Phytoplankton spring blooms dominated by diatoms and often by Thalassiosira rotula and/or Skeletonema costatum are typical features of coastal seas in the temperate zone. They are usually terminated by nutrient limitation, aggregation and sedimentation events. In the course of climate change increasing temperatures in spring may affect the development of diatom blooms and in particular aggregation, a process in which heterotrophic bacteria play an active role. As heterotrophic microbial processes are favoured more relative to autotrophic processes by increasing temperature, we hypothesize that aggregation will be enhanced with increasing temperature and in particular towards the end of the spring bloom with increasing senescence of the diatoms. To test this hypothesis we examined aggregation in the course of a spring bloom in a mesocosm experiment in the southern North Sea at the in situ temperature of 6°C and at 11°C. Water was transferred to a mesocosm of 720 liter and incubated at the in situ temperature and a 12:12 h light-dark cycle for 3 weeks. Aggregation was assessed in the pre-bloom, bloom and late bloom phase by measuring the formation of aggregates in subsamples incubated for 48 h in rolling tanks at 6°C and 11°C in a 12:12 h light-dark cycle. Aggregation was determined in subsamples withdrawn directly from the mesocosm and in subsamples to which axenic T. rotula cells were added in a concentration of ca. 5000 cells ml-1. Transparent exopolymer particles (TEP), dissolved organic carbon and dissolved carbohydrate as well as the bacterial abundance and community composition by DGGE and FISH were analysed during the bloom and the aggregation experiments. The addition of T. rotula enhanced aggregation relative to the untreated samples. For the latter, aggregation was rather independent of temperature whereas aggregation was enhanced at the higher temperature when T. rotula was added and in particular towards the end of the bloom. The results suggest that increasing temperature enhances aggregation of phytoplankton spring blooms dominated by T. rotula. This enhanced aggregation may have implications for the trophic transfer of diatom blooms in the food web by reducing the available amount of diatom-derived organic matter to zooplankton.