Phytoplankton community structure in the VAHINE mesocosm experiment

• Main Authors: K. Leblanc, V. Cornet, M. Caffin, M. Rodier, A. Desnues, H. Berthelot, K. Turk-Kubo, J. Heliou
• Format: Article
• Language: English
• Published: Copernicus Publications 2016-09-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/13/5205/2016/bg-13-5205-2016.pdf

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 <i>Rhizosolenia/Richelia intracellularis</i>, and by <i>Trichodesmium</i>, which fueled enough DDN to sustain the growth of other diverse diatom species and <i>Synechococcus</i> 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 <i>Synechococcus</i> and diatom populations, while <i>Prochlorococcus</i> 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 <i>Cyanothece</i>-like (UCYN-C) populations, which were accompanied by a monospecific bloom of the diatom <i>Cylindrotheca closterium</i>. During the second period, biomass increased sharply together with primary production and N₂-fixation fluxes near tripled. Diatom populations, as well as <i>Synechococcus</i> and nanophytoeukaryotes, showed a re-increase towards the end of the experiment, showing efficient transfer of DDN to non-diazotrophic phytoplankton.