Earlier onset of phytoplankton bloom in the Southern Bight of the North Sea in response to climate variability

Phytoplankton are essential to marine ecosystems, driving biogeochemical cycles and supporting diverse marine species and systems. Shifts in bloom timing of these species can disrupt food webs and carbon cycling. This study investigates changes in the timing and composition of phytoplankton blooms in the Belgian Part of the North Sea (BPNS) and examines the environmental drivers behind these shifts. We analyzed data from 2018, 2019, and 2021, combining remote sensing, in situ measurements, flow imaging microscopy, and DNA metabarcoding to assess phytoplankton community structure and bloom phenology. Typically, two major annual blooms occur in this region: a mid-spring bloom dominated by Phaeocystis globosa and an early-summer diatom bloom primarily consisting of Thalassiosira species. In 2021, both blooms occurred approximately one month earlier than in 2018 and 2019. Contrary to expectations, this advance in blooming was not associated with higher sea temperatures or increased light availability (quantified by photosynthetically active radiation). Instead, we observed increases in phosphate concentrations and reductions in salinity preceding the blooms. These changes suggest that alterations in nutrient dynamics and salinity acted as bottom-up controls, triggering the earlier onset of phytoplankton blooms and supporting the Disturbance Recovery Hypothesis.