Temperature effects on aggregation during a spring diatom bloom

Diatom-dominated phytoplankton blooms in neritic seas are typically terminated by aggregation. Despite extensive studies, it is still unknown how temperature controls aggregation. Therefore, we studied aggregation in a 750 liter mesocosm experiment under simulated in situ conditions during a spring diatom bloom in the southern North Sea at Helgoland Roads. In the early phase, at the peak, and in the late phase, aggregation was examined in rolling tanks at the ambient temperature (6°C) and at 11°C. In a second set of rolling tanks, cells of Thalassiosira rotula were added to examine how this readily aggregating diatom affects aggregation. In the treatments with the natural phytoplankton, a significant effect of temperature on aggregation was detected only in the late bloom, when the aggregate area increased at 11°C relative to 6°C. In contrast, in the T. rotula–supplemented treatments, aggregation was enhanced by the elevated temperature at the peak but not in the late bloom. The aggregate area per microgram particulate organic carbon, a normalized measure to assess the aggregation potential, indicated that the aggregation potential in the natural phytoplankton community decreased from the peak to the late bloom, independent of temperature. This was presumably due to the reduced stickiness of the particles and lower concentrations of transparent exopolymer particles in the late phase. In the T. rotula–supplemented treatments, aggregation potential increased from the peak to the late bloom at the ambient temperature, whereas it decreased at the elevated temperature. Flavobacteria and Sphingobacteria of the Bacteroidetes phylum and Gammaproteobacteria were the dominant bacterial groups colonizing the aggregates.