Abstract
Abstract. The significant impact of dust deposition on heterotrophic bacterial dynamics in the surface oligotrophic ocean has recently been evidenced. Considering the central role of bacteria in the microbial loop, it is likely that dust deposition also affects the structure and the functioning of the whole microbial food web. In the frame of the DUNE project, aiming to estimate the impact of dust deposition on the oligotrophic Mediterranean Sea through mesocosm experiments, the main goal of the present paper was to assess how two successive dust deposition events affect the dynamics of the microbial food web. The first dust seeding delivered new P and N to the amended mesocosms and resulted in a pronounced stimulation of bacterial respiration. It also induced pronounced, but transient, changes in the bacterial community composition. No significant effects were observed on the abundances of viruses and heterotrophic nanoflagellates. The second dust seeding also delivered new P and N to the amended mesocosms, but the effect on the microbial food web was very different. Bacterial respiration remained constant and bacterial abundance decreased. Compositional changes following the second seeding were minor compared to the first one. The decrease in bacterial abundance coincided with an increase in virus abundance, resulting in higher virus:bacteria ratios throughout the second seeding period. Our study shows that dust deposition to the surface oligotrophic ocean may involve important modifications of the trophic links among the components of the microbial food web with presumed consequences on C and nutrient cycling.
Highlights
Nutrient cycles control the strength of the biological carbon pump through which CO2 is consumed in surface water and transported as sinking organic carbon to the deep sea (Balino et al, 2001)
Increasing evidence indicates that phytoplankton and heterotrophic bacteria are limited by inorganic nutrients, mainly P, in oligotrophic oceanic systems (Rivkin and Anderson, 1997; Thingstad et al, 1998; Sala et al, 2002; Obernosterer et al, 2003; Zohary et al, 2005) suggesting that the supply of these resources could explain variability in bacterial activity
Our study shows that the effects of dust deposition on ocean carbon cycle are not restricted to a simple response of autotrophic phytoplankton and/or heterotrophic bacteria
Summary
Nutrient cycles control the strength of the biological carbon pump through which CO2 is consumed in surface water and transported as sinking organic carbon to the deep sea (Balino et al, 2001). Increasing evidence indicates that phytoplankton and heterotrophic bacteria are limited by inorganic nutrients, mainly P, in oligotrophic oceanic systems (Rivkin and Anderson, 1997; Thingstad et al, 1998; Sala et al, 2002; Obernosterer et al, 2003; Zohary et al, 2005) suggesting that the supply of these resources could explain variability in bacterial activity. Dust deposition is recognized as a significant source of macro- and micro-nutrients to the surface ocean (Jickells et al, 2005; Mahowald et al, 2008). Recent studies combining field and experimental approaches have demonstrated significant increase in heterotrophic bacterial abundance and respiration following dust deposition in oligotrophic ecosystems (Pulido-Villena et al, 2008; Lekunberri et al, 2010; Romero et al, 2011). No studies have so far documented the consequences of dust pulses on the dynamics of the microbial food web (including viruses, heterotrophic nanoflagellates) and, the fate of bacterial production after a dust pulse remains unexplored
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