Effect of phytoplankton bloom deposition on benthic bacterial communities in two contrasting sediments in the southern North Sea

The response of benthic bacterial community composition, diversity, and biomass to phytoplankton deposition was investigated in 2 different sediment horizons at 2 contrasting sites in the southern North Sea. Differences in bacterial community composition between stations were observed. Seasonal differences in bacterial community composition were significant and were stronger in fine sediments, probably related to stronger fluctuations in food availability. Variation in community composition over the vertical sediment profile was different for both stations. In coarser sediment, the difference was mostly due to the absence of certain operational taxonomic units (OTUs) at the surface, while in fine sediment, 2 distinct communities were present. A RELATE test revealed that bacterial community composition was influenced by the amount of labile organic matter (estimated through chl a concentration in the sediment). Diversity in terms of OTU richness and Shannon-Weaver diversity index was higher in finer grained sediments. In coarser sediments, diversity at the surface layer was lower, which might be related to stronger hydrodynamic pressure at this station. These differences were not observed at the other station. Seasonal changes in diversity were not detected at either station. Bacterial biomass was slightly higher in finer sediments and was not correlated with either chl a or temperature. Seasonal differences in bacterial biomass followed those observed for community composition, while no vertical differences were detected.