Inflows of coloured terrestrial organic matter cause seawater browning and reduced phytoplankton production in subarctic coastal ecosystems, potentially deteriorating the nutritional quality of marine food webs. We analyzed the fatty-add (FA) compositions of seston and the zooplankton taxa Eutytemora affinis and cladocerans at three locations of the northern Baltic Sea. At the coastal and northerly locations, salinity and phosphorus concentrations were low, while concentrations of humic substances (i.e., terrestrial organic matter) were high. The southerly location showed the opposite trend. The ratio between alga-specific omega 3 polyunsaturated FA and terrigenous monounsaturated FA (MUFA) in Fun/femora decreased from south to north, as did the ratio between the alga-specific docosahexaenoic add (DHA) and terrigenous MUFA in cladocerans. With increasing humic substances, the biomass of DHA-rich phytoplankton decreased and the zooplankton MUFA content increased. Our results indicate that coloured terrestrial organic matter alters the phytoplankton composition, consequently affecting the zooplankton nutritional quality
