The impact of anticyclonic mesoscale structures on microbial food webs in the Mediterranean Sea

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 the east gradient of deepening of DCM (deep chlorophyll maximum) 5 and increasing oligotrophy reflected in gradients of heterotrophic microbial biomass and production. However, within this longitudinal trend, hydrological mesoscale features exist and likely influence microbial dynamics. We show here the importance of mesoscale structures by a description of the structure and function of the microbial food web through an investigation of 3 geographically distant eddies within a longitudinal 10 transect. Three selected sites each located in the center of an anticyclonic eddy were intensively investigated: in the Algero-Provencal Basin (St. A), the Ionian Basin (St. B), and the Levantine Basin (St. C). The 3 geographically distant eddies showed the lowest values of the different heterotrophic compartments of the microbial food web, and except for viruses in site C, all stocks were higher in the neighboring stations outside 15 the eddies. During our study the 3 eddies showed equilibrium between GCP (Gross Community Production) and DCR (Dark Community Respiration); moreover, the west-east (W-E) gradient was evident in terms of heterotrophic biomass but not in terms of production. Means of integrated PPp values were higher at site B (∼190 mg C m −2 d −1) and about 15% lower at sites A and C (∼160 mg C m −2 d −1). Net community production 20 fluxes were similar at all three stations exhibiting equilibrium between gross community production and dark community respiration.