Phytoplankton community structure in the VAHINE mesocosm experiment

Karine Leblanc,Karine Leblanc,Karine Leblanc,Véronique Cornet,Hugo Berthelot,Hugo Berthelot,Hugo Berthelot,Martine Rodier,Kendra Turk-Kubo,Mathieu Caffin,Mathieu Caffin,Mathieu Caffin,Anne Desnues,Jules Heliou

doi:10.5194/bg-13-5205-2016

Abstract

Abstract. The VAHINE mesocosm experiment was designed to trigger a diazotroph bloom and to follow the subsequent transfer of diazotroph-derived nitrogen (DDN) in the rest of the food web. Three mesocosms (50 m3) located inside the Nouméa lagoon (New Caledonia, southwestern Pacific) were enriched with dissolved inorganic phosphorus (DIP) in order to promote N2 fixation in these low-nutrient, low-chlorophyll (LNLC) waters. Initially, the diazotrophic community was dominated by diatom diazotroph associations (DDAs), mainly by Rhizosolenia/Richelia intracellularis, and by Trichodesmium, which fueled enough DDN to sustain the growth of other diverse diatom species and Synechococcus populations that were well adapted to limiting DIP levels. After DIP fertilization (1 µM) on day 4, an initial lag time of 10 days was necessary for the mesocosm ecosystems to start building up biomass. However, changes in community structure were already observed during this first period, with a significant drop of both Synechococcus and diatom populations, while Prochlorococcus benefited from DIP addition. At the end of this first period, corresponding to when most added DIP was consumed, the diazotroph community changed drastically and became dominated by Cyanothece-like (UCYN-C) populations, which were accompanied by a monospecific bloom of the diatom Cylindrotheca closterium. During the second period, biomass increased sharply together with primary production and N2-fixation fluxes near tripled. Diatom populations, as well as Synechococcus and nanophytoeukaryotes, showed a re-increase towards the end of the experiment, showing efficient transfer of DDN to non-diazotrophic phytoplankton.

Highlights

Atmospheric dinitrogen (N2) fixation by marine planktonic diazotrophic organisms is a major source of new N to the ocean, and this process is important in sustaining primary productivity in oligotrophic N-limited environments at low latitudes (Capone et al, 2005)
In support of the other main results presented in this VAHINE special issue, this paper presents the evolution of the phytoplanktonic community structure during this experiment
We show that the elevated N2-fixation rates, stimulated by a dissolved inorganic phosphorus (DIP) fertilization in enclosed mesocosms in LNLC waters benefited the entire planktonic community with clear stimulation of both diazotrophic and nondiazotrophic groups mainly observed by Synechococcus and diatom species other than diazotrophs in symbiotic association with diatoms (DDAs), which has clear implications for the efficiency C export fueled by derived nitrogen (DDN)

Summary

Introduction

Atmospheric dinitrogen (N2) fixation by marine planktonic diazotrophic organisms is a major source of new N to the ocean, and this process is important in sustaining primary productivity in oligotrophic N-limited environments at low latitudes (Capone et al, 2005). The increase in primary productivity through diazotroph-derived nitrogen (DDN) has been shown to increase carbon (C) export to depth (White et al, 2013). Diazotrophs have been seen to contribute directly to C export (Subramaniam et al, 2008; Karl et al, 2012) and together these processes are capable of significantly impacting the biological C pump (Dore et al, 2008; Karl et al, 2012). Each group possesses different growth and N2-fixation potential uptake rates and responds differently to environmental factors, depending on their ecological niches

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Conclusion