estimates the population growth, contrary to carbon and phosphorus that are also turned over in energetic cellular processes. Moreover, nitrogen uptake experiments allow the distinction between new (based on newly available nitrogen from outside the euphotic zone) and regenerated (based on recycled nitrogen inside the euphotic zone) primary production. This distinction has important implications for studies concerning the ocean carbon cycle and, in consequence, the carbon dioxide exchange with the atmosphere. The ratio of new to total production, integrated over a long time-scale, represents an estimate of the exportable production from the euphotic zone toward deeper waters. However, the analysis of nitrogen uptake data is complicated by the fact that they are not representative only of phytoplanctonic primary production as also bacteria assimilate several important nitrogen forms (i.e. nitrate, ammonium and urea). Bacteria production should not be accounted as exportable production since it basically remains in the euphotic zone being processed through the microbial loop. The partitioning of the total nitrogen uptake between phytoplankton and bacteria is usually obtained by size-fractionation experiments, but the serial filtration technique has several disadvantages. Size fractions do not exactly correspond to functional plankton classes, filters can retain bacteria because of their tightly association to large particles and high concentrations of suspended particulate matter alter filter porosity. The utilization of bacterial or algal inhibitors is another potentially powerful approach to the partitioning of the total nitrogen uptake that has not been largely used to date. In this work, we present preliminary results of two nitrogen uptake experiments carried out in August and October 2003 in the Gulf of Trieste (Northern Adriatic Sea, ITALY) in conditions of high and low primary production, respectively. The total (nitrate + ammonium + urea) nitrogen uptake was partitioned among some functional plankton classes with a combined utilisation of a treatment with bactericidal agents and light-dependent incubations. In the former experiment, phytoplankton, heterotrophic bacteria and photosynthetic bacteria contributed to the total nitrogen uptake (104 nmol-N/dm3 h) by 71, 7 and 22%, respectively. During the second experiment, they contributed to the total nitrogen uptake (24 nmol-N/dm3 h) by 94, 1 and 5%.

Metodologia sperimentale per la misurazione dell'assimilazione del nitrato, nitrito, ammonio e urea nel particellato vivente marino mediante tecnica di spettrometria di massa.