Annual cycles and spring blooms in phytoplankton: don’t abandon Sverdrup completely

Abstract

The critical-depth model for the onset of the spring phytoplankton bloom in the North Atlantic has recently been called into question by several researchers. The critical-depth model considers that the spring bloom starts when the mixed layer shoals to become shallower than a critical depth. Satellite and in situ measurements of chlorophyll are used here to show that the critical-depth model is indeed flawed. It is shown that the critical-depth model does not apply in the spring because the basic assumption of an upper layer that is well-mixed in plankton is not met. Instead, the spring bloom forms in shallow near-surface layers that deepen with the onset of thermal stratification. A stratification-onset model for the annual cycle in plankton is proposed that adheres to the conventional idea that the spring bloom represents a change from a deepmixed regime to a shallow light-driven regime, but where the upper layers are not well mixed in plankton in spring and so the critical-depth model does not apply. Ironically, perhaps, the criticaldepth model applies in the autumn and winter when plankton are well-mixed to the seasonal thermocline, so that the critical-depth model can be used to determine whether net winter production is positive or negative.

Drivers of the winter–spring phytoplankton bloom in a pristine NW Mediterranean site, the Bay of Calvi (Corsica): A long-term study (1979–2011)

“…Our observations and those reported above indicate instead that phytoplankton blooms may be diverse in terms of patterns and formation mechanisms (e.g. Townsend et al, 1992;Behrenfeld, 2010;Chiswell, 2011). High phytoplankton biomass in the absence of thermal stratification could be explained by (1) the close-to-the-surface bottom, which keeps phytoplankton in layer that is always well illuminated , (2) sustained periods of calm weather during which vertical mixing would be weak (Townsend et al, 1994), (3) rate of phytoplankton photoacclimation faster than vertical mixing (Morán and Estrada, 2005), and (4) decoupling between phytoplankton growth and grazing, which would make the loss term (grazing) smaller than the gain (growth) (Behrenfeld, 2010;Herrmann et al, 2013).…”

Influence of anticyclonic eddies on the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM cruise)

“…Outside the eddies, D NO 3 is determined by nitrate biological assimilation during the spring-summer-autumn period, and a D NO 3 near the beginning of winter EZD is expected for a stratified ocean. If, during winter, W-MLD is lower than D NO 3 , there is no new supply of nitrate into the photic zone and the oligotrophic status of the system will increase, even if primary production may temporarily increase because of increasing irradiance following the winter solstice (Behrenfeld, 2010;Chiswell, 2011 2. Inside an eddy core formed during winter, water less dense and exhausted in nutrients accumulates.…”

Mapping and assessing variability in the Antarctic marginal ice zone, pack ice and coastal polynyas in two sea ice algorithms with implications on breeding success of snow petrels

“…Similarly, within polynyas there is a narrow opportunity for phytoplankton growth, the timing of which plays an important role in both biogeochemical cycles (Smith Jr. and Barber, 2007) and biological production (Arrigo and van Dijken, 2003;Ainley et al, 2010). However, while studies have suggested that the timing of sea ice retreat is synchronized with the timing of the phytoplankton bloom, other factors such as wind forcing (Chiswell, 2011), thermal convection (Ferrari et al, 2014) and iron availability (Boyd et al, 2007, and references therein) play important roles as well.…”