Cobalt (Co) nanoparticles (NPs) may be diffusely dispersed to natural ecosystems from various sources such as traffic settings, and hence eventually end up in aquatic systems, such as lakes and oceans. This has motivated our studies on the transfer of Co NPs through an aquatic food web including algae (Scendesmus sp), zooplankton (Daphnia magna) and fish (Crucian carp, Carassius carassius). The influence of excreted biomolecules from D. magna, potentially adsorbing and forming an eco-corona at the surfaces of the Co NPs, was also investigated from an environmental fate perspective. ATR-FTIR measurements show adsorption of algae constituents and excreted biomolecules onto the Co NPs. The initially adsorbed smaller molecules of the excreted biomolecules were eventually replaced with larger molecules of higher surface affinity, originating from the algae. In tap water (TW) containing algae, less than 1.5% of the Co NPs mass was dissolved within 24 h, mainly as a result of the near neutral pH. Less than 5% of the Co NPs formed heteroagglomerates with algae, which was partly an effect of agglomeration and settling of the Co NPs. The presence of excreted biomolecules in solution did not affect the extent of heteroagglomeration between the Co NPs and the algae. Some Co NPs were transferred to the next trophic level (D. magna) despite the low extent of heteroagglomeration of Co NPs to algae. The Co uptake in D. magna was 300 times larger compared to the control samples (without Co NP), results that were not influenced by the addition of excreted biomolecules to the solution. A significant uptake of Co by the fish was observed in the intestine, without any evident effect of the presence of excreted biomolecules in the solution. No bioaccumulation of Co was observed in the fish feeding on D. magna containing Co NPs, and 10-20% of the Co NP mass was dissolved after 24 h in its simulated gut solution (pH 7.4). In all, extensive agglomeration and sedimentation of the Co NPs and small extent of heteroagglomeration between the Co NPs and the algae resulted in only a small fraction of Co transferred between the algae and D. magna. No significant bioaccumulation of Co NPs was observed in any of the fish organs, even though a small difference was observed in the fish stomachs and intestines. The dissolution findings in the simulated fish stomach solution (pH 7.4) imply that 60% of the Co NPs will dissolve within 24 h if reached this trophic level. QC 20200831