Surface coastal Baltic Sea water for the mesocosms was collected in June 2011 1 km offshore of Warnemünde (54°11'22"N; 12° 4'45"E). The water was filtered through a 100 µm net to eliminate larger zooplankton and particles.
Mesocosms were prepared by mixing pretreated tDOC and Baltic Sea water together in nine mesocosms according to the following scheme : (i) Baltic Sea water + particle free river water (RB), (ii) Baltic Sea water + ultrafiltrate (ULTRA), (iii) Baltic Sea water + lyophilized river water (LYO). Control mesocosms included untreated Baltic Sea water)(cBS), particle free (0.2 µm filtered RB) and unfiltered river water (cRW). The volumes of the mesocosms was 25 L except the LYO treatment and the cRW controls which was 5 L. Before adding the microbial inoculum (Baltic Sea water) to the mesocosms the salinity of the pretreated Kalix water was adjusted to the in situ salinity of 8.1 using 100x enriched artificial seawater based on Difco Marine Broth 2216 where organic substrates were replaced or omitted. 10 µM NO3NH4 and 1 µM NaHPO4were added to all experiments. The mesocosm were held at constantly 10°C in dark climate chambers. The pH and ionic strength in each treatment was permanently controlled but did not change during the incubation, phosphate concentration was low (0.03 µM) in all mesocosm where artificial seawater was added.