Influence of allochthonous matter on microbial community structure and function in an upwelling system off the northwest Iberian Peninsula

The input of allochthonous matter of continental origin to coastal zones globally may cause changes in the activity (function) as well as the taxonomic composition (structure) of the micro- bial plankton community. The goal of the present study was the simultaneous analysis of microbial plankton community structure (size-fractionated phytoplankton biomass, bacterial community com- position) and function (particulate and dissolved primary production, bacterial production, microbial plankton community respiration) in the northwest Iberian coastal transition zone during a dry (Feb- ruary 2005) and a rainy (October 2005) period. An influence of freshwater input was observed in October, even at an offshore site, but not in February. We found an autotrophic community domi- nated by picophytoplankton during both sampling periods. In contrast, the bacterial groups Beta- and Gammaproteobacteria were significantly more abundant during the high precipitation period. Pri- mary production rates were low and similar during both sampling periods; however, bacterial pro- duction was 8-fold and community respiration was 3-fold higher in October than in February. Conse- quently, the microbial community metabolism was net autotrophic in February and net heterotrophic in October. The high precipitation and the significant presence of bacteria belonging to the Betapro- teobacteria, typical for freshwater systems, in October compared to February, strongly suggest an influence of material of continental origin on microbial metabolism in this coastal transition zone.