Nutrient regulation of late spring phytoplankton blooms in the midlatitude North Atlantic

Thomas J Browning,Ewan D Wakefield,Ali A Al‐Hashem,Eric P Achterberg,Mark J Hopwood,Anja Engel

doi:10.1002/lno.11376

Thomas J Browning, Ewan D Wakefield + Show 4 more

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https://doi.org/10.1002/lno.11376

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Abstract

AbstractThe duration and magnitude of the North Atlantic spring bloom impacts both higher trophic levels and oceanic carbon sequestration. Nutrient exhaustion offers a general explanation for bloom termination, but detail on which nutrients and their relative influence on phytoplankton productivity, community structure, and physiology is lacking. Here, we address this using nutrient addition bioassay experiments conducted across the midlatitude North Atlantic in June 2017 (late spring). In four out of six experiments, phytoplankton accumulated over 48–72 h following individual additions of either iron (Fe) or nitrogen (N). In the remaining two experiments, Fe and N were serially limiting, that is, their combined addition sequentially enhanced phytoplankton accumulation. Silicic acid (Si) added in combination with N + Fe led to further chlorophyll a (Chl a) enhancement at two sites. Conversely, addition of zinc, manganese, cobalt, vitamin B12, or phosphate in combination with N + Fe did not. At two sites, the simultaneous supply of all six nutrients, in combination with N + Fe, also led to no further Chl a enhancement, but did result in an additional 30–60% particulate carbon accumulation. This particulate carbon accumulation was not matched by a Redfield equivalent of particulate N, characteristic of high C:N organic exudates that enhance cell aggregation and sinking. Our results suggest that growth rates of larger phytoplankton were primarily limited by Fe and/or N, making the availability of these nutrients the main bottom‐up factors contributing to spring bloom termination. In addition, the simultaneous availability of other nutrients could modify bloom characteristics and carbon export efficiency.

Highlights

Following experimental supply of either fixed N (Experiments 1–2) or Fe (Experiments 3–6), chlorophyll a (Chl a) and POC concentrations increased significantly beyond untreated controls (Fig. 2a–l)
In the high latitude Irminger Basin, community-level Fe stress has been found in summer—demonstrated by enhanced apparent photochemical efficiencies of photosystem II (PSII) (Fv/Fm) and chlorophyll a (Chl a) biomass following Fe supply (Nielsdóttir et al 2009; Ryan-Keogh et al 2013)
Low concentrations of Silicic acid (Si) have been implicated as limiting to diatoms, and zinc (Zn), cobalt (Co), and vitamin B12 depletion have been suggested as potentially having coregulatory roles (Lochte et al 1993; Sieracki et al 1993; Ellwood and van den Berg 2000, 2001; Allen et al 2005; Henson et al 2006; Panzeca et al 2008; Leblanc et al 2009; Martin et al 2011)

Summary

Introduction

Following experimental supply of either fixed N (Experiments 1–2) or Fe (Experiments 3–6), Chl a and POC concentrations increased significantly beyond untreated controls (Fig. 2a–l). These responses are consistent with prior observations, likely regulated by a mechanism involving phytoplankton accumulating fluorescent light harvesting complexes that are energetically decoupled from PSII under Fe limited, N sufficient conditions (leading to low Fv/Fm) and their rapid connection to an increasing PSII pool following Fe supply (increasing Fv/Fm and decreasing σPSII) (Behrenfeld et al 2006; Moore et al 2006; Schrader et al 2011; Behrenfeld and Milligan 2013; Ryan-Keogh et al 2013; Browning et al 2014; Macey et al 2014; Browning et al 2017; Li et al 2019; Schoffman and Keren 2019).

Results

Conclusion