Abstract
Abstract. Recent observations have shown that phytoplankton biomass increases in the North Atlantic during winter, even when the mixed layer is deepening and light is limited. Current theories suggest that this is due to a release from grazing pressure. Here we demonstrate that the often-used grazing models that are linear at low phytoplankton concentration do not allow for a wintertime increase in phytoplankton biomass. However, mathematical formulations of grazing as a function of phytoplankton concentration that are quadratic at low concentrations (or more generally decrease faster than linearly as phytoplankton concentration decreases) can reproduce the fall to spring transition in phytoplankton, including wintertime biomass accumulation. We illustrate this point with a minimal model for the annual cycle of North Atlantic phytoplankton designed to simulate phytoplankton concentration as observed by BioGeoChemical-Argo (BGC-Argo) floats in the North Atlantic. This analysis provides a mathematical framework for assessing hypotheses of phytoplankton bloom formation.
Highlights
One of the most prominent biological events in the surface ocean is the North Atlantic spring bloom (Boss et al, 2008; Siegel et al, 2014; Cole et al, 2015)
The traditional theory of phytoplankton population dynamics in the North Atlantic attributes the spring bloom to the release of phytoplankton from light limitation, which causes phytoplankton growth rates to increase. This has become known as the “critical depth hypothesis” (Sverdrup, 1953) because the theory states that phytoplankton can begin to grow when the mixed layer has shoaled sufficiently so that the light-dependent phytoplankton growth terms are larger than the phytoplankton loss terms, which are assumed to be constant throughout the winter and into the spring
A reduction in the grazing rate at low phytoplankton concentration has been proposed as the mechanism to explain the emerging observation that biomass often increases, albeit weakly, during the wintertime when mixed layers deepen (Behrenfeld, 2010)
Summary
One of the most prominent biological events in the surface ocean is the North Atlantic spring bloom (Boss et al, 2008; Siegel et al, 2014; Cole et al, 2015). The large annual cycle in the phytoplankton population in the North Atlantic occurs in the context of large seasonal cycles in atmospheric conditions that drive changes in mixed layer depth, surface irradiance, and upper layer temperature How these environmental factors interact with ecological processes to produce a bloom is still being debated (Fischer et al, 2014). The traditional theory of phytoplankton population dynamics in the North Atlantic attributes the spring bloom to the release of phytoplankton from light limitation, which causes phytoplankton growth rates to increase This has become known as the “critical depth hypothesis” (Sverdrup, 1953) because the theory states that phytoplankton can begin to grow when the mixed layer has shoaled sufficiently so that the light-dependent phytoplankton growth terms are larger than the phytoplankton loss terms, which are assumed to be constant throughout the winter and into the spring. Phytoplankton can be released from light limitation while the mixed layer is deep if turbulence is temporarily reduced (Huisman et al, 1999; Taylor and Ferrari, 2011; Paparella and Vichi, 2020)
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