Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.
"The small difference in the M-Chl data for F96 and F97 during WES (0.33 ± 0.045 mg m −3 and 0.31 ± 0.026 mg m −3 , respectively) suggests , however, that horizontal advection was not a primary factor controlling phytoplankton concentrations in this region. Furthermore, the increasing I-Chl and I-Tur with increasing PLD cannot simply be explained by eddy-driven stratification that was proposed by Mahadevan et al. (2012). As in the second case, if ML eddies appeared or disappeared in the upper layer at the float positions (specified by the latitude and longitude ) during when the floats were below the ML, observed variations in PLD, Chl and Tur in the upper layer were primarily attributed to differences in these properties inside and outside the ML eddies. "
[Show abstract][Hide abstract] ABSTRACT: Variability in the chlorophyll a concentration (Chl) in relation to fluctuations in the mixed layer (ML) was investigated together with turbidity (Tur) in the Kuroshio-Oyashio Extension region, using profiling floats. A particular focus was the validity of two hypotheses concerning the spring bloom: the critical depth hypothesis (CDH) and the recently proposed alternative, the disturbance- recovery hypothesis (DRH). During the period from winter to early spring, Chl and Tur integrated over the photosynthetically active layer (PL; defined as the greatest depth of the ML and the euphotic layer) increased with increasing PL depth (PLD), indicating an increase in the phytoplankton biomass. This result is partly consistent with the DRH in that the observed increase in biomass was not explained by an increase in production. Instead, it was more likely attributable to a reduction in the loss rate. However, theoretical analyses revealed that grazer dilution alone could not cause this increase in biomass because such an increase in the ML in the real ocean (as opposed to a dilution experiment within a bottle) would cause a reduction in the mean light intensity. Despite the loss-controlled fluctuation in biomass during the period of low light, a production-driven fluctuation in biomass was also revealed. This occurred when the light intensity was elevated, particularly after late spring, and was consistent with the CDH. Thus, the present study suggests that both the production-driven and loss-driven hypotheses are responsible for the dynamics of the phytoplankton dynamics from winter to spring in the Kuroshio-Oyashio Extension region.
Journal of Marine Systems 07/2015; 91. DOI:10.1016/j.jmarsys.2015.06.004 · 2.51 Impact Factor
"These mechanisms remain much debated despite decades of research (e.g. Sverdrup, 1953; Ryther and Hulburt, 1960; Evans and Parslow, 1985; Townsend et al., 1992; Huisman et al., 1999; Behrenfeld, 2010; Taylor and Ferrari, 2011a, b; Mahadevan et al., 2012; Ferrari et al., 2014). This debate arises from the wide diversity, and often inter-related, factors that control phytoplankton blooms, which range from physical (e.g. "
[Show abstract][Hide abstract] ABSTRACT: In this study, we document the regional variations of bloom phenology in the Southern Ocean, based on a 13-year product of
ocean colour measurements co-located with observation-based estimates of the mixed-layer depth. One key aspect of our work
is to discriminate between mixed-layer integrated blooms and surface blooms. By segregating blooms that occur before or after
the winter solstice and blooms where integrated and surface biomass increase together or display a lag, we define three dominating
Southern Ocean bloom regimes. While the regime definitions are solely based on bloom timing characteristics, the three regimes
organize coherently in geographical space, and are associated with distinct dynamical regions of the Southern Ocean: the subtropics,
the subantarctic, and the Antarctic Circumpolar Current region. All regimes have their mixed-layer integrated onset between
autumn and winter, when the daylength is short and the mixed layer actively mixes and deepens. We discuss how these autumn–winter
bloom onsets are controlled by either nutrient entrainment and/or reduction in prey-grazer encounter rate. In addition to
the autumn–winter biomass increase, the subantarctic regime has a significant spring biomass growth associated with the shutdown
of turbulence when air–sea heat flux switches from surface cooling to surface warming.
"This allows phytoplankton to reside within increasingly shallow depths where they experience higher light levels, and, as a result, begin to bloom (Sverdrup, 1953). Recent studies have challenged this explanation however, proposing instead that bloom initiation is driven by (i) eddyinduced stratification that shoals the MLD to create a favourable light environment before seasonal warming (Mahadevan et al., 2012); (ii) the stratification-onset model where convective overturning shuts down in spring, decreasing the depth of mixing (despite a still deep seasonal MLD) allowing phytoplankton blooms to form in shallow near-surface layers that deepen with the onset of thermal stratification (Chiswell, 2011; Chiswell et al., 2013); (iii) a shutdown of turbulent convective mixing when net heat fluxes become positive, increasing the residence times of phytoplankton in the euphotic zone (Taylor and Ferrari, 2011); (iv) a decrease in the dominant mixing length scales when negative heat fluxes weaken and shift the mixing mechanism from convection to wind, allowing phytoplankton to gain sufficient light exposure to bloom (Brody and Lozier, 2014); and (v) the disturbance-recovery hypothesis , where physical dilution during winter mixed layer deepening decreases grazing pressure driving subtle imbalances between phytoplankton division and loss rates allowing positive biomass accumulation to result in a bloom (Behrenfeld, 2010; Boss and Behrenfeld, 2010; Behrenfeld et al., 2013). In this study, we use 5.5 months of continuous, high-resolution glider data (3 h, 2 km horizontal resolution) from spring to summer in the Atlantic Subantarctic Zone (SAZ) to investigate ; (i) the phytoplankton spring bloom initiation and time-scale sensitivity in the region and (ii) the seasonal evolution of water column production and respiration. "
[Show abstract][Hide abstract] ABSTRACT: In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.
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