Climatic Controls on the Spring Phytoplankton Growing Season in a Temperate Shelf Sea

• Published in: Journal of geophysical research. Oceans 2022-05, Vol.127 (5), p.n/a
• Main Authors: Jardine, J. E., Palmer, M., Mahaffey, C., Holt, J., Wakelin, S., Artioli, Y.
• Format: Article
• Language: English
• Subjects: Algae; AMO; Atlantic Oscillation; Atmospheric pressure; Biogeochemistry; Biology; Blooms; Chlorophyll; Chlorophylls; Climate control; Confidence limits; Fish; Geophysics; Growing season; Growth; Light; Light levels; Northwest European Shelf; Oases; Oceans; Oscillations; Physics; Phytoplankton; Phytoplankton bloom; Plankton; Response time; Seasons; Shelf seas; Spring; Spring (season); spring bloom; Stocks; Storm tracks; Storms; Stratification; Underwater gliders; Water circulation; Water column; Water stratification; Water temperature; Winds; Winter
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC017209

The Northwest European Shelf is positioned directly beneath the North Atlantic Storm Track, within which the frequency and intensity of transient storms are modulated by large‐scale climatic oscillations. In temperate shelf seas, the impact of storms on the physical environment has received considerable attention, but the effect on biogeochemistry is less studied. Here, we use output from a multidecadal (1982–2015) coupled physical‐biogeochemical model supported by observations from ocean gliders to investigate phytoplankton growth throughout the winter‐spring transition. We define two separate phytoplankton growth events: the spring bloom, defined as the exponential growth following seasonal stratification, and the prebloom, occurring before stratification, and accounting for up to 22% of the total spring growth. Our results support the paradigm that light is a first‐order control, with the spring bloom initiating up to 22 days after stratification onset should light levels be too low to trigger the bloom. The prebloom is heavily influenced by the phase of the Atlantic Multidecadal Oscillation (AMO), demonstrated by an acceleration in the rate of increase of total chlorophyll concentrations (±90% confidence limit) from 7.6 ± 2.8 mg m−2 d−1 (during a positive AMO) to 13.1 ± 4.3 mg m−2 d−1 (negative AMO), due to modulation of periods of ephemeral stratification that occur between successive storms. We propose that phytoplankton growth in prebloom events might help buffer the lag between phytoplankton supply and larval recruitment, particularly during years when the spring bloom is delayed. Plain Language Summary In temperate shelf seas, the seasonal onset of stratification is usually considered the precursor for the spring phytoplankton bloom: an exponential growth of algae that is of key biological importance to fish stocks. Seasonal stratification, whereby the water column becomes layered with warmer water on top of colder water, is initiated by more heat going into the ocean toward spring. Increasing winds, due to passing storms, delay the onset of stratification due to increased mixing. Changes in water temperatures and atmospheric pressure across the North Atlantic change the frequency and intensity of storms across Northwest Europe and influence stratification onset. Using a model that incorporates both physics and biology, we investigated how changing storm patterns from 1982 to 2015 influenced phytoplankton variability during the winter‐spring trans