Winter‐mixing preconditioning of the spring phytoplankton bloom in the Bay of Biscay

Abstract

The spring phytoplankton bloom plays a key role in the dynamics of temperate and polar seas. Nevertheless, the mechanisms and processes behind these blooms remain a subject of considerable debate. We analyzed the influence of deep mixing during winter on the spring phytoplankton bloom in the Cantabrian Sea (southern Bay of Biscay). To this end, we combined long‐term physical and biogeochemical in situ data (1993–2012) and satellite observations (1997–2012). Deeper winter mixing led to higher nitrate and chlorophyll concentrations through the water column during the spring bloom. However, this effect was modified by short‐term variability in near‐surface stratification in spring. Winter‐mixing preconditioning also influenced different spring bloom metrics: deeper and later mixing in winter was followed by later blooms with a larger peak. In these enhanced blooms, nitrate was taken up at faster rates, indicating higher rates of phytoplankton production. Winters with weaker mixing (that led to weaker spring blooms) were associated with warmer surface temperatures. This relationship suggests that the multi‐decadal trend toward warmer surface temperatures in the Bay of Biscay may promote a decrease in the magnitude of the spring bloom, which could impact upper trophic levels and also deep carbon export in the future.

Large scale patches of Calanus finmarchicus and associated hydrographic conditions off the Lofoten archipelago

“…Nevertheless, the location of deep overwintering habitats from which copepods emerge in early spring and to which they return in late summer could determine surface distributions regardless of spring-time hydrographic structures. Winter pre-conditioning is known to affect the spatial distributions of many pelagic species from phytoplankton to seabirds in spring (Schroeder et al, 2009;Black et al, 2010;Batchelder et al, 2013;Gonzalez-Gil et al, 2017). Overwintering habitats of Calanus spp.…”

Seasonal and vertical dynamics in the trophic structure of a temperate zooplankton assemblage

“…S1). Conditions during BIOCANT 1 (March 2012) were typical for the onset of the spring phytoplankton bloom, with high concentrations of Chl a that reflected the high, newly produced phytoplankton biomass (González‐Gil et al ). During that period, the food web in SW was highly size structured, as shown by the high b (2.37‰ g −1 ; Fig.…”

“…Seasonal fluctuations in primary production, especially accentuated in temperate seas with a marked annual cycle, are determinant for the dynamics of the deep ocean (Gooday ). In these temperate regions, the transition from mixed to stratified conditions in spring and back to mixing in autumn releases phytoplankton cells from light/nutrient limitation, leading to two major production pulses known as the spring and autumn blooms, respectively (e.g., Fernández and Bode ; González‐Gil et al ). Stratification favors the prevalence of a microbial food loop (Cushing ; Legendre and Rassoulzadegan ), where heterotrophic bacteria recycle the OM while feeding a complex community of heterotrophic or mixotrophic microbes, protozoans, and copepods (Azam et al ).…”

Evolution of Phytoplankton in Relation to Their Physiological Traits

“…Perhaps winter survival of sufficient phytoplankton cells to 'seed' the spring bloom is enhanced by limited photosynthesis when cells in the upper mixed layer are briefly close to the sea surface in their vertical circulation. Whereas the chlorophyll a per volume of seawater in the upper mixed layer can be an order of magnitude lower at the lowest value in winter than at the peak of the spring bloom [55], it is the chlorophyll a per cell and the functionality of photochemistry and downstream metabolism that is important in providing the 'seed' for the spring bloom.…”

“…Arctic marine phytoplankton must be able to survive months without sunlight [54]. Phytoplankton in high latitude temperate open oceans have very limited productivity in winter with lower temperatures, shorter photoperiods and deeper mixing (see, e.g., [55]). Perhaps winter survival of sufficient phytoplankton cells to 'seed' the spring bloom is enhanced by limited photosynthesis when cells in the upper mixed layer are briefly close to the sea surface in their vertical circulation.…”