Abstract
BIO-Argo float (chlorophyll a (Chl-a), temperature, and salinity profiles) and remote sensing data (Chl-a, photosynthetic available radiation (PAR), and wind) located south of the Kuroshio current near Shikoku from September 2018 to May 2019 were used to study phytoplankton bloom and their mechanisms of development in open oceans. Results show that higher (lower) Chl-a concentrations are correlated with a deeper (shallower) mixed layer (RPearson = 0.77, Rcrit = 0.12 (alpha = 0.05, n = 263)) compared to the average of Chl-a and mixed layer depth (0.13 mg/m3 and 105 m). The average net accumulation rates (r) of phytoplankton were close to 0.08 d−1. An increasing r corresponds to a gradually increasing surface Chl-a (S (Chl-a): 0–20 m average Chl-a) and integrated Chl-a inventory (I (Chl-a): integrated Chl-a from surface to euphotic depth). These phenomena indicate that the mechanism of winter-spring phytoplankton blooms is consistent with the dilution-recoupling hypotheses (DRH). During the bloom formation, winter deep mixing and eddy-wind Ekman pumping are enhanced by a strong winter monsoon. The enhancement may disturb predator–prey interactions and dilute zooplankton in deep mixed layers. Moreover, winter deep mixing and eddy-wind Ekman pumping can cause the nutrients to be transported into the euphotic layer, which can promote the growth of phytoplankton and increase grazing. During the bloom extinction, the stratification strengthens and the intensity of light increases; this increases grazing and nutrient consumption, and decreases the phytoplankton bloom significantly (S (Chl-a) and I (Chl-a) increase by 0.3 mg/m3 and 27 mg/m2, respectively). The output from a biogeochemistry model shows that nutrients are consistent with the temporal distribution of S (Chl-a) and I (Chl-a). Our results suggest that physical processes (deep winter mixing and eddy-wind Ekman pumping) under the DHR framework are critical factors for winter-spring blooms in open oceans with an anticyclone eddy.
Highlights
Chlorophyll a (Chl-a) concentration is affected by the changes in intracellular pigment levels driven by light and nutrients
We studied the winter-spring bloom initiation and development using biogeochemical observations from BIO-Argo floats (2902750) south of the Kuroshio current near Japan and tested three commonly discussed hypotheses (CDH, critical turbulence hypothesis (CTH), or dilution-recoupling hypothesis (DRH)) to determine which can best explain the observed results
According to Sverdrup’ s theory of critical depth and growth time scales, we ruled out two hypotheses (CDH, CTH)
Summary
Chlorophyll a (Chl-a) concentration is affected by the changes in intracellular pigment levels driven by light and nutrients. The mechanism of the spring bloom was initially explained using CDH [9] According to this theory, strong ocean convection and turbulence can deepen the mixing layer in winter, which can carry abundant nutrients into the mixed layer, increasing the phytoplankton concentrations. The CTH theory suggests that at the end of winter, wind and net cooling decrease, which may weaken the intensity of turbulent mixing in the ocean upper layer, and the residence time of phytoplankton increases in the euphotic layer [10]. This can promote the propagation of phytoplankton and induce phytoplankton blooms. The above hypotheses suggest that bloom dynamics is closely related to detailed physical and biological processes [9,10,11,12] and, it is necessary to identify these mechanisms to fully understand the bloom dynamics
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