Blueprint Kalmar DOM mesocosms

In this project BLUEPRINT, we designed and conducted mesocosm experiments to investigate the bacterial growth and gene expression responses in which river water from a boreal forest- (enriched in humic substances) or agriculture-influenced catchment area (enriched in nitrogen), or a mix of inorganic nutrients was added to Baltic Proper seawater. Approximately 120 L of water from each of the two rivers flowing through an agriculture-dominated catchment area (Lielupe River, Latvia, 56°48'42" N 23°35'5" E) and a boreal forest catchment area (Lapäarti River, Finland, 62°14'21" N 21°34'38" E), were collected on May 26 and May 28, 2016, respectively. The river water was collected in 50 L polypropylene containers and transported to Linnaeus University in Kalmar, Sweden. On May 30, 2016, around 3000 L of seawater was collected from two meter depth at the Linnaeus Microbial Observatory (LMO) site in the Baltic Sea Proper (~11 km off the coast of Öland, Kårehamn; 56°55'51.24" N 17°3'38.52" E and transported o the Linnaeus University mesocosm facility within 1.5 h after sampling. Twelve mesocosm tanks (each 300 L, Nalgene) were placed in a temperature-controlled room (at ~14 °C) and filled with 160 L of 100 µm filtered LMO seawater. On June 1, representing day 0, the mesocosm experiment was started by adding portions of river water or inorganic nutrients to the mesocosms as follows. One treatment (in triplicates) consisted of an addition of 40 L (20% of final volume) of 0.22 µm filtered (Opticap XL filter, Millipore) agricultural-influenced Lielupe river water (hereafter Agri). A second treatment (in triplicates) consisted of an addition of 40 L (20% of final volume) of 0.22 µm filtered (Opticap XL filter, Millipore) Lapäarti river water (forest river with humic influence; hereafter Humic). The remaining six mesocosms each received 40 L (20 % of final volume) of 0.22 µm filtered (Opticap XL filter, Millipore) LMO seawater (to obtain the same dilution of the LMO water microbial community in all twelve mesocosms). Of these six mesocosms, triplicate mesocosms received enrichments with 16 µM N in the form of NaNO3, 1 µM P in the form of Na2HPO4, and 32 µM Si in the form of NaSiO3 (final concentrations) (hereafter referred to as NPSi). The remaining three mesocosms were left untreated and served as controls. Light was supplied in a 12 h light:dark cycle, using 250 W halogen lamps (HQI –E 250 W/D Pro E40, Powerstar) placed 2 m above the mesocosms that gave a photosynthetically active radiation (PAR) light level of ~400 µmol photons m-2 s-1.