Abstract
AbstractThe annual phytoplankton bloom is a key event in pelagic ecosystems. Variability in the timing, or phenology, of these blooms affects ecosystem dynamics with implications for carbon export efficiency and food availability for higher trophic levels. Furthermore, interannual variability in phytoplankton bloom timing may be used to monitor changes in the pelagic ecosystem that are either naturally or anthropogenically forced. The onset of the spring bloom has traditionally been thought to be controlled by the restratification of the water column and shoaling of the mixed layer, as encapsulated in Sverdrup's critical depth hypothesis. However, this has been challenged by recent studies which have put forward different mechanisms. For example, the critical turbulence hypothesis attributes bloom initiation to a reduction in turbulent mixing associated with the onset of positive net heat fluxes (NHFs). To date, the majority of studies on bloom initiation mechanisms have concentrated on North Atlantic datasets leaving their validity in other subpolar regions unknown. Here, we use chlorophyll output from a model that assimilates satellite ocean colour data to calculate bloom initiation timing and examine the basin-wide drivers of spatial and interannual variability. We find that the date that the NHF turns positive is a stronger predictor for the date of bloom initiation, both spatially and interannually, across the North Atlantic than changes in the mixed layer depth. However, results obtained from the North Pacific and Southern Ocean show no such basin-wide coherency. The lack of consistency in the response of the subpolar basins indicates that other drivers are likely responsible for variability in bloom initiation. This disparity between basins suggests that the North Atlantic bloom initiation processes are unique and therefore that this region may not be a suitable model for a global, theoretical understanding of the mechanisms leading to the onset of the spring bloom.
Highlights
Phenology is the study of the timing of periodic biological events, such as the annual phytoplankton bloom, and has led to a number of ecological and biogeochemical insights
This study aims to use satellite-derived chlorophyll data and output from a data-assimilating model to quantify variability in the date of bloom initiation in the subpolar North Atlantic, North Pacific, and Southern Oceans
Bloom initiation occurred just after the net heat fluxes (NHFs) turns positive, the mixed layer depth (MLD) was shoaling and PARML was increasing in all three basins except for the Southern Ocean
Summary
Phenology is the study of the timing of periodic biological events, such as the annual phytoplankton bloom, and has led to a number of ecological and biogeochemical insights. The timing of greatest food availability is important for grazers in addition to food abundance as summarized in the match–mismatch hypothesis (Cushing, 1990). This hypothesis states that interannual variability in the timing of the bloom results in years where the bloom coincides with larval hatching (a match) and years when their timing is not synchronous (mismatch). The bloom duration has been linked to carbon export efficiency with short, but highly productive, blooms producing less refractory material (Lutz et al, 2007), though exporting larger quantities of organic carbon (Eppley and Peterson, 1979; Francois et al, 2002)
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