Seasonal variability and export of dissolved organic nitrogen in the northwestern Mediterranean Sea

Abstract

[1] Hydrodynamic and biogeochemical monthly time series were carried out in the Gulf of Lions (northwestern Mediterranean) between 1993 and 1994. Detailed analysis of vertical profiles of dissolved organic nitrogen (DON) revealed an accumulation of nitrogen in the Upper Mixed Layer (UML) above the seasonal thermocline. DON was not corrected for ammonia concentration because of negligible concentrations of this element during the whole year in the studied area. In midwinter (January-February), DON concentrations were quasi-homogeneous throughout the water column (about 3.5 mM). Subsequently, surface DON concentrations increased up to autumn, with a subsurface maximum greater than 6.0 mM observed in September. During this time, inorganic nutrients became virtually undetectable in the UML. The total DON content (0-1000 m) increased by about 784 mmol m À2 from winter through to the end of summer. At this time, labile and semilabile DON represented a maximum of 20% of the bulk DON. Assuming that DON was essentially removed from surface water by diffusive process during the stratified period and by vertical mixing during the rest of the year, 210 to 723 mmol m À2 a À1 was exported as DON, which represented 40-90% of the total new production. Following this DON export enabled us to balance the nitrogen budget of the area studied over the duration of the experiment.

Nitrogen and Phosphorus Budgets in the Northwestern Mediterranean Deep Convection Region

“…According to the comparison exercise (section 3), it may have been underestimated. The simulated vertical nutrients import is of the same order of magnitude as previous estimates for the NW Med (Pujo‐Pay & Conan, ; Severin et al, ; Table ). It is also of the same order of magnitude as the estimation of 1400 mmol N m −2 yr −1 in the convective Atlantic subpolar region (Williams & McLaren, ).…”

“…The vertical export to the deep layer of dissolved and particulate organic nitrogen and phosphorus is estimated at 724 mmol N m −2 yr −1 and 52 mmol P m −2 yr −1 , respectively. These estimates are in the same order of magnitude as the estimates in Pujo‐Pay and Conan (; Table ). During the convection period (17 January to 17 March), the export represents 55% of the annual amount of organic nitrogen and 58% of the annual amount of organic phosphorus.…”

Impact of atmospheric and oceanic interannual variability on the Northwestern Mediterranean Sea pelagic planktonic ecosystem and associated carbon cycle

“…[] and 3 mmolN m −3 for DON, Pujo‐Pay and Conan []), the minimum modeled average concentrations of DOM in the surface layer vary in March around 45–50 mmolC m −3 and 3.9–4.5 mmolN m −3 . Copin‐Montégut and Avril [], Avril [], and Pujo‐Pay and Conan [] observed minimum concentrations around 50–60 mmolC m −3 and 3.5 mmolN m −3 . DOM concentration then increases due to the intensification of the microbial loop and the stratification of the water column.…”

“…Locally, maximum modeled values can exceed 90 mmolC m −3 , 7 mmolN m −3 , and 0.15 mmolP m −3 in the open sea and 150 mmolC m −3 , 8 mmolN m −3 , and 0.18 mmolP m −3 over the shelf (Figures not shown). Copin‐Montégut and Avril [], Avril [], and Pujo‐Pay and Conan [] observed maximum summer concentrations around 90–100 mmolC m −3 and 6 mmolN m −3 in the open Ligurian Sea. The modeled DON and DOC may thus be slightly overestimated probably owing to the bacterial biomass underestimation that may induce an underestimation of DOM assimilation (see section 3.2.5).…”

A New Glider-Compatible Optical Sensor for Dissolved Organic Matter Measurements: Test Case from the NW Mediterranean Sea

“…The accumulation of DOM in surface Mediterranean oligotrophic waters in summer is an important driver for the efficiency of carbon and nitrogen export to the deep ocean. In the NW Mediterranean, this pattern has however mostly been assessed using variations in bulk DOC and DOM (Avril, 2002;Pujo-Pay and Conan, 2003;Santinelli, 2015). Only few studies have addressed the dynamics of DOC components (Bourguet et al, 2009;Goutx et al, 2009;Organelli et al, 2014;Xing et al, 2014), and the relative importance of diagenetic processes (microbial production, microbial degradation and phototransformation) responsible for production and removal of the DOM pool remains difficult to discern.…”