Effects of the physical properties of water masses on microbial activity during an Ice Shelf Water overflow in the central Ross Sea

During the 1997-98 Italian Expedition to Antarctica a five-day mesoscale experiment was carried out on the continental shelf-break in the central Ross Sea. This area is oceanographically characterized by shelf/slope interactions, through intense mixing processes, between the Circumpolar Deep Water (CDW) and the Ice Shelf Water (ISW), coming from beneath the Ross Ice Shelf and spilling over the shelf edge. The export of dense shelf waters is of crucial importance not only for the mass balance of the basin, but also for carbon export from the upper layers into the abyssal ones. The study investigated how the ISW interactions with the CDW may influence bacterial metabolism during an ISW downslope event. In particular, what effect does this have on the bacterial activities related to the utilization and transformation of the organic carbon substrate (ectoenzymatic activities, carbon production, growth rate) within the ISW and the CDW cores? Our data show that in the CDW the metabolic response was to increase the biomass and enzymes were less active due to a higher nutritional value for the substrate. In the ISW the bacterial metabolic activity shifted towards degradative processes. These results suggest differences in the quality of the organic carbon pool with a greater concentration of labile organic matter in the CDW and of low-degradable compounds in the ISW. The use of microbial parameters seems to be very promising in the evaluation of the carbon export during mixing processes, when the refractory fraction of the organic carbon pool might play a key role

Short-term variability of primary production and inorganic nitrogen uptake related to the environmental conditions in a shallow coastal area (Gulf of Trieste, N Adriatic Sea)

Primary production (PP) and, nitrate (QNO(3)) and ammonium (QNH(4)) uptakes were measured together with other environmental parameters from October 1999 to February 2001 in the Gulf of Trieste (N Adriatic Sea). Their trends showed a high variability because of the combined effects of meteorological conditions, water circulation and river discharges. PP ranged from 0.2 to 1.5.9 mumol C dm(-3) d(-1), whereas QNO(3) varied from 0.8 to 442 nmol N dm(-3) d(-1), showing a trend similar to that of carbon basically ascribable to the autotrophic activity. QNH(4) ranged from 20 to 1308 nmol N dm-3 d-' and it reached the highest values during the declining phases of phytoplankton blooms, indicating that bacterial community can also be involved in its uptake. Regenerated PP generally prevailed over the new production (depth-integrated f-ratios from 0.05 to 0.50). C/N uptake ratios by planktonic community (annual average of 16 +/- 11) showed the repetitive carbon overconsumption (23-33) during periods of high production, and lower values (2-13) during: the post-bloom phases and in the months of scarce autotrophic activity. Residence time of freshwater in the area (1 d in November and January, up to 23 d in July) indicated the fast export of low salinity waters in winter and their longer permanence in summer. These values were closer to the ammonium turnover times (1-34 d) than to the nitrate ones (2-831 d). Riverine nitrate load (3-67 t N d-') generally exceed the biological demand of this nutrient (uptakes from 0.2 to 8 t N d-'), whereas the ammonium load (0.1-33 t N d(-1)) was almost always insufficient (uptakes, from 2.6 to 33 t N d(-1)). These results evidenced the major role of physical transport and recycling processes to regulate, respectively, nitrate and ammonium availability in this shallow ecosystem.La production primaire et l’assimilation de nitrates et d’ammonium ont été mesurées en même temps que les facteurs du milieu entre octobre 1999 et février 2001 dans le golfe de Trieste. La variabilité est élevée en raison de l’action combinée des conditions météorologiques, de la circulation et des apports des rivières. La production primaire varie entre 0,2 et 15,9 μmol C dm–3 j–1 alors que l’assimilation de nitrates va de 0,8 à 442 nmol N dm–3 j–1, montrant une tendance identique au carbone lié à la production autotrophe. L’assimilation d’ammonium varie entre 20 et 1308 nmol N dm–3 j–1 et elle atteint ses valeurs maximales durant le déclin de la floraison planctonique, indiquant que la communauté bactérienne est impliquée dans cette assimilation. La production primaire régénérée surpasse la production nouvelle (le facteur f intégré en fonction de la profondeur varie entre 0,05 et 0,5). Les taux C/N d’assimilation de la communauté planctonique (moyenne annuelle de 16 ± 11) soulignent la surconsommation répétitive de carbone (23–33) durant les phases de production actives ; les valeurs minimales (2–13) caractérisent les phases postérieures à la floraison et les mois de faible activité autotrophe. Le temps de résidence d’eau douce dans la zone (d’un jour en novembre et janvier à 23 jours en juillet) indiquent l’exportation rapide d’eau de basse salinité en hiver et leur permanence en été. Ces valeurs sont plus proches des temps de renouvellement de l’ammonium (1 à 34 jours) que des nitrates (2 à 831 jours). L’apport de nitrates par les fleuves (3–67 t N j–1) surpasse les besoins (0,2 à 8 t N j–) alors que l’apport d’ammonium (0,1–3,3 t N j–1) est presque toujours insuffisant (assimilation de 2,6 à 33 t N j–). Ces résultats mettent en lumière le rôle majeur du transport et du recyclage dans la régulation de la disponibilité en nitrate et ammonium dans des écosystèmes de faible profondeur