Phytoplankton spring blooms in Arctic coastal ecosystems are affected by a drastically changing climate; making modelling their responses to these changes an important challenge to model ecosystem responses. These spring blooms are mostly dominated by diatoms growing exponentially in spring until the nutrients silicon or nitrogen are depleted and the bloom terminates. Regenerated production fueled by ammonium recycled by bacterial activity may extend this Bloom. Ecosystem models currently used mostly ignore or oversimplify multinutrient co-limitations and the role of bacteria recycling. Hence, we recreated a typical spring Bloom scenario in a cultivation experiment with the aim to develop a dynamic model that describes these dynamics accurately with the lowest possible complexity. In the cultivation study, the diatom Chaetoceros socialis grew either under axenic conditions or in co-culture with bacteria cultures of Pseudoalteromonas elyakovii. In co-culture, the diatoms grew longer, faster and to higher biomass due to ammonium regeneration.