In order to investigate the influence of organic matter (OM) on rare earth element (REE) distributions and patterns in the marine environment we monitored concentrations of dissolved REEs (dREEs) during an artificially induced spring bloom. Our mesocosm approach mimicked a neritic North Sea water body. Three biological replicates (P2-P4) were inoculated with a phytoplankton and associated bacterial community, which was retrieved in March 2018 from the southern North Sea. The incubation was monitored for 38 days. The experiment additionally covered the investigation of two biota-free controls. A variety of parameters were sampled, the results of some are published by Mori et al. (2021). Samples for dREE analyses were taken at intervals of 1-5 days. Preconcentration, analysis and quantification of dREEs followed the method described by Behrens et al. (2016). In order to investigate possible complexation of dREEs with components of the dissolved OM pool, a PHREEQC model was written that simulated chemical speciation of the dREEs in the mesocosms. A new databank was created that includes stability constants for complexes of dREEs with the main inorganic ligands (Cl⁻, SO₄⁻, OH⁻, CO₃⁻) as well as with the strong organic ligand desferrioxamine B (DFOB) after Christenson and Schijf (2011). The model outcome includes concentrations of inorganic and organic dREE complexes as well as abundances as free ions (REE3+) and total dREE concentrations. Additionally, we calculated the proportions of the different complexes to the total dREE pool. We used two different approaches for the PHREEQC model approach that followed Schijf et al. (2015) and were characterized by the concentration of the strong organic ligand and the resulting proportion of organic complexes to the dREE pool. The 'High-DOC' approach results in a maximal proportion of organic REE-DOC complexes of 40%, the 'Low-DOC' approach results in maximum of 10% organic complexes. To keep an eye on variations in carbonate complexes, total alkalinity (TA) was monitored as well. TA was sampled daily, for the analysis we used a multiscan GO microplate spectrophotometer (Thermo Scientific) and followed the method described by Sarazin et al. (1999).