Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

Brivaëla Moriceau (1) , Morten Iversen (2, 3) , Morgane Gallinari (1) , Antti-Jussi Evertsen (4) , Manon Le Goff (1) , Beatriz Beker (1) , Julia Boutorh (1) , Rudolph Corvaisier (1) , Nathalie Coffineau (1) , Anne Donval (1) , Sarah Giering (5) , Marja Koski (6) , Christophe Lambert (1) , Richard Lampitt (5) , Alain Le Mercier (1) , Annick Masson (1) , Herwig Stibor (7) , Maria Stockenreiter (7) , Christina L. de La Rocha (1)

1 LEMAR - Laboratoire des Sciences de l'Environnement Marin (LEMAR)
2 AWI - Alfred Wegener Institute for Polar and Marine Research
3 MARUM - Center for Marine Environmental Sciences [Bremen]
4 IBI NTNU - Department of Biology [Trondheim]
5 NOC - National Oceanography Centre [Southampton]
6 DTU Aqua, National Institute of Aquatic Resources
7 LMU - Ludwig Maximilian University [Munich] = Ludwig Maximilians Universität München

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the −Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the −Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency.

Mots clés: biogenic silica; POC; marine snow; zooplankton; mesocosm; Bay of Hopavagen; plankton community; biological pump; ACL

Domaines: Océan, Atmosphère; Biodiversité et Ecologie

Brivaëla Moriceau, Morten Iversen, Morgane Gallinari, Antti-Jussi Evertsen, Manon Le Goff, et al.. Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms. Frontiers in Marine Science, 2018, 5, pp.UNSP 82. ⟨10.3389/fmars.2018.00082⟩. ⟨hal-02382940⟩