Wind Intensity Is Key to Phytoplankton Spring Bloom Under Climate Change

Frode B Vikebø,Kjersti Opstad Strand,Svein Sundby

doi:10.3389/fmars.2019.00518

Abstract

The onset of the spring bloom (OSB) occurs when phytoplankton growth exceeds losses and is promoted by a transition from deep convection to a shallow mixing layer concurrent with increasing light intensities in nutrient-enriched waters. We have combined remotely sensed chlorophyll-a data and high-resolution sea-surface winds to quantify and understand high-latitude spring-bloom dynamics and the effect of varying winds. Increasing winds strengthen turbulent mixing and may eventually cause the mixing depth to extend beyond the depth at which light is favorable for net growth and delay the OSB. We find that wind intensity accounts for up to 60 % of the interannual variation in the OSB as revealed by remotely-sensed chlorophyll-a values at the key spawning ground (62-63 oN) of one of the worlds’ largest herring stocks. The OSB is, on average, one month later and with about half the variability farther north at the main spawning ground (67-68 oN) of one of the world’s largest cod stocks. Since the atmospheric reanalysis considered here extends wind time series much further back in time (1958) than remote sensing (1998), the former may act as a good proxy for investigating OSB trends on the time scales of multi-decadal variability and climate change. We find a weak but non-significant signal of delay in the OSB across these extended time periods. More importantly, our results clearly show that predictions of future productivity and ecosystem dynamics under global warming based on earth system models require accurate representation of winds.

Highlights

Timing of fish spawning to match the onset of the spring bloom (OSB), that in turn triggers the egg production of the herbivorous Calanus finmarchicus (Melle and Skjoldal, 1998; Kaartvedt, 2000), is a key to secure sufficient and suitable prey for the larval fish (Ellertsen et al, 1989)
High-latitude ecosystems are dominated by a strong seasonal variation in primary productivity with a peak during spring and with a limited duration that sets the scene for the regional trophodynamics (Sundby et al, 2016)
According to Sverdrup’s (1953) Critical-Depth-Hypothesis the main forces that regulate the spring bloom are the vertical flux of nutrients from the ocean deep to the upper layer during winter mixing, the increase of light after winter darkness and the subsequent stabilization of the water column with seasonal heating and freshwater melting

Summary

Introduction

Timing of fish spawning to match the onset of the spring bloom (OSB), that in turn triggers the egg production of the herbivorous Calanus finmarchicus (Melle and Skjoldal, 1998; Kaartvedt, 2000), is a key to secure sufficient and suitable prey for the larval fish (Ellertsen et al, 1989). The remote sensing data are compared to events of strong winds during the initiation of spring bloom.

Results

Conclusion