Suspended particles and particle aggregates, which formed from concentrated field samples on the roller table, were characterized biologically and chemically along a transect through the Baltic Sea in summer 1999. Phytoplankton composition in field samples was dominated by cyanobacteria, including the filamentous diazotrophic cyanobacteria Aphanizomenon ‘ baltica’, Nodularia spumigena and Anabaena spp. These species formed aggregates together with diatoms, mainly Skeletonema costatum and Chaetoceros spp. and with dinoflagellates, mainly withDinophysis norvegica . Compared to the Redfield ratio, concentration ratios of particulate organic carbon, nitrogen and phosphorus, [POC]:[PON]:[POP], indicated an enrichment of carbon, especially in aggregates. However, regression analysis indicated a higher production rate of PON relative to POP and POC and significant background concentrations of POC. In field samples the concentration of transparent exopolymer particles (TEP) varied around 200 μg Xanthan Equiv. l−1 and comprised a volume fraction of 2–7 ppm and an abundance of about 105 TEP ml−1. TEP were enriched in aggregates as inferred from volume ratios of TEP to conventional particles. It is suggested, that TEP contribute substantially to the background concentration of POC, while the high production rate of PON is attributed to nitrogen fixation of diazotrophic cyanobacteria.