Microbial community structure and function in the Levantine Basin of the eastern Mediterranean
Résumé
During May 2001 and May 2002, the structure and function of the microbial community within and outside the Cyprus quasi-stationary warm-core eddy in the Levantine Basin of the eastern Mediterranean was studied down to the depth of the bathypelagic layer. We present here the detailed description of the microbial food web in one of the most oligotrophic and P-starved marine systems on earth. The isothermal layer was at the depth between 20 and 260/300m at the core of the eddy, and between 20 and 100/110m outside. Nitrate and phosphate were found at higher concentration between 100 and 500/800 m outside the eddy compared within the core of the eddy, but the vertical diffusive flux of nitrate and phosphate across the pycnocline was higher within the core of the eddy. There were only minor differences in microbial abundance in the euphotic, layers of the two sites. It is suggested that the differences in the areal supply of nutrients to the isothermal layer, between the two sites, resulted in essentially a similar volumetric supply of nutrients to the euphotic layer. This suggests that the results of this study can be applied to describe the microbial food web within the euphotic layer over the larger area of the Levantine Basin, which exhibits ultra-oligotrophic and P-starved conditions. Primary production and abundances of the microbial community were somewhat higher in May 2001 than in May 2002, possibly because of higher nutrient fluxes in the euphotic layer, which are probably the result of deeper winter mixing in 2001, although a later onset of winter mixing or increased dust supply could not be discounted. In the euphotic layer, heterotrophs (bacteria, heterotrophic nanoflagellates (HNF), and ciliates) dominated (60-70%) the microbial carbon biomass. Heterotrophic ciliates were found to be much more abundant in the upper 50 m of the water column, while no consistent pattern wasfound for bacteria and HNF throughout the euphotic layer. Autotrophs showed a maximum distribution at the deep chlorophyll maximum found between 100 and 130m. In the euphotic layer, the relationships between biomass and production for phytoplankton and bacteria suggested a higher top-down control on the phytoplankton in the upper similar to 50 in and a consistently tight top-down control on the bacterial biomass throughout the euphotic layer. The phosphate addition experiment in the Cyprus Eddy suggests that the close predator-prey relationships within the microbial heterotrophic community were required for the rapid transfer of a limiting element to higher trophic levels without biomass oscillations in the P-fertilized surface mixed layers (0-20m). The results from the unmodified system in this study suggested that the rapid P transfer mechanisms would function only in the upper similar to 50m, while the element transfer would be based on a predator-prey relationship with more conceivable biomass oscillations in the deeper waters (similar to 100-160m) of the euphotic layer. In the Mediterranean Sea, nutrient concentrations, POC export, and integrated chlorophyll and primary production all tend to decrease toward the east. Our results together with a literature survey showed that abundances of the microbial components in the euphotic layer were not consistently lower in the study area than in the northwestern Mediterranean and that abundances of bacteria and HNF found in the mesopelagic and bathypelagic layers of the study area were within the reported ranges and quite similar to those found in the northwestern Mediterranean. This suggests that the oligotrophic status and the lw export production are not reflected in the abundance of the microbial components down to the bathypelagic layer of the eastern Mediterranean. (C) 2007 Elsevier Ltd. All rights reserved.