Abstract
The spring phytoplankton bloom plays a major role in pelagic ecosystems; however, its dynamics are not well understood due to insufficient, highly resolved observational data. Here we investigate the start, peak, and decline of a 2-week phytoplankton spring bloom in Frohavet located in a biological hotspot of the coast of mid-Norway. We used observations from an uncrewed surface vehicle (USV) combined with buoy measurements, satellite images, discrete water sampling and modeling approaches. The spring bloom (March–June 2022) consisted of multiple peaks (up to 5 mg Chla m−3), with a long peak in April dominated by the diatom Skeletonema, coincident with the period when the USV captured the temporal and spatial dynamics of the bloom. Short-term episode of calm weather in spring, such as clear skies and consistent low wind speed (<7 m s−1) shoaled the mixed layer depth (<15 m) and promoted the rapid development of the spring bloom - from 1 to 5 mg Chla m−3 in 5 days. Likewise, the collapse of the bloom was rather quick, 1–2 days and coincides with low nitrate values and rapid increase in wind speed (>10 m s−1), suggesting strong influence of the environment on phytoplankton dynamics. Understanding the dynamics of the spring bloom is crucial for predicting its impact on marine trophic web and biogeochemical cycles. Integration of distinct observational platforms has the potential to unveil the environmental factors underlying phytoplankton bloom dynamics.