Nutrient control of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation in the oligotrophic Mediterranean Sea and the impact of Saharan dust events

C Ridame,M Pujo-Pay,C Guieu,S L'Helguen,M Le Moal,E Ternon,I C Biegala

doi:10.5194/bg-8-2773-2011

Abstract

Abstract. A better understanding of the factors controlling N2 fixation is a pre-requisite for improving our knowledge on the contribution of N2 fixation process in the nitrogen cycling. Trace-metal clean nutrient/dust addition bioassays (+P, +PFe, +dust) were performed at three stations located in the western, central and eastern Mediterranean Sea, in summer 2008 as part of the BOUM cruise. The main goals were (1) to investigate the nutrient factor(s) limiting N2 fixation (uptake of 15N2) and (2) to evaluate the potential impact of a Saharan dust event on this biological process during the stratification period. Initially, surface waters at the three stations were DIP-depleted (<10 nM) while the DFe concentrations were relatively high (from 1.2 to 2.3 nM) most likely due to atmospheric iron accumulation in the surface mixed layer. At all stations, Saharan dust input relieved the ambient nutrient limitation of the diazotrophic activity as demonstrated by the strong stimulation of N2 fixation (from 130 % to 430 %). The highest dust stimulation of N2 fixation was recorded at the station located in the eastern basin. The response of diazotrophic activity to nutrient additions was variable between the sampled stations suggesting a spatial variability of the factor controlling N2 fixation over the whole basin. At all stations, N2 fixation was not limited by Fe nor co-limited by P and Fe. At the western station, N2 fixation was DIP limited while at the eastern one, N2 fixation was first DIP limited, then was limited by one or several chemical element(s) released by dust. Our results demonstrated that a Saharan dust input was able to relieve these successive on going limitations. Very interestingly, at the station located in the central basin, N2 fixation was not limited by the availability of P yet it was strongly stimulated by dust addition (x3.1). A chemical element or a combination of several, released by the added dust may have been responsible for the observed stimulations of N2 fixation. These results indicated that Saharan dust pulses to the surface Mediterranean waters, in addition to P and Fe, could be a source of chemical(s) element(s) that are necessary for metabolic processes and therefore influence rates of N2 fixation.

Highlights

The Mediterranean Sea is one of the most oligotrophic areas of the world’s ocean (Krom et al, 2004; Bosc et al, 2004) characterized by a strong gradient of nutrients and primary production decreasing eastward (Bethoux et al, 1998; Moutin and Raimbault, 2002; Lagaria et al, 2011; PujoPay et al, 2011)
Initial N2 fixation rates in surface waters measured in the control treatment (0–24 h) were very similar at the three study sites (Table 1) as demonstrated by the low standard deviation
As DIP turnover times measured in the surface waters of stations A, B and C were lower than 10 h (Tanaka et al, 2011; Mauriac et al, 2011), the very low DIP availability could explain the absence of Trichodesmium at these stations

Summary

Introduction

The Mediterranean Sea is one of the most oligotrophic areas of the world’s ocean (Krom et al, 2004; Bosc et al, 2004) characterized by a strong gradient of nutrients and primary production decreasing eastward (Bethoux et al, 1998; Moutin and Raimbault, 2002; Lagaria et al, 2011; PujoPay et al, 2011). Ridame et al.: Nutrient control of N2 fixation in the oligotrophic Mediterranean Sea phosphorus (DIP) turnover times, the Mediterranean Sea can be considered as a Low Phosphate Low Chlorophyll (LPLC) environment (Moutin et al, 2008) Another interesting feature is the unusually high dissolved inorganic N:P ratios of 22–23:1 and 28–30:1 for the western and eastern basins, respectively (Marty et al, 2002; Pujo-Pay et al, 2011) which may result from an intense N2 fixation (Gruber and Sarmiento, 1997; Ribera d’Alcala et al, 2003). Apart from the extremely high N2 fixation rate (129 nmol N L−1 d−1) measured in spring in the Eastern basin (Cyprus eddy, Rees et al, 2006), N2 fixation rates in the Mediterranean Sea are low when compared to other oligotrophic environments (see review in a companion paper, Bonnet et al, 2011)

Objectives

Methods

Results

Discussion

Conclusion