Seasonal development of iron limitation in the sub-Antarctic zone

Thomas J Ryan-Keogh,Thato N Mtshali,Sandy J Thomalla,Hazel J Little,Natasha R Van Horsten

doi:10.5194/bg-15-4647-2018

Thomas J Ryan-Keogh, Thato N Mtshali + Show 3 more

Open Access

https://doi.org/10.5194/bg-15-4647-2018

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Abstract

Abstract. The seasonal and sub-seasonal dynamics of iron availability within the sub-Antarctic zone (SAZ; ∼40–45∘ S) play an important role in the distribution, biomass and productivity of the phytoplankton community. The variability in iron availability is due to an interplay between winter entrainment, diapycnal diffusion, storm-driven entrainment, atmospheric deposition, iron scavenging and iron recycling processes. Biological observations utilizing grow-out iron addition incubation experiments were performed at different stages of the seasonal cycle within the SAZ to determine whether iron availability at the time of sampling was sufficient to meet biological demands at different times of the growing season. Here we demonstrate that at the beginning of the growing season, there is sufficient iron to meet the demands of the phytoplankton community, but that as the growing season develops the mean iron concentrations in the mixed layer decrease and are insufficient to meet biological demand. Phytoplankton increase their photosynthetic efficiency and net growth rates following iron addition from midsummer to late summer, with no differences determined during early summer, suggestive of seasonal iron depletion and an insufficient resupply of iron to meet biological demand. The result of this is residual macronutrients at the end of the growing season and the prevalence of the high-nutrient low-chlorophyll (HNLC) condition. We conclude that despite the prolonged growing season characteristic of the SAZ, which can extend into late summer/early autumn, results nonetheless suggest that iron supply mechanisms are insufficient to maintain potential maximal growth and productivity throughout the season.

Highlights

The Southern Ocean is an important region for atmospheric CO2 drawdown, with 30–40 % of global anthropogenic carbon uptake (Khatiwala et al, 2009; Mikaloff Fletcher et al, 2006; Schlitzer, 2002), which is driven by local phytoplankton community production and the biological carbon pump (BCP)
This paper aims to test whether the phytoplankton community in the sub-Antarctic zone is seasonally limited by iron availability
The bloom initiation date was calculated as 2–3 November for the mixed layer and euphotic zone, with the peak of the bloom calculated as 10–11 December

Summary

Introduction

The Southern Ocean is an important region for atmospheric CO2 drawdown, with 30–40 % of global anthropogenic carbon uptake (Khatiwala et al, 2009; Mikaloff Fletcher et al, 2006; Schlitzer, 2002), which is driven by local phytoplankton community production and the biological carbon pump (BCP). The BCP is, sensitive to environmental influences that are associated with climate change, which include an intensification of the westerly winds (Le Quéré et al, 2009), and altered upwelling and mixed-layer stratification (Bopp et al, 2005; Boyd, 2002). Together, these changes will impact the light and nutrient supply to the phytoplankton community, which could in turn alter the efficiency and extent of the BCP in the future. Further controls on the seasonal evolution and extent of the phytoplankton bloom include potential silicate limitation (Boyd et al, 2010; Hutchins et al, 2001), top–down controls by meso- and micro-zooplankton grazing (Dubischar and Bathmann, 1997; Moore et al, 2013; Pakhomov and Froneman, 2004; Smetacek et al, 2004), and seasonal– sub-seasonal changes in the critical and mixed-layer depths (Fauchereau et al, 2011; Nelson and Smith, 1991)

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