The Role of Rapid Changes in Weather on Phytoplankton Spring Bloom Dynamics Captured by an Autonomous Uncrewed Surface Vehicle

The spring phytoplankton bloom plays a major role in pelagic ecosystems; however, its dynamics is overlooked due to insufficient, highly-resolved observational data. Here we investigate the start, peak and decline of a two-week phytoplankton spring bloom in Frohavet, located at the coast of mid-Norway. We used observations from an uncrewed surface vehicle (USV) combined with buoy measurements, satellite images, discrete water sampling and modelling approaches. The spring bloom (March-June 2022) consisted of multiple peaks (up to 5 mg m-3), with a long peak in April, coincident with the period when the USV captured the temporal and spatial dynamics of the bloom. Short-term (5 days) episode of calm weather in the spring, such as clear skies and consistent low wind speed (< 7 m s-1) shoaled the mixed layer depth (< 15 m), after strong wind speed (average wind speed up to 20 m s-1 in March) and mixing events in winter. These rapid changes in the environment promoted the rapid development of the spring bloom - from 1 to 5 mg 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 during early stages of the spring bloom. Understanding the dynamics of the spring bloom is crucial for the management of marine resources. Integration of distinct observational platforms has the potential to unveil the environmental factors underlying phytoplankton bloom dynamics.