The Role of Rapid Changes in Weather on Phytoplankton Spring Bloom Dynamics From Mid‐Norway Using Multiple Observational Platforms

• Published in: Journal of geophysical research. Oceans Vol. 129; no. 3
• Main Authors: Fragoso, G. M., Dallolio, A., Grant, S., Garrett, J. L., Ellingsen, I., Johnsen, G., Johansen, T. A.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-03-2024
• Subjects: Biodiversity hot spots; Biogeochemical cycle; Biogeochemical cycles; Blooms; Buoy measurements; Clear sky; Diatoms; Dynamics; Environmental conditions; environmental controls; Environmental factors; Food chains; Food webs; Hot spots; Impact prediction; instruments, sensors, and techniques; Low wind speeds; Marine technology; Mixed layer; Mixed layer depth; physical and chemical properties of seawater; Phytoplankton; Phytoplankton bloom; phytoplankton dynamics; phytoplankton spring bloom; Plankton; Platforms; Satellite imagery; Skeletonema; Spring (season); Surface vehicles; Temporal variations; uncrewed surface vehicles; Water analysis; Water sampling; Weather; Wind; Wind speed; Winds; Norway
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2023JC020415

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.
Plain Language Summary
The phytoplankton spring bloom is an important recurrent phenomenon because it provides food for the marine food web and regulates the climate. Although previous studies were focused on the initiation of the spring bloom, its dynamics, meaning, rapid changes in formation and decline, are usually not observed in detail. Here we used a combination of technological (marine sensors and robots) and traditional methods (water collection, laboratorial and microscopic analyses) to observe the spatial and temporal variation of the spring bloom in a biological hotspot of the coast of mid‐Norway. Small windows of “good weather,” where few days of sunny, clear skies and weak winds in the midst of “stormy spring” promoted the rapid development of second spring bloom peak dominated by the diatom Skeletonema. The bloom collapsed after the wind speed got high again, suggesting the strong influence of environmental conditions in the spring bloom. Here we demonstrated that the use of multiple ocean observation platforms is crucial to understand, in detail, the processes controlling the spring phytoplankton bloom.
Key Points
The spring bloom in coastal high latitudinal regions consisted of multiple peaks associated with gain and loss processes
Relaxation of strong winds and clear skies for 7–10 days in spring allowed phytoplankton accumulation and bloom development
Episodic strong winds interluded spring bloom development after a period of calm, sunny weather