Vertical mixing, critical depths, and phytoplankton growth in the Ross Sea

Walker O Smith,Randolph M Jones

doi:10.1093/icesjms/fsu234

Abstract

AbstractPhytoplankton growth and biomass accumulation vary spatially and temporally in the Ross Sea, largely as a function of ice concentrations, vertical mixing depths, and iron concentrations. To assess the role of vertical mixing in bloom initiation, we used a high-resolution numerical model to estimate changes in mixed layer depths from October 1 through early December, the period where phytoplankton growth begins and biomass accumulates, and estimate critical depths for this period. Mixed layers in October ranged from the complete water column (&gt;600 m) to ca. 200 m; over a 60-day period, the mixed layers decreased on average by 70%. Estimated critical depths were exceeded in October, but would allow growth to proceed in late October due to shoaling of mixed layer depths, consistent with the known onset of the spring bloom in the Ross Sea. We also analysed a series of stations sampled near the Ross Ice Shelf during January 2012. Mean vertical profiles for the stations indicated deep vertical mixing; mixed layer depths averaged 60 m and ranged up to 96 m. Chlorophyll concentrations within the mixed layer averaged 6.60 µg l−1, and the pigment contributions were dominated by Phaeocystis antarctica. We suggest that this mesoscale region near the ice shelf is elevated in phytoplankton biomass due to frequent mixing events that redistribute biomass to depth and replenish nutrients, which in turn are utilized by an assemblage capable of utilizing low mean irradiance levels. Thus, the deep mixed layers and high biomass concentrations represent growth over long periods under reduced mixing punctuated by short periods of deeper vertical mixing that redistribute biomass. Water column vertical mixing and phytoplankton biomass in the Ross Sea are consistent with the critical depth concept as originally proposed by Sverdrup.

Highlights

Southern Ocean phytoplankton experience exceptionally large variations in physical forcing variables, such as irradiance, vertical water column stratification, and nutrient supply, and have relatively low growth rates due to low temperatures
If a daily irradiance of 5 mol photons m22 d21 and chlorophyll concentrations of,0.1 mg l21 are assumed in early October, critical depths equal 27 m [derived from Equation (1)]; when daily integrated irradiance increases by late November and approaches 40 mol Downloaded from https://academic.oup.com/icesjms/article-abstract/72/6/1952/917810 by VIRGINIA INSTITUTE OF Marine Science user on 30 October 2018 photons m22 d21, critical depths exceed 200 m
The role of vertical mixing and irradiance limitation has long been recognized as a dominant factor in controlling the annual productivity of the Southern Ocean (Smith et al, 2014b)

Summary

Introduction

Southern Ocean phytoplankton experience exceptionally large variations in physical forcing variables, such as irradiance, vertical water column stratification, and nutrient supply, and have relatively low growth rates due to low temperatures. The classical concepts formalized by Sverdrup (1953) have long been applied to Southern Ocean phytoplankton (Mitchell and Holm-Hansen, 1991; Nelson and Smith, 1991), in that it has largely been assumed that vertical mixing is deep in winter, and mixed layers shoal as less saline water from ice melt and heat from increased solar irradiance in spring increases vertical stratification in spring and summer These concepts have been questioned by Behrenfeld (2010), who suggested that blooms in the North Atlantic were regulated by the tight coupling between grazers and phytoplankton, as well as the dilution effect due to increased vertical mixing in winter. This is especially true in the Southern Ocean and its most southerly seas, such as the Ross Sea, where thick ice cover makes it difficult for ships to sample during the winter–spring transition

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