Spring phytoplankton blooms in the absence of vertical water column stratification

Abstract

THE spring phytoplankton bloom in temperate and boreal waters represents a pulsed source of organic carbon that is important to ecosystem productivity¹ and carbon flux² in the world ocean. It is widely accepted that the seasonal development of a thermocline, in combination with increasing solar elevation in spring, is requisite for the development of the bloom in shelf and open ocean environments^3–7. Here we report results for the offshore waters of the Gulf of Maine which suggest that the spring bloom can precede the onset of vertical water column stability, and may even be a contributing factor in the development of the thermocline. Deep penetration of light in relatively clear, late-winter waters, and weak, or absent, wind-driven vertical mixing, appear to support cell growth rates that overcome the vertical excursion rates in the neutrally stable water column, leading to a bloom. Phytoplankton forms typical of a spring bloom, including gelatinous colonies and chains, may contribute to the cells' ability to maintain a vertical position in a water column lacking stability.