Grubisic LM, Brutemark A, Weyhenmeyer GA, Wikner J, Bamstedt U, Bertilsson S
Bacterial community composition, DOM, Stratification depth, mesocosm, Brackish, phytoplankton, Umea, Sweden, 2.5 m3
Bacterioplankton growth is often directly or indirectly controlled by external energysubsidies via organic matter inputs or solar radiation. We carried out a mesocosm experiment toassess how bacterioplankton communities responded to elevated levels of dissolved organicmatter (DOM) and experimentally controlled stratification depth. The month-long experimentconsisted of 2500 l mesocosms subjected to 4 experimental manipulations in triplicate: the stratificationdepth was set to either 1.5 or 3.5 m, with or without experimental addition of ambient levelsof chromophoric DOM. DOM addition had a significant effect on bacterial community compositionas assessed by terminal restriction fragment length polymorphism of amplified 16S rRNA genes.In contrast, there were no effects of the DOM amendment on bacterial biomass or production.Mixing depth and the coupled effective light climate in the photic zone also had a significanteffect on bacterial community composition. Furthermore, shallow mixing depth was associatedwith enhanced primary production, whereas DOM addition had a negative effect on phyto -plankton biomass and productivity. Our results suggest that bacterial community composition iscoupled to primary production under the studied coastal nutrient regime, and point to a key roleof DOM quality in controlling bacterioplankton communities.