Microbial food webs and metabolic state across oligotrophic waters of the Mediterranean Sea during summer

U Christaki,L Prieur,S Psarra,J R Dolan,T Sime-Ngando,J.-D Grattepanche,D Lefevre,J Colombet,P Conan,M Pujo-Pay,A Lagaria,T Moutin,F Van Wambeke,M G Weinbauer

doi:10.5194/bg-8-1839-2011

Abstract

Abstract. The abundance and activity of the major members of the heterotrophic microbial community – from viruses to ciliates – were studied along a longitudinal transect across the Mediterranean Sea in the summer of 2008. The Mediterranean Sea is characterized by a west to-east gradient of deepening of DCM (deep chlorophyll maximum) and increasing oligotrophy reflected in gradients of biomass and production. However, within this well documented longitudinal trend, hydrological mesoscale features exist and likely influence microbial dynamics. Here we present data from a W-E transect of 17 stations during the period of summer stratification. Along the transect the production and fate of organic matter was investigated at three selected sites each one located in the centre of an anticyclonic eddy: in the Algero-Provencal Basin (St. A), the Ionian Basin (St. B), and the Levantine Basin (St. C). The 3 geographically distant eddies showed low values of the different heterotrophic compartments of the microbial food web, and except for viruses in site C, all integrated (0–150 m) stocks were higher in reference stations located in the same basin outside the eddies. During our study the 3 eddies showed equilibrium between GPP (Gross Primary Production) and DCR (Dark Community Respiration). Integrated PPp (Particulate Primary Production) values at A, B and C varied from ~140 to ~190 mg C m−2.

Highlights

The Mediterranean Sea is one of the most oligotrophic marine systems in the world
Previous studies have documented the W-E gradients in the Mediterranean Sea but in terms of the individual components of the microbial food web (e.g. Dolan et al, 1999; Christaki et al, 2001; Pitta et al, 2001; Van Wambeke et al, 2002)
We found a weak relationship between Chl-a and Virus-like particles (VLP) compared to the tight correlation of the latter with HBA implying that heterotrophic bacteria are the more probable viral hosts compared to phytoplankton

Summary

Introduction

The Mediterranean Sea is one of the most oligotrophic marine systems in the world. The basin-wide cyclonic circulation of nutrient-depleted water (Dugdale and Wilkerson, 1988), hot, dry climate and low land run-off contribute to the low productivity of the sea. U. Christaki et al.: Microbial food webs and metabolic state across oligotrophic waters establishment of large-scale patterns of abundance and activity for different planktonic food web components (reviewed in Siokou et al, 2010). Microbial heterotrophic activity is an important energy pathway in the planktonic food web and in particular in the eastern Mediterranean, where most of the organic carbon produced is consumed and respired (Regaudie-de-Gioux et al, 2009). The most convincing evidence of P limitation of heterotrophic prokaryotes resulted from the CYCLOPS in-situ P-fertilization experiment conducted in May 2002 in the Cyprus-Eddy. In this experiment, prokaryotic heterotrophic production increased in response to P addition whereas phytoplankton biomass diminished (Thingstad et al, 2005)

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