Microbial Food Web Structure and Its Impact on Primary Production in a Meso-Oligotrophic Coastal Area (Pagasitikos Gulf, Aegean Sea)

Abstract

Seasonal structure and dynamics of the planktonic microbial food web (phytoplankton, bacteria, nanoflagellates and ciliates) were studied in the meso-oligotrophic Pagasitikos Gulf, NW Aegean Sea. Pagasitikos Gulf is exposed to anthropogenic activity and for this, the sampling covered different parts of the system (city of Volos, sewage effluents, central gulf, Trikeri channel, outer gulf). The standing stocks of all the microbial components fell within the range of other similar coastal systems. Depth integrated primary production (PP) indicated a spring phytoplankton bloom in April/May in all sampling sites. A second phytoplankton bloom was recorded in the western and the outer part of the gulf in the fall. An estimation of bacterial carbon demand indicated that in several cases the percentage of PP that is routed through heterotrophic bacteria is close or over 100%. This is consistent with the excess bacterial biomass relative to autotrophic biomass found in the system. Hence recycling processes mediated by heterotrophs and particularly bacteria are crucial for maintaining the structure and functioning of the planktonic community in Pagasitikos Gulf.