Nitrification rates in the NW Mediterranean Sea

Abstract

During spring, ammonium oxidation and nitrite oxidation rates were measured in the NW basin of the Mediterranean Sea, from mesotrophic sites (Ligunan Sea and Gulf of Lions) to oligotrophc sites (Balearic Islands). Nitrification rates (average values for 37 measurements) ranged from 72 to 144 nmol of N oxidised 1-' d-', except in the RhBne River plume area where the rates increased to 264-504 nmol l-' d-' because of the riverine inputs of nitrogen. Maximal rates were located around the peak of nitrite within the nitracline at about 40 to 60 m and just above the phosphacline. At 1 station, relatively high values of nitrification (50 to 130 nmol 1-' d-l) were also measured deep in the water column (240 m). Day-to-day variations were measured demonstrating the response within a few hours to hydrological stress (wind-induced mixing of the water column) and showing the role of hydrological characteristics on the distribution of nitnfication rates. Because of the homogenous temperature (13'C) in the Mediterranean Sea, the spatial (geographical and vertical) fluctuations of nitnfying rates were linked to the presence of substrate due to mineralisation processes and/or Rhbne River inputs. We estimate the contribution of nitrate produced by nitrification to the N demand of phytoplankton to range from 16% at mesotrophic to 61 % at oligotrophic stations

Nitrite oxidation in the upper water column and oxygen minimum zone of the eastern tropical North Pacific Ocean

“…These values are in line with previous studies that have measured rates of 0-600 nmol l À 1 d À 1 across different regions of the ocean (Ward, 1987, Lipschultz et al, 1990Dore and Karl, 1996;Bianchi et al, 1999;Clark et al, 2008;Fü ssel et al, 2012). Higher rates have typically been observed in OMZs and other productive regions of the oceansuch as the ETSP, in the Benguela upwelling region and in the Southern California Bight (Ward, 1987;Lipschultz et al, 1990;Fü ssel et al, 2012)-than within oligotrophic regions (Dore and Karl, 1996;Bianchi et al, 1999;Clark et al, 2008). Overall, 15 NO 2 À oxidation rates were correlated with NO 2 À concentrations (r 2 ¼ 0.344, Po0.05; Figure 4a) in the GOC and ETNP, and relationships between rates and NO 2 À were strongest at stations 2 and 5 (Table 1).…”

“…With our coupled 15 NH 4 þ and 15 NO 2 À oxidation data, we note that differential sensitivity to light does not appear to be a driving factor, because 15 NO 2 À oxidation occurred in the euphotic zone and was correlated with 15 NH 4 þ oxidation in some cases. This is an important distinguishing factor from previous work, which has not measured 15 NH 4 þ and 15 NO 2 À oxidation at high resolution with depth (Ward, 1987, Lipschultz et al, 1990Dore and Karl, 1996;Bianchi et al, 1999;Clark et al, 2008;Fü ssel et al, 2012), as we have here (5-10 m intervals in the upper water column). On the basis of our data, the fact that NH 3 oxidation rates exceed NO 2 À oxidation rates appears to be the key in generating the PNM within the upper water column.…”

First basin‐wide experimental results on N2 fixation in the open Mediterranean Sea

“…Several hypotheses have been proposed to explain this anomaly: i) high nitrate/phosphate ratios in the Adriatic dense waters feeding the deep layer of the Eastern Mediterranean [Civitarese et al, 1998]; ii) significant atmospheric input rich in nitrogen [Herut et al, 1999]; and iii) N 2 fixation [Bethoux and Copin-Montegut, 1986;Sachs and Repeta, 1999]. This last hypothesis is further supported by regional geochemical observations, including (i) decoupling between the phosphocline and the nitracline [Bianchi et al, 1999], (ii) d 15 N of deep nitrate significantly lower (2.4-3‰) [Pantoja et al, 2002] than ocean values (5-6‰) [Sigman et al, 2005] and (iii) the low isotopic signature of suspended particulate organic nitrogen in the upper layer [Pantoja et al, 2002].…”

Sponges as a source of dissolved inorganic nitrogen: Nitrification mediated by temperate sponges

“…At community level, our estimation of the sponge nitrification rate (2.5-7.9 mmol N m 22 d 21 ) was higher than values reported for the potential rate of animal-associated nitrification by Welsh and Castadelli (2004) on the mentioned benthic animals (0.7-3.1 N m 22 d 21 , in which densities for intensive commercial production of bivalves were included). Our estimation of the sponge nitrification rate also appears to be similar or even higher than that reported for sediments from coastal environments in productive waters (range 0.1-8; mean 2.7 6 3 mmol m 22 d 21 , Herbert 1999 (Bianchi et al 1999). Therefore, considering an area of 1 m 2 to affect the cubic meter of water adjacent to the bottom, nitrification by the sponges will reach values from 2,500 mmol N L 21 d 21 to 7,900 mmol N L 21 d 21 , several orders of magnitude higher than the rates reported for Mediterranean plankton.…”