Phytoplankton Phenology in the North Atlantic: Insights From Profiling Float Measurements

Abstract

Phytoplankton division rate (µ), loss rate (l), and specific accumulation rate (r) were calculated using Chlorophyll-a (Chl) and phytoplankton carbon (C phyto) derived from bio-optical measurements on 12 Argo profiling floats in a north-south section of the western North Atlantic Ocean (40 • N to 60 • N). The float results were used to quantify the seasonal phytoplankton phenology and bloom dynamics for the region. Latitudinally varying phytoplankton dynamics were observed. In the north, the C Phyto peak was higher, occurred later, and was accompanied by higher total annual C Phyto accumulation. In contrast, in the south, stronger µ-r decoupling occurred despite smaller seasonal variations in mixed layer depth (suggesting the possibility of other ecological forcing), and was accompanied by an increasing portion of winter to total annual production, consistent with relief of nutrient limitation. The float observations of phytoplankton phenology for the mixed layer were compared to ocean color satellite remote sensing observations and found to be similar. A similar comparison to an eddy-resolving ocean simulation found the model only reproduced some aspects of the observed phytoplankton phenology, indicating possible biases in the simulated physical forcing, turbulent dynamics, and bio-physical interactions. In addition to seasonal patterns in the mixed layer, the float measurements provided information on the vertical distribution of physical and biogeochemical quantities and therefore are complementary to the remote sensing measurements. Seasonal phenology patterns arise from interactions between "bottom-up" (e.g., resources for growth) and "top-down" (e.g., grazing, mortality) factors that involve both biological and physical drivers. The Argo float data are consistent with the disturbance recovery hypothesis over the full, annual seasonal cycle; for the late winter/early spring transition, the float data are also consistent with other bloom hypotheses (e.g., critical photosynthesis, critical division rate, and meso/sub-mesoscale physics) that highlight the importance of brief, episodic boundary layer shoaling for decoupling of division and grazing rates.

Seasonal modulation of phytoplankton biomass in the Southern Ocean

“…The development of this hypothesis has largely been based on observations in the northern hemisphere and strongly biased toward satellite, rather than in situ, data. Inferred support for the ‘Disturbance-Recovery’ hypothesis is also derived from ecological models of the North Atlantic spring bloom 21 , 34 , 35 . Here, a large array of biogeochemical floats deployed over the last 7 years has allowed a detailed and in situ evaluation of phytoplankton bloom dynamics in the Southern Ocean.…”

Exploring Vitamin B1 Cycling and Its Connections to the Microbial Community in the North Atlantic Ocean

“…We hypothesize that under these conditions, TRC concentrations approach a steady state where production and consumption terms are balanced, and the standing stock of TRCs is positively correlated with bacterial productivity as seen at station 1 (Figure 5). In contrast, community stability is disrupted during a bloom, with the microbial community shifting toward dominance by a single group or consortia of species resulting in elevated microbial biomass and metabolic activity (Teeling et al, 2012;Behrenfeld and Boss, 2018;Bolaños et al, 2020b;Kramer and Graff, 2020;Yang et al, 2020). We hypothesize that during bloom conditions, such as those observed at station 2, TRC production and consumption terms become unbalanced and, depending on the TRC auxotrophy status of the blooming organisms, TRC demand could outpace production, and result in TRC drawdown (Figure 5).…”

Chlorophyll‐Based Model to Estimate Underwater Photosynthetically Available Radiation for Modeling, In‐Situ, and Remote‐Sensing Applications

“…The vertical distribution of underwater photosynthetically available radiation (PAR) is a necessary input for models of net primary production (e.g., review by Behrenfeld & Falkowski, 1997) and is key to interpret physiological response of phytoplankton to light, its change in cellular pigmentation (e.g., Behrenfeld et al., 2016) or the associated solar‐stimulated fluorescence (Behrenfeld et al., 2009). In the last two decades, carbon‐based primary production models (CbPM, Behrenfeld et al., 2005; Westberry et al., 2008), and photoacclimation models (Fox et al., 2020), provide novel approaches to estimate marine net primary production (NPP) from satellite measurements that have also been applied to in‐situ platforms (Estapa et al., 2019; Yang et al., 2020). These models derive the vertical distribution of PAR by attenuating surface PAR using an estimate of its diffuse attenuation coefficient, K d (PAR).…”